CALIFORNIA AGRICULTURAL EXPERIMENT STATION
r
RR I G ATI ON
AND
CULTIVATION
OF
LETTUCE
MONTEREY BAY REGION
EXPERIMENTS
MARCH, 1949 BULLETIN 711
F. J. VEIHMEYER
A. H. HOLLAND
THE COLLEGE OF AGRICULTURE
UNIVERSITY OF CALIFORNIA • BERKELEY
■5
The results of the experiments de-
scribed in this bulletin indicate that 3
irrigations only, the first to ger-
minate the seed, the second at the
time of thinning, and the third 30
days after thinning, will produce a
crop without losses in yield or qual-
ity.
The results also indicate that cul-
tivation after planting, other than
to control weeds, is wasted effort.
THE AUTHORS
F. J. Veihmeyer is Professor of Irrigation and Irrigation Engineer in the Experiment Station.
A. H. Holland formerly Associate in the Experiment Station.
Manuscript submitted for publication April, 1948.
J
The Salinas and Pajaro valleys, in the
Monterey Bay region of California, pro-
duce a large portion of the State's com-
mercial lettuce.
Knott and Tavernetti (10) describe
the methods used to grow head lettuce in
California.
Soil Conditions and Climate
Soil conditions in this area vary
greatly in texture, fertility, and drainage.
Generally, however, the soils used in let-
tuce production range from sandy loams
to clay loams. These are fertile, but often
not well drained.
The climate in this area is mild
throughout the year. During the summer
months, cool ocean fogs blanket the val-
leys. Seldom is there a day when fogs are
not present for at least a few hours. This
has the effect of reducing plant transpira-
tion, and evaporation from the soil sur-
face.
Rainfall occurs almost entirely during
the winter and varies greatly in amount
from year to year. Rain, in appreciable
amounts, is rare from May 1 to Novem-
ber 1.
Purpose of the Experiments
Lettuce growers in this area are not
in complete agreement in their reasons
for irrigation and cultivation practices,
even when climatic and soil conditions
are much the same.
Questions often arise as to how much
water is required by lettuce. It would
seem that more water is applied to the
soil than is used through plant transpira-
tion and soil-surface evaporation com-
bined.
Information is also desired to deter-
mine the effects of different soil-moisture
conditions upon growth, and whether a
crop such as lettuce responds to soil mois-
ture in the same manner as the many
kinds of fruit trees, truck and field crops
previously investigated at Davis (3, 4,
5, 6, 7, 8, and 17).
The reasons given for the selection of
an irrigation program vary so much from
grower to grower that the need for care-
fully planned and executed experiments
is obvious.
Grower-preferences for a particular
schedule of cultivation practices are also
based on diverse reasons. The primary
purpose, doubtless, is to destroy weeds;
but other reasons are often given for till-
age, such as the supposed effect on soil
aeration, microbial activity, and water
conservation.
One of the purposes of the experiments
was to determine by careful field tests the
effect of tillage on the yield of lettuce.
Effects of Soil-Moisture
Conditions
Various effects upon lettuce produc-
tion have been attributed to certain soil-
moisture conditions.
Many growers believe that water ap-
plied to the soil when the heads are ma-
turing is apt to make them soft and loose.
They also believe that when the moisture
supply is plentiful, the leaves are crisp
and a lighter green than when it is at a
low level. Premature production of seed
stalks is sometimes believed to be due to
unfavorable soil-moisture conditions.
Schwalen and Wharton (13) believe
that "the highest yield of quality lettuce
is produced with a uniformly high soil-
moisture content throughout the season
for either the winter or spring crop of
lettuce in the Salt River Valley."
Knott, Andersen, and Sweet (9) re-
mark that on New York peat soils, wide
fluctuations in soil-moisture or heavy
rainfall occurring as the heads mature
may make them puffy. Further, they
found on peat with a high water table
that when the water supply was exces-
sive, plant growth was stunted and head-
ing was delayed and poor, although there
was an absence of tipburn even in warm
weather.
[3
Tipburn and Bolting
Dearborn and Hepler (2) state that
heavy applications of water in hot weather
when the crop is heading will cause soft
heads, tipburn, and bolting; also that
withholding water will have a similar
effect in relation to tipburn and bolting.
Andersen ( 1 ) , working with lettuce on
peat, while believing that water deficiency
is the primary cause of tipburn, states:
"The possibility exists that tipburn re-
sults from some physiological disorder
in the plant which accompanies a reduced
moisture supply to succulent tissues
rather than to the reduced moisture sup-
ply itself."
Cultivation
Studies with cultivation of vegetable
crops have been limited, although the
principles involved seem to be well un-
derstood.
Previous cultivation experiments at
Davis have shown that cultivation in the
absence of weed growth does not con-
serve water (16), and the practice of so-
called "non-cultivation of orchards" is
quite common.
Thompson, Wessels and Mills (14, 15)
and Doneen (3) report experiments with
truck and field crops which indicate that
cultivation largely benefits crops because
of weed control.
GROWER IRRIGATION PRACTICES IN THE
MONTEREY BAY REGION
Commercial plants of lettuce are grown
on raised beds on which two rows of let-
tuce are planted. Occasionally a winter-
grown, spring-maturing crop is planted
on the flat and not irrigated.
How Beds Are Irrigated
Beds are irrigated by running water
down each furrow. The general practice
is to keep a small stream of water running
down the furrows for many hours. When
water is applied to germinate the seed, it
is held in the furrow until it soaks into
the beds. It is not unusual to run the
water for long periods at other irriga-
tions. However, the water is generally ap-
plied more efficiently at later irrigations.
Some growers irrigate before thinning
because this practice seems to be facili-
tated when the soil is moist. Others irri-
gate after thinning because they believe
irrigation helps plants recover from the
disturbance of the soil caused by thinning.
Irrigation runs may vary from a few
hundred to more than 1,000 feet long de-
pending upon slope, soil type, and the
practices of the individual grower.
Frequency of Irrigation
The frequency of irrigation varies con-
siderably among growers. However, in
general, irrigations are more frequent on
light sandy soils than on heavy ones. In
addition, sandy soils, being generally
more permeable, are frequently given
more water at each irrigation than are
heavier soils. As many as 6 to 8 irriga-
tions have been made on one crop. In
other cases, as few as 2 or 3 irriga-
tions have been made on crops not re-
ceiving moisture from rainfall. A fairly
common practice is to irrigate a crop
lightly after the first cutting has been
made when 2 or 3 more cuttings are an-
ticipated.
[4]
J
GROWER CULTIVATION PRACTICES
The usual tillage practices previous to
planting are plowing, disking, and listing.
Sled-type implements with planters at-
tached are then used to shape the beds
(10).
Cultivation often begins shortly after
the plants have two true leaves.
A more common practice is to make
the first cultivation shortly before the
plants are thinned.
The first cultivation is generally shal-
low and done with side and top knives
together with shovels. The blades of the
knives are so set that they will cut weeds
between the two rows and on the sides of
each bed. Two and four-bed tractor-
powered cultivators are most commonly
used.
Following the first cultivation, the beds
are frequently chiseled. Two chisel-like
blades are drawn through the soil to a
depth of 4 to 6 inches between the two
rows of each bed. After thinning, when
the plants become larger, 2 to 6 cultiva-
tions are often made. These later practices
may stir the soil from 1 to 3 inches deep.
Seven irrigation experiments, 1938-39,
and 5 tillage experiments, 1937-40, were
conducted in the Pajaro Valley. Two irri-
gation experiments were conducted in the
Salinas Valley in 1940.
LOCATION OF EXPERIMENTS
A small area of each planted field was
used. In several cases, two consecutive
experimental crops were grown in the
same area. Irrigation and cultural prac-
tices were under the authors' direction.
Coileoe £a
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JUNCTION LA,
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7 frr/yo/ion -/938
/^. ^
Sca/e of Af//es
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Irr/oat/on\
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Fig. 1.— Location of irrigation and cultivation experiments in the
Pajaro and Salinas valleys.
[5]
GENERAL PLAN OF THE IRRIGATION EXPERIMENTS
The general procedure was much the
same in all cases.
Seed-bed preparation and planting was
done by the cooperating growers as di-
rected.
Following the usual practice, the field
was plowed, disked, cultivated deeply,
and then listed.
The seed was usually planted on beds
about 6 inches high. Two rows were al-
ways planted on each bed. The beds were
usually spaced 42 inches from center to
center, and the rows on the bed were 14
inches apart. The crop was thinned so
that the plants were spaced about 1 foot
apart in the row.
Lettuce is designated as a spring, sum-
mer, or fall crop according to the time of
maturity. In the nine irrigation experi-
ments, 5 were summer and 4 were fall-
maturing crops. All summer and 2 fall
crops were planted to Imperial 847 and 2
fall crops to Imperial D lettuce.
The usual procedure used in our irri-
gation experiments of basing the treat-
ments on soil-moisture conditions and not
number and frequency of irrigations was
found to be impracticable during the pre-
liminary experiments of 1937. As is
brought out later, it was found that the
root system of lettuce in the field seemed
to be such that soil-moisture samples did
not give correct indications of the avail-
ability of water to the plants.
Each experiment consisted of several
small plots which were given 3 to 4 treat-
ments; that is, some experiments had 3
differential treatments while others had 4.
The treatment differences, including
frequency, amount, and distribution of
water applied, are discussed later in de-
tail. Briefly stated, the treatments were
designated as A, B, C, and D. Treatment
A received the least number of applica-
tions, had the longest period without
irrigation, and may be considered the
driest plot. Treatment B received one
more irrigation than A in 6 of the tests;
in 2, the same number; and in 1 it re-
ceived 2 more applications than did A.
But in every case the length of time with-
out irrigation was shorter than A. Treat-
ments C and D were irrigated more
frequently than either A or B. In all but
one case, D received more irrigations than
C. In that case it received the same
number
Arrangement of plots was randomized
and varied in the different experiments.
Each plot usually consisted of 3 beds 50
feet long bounded on both sides by a
guard bed (fig. 2).
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Fig. 2.— Typical arrangement of plots used in the experiments.
[6]
I
J
Experimental Irrigation
Practices
The first irrigation was applied within
a day or two after planting to germinate
the seed.
There was one exception to this prac-
tice. One summer crop was not irrigated
since rainfall wetted the soil sufficiently
for seed germination.
A small stream was run down the fur-
rows for several hours until water soaked
up the sides and into the beds, finally
reaching the seed. Care was taken to pre-
vent the stream from topping the beds
and washing the seed out of the soil.
By this method, the soil is wetted with-
out causing a crust to form. Crusting
might occur if the beds were flooded.
Crusting of the soil before the seedling
emerges is believed to be detrimental to
obtaining a good stand.
Thinning
In some of our experiments we irri-
gated before thinning. In others, just
after or not at all.
Following thinning, some plots re-
ceived no further irrigation while others
were irrigated once, twice, or three times.
In most of these irrigations water was not
allowed to cover the beds, but in some
others, the beds were purposely flooded
to see if flooding was harmful.
The plants were thinned 3 to 5 weeks
after planting, depending upon their rate
of growth.
Cultivation
One shallow but thorough cultivation
was made on all crops shortly before
thinning, except in the 1940 experiments.
Following thinning, the plots were fur-
rowed and not touched by a cultivator
thereafter. Weeds not destroyed by culti-
vation were hoed by hand.
Data Obtained
Numerous data were obtained during
the experiments. These included the de-
termination of soil-moisture, moisture
equivalents, permanent wilting percent-
ages, measurements of irrigation water,
measurement of plant growth, yields, and
quality. Evaporation, humidity and tem-
perature records were obtained in the
field during some of the experiments.
Measurements of Irrigation
Water
In most cases irrigation water was
measured at the plots with a 1-foot rec-
tangular contracted weir.
In cases where this was not feasible,
the amount of water applied was deter-
mined by timing the flow of water from
the supplying pumps having rated ca-
pacities.
Moisture Determinations
Moisture determinations were made
from samples gathered by a soil tube.
These samples were collected at about
weekly intervals during the growth of the
crops.
Three samples of 6-inch increments
were composited. These were taken to a
depth of 3 feet on each of 2 plots for each
treatment. In other words, there were 3
sampling locations at the plant and 3 in
the center of the furrow (fig. 3).
The moisture determinations were
made by oven drying the soil at 110° C
and calculating the percentage loss on a
dry weight basis.
Moisture Equivalents and
Permanent Wilting
Percentages
These were determined on samples col-
lected in 1-foot increments from 2 plots
of treatment A for each experiment, and
composite samples were made for each
of the 2 plots by collecting samples from
the upper, center, and lower sections.
Moisture equivalents and permanent wilt-
ing percentages are indicated for each
depth of soil on the soil-moisture charts
which follow.
[7]
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c/e/Phs^Jjce/p/er of furrow)
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Fig. 3.— Sampling locations at the plant.
Average Weight of Plants
The average weight of about 100 plants
cut at each sampling date from each treat-
ment was used as an index of growth. Ten
to 20 plants were taken at intervals of
about one week from each end of all plots.
Yields
These were obtained by cutting, trim-
ming, and weighing individually all mar-
ketable heads from each plot. Trimmings
from each plot were weighed. The crops
were harvested in from 2 to 5 cuttings.
At the last cutting, all non-marketable
plants were cut, counted, and weighed,
except in 2 cases.
The areas of the plots were measured
just before cutting since the original areas
had been reduced by cutting plants from
the plots each week.
DESCRIPTIONS OF THE IRRIGATION EXPERIMENTS
The nine irrigation experiments were
designated as:
1. Union Ice Co., Summer, 1938.
2. Union Ice Co., Fall, 1938.
3. Gerrard, Summer, 1938.
4. Gerrard, Fall, 1938.
5. Capitanich, Summer, 1939.
6. Rowe, Summer, 1939.
7. Rowe, Fall, 1939.
8. Stirling, Summer, 1940.
9. Stirling, Fall, 1940.
The above names are those of the
ranches on which the experiments were
conducted.
The locations of these ranches have
been shown in figure 1.
Union Ice Company,
Summer, 1938
The plots occupied an area of about
125 by 225 feet.
The soil is classed as a Botella silty clay
loam.
During the experiments, a water table
stood about 3 feet from the surface.
Water tables within at least 6 feet of the
surface are common in the lettuce grow-
ing areas of the Pajaro and Salinas val-
leys.
The land had not been cultivated
within 2 years previous to the experi-
ment. This resulted in a dense sod condi-
tion. It was plowed, worked into a seed
bed, planted to Imperial 847 lettuce, irri-
gated May 11, and thinned June 10.
Due to the clay-like nature of the sur-
face soil throughout most of the area, the
beds were extremely cloddy.
Twenty-six plots of three 50-foot ex-
perimental beds were laid out and 4 treat-
ments of different irrigation frequencies
were planned. These treatments were des-
ignated as A, B, C, and D.
[8]
Treatment A was irrigated once, at the
time of planting.
Treatment B was irrigated 3 times.
Treatment C was also irrigated 3 times
but at different intervals.
Treatment D was irrigated 5 times.
Dates of irrigation, amounts of water
applied, and weights of lettuce plants for
each treatment are shown in figure 4.
Weekly growth data were obtained by
cutting and weighing 40 plants from each
of 3 plots for each treatment (table 1).
The first weighings were made 4 days
after the lettuce was thinned and the last
about 5 weeks later.
Soil-moisture conditions for 2 plots of
each treatment were determined by col-
lecting samples at weekly intervals to a
depth of 2 feet or more.
Moisture conditions at the plant (as
obtained by averaging data of the 2 plots
for each treatment sampled) are shown
in figure 5. By the time the crop was
mature, many plants had been destroyed
to determine growth. No yields, therefore,
were taken on those plots from which
growth data were obtained.
Yield data were secured from 4 plots
in treatment A; 3 plots in treatment
B ; 4 plots in treatment C ; and 3 plots in
treatment D. Table 2 gives the average
yields of these plots by treatment and
shows the total number of plants and
marketable heads harvested.
In each experiment, all heads classed
as marketable were free of disease, firm,
and would probably have met minimum
Federal standards for head lettuce.
600
500
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£ 400
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200
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Fig. 4.— Dates of irrigation, amounts of water applied and weights of
plants, Union Ice, summer, 1938.
[9]
Table 1: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 40
LETTUCE PLANTS PER PLOT, UNION ICE, WATSONVILLE, SUMMER 1938
Treatment
Plot
Dates Sampled
June 14
June 20
June 28
July 5i
July 11»
July 18i
Aug. 1, 22
A
17
2
15
3
3
3
12
6
9
11
3
6
63
21
45
200
52
84
345
133
135
Average ....
3
9
7
43
112±5.3
204 ±9.2
281
B
13
19
12
4
3
4
13
7
8
11
7
10
72
36
59
194
130
175
405
212
355
Average
4
9
9
56
166 ±4.8
324±8.2
509
C
9
24
20
4
5
3
15
16
10
17
15
10
90
113
77
235
301
155
419
440
326
Average ....
4
14
14
93
230 ±6.9
395 ±8.6
446
D
18
3
16
3
2
4
9
8
8
12
8
8
75
54
56
178
117
150
396
348
315
Average
3
8
9
62
148±5.2
353 ±7.9
559
1 The probable errors are based on weights of individual plants.
1 Weights on August 1, 2 are the mean of the entire crop harvested for final yields. The dates represent
the mean date of maturity. Mean date of maturity is discussed in detail later. This procedure of reporting data
is followed in all other tables of plant weights.
Table 2: NUMBERS AND WEIGHTS OF PLANTS, UNION ICE, WATSONVILLE,
SUMMER 1938
Treatment
Total
plants
Average
total weight
per plant 3
Lettuce
per acre 1
Market-
able
heads
Average weight
marketable
trimmed
heads 2
Marketable
lettuce
per acre
A
B
C
D
978
836
1108
814
grams
281 ±29.0
509 ±69.6
446 ±22.9
559 ± 3.7
pounds
11,900
22,270
19,660
24,650
per cent
8.2
50.4
42.1
65.6
grams 4
371±5.3
450 ±4.0
383 ±2.0
436 ±2.6
pounds
1,340 ±19
10,010 ±89
7,100±37
12,610 ±75
i Based on 20,000 plants per acre.
2 Marketable heads are those cut which weighed over 300 grams.
3 Probable errors based on average weight per plant per plot in each treatment.
* Probable errors are based on weights of individual heads. This applies to all tables of head weights
except tables 16 and 19.
[10]
i
In this experiment, the minimum size
was set at 300 grams, but in all other ex-
periments reported herein, heads classed
as marketable were graded in the field to
be larger than 75 per standard container
classification. Three hundred grams,
however, is about the minimum weight
for heads classified as 75 per standard
container.
The crop was harvested in 3 cuttings,
July 26-27, 30-31, and August 1-2. Two
days were required for each cutting.
Treatment D had a significantly larger
yield of marketable lettuce than the other
treatments.
The yield from C was significantly
larger than that from A.
The yield from B was larger than from
A and C.
It should be noted that when analyz-
ing the results of the experiments de-
scribed in this bulletin, differences are
considered to be significant with odds of
30 to 1 . For these odds, the differences
must be about 3.2 times their probable
error.
Union Ice Company, Fall, 1 938
This experiment was conducted in ap-
proximately the same area as the summer
experiment.
Immediately following the summer
crop, the soil was disked, cultivated,
ridged, planted to Imperial D lettuce, irri-
gated August 5 for seed germination, and
thinned August 29.
Twenty-eight plots were laid out in 4
treatments (A, A-l, C, and D) of 7 plots
each.
Treatment A was irrigated at the time
of planting and once more 26 days later.
Treatment A-l was irrigated the same
as A. A-l was to have been a B treatment,
but a % inch rainfall 58 days after plant-
ing made the proposed irrigation un-
necessary.
Treatments C and D each received a
third irrigation 52 and 42 days, respec-
tively, after planting.
The dates of irrigation and amounts
M.f. 2
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Treatments: A C. O.
Fig. 5.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Union
Ice, summer, 1938. M.E. and P.W.P. are the
moisture equivalent and permanent wilting
percentage for each depth.
of water applied to each treatment are
shown in figure 6.
The soil-moisture data obtained from
2 plots of A, 2 of C, and 2 of D are shown
in figure 7.
Growth was determined by cutting and
weighing 15 plants from each plot and
calculating the average plant weight for
each treatment (table 3 and fig. 6).
The crop was harvested in 3 cuttings,
October 24-25, 26-27, and 28.
Yield data, including the total number
of plants and marketable heads harvested
for each treatment, are shown in table 4.
The yield of marketable lettuce per
acre from C is greater than that from any
of the other treatments.
The yield for A-l is greater than for
D or A, even though A-l and A were irri-
gated the same.
The yield for D is greater than A.
[ii
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August
30
19 29
September
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October
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Dotes of Irrigation
o - Dates of tfatnfatt
Fig. 6.— Dates of irrigation, amounts of water applied and weights of plants,
Union Ice, fall, 1938.
29
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May June
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Fig. 7.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Union
Ice, fall, 1938. M.E. and P.W.P. are the mois-
ture equivalent and permanent wilting per-
centage for each depth.
[12]
Table 3: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 15
LETTUCE PLANTS PER PLOT, UNION ICE, WATSONVILLE, FALL 1938
Treatment
Plot
Dates sampled
Sept. 16
Sept. 21
Sept. 29i
Oct. 72
Oct. 263
A
2
8
10
15
17
23
25
20
22
22
25
39
22
35
52
40
51
48
71
41
69
105
96
115
116
142
117
133
240
184
217
225
290
216
304
613
499
555
543
589
509
560
Average
26
53
118±3.8
239 ±6.3
553 ±10.3
A'
3
6
12
13
19
21
28
25
22
24
31
27
35
33
132
125
117
185
118
190
124
236
196
221
293
282
360
234
594
542
575
631
557
621
572
Average
28
142±5.9
260 ±7.9
585 ±8.3
C
1
7
9
14
20
22
24
19
26
29
25
39
20
36
158
100
158
155
128
101
209
285
201
324
274
282
253
404
616
571
615
568
638
574
689
Average
28
144 ±6.9
289 ±8.0
610±11.3
D
4
5
11
16
18
26
27
20
32
24
29
31
24
41
52
80
44
47
57
45
96
110
146
136
130
149
128
172
229
296
224
227
387
215
296
505
619
542
491
553
540
653
Average ....
29
60
139 ±4.8
268 ±8.2
558 ±15.0
1 Probable error for Sept. 29 was determined from mean weights of 4 groups of 4 plants per plot instead
of individual plants.
2 Probable error based on weights of individual plants.
3 Probable errors based on average weight per plant per plot in each treatment.
[ 13 ]
Tabic 4: NUMBERS AND WEIGHTS OF PLANTS, UNION ICE, WATSONVELLE,
FALL 1938
Treatment
Total plants
Average
total weight
per plant 2
Lettuce
per acre 1
Market-
able
heads
Average weight
marketable
trimmed
heads 2
Marketable
lettuce
per acre
A
A'
C
D
443
517
563
484
grams
553 ±10.3
585 ± 8.3
610±11.3
558±15.0
pounds
24,380
25,790
26,900
24,600
per cent
90.5
94.4
90.9
92.4
grams
419 ±2.0
429 ±2.0
463 ±2.7
434±2.4
pounds
16,710 ± 80
17,860 ± 83
18,560±108
17,680 ± 98
» On the basis of 20,000 plants per acre.
2 Probable errors based on average weight per plant per plot in each treatment.
700
^600
I
A 500
\
k 400
%3°°
Y°°
WO
c
It*
/
/:.
A
/
i
/ /
/
/ /
1
// /
A
V
zo
& A.
I B.
JO
Apr/7
6.o"
10
eo
May
JO
/9
June
£9
02
9 J9
6.0"
4.0"
02"
3.8
6.Q"
_ . — _ i
4LO"
I f£l
£g u m
__. — _-
Jg
/£"
/.f>"
Dates of Irrigation.
o - Dates of Pain fa//.
Fig. 8.— Dates of irrigation, amounts of water applied and weights of plants,
Gerrard, summer, 1938.
[14]
Gerrard, Summer, 1938
These experiments were conducted on
a level, rich alluvial loam of the Pajaro
series. It drains slowly but had no water
table within 6 feet of the surface.
On April 26, the field was planted to
Imperial 847 lettuce, irrigated, and
thinned May 26.
The area was divided into 6 plots of 3
treatments (A, B, and C).
Treatment A was irrigated twice, in-
cluding the initial application for seed
germination.
B and C were irrigated 3 and 6 times,
respectively.
The dates and amounts of water ap-
plied are shown in figure 8.
Soil-moisture samples were collected
from all plots. Average moisture condi-
tions at the plant are shown for each
treatment in figure 9. Soil samples for
determining permanent wilting percent-
ages were taken in 1-foot increments.
These percentages are shown in figure 9.
Beginning May 26, 20 plants from each
plot were cut and weighed each week.
Data are shown in table 5. Average plant
weights per treatment are plotted in fig-
ure 8.
The crop was harvested in 4 cuttings,
July 8, 11, 14, and 18.
Yield data are presented in table 6.
The yield of marketable lettuce from
C is significantly greater than that from
A but not from B.
'A' yielded significantly less than the
other two treatments.
Gerrard, Fall, 1938
This experiment was conducted on the
same field as the summer one.
Imperial 847 seed was planted and irri-
gated August 1, and thinned August 20.
Irrigations, including rainfall, and the
average weights of plants for 3 treatments
(A, B, and C) are shown in figure 10.
Including the initial seed wetting, A
•S 25
,
m M£.
29.5
1
-/'//ft.
-Zi
P.W.k //.a
26 2 9/6 23 JO 7 /4
May June Ju/y
Treatments: d. B C.
Fig. 9.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Gerrard,
summer, 1938.
was irrigated twice, and B and C were
irrigated 3 and 5 times, respectively.
Soil-moisture conditions were deter-
mined by taking samples from 2 plots of
each treatment. Figure 11 shows these
conditions and the permanent wilting
percentages.
Growth was measured by weekly cut-
ting and weighing 15 plants from each
plot (table 7 and figure 10).
The crop was harvested in 3 cuttings,
October 14, 17, and 21.
Yields are shown in table 8.
Treatment C had significantly higher
yields of marketable lettuce than A or B,
and B greater than A.
[15]
Table 5: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 20
LETTUCE PLANTS PER PLOT, GERRARD, WATSONVILLE, SUMMER 1938
Treatment
Plot
Dates sampled
May 26
June 3
June 9
June 16
June 23
June 30i
July 12, 13
A
1
6
14
18
41
54
78
127
171
236
317
384
455
603
Average
3
16
48
102
204 ±17.8
350 ±13.7
553 2
B
3
4
16
17
37
44
81
127
211
275
435
471
640
694
Average
3
17
41
104
243±10.6
453 ±19.1
667 2
C
2
5
14
16
45
47
107
114
266
257
505
402
674
700
Average
3
15
46
110
261±2.7
453 ±16.1
690 2
1 Probable errors are based on weights of individual plants.
2 Probable errors could not be obtained from the weights taken on July 12 and 13.
soo
1
^ 400
k
\
<5
f
§>/00
i
tc
Is
t/
t*
/
//
J]
Y
J/
Jc/fo
1
£0
JO
duyusf
/9
^September
o./"
29
9 /9
October
08"
3 -^
^^ L_.
J£ £^"
J'' £.P" O.J" V.9'
— Dates of Zrr/yof/on
— #o//?fo//
0.8'
j.o" o.jr
Fig. 10.— Dates of irrigation, amounts of water applied and weights of plants,
Gerrard, fall, 1938.
[16]
Table 6: NUMBERS AND WEIGHTS OF PLANTS, GERHARD, WATSONVILLE,
SUMMER 1938
Treatment
Total
plants
Average
total weight
per plant
Lettuce
per acre
Market-
able
heads
Average weight
marketable
trimmed
heads
Marketable
lettuce
per acre 1
A
130
142
138
grams
553
667
690
pounds
24,380
29,410
30,420
per cent
91.5
95.8
97.8
grams
390±6.2
466 ±6.6
467 ±6.2
pounds
15,720 ±250
B
19,660 ±279
C
20, 120 ±267
1 Based on 20,000 plants per acre.
Table 7: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 15
LETTUCE PLANTS, PER PLOT, GERRARD, WATSONVILLE, FALL 1938
Treatment
Plot
Dates sampled
Aug. 25
Sept. 2
Sept. 8
Sept. 15
Sept. 22i
Sept. 30i
A
4
7
3
2
15
10
46
35
108
134
230
237
434
355
Average
3
13
41
121
234±8.2
395 ±17.6
B
2
5
2
3
13
14
34
41
99
100
246
263
493
411
Average
3
12
38
100
255 ±10.5
452 ±19.5
C
1
6
2-
3
13
11
40
43
120
118
225
249
532
490
Average
3
12
42
119
237 ±9.4
511±21.8
1 Probable errors based on weights of individual plants.
Table 8: NUMBERS AND WEIGHTS OF PLANTS, GERRARD, WATSONVILLE,
FALL 1938
Treatment
Total
plants
Average weight
per untrimmed
marketable
head
Marketable
heads
Average weight
marketable
trimmed
heads
Marketable
lettuce per acre x
A
B
C
327
277
365
grams
755
784
764
per cent
68.8
69.7
68.2
grams
518±4.8
547 ±4.5
605±5.5
pounds
15,700 ±146
16,800 ±138
18,190±165
i Based on 20,000 plants per acre.
[17]
M£.
29.0 \
o-% rr.
^<b—
^'^■'
******^
—
■--».
P.W.P 15.5
ML.***
•
-J_
/z
•/ft.
PW.P /5
5
r~
r
M.E. 29.1^
■S?-*-
«s
._/ -/%rt.
PW.P /7.8
^H^=
ME. 29 7
^z^L
'~j£z*JL
PW.P /7.
8
— -
M.E.30.0
I £- 9</, ft
P">^
PW.P 17.
2
Fig. 11.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Gerrard,
fall, 1938. M.E. and P.W.P. are the moisture
equivalent and permanent wilting percentage
for each depth.
Capitanich, Summer, 1939
The soil upon which this experiment
was conducted is classed as a Botella silty
clay loam. It is highly productive, though
a 3-foot water table was present during
the experiment.
Previous to planting, the land had been
cover-cropped with vetch and fertilized
with 2% tons per acre of chicken manure.
The field was planted to Imperial 847
lettuce April 22, and thinned May 30.
The area was divided into 28 plots
which were given one of 4 treatments.
A received 2 irrigations, including the
one at planting.
B and C received 3 irrigations.
D received 4 irrigations.
Dates of irrigation, amounts of water
applied, and average weights are shown
in figure 12.
Growth was determined by cutting, at
approximately weekly intervals, and
weighing 10 plants from each plot
(table 9).
Treatment B was irrigated in the usual
manner, that is by furrows without flood-
ing the beds, but at the irrigation made
on June 12, the beds were intentionally
flooded. Since the beds were quite high, it
was difficult to flood them, but sufficient
water was applied so that the soil surface
appeared sealed.
During the period June 13-19, the
temperature and evaporation conditions
were unusually high.
Soil-moisture conditions under the dif-
ferent irrigation treatments were deter-
mined by sampling from 2 plots of each
treatment. These, and the permanent wilt-
ing percentages, are shown in figure 13.
The crop was harvested in 4 cuttings
made July 2, 11, 14, and 17. Due to tip-
burn damage, there were no marketable
heads harvested after the second cutting.
Yields are shown in table 10.
The yield of marketable lettuce from
B is significantly higher than the others;
that for D higher than from A but not
from C ; and from A lower than the others.
18]
850
800
700
\600
k
^500
1
1
^00
100
p
i
if'
IF
ll
t
ft'
ill
9!
i
1
/
iff
if
*
3
if
if
20 JO
Jpr//
/<?
I'
Tf~
£0
May
30
/9
June
£9
8
Jlt/y
O «sr
V
2.z-
fl(?"
j.r
55"
— ^£
0.*"
.£/"
5.5"
.-».- ......
JZ.2"
..... — -4-
o.e
/.5"
2.7"
• - Da/es of Irr/gaf/or?.
Q-Dafes of Pa /r? fa//
Fig. 12.— Dates of irrigation, amounts of water applied and weights of plants,
Capitanich, summer, 1939.
[19]
Table 9: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 1
LETTUCE PLANTS PER PLOT, CAPITANICH, WATSONVILLE, SUMMER 1939
Treatment
Plot
Dates sampled
June 6
June 12
June 192
June 26 1
July 10
A
22
2
10
18
5
20
14
18
23
23
19
22
22
18
41
53
57
56
63
55
49
144
112
198
137
145
142
150
281
288
278
308
329
301
357
635
644
590
640
708
685
634
Average ....
21
53
147±6.9
306 ±8.8
648
B
15
23
3
11
26
13
7
16
23
22
22
27
19
26
50
48
55
65
68
52
60
119
106
162
159
176
134
131
266
275
340
419
392
349
331
735
698
744
848
844
735
776
Average
22
57
141±5.5
339 ±9.5
768
C
8
16
24
4
12
27
21
25
18
24
23
22
23
14
61
47
48
60
72
56
59
148
118
149
150
180
154
136
252
263
337
350
410
350
316
694
698
789
826
826
726
685
Average
21
58
148 ±6.0
325 ±9.2
748
D
1
9
17
25
19
6
28
19
17
16
23
30
17
21
54
49
48
69
68
54
67
161
161
146
183
164
184
166
378
349
285
378
360
330
308
766
753
717
894
875
962
847
Average
20
58
166 ±4.0
341±9.8
831
1 Probable error based on weights of individual plants.
2 Probable error based on average of two groups of 5 plants for each of 7 plots per treatment.
[20
<8 26
X *<
zo
/W. £*. 7
O-'/z-ff.
\
5
>^
is.
^ \
^s^,
"V
■**•».„
■^^^
/?KK/r /o. ^
N
Mf.2>8.7
/z-/-rt.
^
*v %
^
— — — .
X
^»^
PW.R /6. 9
, i
26
34
32
30
28
26 2
May
Treatments : __
s
M£.30.7
t/2-
2 ft.
^fcj
^5^r—
*^»
'
PW.f-
> 19.7
/2 /9 28 5
June Ju{</
.A- B. C D.
Fig. 13.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Capi-
tanich, summer, 1939. M.E. and P.W.P. are
the moisture equivalent and permanent wilt-
ing percentage for each depth.
Rowe, Summer, 1939
These experiments were conducted on
soil classed as Elder loam. It is deep and
well drained. Imperial 847 lettuce was
planted and irrigated May 11. Since the
beds were quite high and not completely
wetted by the first irrigation, a second
irrigation was made May 20. Thinning
was done June 5. Twelve plots were laid
out into 3 treatments (A, B, and C). All
plots were irrigated 4 days before thin-
ning.
After thinning, A was irrigated once
and all beds flooded, water covering the
top of the beds. The beds in B and C were
not flooded. B was given one irrigation,
but later than A. C was irrigated twice.
Dates of irrigation, amounts of water
applied, and average weights of plants
are shown in figure 14.
Growth data were obtained by weekly
cutting and weighing 25 plants for each
plot (table 11).
Soil-moisture conditions were obtained
by weekly sampling from 2 plots of each
treatment. These conditions and the per-
manent wilting percentages are shown in
figure 15.
Yields were obtained from 3 cuttings
made July 22, 25, and 28 (table 12) .
The yield of marketable lettuce from
treatment C is significantly greater than
for A or B, and that from B is greater
than from A.
Table 10: NUMBERS AND WEIGHTS OF PLANTS, CAPITANICH, WATSONVILLE,
SUMMER 1939
Treatment
Total
plants
Average
total weight
per plant 1
Lettuce
per acre 2
Market-
able
heads
Average weight
marketable
trimmed
heads
Marketable
lettuce
per acre
A
B
C
D
1563
1597
1716
1611
grams
648 ± 9.8
769±14.7
749 ±16.0
831 ±22.5
pounds
28,570
33,910
33,030
36,640
per cent
47.6
69.8
63.5
58.9
grams
509 ± 9.3
561±10.7
542 ± 8.8
590±13.3
pounds
10,670 ±195
17,240±329
15,170±246
15,320 ±345
1 Probable errors based on average plant weights per plot in each treatment.
* Based on 20,000 plants per acre.
[21]
-Dates of frr (pat/on
\?.o\
o- Dates of /?a in fa//
Fig. 14.— Dates of irrigation, amounts of water applied and weights of plants,
Rowe, summer, 1939.
[22]
Table 1 1: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 25
LETTUCE PLANTS PER PLOT, ROWE, WATSONVILLE, SUMMER 1939
Treatment
Plot
Dates sampled
June 12
June 23
June 30 »
July 10i
July 23, 24
A
1
4
9
12
7
9
11
7
82
76
89
82
213
210
221
186
593
654
540
511
840
896
717
803
Average
9
82
208±1.4
575±11.6
814
B
2
5
7
10
7
7
8
8
69
70
76
62
193
178
223
147
614
566
571
543
1055
1085
1003
1040
Average. . . .
7
69
185 ±5.0
573 ±11.3
1046
C
3
6
8
11
8
8
9
8
90
65
87
70
222
174
231
178
659
636
697
578
1097
1151
1109
1063
Average
8
78
201 ±4.9
643±11.1
1105
1 Probable errors based on weights of individual plants.
Table 12: NUMBERS AND WEIGHTS OF PLANTS, ROWE, WATSONVILLE,
SUMMER 1939
Treatment
Total
plants
Average
total weight
per plant 1
Lettuce
per acre 2
Market-
able
heads
Average weight
marketable
trimmed
heads
Marketable
lettuce
per acre
A
B
C
1137
1083
1106
grams
814 ±25.4
1046±11.5
1105 ±23.0
pounds
35,890
46,120
48,720
per cent
76.0
80.7
86.9
grams
631 ±2.7
771 ±3.2
817±3.2
pounds
21,160± 90
27,420±114
31,300 ±123
1 Probable errors based on average plant weight per plot in each treatment.
2 Based on 20,000 plants per acre.
[23]
25
J*.£.28.j\
1
1
20
--^
—
<X
P.W.P (7.0
— —
30
25
20
*25
% 20
^0
/^<?<*J~~P^;
J&'irt
/?W>? (7.0
— — — .
— ■-=*■- — =-
^■^^^
J 1-1/2 ft
M.E.29.3
P.W.P (7.Z
^^
= — L^r!~
ct^T-
. l/z-2ft.
M.5.29.J
*^-
'
^^
r*^
P.W.P 1
?,?
^>
V
2-2/2 ft.
Ml '28./
^^^^s
.— •-"
P.W.P. /
7.5
1
Af.£.28.(
.
_
2/z'Jft.
>-■ —
^^=r
~~-~^Il~—
PW.P /
7T5
~~~-
^^ — .
<5
/<f i>J J<? A? /7
June S u d/
?<7//7
>enfs • A ft . . C
Fig. 15.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Rowe,
summer, 1939. M.E. and P.W.P. are the mois-
ture equivalent and permanent wilting per-
centage of each plant.
Rowe, Fall, 1939
This experiment, made in the same
field as the summer experiment, and with
the same lettuce, was given 3 treatments
(A, B, andC).
It required two irrigations to wet the
beds for seed germination, one on August
9, the date of planting, and the other on
August 19. A second irrigation probably
would have been unnecessary if the soil
had been thoroughly wetted at the first
irrigation.
A third irrigation was made September
8 to all plots. They were thinned Septem-
ber 12.
Treatment A received one more irriga-
tion, 44 days after planting.
B also received one more irrigation,
but it was 63 days after planting.
It took 91 days before any heads were
ready for harvest, a longer period than
[24
that for any other crop during the irriga-
tion experiments.
Irrigations were made in the usual
manner except for the last application of
the A and B treatments. In these, the en-
tire beds were quickly flooded by sub-
merging them with a large stream of
water.
Dates of irrigation and amounts of
water used are shown in figure 16.
Growth was determined by weighing
20 plants from each plot (fig. 16 and
table 13).
During the period September 14-26,
the temperature and evaporation condi-
tions were extremely high.
Moisture determinations were made on
soil samples from 2 plots of each treat-
ment. These, and the permanent wilting
percentages for the summer crop (also
used for the fall crop), are shown in fig-
ure 17.
]
/oso
/ooo
900
600
600
500
X
P*oo
5 JOO
£00
/OO
oo
Hi. 9
1
:•
*j
If
1!
^7
1
A
h
11 /
I
V
it
ft
tjf
ij
V
III
//
/
ft
1
V
/
MO"
fe C -
/9
/ftsyasf
so" 4.p"
I/O 20 JO /O 20
September October
4.0" 2.8" 0.5"
JO
4-.Q'\
2J"
O.J
•-Dates of Irrtyat/on
0^5" I I J. % 0"
Dotes of Po/nfaf/
9 /4
A/or.
Fig. 16.— Dates of irrigation, amounts of water applied and weights of plants,
Rowe, fall, 1939.
[25]
Three cuttings were made: the first on
November 8, 9, and 10, the second on
November 14, and the last on Novem-
ber 17. Yields are given in Table 14.
The yield of marketable lettuce is sig-
nificantly greater for C than for B but
not for A, and that for A is greater than
that for B.
Shortly before the crop was harvested,
a ditch was dug across a bed in treatment
A so that the face bisected the tap root of
a lettuce plant. The face was marked off
into twenty-four 2-inch squares from
which a sample of soil was taken by cut-
ting 2 inches into the face.
After obtaining the moisture losses
from the 2-inch cube samples, they were
screened through a 2-millimeter screen.
The weight of the soil particles larger
than 2 millimeters was subtracted from
the wet and dry weights of the respective
samples, and moisture determinations
were determined on that basis.
Moisture equivalents were determined
on the screened samples. Using the data
for the moisture equivalent and perma-
nent wilting percentage determinations
made for the experiment, the average
r
^ 20
J\^- /rA .A^Z
- k~u._
— ^_
M.£. 29.J
--"^
,y' ~~"
^--i
PWP
17. Z
i
>
* — ..
1 mF
ea./
■^=--==.
PWP /75
Z-2'/zft.
1
\_.^J_^S.I ,
H—
r— - r
PW.P. IT5 1
1
1
C /4
Sept.
Freafmenfj .
9 /6
Oct
-A
6
Mov.
Fig. 17.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Rowe,
fall, 1939. M.E. and P.W.P. are the moisture
equivalent and permanent wilting percentage
of each plant.
r Sea
' Surfa
ce
7
-.6
.o
/.j\*.e
A 2
.O
2.5
2.9
J. 8 J4.2
4.0
J./
4.4
4.4
5Z
2.7
5.0
4.6
6.7
69
7/
6.7
7.2
63
^_ /2"
"
Furrow
Fig. 18.— Distribution of moisture under a lettuce plant 35 days after irrigation in treatment A.
The values are the percentages of moisture above the permanent wilting percentage, and in one
case below this percentage as is indicated by the minus sign.
[26]
Table 13: AVERAGE WEIGHT PER PLANT IN GRAMS BASED ON SAMPLES OF 20
LETTUCE PLANTS PER PLOT, ROWE, WATSONVILLE, FALL 1939
Treatment
Plot
Dates sampled
Sept. 19
Oct. 2
Oct. 9i
Oct. 16^
Oct. 27i
Nov. 11»
A
1
4
6
10
12
16
19
10
13
16
14
21
14
16
86
111
108
114
117
104
112
159
208
219
225
235
215
213
260
381
386
323
415
312
343
509
661
716
480
605
456
613
Average
15
107
211±16.8
346 ±8.0
577 ±12.5
809
B
2
5
8
11
14
17
20
12
15
12
16
13
18.
18
82
111
87
112
120
102
110
156
209
145
226
131
188
162
262
418
280
405
282
317
342
511
801
538
695
441
497
595
(Nov. 12)2
Average
••
15
104
174±6.1
329 ±8.7
583±11.6
1045
C
3
7
9
13
15
18
21
12
13
12
14
14
16
15
98
103
97
111
96
128
98
176
232
181
228
182
209
156
364
347
360
355
344
413
315
562
619
581
561
636
638
527
(Nov. II) 2
Average
14
104
195 ±4.9
357±7.0
589±10.8
961
1 Probable errors based on weights of individual plants.
8 Probable errors could not be obtained from the weights taken on Nov. 11 and 12.
Table 14: NUMBERS AND WEIGHTS OF PLANTS, ROWE, WATSONVILLE,
FALL 1939
Treatment
Total
plants
Average weight
per untrimmed
marketable
head 1
Marketable
heads
Average weight
marketable
trimmed
heads
Marketable
lettuce per acre*
A
B
C
1170
1156
1342
grams
863 ±24.1
828 ±27.4
868±16.2
per cent
79.8
65.0
77.1
grams
503 ±4.5
484±4.6
537 ±4.9
pounds
17,720 ±158
13,930±132
18,430±166
» Probable errors based on average plant weights per plot in each treatment,
s Based on 20,000 plants per acre.
[27]
Fig. 19.— The sparse root development of lettuce. Photograph of face of trench cut across a
bed at time of harvest. The absence of lateral roots and large masses of soil not occupied by
roots is evident.
[28
ratio of permanent wilting percentages
to moisture equivalents was found to be
0.60. This indicates that 60 per cent of
the total water held by that soil is not
readily available to the plant. With this
knowledge, the per cent of available mois-
ture remaining in each soil sample was
calculated. These percentages, shown in
their respective locations from the plant
(figure 18), are the moisture contents in
percentages above the permanent wilting
percentage. The indication is that mois-
ture was available for plant use except in
areas of the surface 2 inches of soil.
The sparse root development of lettuce
under the conditions of these experiments
is further shown in figure 19.
Stirling, Summer, 1 940
This experiment was made on a field
which had been winter cover-cropped to
vetch.
The soil is a Salinas silty clay loam,
and drains quite rapidly, although dur-
ing the experiment, the water table re-
mained within 3 to 4 feet of the surface.
The crop was planted April 7 to Im-
perial 847 and was thinned May 1. No
irrigation was necessary to germinate the
seed because the soil was already mois-
tened by spring rains.
The area was divided into 4 treatments
(A, B, C, and D) of 10 plots each, with
relatively low beds, 4 or 5 inches high.
700
600
\
<\soo
k
§400
i
^JOO
I
^200
j
4
ff >
V
a
y
13
1
i
/
V
/5
/Jpr/7
25
0.4
O./"
/5
Afqy
25
June
24
44" ar
££'
a4" 0£
o.4" o.r
2 J"
5.0"
~~
ss:
tmmm 2.4^ m
Dotes of frr/yof/on
*- Dates of /?o/nfa//
Fig. 20.— Dates of irrigation, amounts of water applied and weights ot plants,
Stirling, summer, 1940.
[29]
Table 15: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 10
LETTUCE PLANTS PER PLOT, STIRLING, SALINAS, SUMMER 1940
Treatment
Plot
Dates sampled
May 11
May 20
May 29
June 8>
June 15'
June 24, 25
A
1
5
17
19
23
26
29
33
37
40
7
5
6
4
5
6
4
6
6
4
23
29
22
15
24
27
18
24
21
19
88
76
81
46
60
90
50
64
70
59
243
225
197
186
186
309
192
196
217
175
509
426
411
355
424
348
328
380
391
365
615
642
642
624
689
674
655
646
633
604
Average . . .
5
22
68
213±5.6
394 ±10.6
642
B
2
6
8
10
11
20
22
24
28
35
7
4
6
4
4
6
5
5
5
4
28
22
24
20
24
23
22
18
22
18
40
34
37
36
72
61
72
64
52
49
274
240
246
235
273
207
237
189
226
182
510
447
389
375
444
351
482
336
350
326
633
688
595
542
611
558
551
566
598
557
Average...
5
22
52
231 ±6.2
410±9.9
590
C
4
9
12
14
16
18
25
27
31
39
5
5
5
3
5
5
4
6
4
5
30
26
20
19
22
19
21
23
25
23
74
61
68
73
83
60
56
92
66
64
278
211
191
264
250
176
207
266
226
188
412
337
426
385
415
327
408
478
403
466
697
701
696
641
700
645
596
670
699
673
Average...
5
23
70
226±5.9
406±11.4
672
D
3
7
13
15
21
30
32
34
36
38
6
5
5
5
6
5
5
5
6
4
22
27
24
21
22
18
25
21
29
32
97
58
80
58
62
56
79
50
77
72
253
278
237
183
249
189
283
187
215
242
420
460
366
419
503
314
460
387
486
442
616
615
674
598
626
580
643
602
681
635
Average...
5
24
69
232±5.7
426 ±10.6
627
i Probable errors based on weights of individual plants.
[30]
Treatment A received no irrigation.
B, C, and D received 1, 2, and 3 irriga-
tions, respectively.
Irrigations on the B and C plots were
made by lightly flooding and submerging
the entire beds. Other irrigations were
made in the usual manner.
Plant weight data are shown in table
15, and figure 20 shows the dates of irri-
gation, amounts of water applied, dates
of rainfall, and the average weights of
plants.
Weekly soil-moisture determinations
made on samples from 2 plots of each
treatment, and permanent wilting per-
centages are shown in figure 21.
Yields were obtained in 5 cuttings.
These were made June 18, 22, 25, 29, and
July 5.
Yields are shown in Table 16.
The largest yield of marketable lettuce
is from treatment C, but it is not signifi-
cantly greater than the yield from A.
The second largest yield, that from A,
is significantly larger than from B and
D, and that from C is significantly larger
than from B.
On June 22, date of the second cutting,
five trimmed and five untrimmed heads
were cut from one plot in A, C, and D.
Individual heads were weighed, dried at
70 °C, and the moisture contents deter-
mined on a fresh weight basis (table 17) .
1
\Af.£
24.9
Sr-»
>s
o-/z rt
^
/
_. — ■ *■
P.WP.
132
*■<
5&QJ
^TV-
25
u JO
g» 25
Af.£..
■ 1
24.9
y*-
/rt.
S
P.W.P 13.2
=i£^j
^v
^
20
M£.
25.3
^"^n^.
/-//2 ft.
fe>
P.W.P /2.9
^
20
MF.
24.4
~^r+~^z
S^
l/2"2 ft
^9
P.W.f
/2.9
2 /O /d 25 31 5 (Z 20
May June
Treatments: A 3. C. D.
Fig. 21.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Stirling,
summer, 1940. M.E. and P.W.P. are the mois-
ture equivalent and permanent wilting per-
centage for each plant.
Table 16: NUMBERS AND WEIGHTS OF PLANTS, STIRLING, SALINAS,
SUMMER 1940
Treatment
Total
plants
Average
total weight
per plant 1
Lettuce
per acre 3
Market-
able
heads
Average weight
marketable
trimmed
heads 2
Marketable
lettuce
per acre 3
A
B
1988
1803
1859
2031
grams
642±5.5
590 ±9.7
672 ±7.4
627 ±7.1
pounds
28,300
26,010
29,630
27,650
per cent
88.9
83.9
88.6
86.4
grams
430±2.8
401 ±6.0
445 ±4.7
426±3.5
pounds
16,840 ±110
14,820 ±222
C
17,390 ±184
D
16,220 ±133
1 Probable errors based on average plant weights per plot for each treatment.
2 Probable errors are based on the 10 plot average for each treatment. In all tables except tables 16 and
19 the probable errors for marketable heads are based on individual weights of heads.
s Based on 20,000 plants per acre.
[31]
Table 17: MOISTURE IN LETTUCE HEADS, HARVESTED JUNE 22, 76 DAYS AFTER
PLANTING, STIRLING, SALINAS, SUMMER 1940
Plot and treatment
A
Ci
D*
Untrimmed
heads
Trimmed
heads
Untrimmed
heads
Trimmed
heads
Untrimmed
heads
Trimmed
heads
per cent
95.4
per cent
95.4
per cent
96.0
per cent
96.8
per cent
94.5
per cent
96.4
95.6
96.1
95.5
96.9
96.2
96.6
95.7
95.8
95.3
97.0
95.4
96.8
94.3
96.4
96.2
97.2
95.2
96.6
95.7
97.4
95.6
97.1
96.5
96.6
Average 95.3
96.2
95.7
97.0
95.6
96.6
1 Harvested 17 days after irrigation. * Harvested 2 days after irrigation.
c/
V
/
/ A
/ 'A
f
Jfl
ff
/
//
1/
7O0
600
\
^■500
k
%400
I
300
X
„ 200
I
\/00
// £/
3/ /O £0 JO
September
2.9" C.2"
/O
£0 30
October
o.j"' o./'
2.9"
4.6""
0.2'
"o.2"
Dates of Irr/jpaf/on
o - Dates of /?o/nfa//
Fig. 22.— Dates of irrigation, amounts of water applied and weights of plants,
Stirling, fall, 1940.
Stirling, Fall, 1940
This experiment was conducted in the
same field as the summer experiment.
The crop was planted to Imperial D
lettuce and irrigated August 6. Plants
were thinned September 9.
Twenty-nine plots were laid out in 3
treatments (A, B, and C). All treatments
were irrigated at planting and shortly
before thinning. A received no further
irrigation. B and C received 1 and 2 irri-
gations, respectively, after thinning.
After the initial seed-wetting irrigation,
A and B were irrigated so that the entire
beds were flooded and submerged.
C was irrigated in the usual manner.
Dates of irrigation, amounts of water,
and average weights of plants obtained by
weekly cutting and weighing 10 plants
from each plot, are shown in figure 22.
Table 18 gives the weight of plants by
plot at various times during the growing
period.
M.£.
■
24.9
\-..\
\
O-/2
//
\
*v
\
■
-^
'*--..
PW.fi
/J. 2
Us
%
30
M£.
24.9
£-/
N
PWP
tj.2
t=
■— «
Mr
25-3
A/4
»..
^^.
-»-...
—-
—^
! /2.9
«
-«^-
1
M.£. 24.4
1
~~' 1 -^:
*=^=;
J^y
^.^
—
"~*--~-.
fiW.fi
/2.9
Jeflt.
Treatments
Oct.
Nov.
C.
Fig. 23.— Soil-moisture conditions as deter-
mined by samples taken at the plant, Stirling,
fall, 1940. M.E. and P.W.P. are the moisture
Soil-moisture conditions and perma-
nent wilting percentages are shown in
figure 23. The latter percentages are those
determined for the summer crop.
Soil-moisture conditions directly at the
plant as compared with those at the center
of the furrow are shown in figure 24. It
should be remembered that in this experi-
ment the soil surface at the plant, which
is on the bed, is approximately 4 inches
higher than in the bottom of the furrow.
24
/8
/6
26
24
22
20
% 24
\22
N
^24
I"
K
^26
24
22
JO
28
26
24
1
M.E
■ m
24.9
\
1 1 1
O-/2 ft.
V
\
\
\
^x
-«».
PW.P. /J. 2
""
1
\M£. 24.9
S v
1 1 1
Si
N
--^
PA
V.P /J.
2
— .1^:
M£.
2 S.J
S.
N
1 L '
•^
~^
D
VP. J2.
M.£.
24.4
■s.
1 1 1
/%-2ft.
s *»
^^
N
V.P /2.
■
s.
—
7
M.£.
24.4
-«
'-«.,
^2-2'/zff.
N.
^
■»»■
f>\
V.P. 12.,
?
1
M.£. 24.4
■s,
X
2%-3ft.
^H
\
P\
1
V.P /2.2
J tJ
29
9 /6 24 2 tO t7 24 30 9
Sept. Oct. A/ov.
So/vp/tny Loc of /on s
At P/onr :
Center of Farrow. — — —
Fig. 24.— Soil-moisture records taken at the
equivalent and permanent wilting percentage plant and at the center of the furrow, Stirling,
for each plant. fall, 1940.
[33]
Table 18: AVERAGE WEIGHT PER PLANT IN GRAMS, BASED ON SAMPLES OF 10
LETTUCE PLANTS PER PLOT, STIRLING, SALINAS, FALL 1940
Treatment
Plot
Dates sampled
Sept. 16
Sept. 24
Oct. 2
Oct. 9
Oct. 17i
Oct. 24i
Nov. 7, 9»
A
3
6
8
12
15
19
22
25
27
18
17
14
14
14
14
12
10
12
74
49
62
75
52
37
45
34
39
134
120
145
113
136
128
103
82
84
242
208
230
258
242
195
189
196
153
474
379
346
376
334
334
408
320
244
584
616
668
583
460
460
444
525
514
797
808
717
730
679
708
685
642
Average .
14
52
116
213
357 ±8.4
547±12.0
721
B
2
4
7
10
13
17
20
21
23
28
11
15
14
18
14
14
14
14
10
11
48
65
40
64
40
51
48
48
35
30
116
159
98
158
144
156
98
106
79
68
219
266
202
239
250
291
191
171
159
188
366
450
375
475
525
456
363
299
351
273
462
673
491
516
586
569
494
583
450
389
775
778
757
763
807
796
740
737
689
636
Average .
13
47
118
218
393±10.1
521 ±12.7
748
C
1
5
9
11
14
16
18
24
26
29
14
14
16
17
14
12
13
12
12
10
35
51
64
67
50
52
34
36
37
36
106
121
150
159
149
166
93
105
127
93
159
190
321
306
236
300
210
179
194
194
382
344
500
464
410
457
365
356
307
260
501
610
630
651
709
600
304
500
509
567
826
830
767
812
838
755
727
667
821
722
Average .
13
46
127
229
384 ±3.0
578±13.2
777
1 Probable errors determined on weights of individual plants.
2 Probable errors could not be obtained from the weights taken on Nov. 7 and 9.
[34]
Table 19:
NUMBERS AND WEIGHTS OF PLANTS, STIRLING, SAUNAS, FALL 1940
Treatment
Total
plants
Average
total weight
per plant 1
Lettuce
per acre
Marketable
heads
Average weight
marketable
trimmed
heads 2
Marketable
lettuce
per acre
A
B
C
1184
1501
1482
grams
721 ±14.1
748 ± 8.4
777 ±10.1
pounds
31,970
33,030
34,300
per cent
82.8
84.5
90.6
grams
447 ±6.5
468 ±5.6
476 ±6.3
pounds
16,310 ±237
17,410 ±209
19,000 ±252
1 Probable errors based on average plant weights per plot for each treatment.
1 Probable errors are based on the 10 plot average for each treatment. In all tables except tables 16 and
19 the probable errors for marketable heads are based on individual weights of heads.
Five cuttings to harvest the crop were
made October 31, November 4, 8, 14, and
19.
Yields are shown in table 19.
The yield of marketable lettuce from
treatment C is significantly larger than
that from each of the other two treat-
ments, and that from B is just signifi-
cantly larger than from A.
On October 31, 10 untrimmed heads
from 2 plots of A and 2 plots of C were
harvested by cutting each head at the base
of the lowest leaf. Moisture percentages
on a fresh weight basis were then deter-
mined. The data gave an average percent-
age of 94.9 ± 0.4 for the 20 heads from
A, and 95.9 ± 0.4 for C.
On November 1, 6 marketable trimmed
heads from each plot of the A and C treat-
ments (or 60 heads per treatment) were
selected at random. Heads from each of
these treatments were packed in 2 crates.
These crates were iced and held in cold
storage until November 12. This would be
about the usual time of transit by rail.
When they were opened the cut surface
at the base of the heads from the C treat-
ment showed a pinkish tinge whereas
those from the A treatment lacked this
tinge. Otherwise there was no distinguish-
able difference.
Tests for firmness and flavor likewise
showed no differences.
On November 15, after the fourth cut-
ting, the same procedure was followed on
all treatments. When the crates were
opened 10 days later there was no differ-
ence in the heads from the various treat-
ments. The slight difference in color of the
cut ends of the lettuce observed in the
first test could not be detected in the sec-
ond test.
[35
MATURITY OF HEADS
The term maturity is used to mean the
state of growth at which the plants become
marketable, even though the plants are
not physiologically mature.
It is important to know whether irriga-
tion delays or advances the time of matu-
rity or marketability. The mean period of
maturity was calculated for the different
treatments in each of the nine experi-
ments. The mean period of maturity was
determined by the following equation :
M *t (^1 x CJ + (X 2 X C 2 ) + (^ n x C n )
Total number of marketable heads
where X x — number of marketable heads obtained at first cutting
X 2 = number of marketable heads obtained at second cutting
X n = number of marketable heads obtained at last cutting
C x = number of days between time of planting and first cutting
C 2 = number of days between time of planting and second cutting
C n = number of days between time of planting and last cutting
Data for mean maturity are given in
Table 20. They show there is very little
difference in time of maturity between
treatments. The differences are not great
enough to indicate that the treatment has
any effect on the time the lettuce reaches
a marketable stage. The greatest differ-
ence was 2.4 days between the mean ma-
turity of the A and C plots in the 1940
Stirling fall crop.
DISCUSSION OF IRRIGATION EXPERIMENTS
Root Distribution
In these experiments soil-moisture ex-
traction curves indicate that roots of let-
tuce penetrate the soil and use some water
to a depth of at least 2 feet. This does not
mean, however, that the soil is completely
permeated by roots as is the case with
most of the crops previously studied ( 18) .
Figure 18 shows that even after the
plant is 79 days old and 35 days after the
soil was wetted, there were appreciable
amounts of readily available water still
present in the soil, even in the surface 8
inches of soil. There was extreme varia-
tion, furthermore, in the amounts of water
in the soil at this time. This indicates that
the plant had a sparse root development
which was unevenly distributed in the
soil.
Strangely enough, the same kind of
plants grown in cans in laboratory trials
containing approximately 600 grams of
soil seemed to have a uniform distribution
of roots which dried the soil to the per-
manent wilting percentage throughout.
Further indication of uneven root dis-
tribution is shown in figure 24. This
condition is typical of all the experiments.
The amount of water used at the plant
was quite different from that used 14
inches away until October 17 when the
plant was nearly mature. After October
17, all of the moisture extraction curves
became nearly horizontal. This may be
due to cloudy weather. Standard atmome-
ter data (table 21) indicate that evapora-
tion conditions were low between October
22 and November 7, 1940.
Soil-Moisture Conditions
The differences in moisture contents of
the various experiments, regardless of the
frequency and time of irrigation, are not
great. In no case was the average soil-
[36
Table 20: NUMBER OF DAYS ELAPSING AFTER PLANTING BEFORE
LETTUCE REACHES MEAN TIME OF MATURITY
Experimental
plots
Treatment
Mean time
of maturity
(days after
planting)
Experimental
plots
Treatment
Mean time
of maturity
(days after
planting)
Union Ice,
A
82.2
Rowe,
A
73.5
Summer
B
82.2
Summer
B
73.5
C
81.6
C
73.3
D
80.6
Rowe,
A
93.9
Union Ice,
A
81.7
Fall
B
94.8
Fall
A'
C
81.6
81.6
C
93.7
D
81.5
Stirling,
A
79.3
Summer
B
79.1
Gerrard,
A
78.5
C
79.1
Summer
B
C
77.8
78.1
D
77.9
Stirling,
A
95.3
Gerrard,
A
78.9
Fall
B
95.2
Fall
B
C
78.9
78.3
C
92.9
Capitanich,
A
78.5
Summer
B
C
D
79.1
79.0
78.5
moisture condition reduced to the per-
manent wilting percentage in the 6-inch
layers sampled. In four experiments, how-
ever, the moisture in the surface 6 inches
was reduced to within 2 per cent of the
permanent wilting percentage. Therefore,
the time of reduction of growth in the
least frequently irrigated plots as com-
pared to that in the frequently irrigated
plots cannot be related to the exhaustion
of the soil-moisture content. The uneven
distribution of the roots apparently is
such that even samples close to the plant
do not give an exact measure of the mois-
ture content of the soil in contact with the
roots. Certainly the differences found by
sampling, in view of previous work on
the effect of soil-moisture on plant growth,
are not great enough to be the cause of the
differences in growth and yields obtained
in these experiments.
Bringing Water to the Plant
It seems that the roots of these lettuce
plants did not extend themselves into
moist soil as is usually the case with other
plants, but that it is necessary to bring
water to them by irrigating.
A similar result was reported by Work
and Lewis (19) who explained the neces-
sity for maintaining the soil at a high
level in a pear orchard on a clay adobe
soil on the basis that "The roots do not
seem to occupy the entire soil mass," and
that "The soil-moisture content of the
soil in contact with the feeding roots may
be at or near the permanent wilting per-
centage, while at the same time the mois-
ture content at some distance, perhaps
only a few centimeters away, may be
much higher, thus allowing the average
content for an ordinary soil sample to be
well above the wilting percentage at the
[37]
Table 21. EVAPORATION FROM STANDARD ATMOMETERS ON LETTUCE IRRIGA-
TION EXPERIMENTAL PLOTS IN CUBIC CENTIMETERS PER HOUR BETWEEN
DATES, FOR BLACK AND WHITE ATMOMETERS
Union Ice — Watsonville,
1938
Date
June
July
August
September
October
6
14
24
7
15
23
30
4
19
31
6
19
29
7
21
Black
White
1.28
.89
1.53
.94
1.15
.72
1.28
1.09
.69
1.61
.97
1.98
1.32
1.24
1.04
1.54
1.00
1.45
.61
1.09
.62
1.01
.64
.96
.51
.84
.54
Capitanich
— Watsonville, 1939
Date
March
April
June
July
28
4
ll
18
25
6
13
19
28
8
17
28
Black
White
1.03
.76
1.56
1.11
1.04
.71
1.02
.71
1.03
1.22
1.99
1.29
1.44
.88
1.41
.79
1.47
.97
Rowe —
Watsonville,
1939
Date
August
September
October
November
14
24
6
14
19
26
9
16
23
30
6
17
Black
White
1.30
.89
1.69
1.19
1.22
.81
2.64
2.06
2.46
2.08
1.26
.89
1.67
1.19
1.58
1.53
1.27
.96
.66
1.04
.77
Stirling — Salinas, 1940
Date
May
June
July
Sept.
October
November
2
12i
20
29
5
12
20
3i
23
2
10
17
22
7
16
Black
White
1.60
1.60
1.82
1.49
1.66
1.56
2.17
1.99
1.57
1.29
1.89
1.61
1.66
1.51
1.74
1.62
1.51
1.39
1.52
1.42
1.61
1.50
.97
.95
1.58
1.72
Bottles ran dry before being refilled.
time the tree shows serious distress for
water."
Method of Sampling
Our method of sampling was not pre-
cise enough to measure the actual mois-
ture content of the soil in contact with
the absorbing parts of the roots. The soil-
moisture curves cannot be interpreted in
the same way as in our previous studies
on plant-soil-moisture relations. They do,
however, serve as a basis for calculation
of losses of moisture.
The very small amount of water taken
by transpiration and the short time the
plants were allowed to develop in relation
to maturity; that is, to finish their com-
plete cycle of growth, together with a
sparsely developed root system, makes it
impossible to use soil moisture records,
[38]
either as an indication of growth or
availability of water.
The only criterion we find to be practi-
cal for the time to irrigate lettuce is the
interval between applications.
Weights of Plants
Examination of the data on mean
weights of plants reveals that if the plants
were irrigated more than 30 days before
the weights of plants were taken, in some
cases, but not all, there are significant dif-
ferences in weights. However, unless 30
days elapsed without irrigation there are
no significant differences.
There are instances where significant
differences were obtained, but they could
not be accounted for by differences in irri-
gation treatments. They are probably due
to inherent variability of the plots.
Thirty Days May Elapse
Without Irrigation
We believe, therefore, that there is a
period of 30 days which may elapse with-
out irrigation or appreciable amounts of
rain without adversely affecting the plants
when grown under the conditions of these
experiments.
A Seeming Exception
One instance which would seem to be an
exception, but which actually is not, is
that of the Rowe summer crop wherein a
difference occurred in the weights of
plants of the C treatment as compared to
A and B. This took place on July 10,
which was 31 days after irrigation of A
and 39 days after irrigation of B.
It would seem unlikely that depression
of growth could have occurred in one day,
but since we did not have measurements
on the thirtieth day, this cannot definitely
be decided. A case where no differences
were obtained, which could be related to
the irrigation treatments, was the Stirling
summer experiment of 1940.
Yields
The yields of pounds of marketable let-
tuce per acre for the treatment receiving
the greatest number of irrigations, D, are
highest in 1 out of 4 cases, but in 1 case
they are lower than those from A which
received the fewest irrigations. In 4 trials
C received the second highest number of
irrigations, while D was the most fre-
quently irrigated.
But in the remaining 5 trials there was
no D treatment, therefore C had the most
number of irrigations.
The yields in pounds of marketable let-
tuce are greatest in C in 6 out of 9 experi-
ments.
In 6 out of 8 experiments, yields from
B are less than those from C. It should be
noted that the interval between irrigations
for B, with one exception, was greater
than 30 days.
Yields from A are lowest in 7 out of 9
experiments but in one, A-l, which was
essentially the same as A, the yield is not
the lowest and, in fact, is greater than D.
In all of the experiments, more than 30
days elapsed between the last irrigation
and the first cutting in A.
Yield Differences Are Mostly
Negligible and Inconsistent
In many instances the differences in
yields are negligible. Furthermore, they
are inconsistent in that the most fre-
quently irrigated treatments did not al-
ways produce the maximum yields. For
instance, in the Stirling summer 1940 ex-
periment, the yield from A is greater than
from D. In fact, it would seem that the
differences obtained are the result of
normal variability in plot yields, except
in some cases for A.
In the Stirling summer experiment
there were 10 replications for each treat-
ment.
Further evidence of variability in
yields is that of treatments A and A-l of
the Union Ice fall 1938 experiment. While
[39]
the irrigation treatment was the same, the
difference between the treatments is as
great as the difference between A-l and
the other treatments.
Effect of the Number of
Irrigations on Yields
The number of irrigations and amounts
of water applied cannot be used as the
only means for determining the effective-
ness of irrigation on yields.
As mentioned before, the time elapsed
between irrigations (30 days or more as
was the case in treatment A) seemed to
be the controlling factor affecting yields.
This is especially true with plants having
relatively shallow and sparsely developed
root systems as in the case of lettuce. This
statement as to the effect of interval be-
tween irrigations upon yields is based
on the fact that the yields from treatment
A are the lowest in so many of the exj
periments.
Effect of Irrigation on the Time
of Marketability
It is claimed that some irrigations ad-
vance or delay the time at which heads
become marketable.
Marketability is determined by size
and firmness, and conditions affecting
quality such as disease and shape.
An irrigation is frequently given within
a few days before cutting in the belief
that the addition of water at this time
results in firm heads and an earlier cut-
ting date.
The records of the dates of maturity
(table 20) show that the differences are
too small and not consistent enough to
indicate that the treatments had any effect
on the time the heads became marketable.
The maximum difference was found to be
2.4 days.
The time to cut a crop depends upon
the judgment of the individual grower
and might vary as much as the 2.4 days.
Moisture Content and Keeping
Qualities
The moisture content and the keeping
qualities of the heads were determined at
the time of harvest.
The results indicate that there are no
marked differences which can be attrib-
uted to the differences in irrigation treat-
ments.
Tests of quality were made by tasting
in all experiments. This failed to show
any marked differences.
Irrigation Close to Harvest
The belief that lettuce requires an irri-
gation close to harvest time to produce
firm heads is without foundation since
our experiments do not show any differ-
ence in moisture content of the heads
from the various treatments, nor in firm-
ness, as measured by hand pressure,
visual condition, and packing house in-
spection.
Bolting and Tipburn
There were few cases of tipburn or
bolting in any of the experiments.
Bolting (production of seed stalks)
was noticeable in only the Rowe 1939 fall
crop. In this case temperatures above
100° F occurred about 44 days after
planting. The bolting appeared to be more
prevalent in the B than in the A and C
treatments which may account, in part,
for the lower production of marketable
lettuce as shown in table 14. In spite of
the loss due to bolting, the B treatment
yielded over 6% tons per acre, or about
191 crates, of marketable lettuce.
Tipburn, like bolting, was not exten-
sive enough to be a decisive factor in
production in any of the experiments ex-
cept on the Capitanich ranch. In this
experiment, however, the occurrence of
tipburn did not seem to be related to the
differences in irrigation treatments.
Bolting and tipburn did not occur fre-
quently enough during the experiments
to give sufficient data to permit drawing
[40]
♦definite conclusions as to the effect of
irrigation on these conditions. It seems
that with the climatic and soil conditions
of these experiments, and with the varia-
tions in irrigation practice, irrigation is
not the causal factor for bolting and tip-
burn.
Water Losses
In general, the rates of use of moisture
as shown by the slopes of the moisture-
extraction curves are about the same re-
gardless of the irrigation treatment.
The calculation of the water losses from
the soil taken from the soil-moisture
curves in treatment B of each experiment
shows that from 1.75 to 5.61 inches of
water were taken from the soil by trans-
piration, evaporation, and drainage
(table 22).
Drainage and evaporation directly
from the soil surface probably account
for a large portion of the losses, and
transpiration must have been very small.
These amounts are small in relation to
those applied. For instance, the average
total amount of water used for the summer
in the frequently irrigated plots is 14.1
inches, and for the fall crops, 14.3 inches
(table 23).
The average losses of water from the
soil, on the other hand, for the B treat-
ments in the summer crops, are 3.95
inches and for the fall crops, 3.16 inches.
The amounts of water applied, further-
more, are much less than those generally
used in commercial practice.
Measurements of Water
Measurements of water applied to com-
mercial lettuce fields are subject to con-
siderable error. One pumping plant
generally supplies water to crops in differ-
ent stages of growth, and the individual
attention necessary to segregate the use
on the various portions of the field usually
cannot be given.
Our measurements show that 6 to 10
Table 22: LOSSES OF SOIL MOISTURE AS INCHES OF WATER FROM
TIME OF THINNING TO MATURITY*
Experiment
Depth of soil, feet
0-H
M-l
l-i^
1H-2
2-2^
2^-3
Total
Union Ice..
Gerrard . . .
Rowe
Capitanich .
Stirling. . . .
Summer
1938
1938
1939
1939
1940
Average
1.34
1.15
1.27
1.30
1.19
1.30
1.42
.73
.89
1.19
.59
.46
.76
.70
.85
.19
.42
.82
.31
.74
.44
.89
.0
.82
3.42
3.89
5.29
3.20
3.97
1.25
1.11
.67
.50
.66
.41
3.95
Union Ice..
Gerrard . . .
Rowe
Stirling
Fall
1938
1938
1939
1940
Average
.48
1.33
1.46
1.13
.50
.28
.73
.55
.32
.28
1.16
.43
.45
.38
1.10
.36
.79
.28
.37
.24
1.75
2.27
5.61
2.99
1.10
.51
.55
.57
.54
.31
3.16
* Extraction includes evaporation, drainage and transpiration as determined from soil-moisture records
taken close to plants in the B treatments.
[41
Table 23: INCHES OF WATER APPLIED ON IRRIGATION EXPERIMENTAL PLOTS
Experiment
Treatment
Water applied to
most frequently
irrigated
treatment
A
B
C
D
Union Ice
Summer
1938
1938
1939
1939
1940
Average
2.4
10.0
15.3
7.7
.0
10.1
13.8
13.4
10.8
2.0
8.3
19.0
16.0
10.8
4.1
14.3
11.9
9.4
14.3
19.0
16.0
11.9
9.4
Gerrard
Rowe
Capitanich
Stirling
7.1
10.0
11.6
11.9
14.1
Union Ice
FaU
1938
1938
1939
1940
Average ....
6.2
8.0
15.8
6.9
9.9
17.8
9.9
8.6
14.9
18.4
14.7
9.3
9.3
14.9
18.4
14.7
Gerrard
Rowe
Stirling
9.2
12.5
14.2
9.3
14.3
inches were usually applied to germinate
the seed. Much greater applications have
been measured. In one case as many as 26
inches were applied. In this case, the runs
were excessively long and the soil was
highly permeable.
At the time of thinning from 4 to 6
inches of water are usually applied. Ex-
cessive applications have also been meas-
ured at this time. A measurement of 11
inches was taken on one field. Commonly,
the applications following thinning are
from 2 to 4 inches.
It is obvious that much water is wasted
by deep percolation in growing lettuce.
The presence of free water at depths of
6 feet or less is frequent in these lettuce
growing areas and may be an indication
of excessive use of water.
Flooding
In the experiments where the beds were
flooded, the heads appeared to be as firm
as when the beds were not flooded. Also,
there was no difference in color or in the
percentage of heads having butt slime.
This does not mean that flooding may not
be harmful from other considerations and
under other conditions.
Height of Beds
The excessive amount of water used to
germinate the seed is due in many in-
stances to the use of high beds; for
instance, those which are 6 inches or more
in height. High beds are necessary, of
course, on land not properly leveled.
Water must be kept in the furrows
until the soil around the seed is moistened.
The greater the distance the seed is above
the water level in the furrow, the longer
will be the time required to wet the soil
around it.
In an experiment on a sandy soil, let-
tuce was grown on 24 low beds to serve
as a comparison to adjoining high-bed
plantings. The low beds were made by
driving a broad-track tractor over the
furrows. This made the furrows broad,
shallow, and compacted instead of nar-
row, deep, and loose.
The amount of water needed to wet
[42]
the beds to germinate the seed was 11
inches for the low beds and 26 inches for
the high beds.
In the case of the low beds, the furrows
were completely filled and water partially
flooded the beds. No dependence was
placed on capillarity to wet the soil
around the seed.
Both applications were excessive due
to the highly pervious nature of the sandy
soil and to the unevenness of the grade.
A second irrigation was made 44 days
later. Eight and 11 inches were used for
the low beds and high beds respectively.
These measurements show considerable
saving in water when low beds are used.
Shortly before the second irrigation, a
side dressing of fertilizer containing 30
pounds of nitrogen per acre in the form
of nitrate was applied to the sides of the
furrow, and the low beds flooded. Not
long after this the plants in the low beds
appeared smaller and yellower than those
in the high beds. They were poorest in
areas of the plot where greatest leaching
of the fertilizer took place.
It should be remembered that a bed
need not be any higher than is necessary
to compensate for the unevenness of the
land. It should also be high enough to
prevent it from being flooded in cases
where flooding should be avoided; for
instance, when crusting of the soil surface
occurs and interferes with seed emergence
or where leaching is excessive. In this
regard, when fertilizer is used it should
be placed high enough on the shoulder
of the bed to prevent leaching.
Crop Water Needs
Even though it has been shown that
only about 4 inches of water are required
for evaporation, transpiration, and drain-
age, it must not be assumed that this
amount is all that is required to produce
a crop of lettuce in the Monterey Bay
region, because there is an inevitable
waste in applying water.
As pointed out previously, large
amounts of water are wasted in wetting
the beds for seed germination, especially
with high beds. Furthermore, the irriga-
tion at thinning is applied only to wet a
shallow layer of soil at the surface at a
time when transpiration has only begun.
Four to 6 inches of water at planting,
and 3 to 4 inches for the other two irriga-
tions, or a total of 10 to 14 inches should
be enough under good irrigation practice,
and with well-leveled land, for summer
and fall crops.
Data Provide Basis for
Satisfactory Irrigation
Schedule
The results of these experiments furnish
data which may be used as a basis for a
satisfactory irrigation schedule for sum-
mer and fall lettuce in the Monterey Bay
region of California.
After the beds are prepared and seeded,
and the land is in shape for irrigation, the
first irrigation seems to be justified be-
cause the soil at this time is generally too
dry for seed germination.
The irrigation which is usually given
just before or just after thinning may be
necessary even though there may be ample
water in the soil at this time.
After the second irrigation, the third
irrigation may be delayed for 30 days.
Three irrigations will be enough for the
strains of lettuce grown under the con-
ditions of our experiments if applied at
the above stated times.
We believe that these recommendations
can be made without loss of yield or
quality.
[43]
CULTIVATION EXPERIMENTS
Five lettuce cultivation experiments
were conducted in the Pajaro Valley
(figure 1). These were carried out on a
summer and fall crop in 1937, a fall crop
in 1938, and spring crops in 1939 and
1940.
Plots were selected in commercial
fields, and with the exception of the num-
ber of cultivations after the field was
planted, they received the same cultural
practices as did the remainder of the
field.
A brief description of the general culti-
vation practices has already been given.
In every experiment there were two
treatments.
One set of plots received the same
number of cultivations as practiced by
the grower.
The second set was cultivated only for
the control of weeds. This resulted in a
considerably larger number of cultiva-
tions for the first set.
In the first 3 experiments, yields were
obtained by counting the number of
packed, marketable heads from each plot.
The heads were packed by commercial
shippers.
In the last 2 experiments yields were
obtained by trimming and weighing each
marketable head.
DESCRIPTIONS OF THE CULTIVATION EXPERIMENTS
The five cultivation experiments were
designated as :
1. Randolph, Summer, 1937.
2. Randolph, Fall, 1937.
3. Capitanich, Fall, 1938.
4. Loveless, Spring, 1939.
5. Loveless, Spring, 1940.
The above names are those of the
ranches on which the experiments were
conducted.
Randolph, Summer, 1937
Four plots, A, B, C, and D, of 4 beds
each 1100 feet long were selected.
The soil is classed as a Metz fine sandy
loam.
A and C received 4 cultivations.
B and D were given only 2 cultivations.
A chronological list of cultural prac-
tices follows:
April 18— Planted to strain 615-x and
irrigated.
May 3— All plots cultivated on beds with
weed-cutting knives and with chisels in
the furrows.
May 10— Thinned.
May 26— All plots irrigated.
June 4— All plots cultivated with side
knives and chisels on beds and in fur-
rows, then hoed for weeds.
June 12— All plots irrigated.
June 14— Differential treatment started.
Knives used on ridge and in furrow of
Plots A and C.
June 1 8— Furrow and side knives used
on Plots A and C.
Table 24: NUMBER OF LETTUCE
HEADS PER PLOT PACKED FOR SHIP-
PING, RANDOLPH, WATSONVILLE,
SUMMER CULTIVATION, 1937
Plot
Size as represented
in heads per crate
Total
60
75
4 Cultivations
A
1046
991
352
357
1398
1348
C
Total....
2037
709
2746
2 Cultivations
B
860
975
498
445
1358
1420
D
Total
1835
943
2778
[44]
June 22— All plots irrigated.
July 1— First cutting.
July 6— Second cutting.
Rainfall Record— A inch rainfall April
26 to 28. .17 inch rainfall May 18.
Yields are given in table 24, the data
being secured from the two center beds
of each plot.
Randolph, Fall, 1937
This experiment was conducted on a
field different to the one used in the sum-
mer experiment.
The soil was very much the same as the
soil in the summer experiment.
Four beds, 1100 feet long, were se-
lected. These were bounded on either side
by four additional beds which were culti-
vated 3 times.
Yields were obtained from the two
center beds of the plots cultivated 3 times.
Two plots were made from the 4 beds
cultivated 6 times.
The four plots were numbered in con-
secutive order, thereby making plots A
and D those which were cultivated 3
times, and plots B and C those cultivated
6 times.
Weeds were quite prevalent on the plots
cultivated only 3 times, and they became
quite weedy just before cutting.
Cultivation practices in chronological
order follow:
July 1 6— Planted to strain 515-x and irri-
gated.
July 30— Cultivated bottom of furrows
with chisels.
July 31— Top of beds rolled.
Aug. 5— Disked top of beds and used side
knives.
Aug. 9— Thinned.
Aug. 70-Applied 300 lbs. of 12-6-4
mixed fertilizer per acre.
Aug. 11— Furrowed out after thinning.
Aug. 16— Irrigated. Differential treatment
started.
Aug. 23— Plots B and C cultivated on beds
and furrows with knives and chisels.
Aug. 29— Hoed weeds from all plots.
Table 25: NUMBER OF LETTUCE
HEADS PER PLOT PACKED FOR SHIP-
PING, RANDOLPH, WATSONVILLE,
FALL CULTIVATION, 1937
Plot
Size as represented
in heads per crate
Total
60
75
6 Cultivations
B
1818
1378
238
75
2056
1453
C
Total. . . .
3196
323
3509
3 Cultivations
A
1446
1696
75
75
1521
1771
D
Total...
3142
150
3292
Chiseled top of beds and furrowed out
on Plots B and C.
Sept. 1— Irrigated.
Sept. 7— Cultivated furrows with chisels
on Plots B and C.
Sept. 14— Irrigated.
Sept. 24— First cutting.
Sept. 30— Second cutting.
Oct. 6— Third cutting.
Rainfall Record— .09 inch rainfall Octo-
ber 2.
Yields in number of packed commer-
cial heads are shown in table 25.
Capitanich, Fall, 1938
In this experiment the soil is classified
as a Botella silty clay loam, and is quite
heavy.
Four adjacent plots 150 feet long,
separated by guard beds, were used.
Plot A had two experimental beds. B
had three. C and D each had four.
A and C were cultivated 4 times. B and
D were cultivated once.
[45]
Table 26: CRATES OF LETTUCE PER ACRE BY PLOT AND BY TREATMENT,
CAPITANICH, WATSONVILLE, FALL CULTIVATION, 1938
Plot
Total
marketable
heads
per plot
Size as represented in heads per crate
Total
48
60
75
4 Cultivations
A
182
486
21.4
35.2
140.3
119.0
21.0
17.5
182.7
171.8
C
Average
31.7
124.4
18.4
174.5
1 Cultivation
B
405
353
41.9
21.1
136.5
112.6
10.9
12 9
189.3
146.6
D
Average
31.0
123.9
11.9
166.9
Table 27: AVERAGE WEIGHT IN GRAMS BY SIZES OF COMMERCIAL LETTUCE HEADS
CUT AT FIRST CUTTING, CAPITANICH, WATSONVILLE, FALL CULTIVATION, 1938
4 doz. size
5 doz. size
6 doz. size
Totals
Plot
No.
heads
Average
weight
grams
No.
heads
Average
weight
grams
No.
heads
Average
weight
grams
No.
heads
Average
weight
grams
4 Cultivations
A
16
75
91
680
703
57
178
579
577
7
20
27
477
497
80
273
590
C
606
Totals
699
235
578
492
353
602
1 Cultivation
B
D
50
32
82
707
806
129
138
601
592
4
7
11
490
497
183
177
627
627
Totals
745
267
596
494
360
627
[46]
A chronological list of cultural prac-
tices follows:
July 22— Planted to strain 847 and irri-
gated.
July 29— All plots ring rolled.
Aug. 14— All plots irrigated.
Aug. 18— All plots thinned.
Aug. 20— All plots cultivated on ridges
and in furrows with knives and chisels.
Aug. 23— Side dressing of commercial
fertilizer applied to all plots.
Aug. 25— All plots irrigated. Differential
practice begun.
Sept. 2— Cultivated ridge and beds on
plots A and C with chisels.
Sept. 5— Cultivated ridge and beds on
plots A and C with chisels.
Sept. 12— Chiseled and furrowed ditches
on plots A and C.
Sept. 1 7— Irrigated all plots.
Oct. 5— First cutting on all plots.
Oct. 10— Second cutting on all plots.
Oct. 13— Third cutting on all plots.
Rainfall record— .75 inch rainfall Octo-
ber 2.
Yields are shown in table 26.
Table 28: CULTURAL PRACTICES AND RAINFALL ON LETTUCE, LOVELESS,
WATSONVILLE, SPRING CULTIVATION, 1939
Date
Rainfall,
inches
Cultural practices
1938
Dec. 6
Fertilized with 2^ tons chicken manure.
12
Planted to strain 615 lettuce.
1939
Jan. 16
Cultivated beds and furrows with knives and chisels.
25
Thinned.
27
Cultivated with knives, chisels, and mulcher, and applied 250 lbs.
fertilizer 12-10-7.
Feb. 8-11
1.08
Intermittent showers.
A Treatment B Treatment
21
Cultivated ridge and furrows with
knives and chisels.
23
Hoed weeds by hand. Scraped weeds by hand.
Mar. 1
Cultivated ridge and furrows with
knives and chisels.
4-14
2.45
Intermittent showers.
14
Hoed weeds by hand. Scraped weeds by hand.
20
Cultivated beds and furrows with
knives and chisels.
25-26
.50
29
Cultivated beds and furrows with
knives and chisels.
April 1
.41
4
Scraped weeds by hand.
12
.09
19
First cutting. First cutting.
24
Second cutting. Second cutting.
28
Third and last cutting. Third and last cutting.
[47]
Table 27 gives the average weights of
marketable heads at the first cutting. The
data show that yields based on the num-
ber of heads per packed crate are in close
relation to the weights of the heads.
Either method may then be considered a
satisfactory basis for determining yields.
Loveless Experiments,
Spring 1939 and 1940
The procedure in these experiments
was different.
In the plots of the B treatment weeds
were kept down by carefully scraping
them with a sharp hoe with as little dis-
turbance of the soil as possible.
Loveless, 1 939
This experiment was made on a soil
classed as a Pinto loam which cracks very
little but becomes hard when dry.
In the 2 treatments, each having 12
plots, A received 6 cultivations and B
received two.
Each plot comprised one bed 35 feet
long with guard beds on each side. Beds
were spaced 42 inches apart.
Cultivation practices and the rainfall
record are shown in table 28.
Yields are shown in table 29.
Loveless, 1 940
This experiment was conducted on a
more fertile soil than that in the 1939 ex-
periment.
The arrangement of plots was similar
to the 1939 experiment, but the plots were
25 instead of 35 feet long.
Cultural practices and the rainfall rec-
ord are shown in table 30.
Yields from this spring cultivation are
shown in table 31.
Table 29: YIELDS OF LETTUCE, LOVELESS, WATSONVILLE,
SPRING CULTIVATION, 1939
A treatment
(Cultivated 6 times)
B treatment
(Cultivated 2 times and weeds scraped)
Plot
Total
plants
Total
marketable
heads
Average weight
marketable
head 1
Plot
Total
plants
Total
marketable
heads
Average weight
marketable
head 1
1
3
5
7
9
11
13
15
17
19
21
23
70
67
68
58
69
66
62
68
61
62
55
60
60
50
60
56
54
61
56
59
49
46
45
47
grams
551
520
552
563
470
529
593
560
591
573
612
528
2
4
6
8
10
12
14
16
18
20
22
24
64
65
63
67
67
60
66
62
60
55
67
59
56
59
53
61
56
49
52
55
48
39
51
46
grams
585
648
561
536
521
487
465
588
612
576
637
659
766
643
552 ±7.5
755
625
581 ±12.2
Probable error is based on mean weight of marketable heads per plot.
[48]
Table 30: CULTURAL PRACTICES AND RAINFALL ON LETTUCE, LOVELESS,
WATSONVILLE, SPRING CULTIVATION, 1940
Date
Rainfall,
inches
Cultural practices
1939
Dec.
Field listed for planting
11
.70
15
260 lbs. fertilizer 11-11-0 applied on beds.
20
Planted to Imperial 615 strain seed.
24 to
Feb. 7, 1940
15.68
Intermittent showers.
Differential Treatment
1940
A Treatment B Treatment
Feb. 13
Thinned and cultivated beds and Thinned,
furrows with knives and chisels.
14-28
6.32
Intermittent showers.
Mar. 7
Cultivated beds and furrows with Applied 250 lbs. 10-7-14.
knives and chisels.
9
Applied 250 lbs. per acre of 10-7-14.
12
Cultivated beds and furrows with
knives and chisels.
Mar. 16 to
Apr. 8
4.16
Intermittent showers.
Apr. 16
First cutting. First cutting.
19
Second cutting. Second cutting.
22
Third cutting. Third cutting.
26
.48
29
Fourth cutting. Fourth cutting.
Table 31: YIELDS OF LETTUCE, LOVELESS, WATSONVILLE,
SPRING CULTIVATION, 1940
A treatment
(Cultivated 3 times)
B treatment
(No cultivation)
Plot
Total
plants
Total
marketable
* heads
Average weight
marketable
head 1
Plot
Total
plants
Total
marketable
heads
Average weight
marketable
head 1
1
41
29
grams
402
2
41
39
grams
431
3
50
47
423
4
38
38
415
5
40
38
426
6
54
50
441
7
51
49
425
8
43
41
429
9
46
44
441
10
47
46
518
11
47
45
450
12
44
43
413
13
48
43
424
14
44
43
428
15
42
36
418
16
42
40
403
17
50
46
410
18
46
44
401
19
44
37
365
20
38
34
399
21
32
28
380
22
34
34
432
23
31
27
392
24
41
40
403
522
479
416 ±4.7
512
492
428 ±7.3
»Prc
bable error is based on mean weight of marketable heads per plot.
DISCUSSION OF CULTIVATION EXPERIMENTS
Lettuce is a crop which has long re-
ceived frequent cultivations. That some
of this work may be saved is indicated
by the results of these experiments on
plots which were cultivated only for the
control of weeds; or which were not cul-
tivated after thinning but on which weeds
were destroyed by scraping. This practice
produced comparable yields to the fre-
quently cultivated plots. Since cultivation
does not save water in the absence of weed
growth (16) it is clear that the primary
purpose of cultivation after the crop is
planted is to control weeds, and that culti-
vation of lettuce in the absence of weeds
is wasted effort.
SUMMARY
Readily Available Water
The roots of lettuce plants under the
conditions of these experiments penetrate
the soil to a depth of at least 2 feet.
The soil is not thoroughly permeated
by the roots of lettuce as is the case with
most other crops investigated. For this
reason, it is impossible to determine when
the soil moisture in contact with the roots
reaches the permanent wilting percent-
age.
The moisture content of the soil in con-
tact with the roots may be reduced to the
permanent wilting percentage, but that
taken in the soil tube may contain some
soil in which there are no roots, and the
average moisture content of the sample
would be higher than the permanent wilt-
ing percentage indicating that water is
available to the plant; but this may not
be the case. Furthermore, the very small
amount of water taken by transpiration
and the short time the plants were al-
lowed to develop in relation to maturity
added further difficulties in the interpre-
tation of the soil-moisture data.
Soil-moisture records cannot be used
as bases to determine when to irrigate
lettuce. They are, however, the bases for
determining the use of water.
Losses of Water
The loss of water from the soil by evap-
oration, transpiration, and drainage for
any one crop amounted to a depth of
water which may be equivalent to a rain-
fall of from 1.75 to 5.61 inches.
The amount of water taken from the
soil by evaporation, transpiration, and
drainage, averaging 3.95 inches for a
summer crop, and 3.16 inches for a fall
crop, is surprisingly small when com-
pared to the amounts of water applied in
commercial practice.
It should also be remembered that
much water may be saved in using low
beds.
Flooding
Flooding the beds did not reduce the
firmness of the heads. In some cases,
however, if the water is allowed to flood
over the beds, detrimental conditions
may result, especially from excessive
leaching.
Differences in Weights
of Plants and
Marketable Lettuce
Decisive differences in weights of
plants and marketable lettuce in the fre-
quently and infrequently irrigated plots
were obtained in most of the cases when
the interval between irrigations exceeded
30 days.
Frequency of Irrigation
Irrigation need not be more frequent
than once every 30 days after thinning,
and under good irrigation practice, a
total of about 14 inches should be ample
to produce a summer or fall crop.
[50]
Time of Marketability
The time at which the heads became
marketable was neither advanced nor de-
layed by the different irrigation treat-
ments in these experiments.
The firmness of the heads, their mois-
ture content, and their apparent eating
quality also were not changed.
Keeping quality showed no differences
which could be attributed to the differ-
ences in irrigation.
Bolting and Tipburn
Irrigation is not the causal factor for
bolting and tipburn under the conditions
of these experiments.
Yield Differences
There were no real differences in yield
on the frequently cultivated plots com-
pared to plots cultivated only to control
weeds.
Recommendations
Results of these experiments warrant
the following recommendations: that
summer and fall-maturing lettuce in the
Monterey Bay region be irrigated three
times— to germinate the seed, at the time
of thinning, and 30 days thereafter.
It is also recommended that cultiva-
tions after planting be limited in number
and depth only to control weeds.
Acknowledgement: The assistance rendered by Henry Washburn and A. A. Tavernetti throughout
the course of these experiments is gratefully acknowledged.
[51]
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8£m-3,'49(B1387)