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 \ Vfetlt/ Late. r jLove/ess Cu/rivof/on \Z93d 4 /9J9 IN 1 "\ 1 Cofl/tonich " \3~p~ /9JS-/9J9 wa J so N yi L L E; XArr/o. ■ Cv/t/v. JUNCTION LA, I - — l/n/on Ice Co. 7 frr/yo/ion -/938 /^. ^ Sca/e of Af//es o & / Z Irr/oat/on\ - /9J9 i \ \ If 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). OOOOOOOOOOO SZ. /sr^s-/ /? sos-/ oooooooooo oooooooooo ooooooooooo^c/c//v CS(*U ooo o o o oooooo o ooooooo SO' 30O ooo ooo oo oo o o ooo ooo o o ooo oooooooooo oo oooo ooooooo. ^ sV o o o o ooo o oooooo ooooooo. b oooooo oooo OOOOOOOOOOOOOOOOOOO OOOOOOOOOOO^ Q oooooooo ooooooooooooooooooo ooo ooooooo ft N* oooooooo * ^1 ooo o oo oo ooooooo ooo oooooo oo oo/o ooooooo oo oooooo ooo or o ooooooo ooo/o o oo oo o **. PW.P. /5.S M.£.25.4 /-/£ // ~~%~~~- ^-< ^-*C "**»»« >«r — 1 1 *^~" f*.^ /?*tf>? /<5 .-^ ^ 30 y° \ l 30 ^ 25 20 3/ 6 (3 20 27 i May June Treo/menfr: A Af.£. 25.7 t%-2ft. — I— ™- «.t — 'j— < 5=ia ^£3= r-.«^ '"^. \ 1 PW.P 16 5 // 18 25 2 July dug. 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. ^^ 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// /^ 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{? (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 /J Jenfs • 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] Literature Cited 1. Andersen, E. M. 1946. Tipburn of lettuce. New York Agr. Exp. Sta. Bui. 829:1-14. 2. Dearborn, R. B., and J. R. Hepler 1932. Head lettuce in New Hampshire. New Hampshire Agr. Exp. Sta. Cir. 39:1-7. 3. Doneen, L. D. 1947. Seed-bed preparation and cultivation for sugar beets. California Agr. Exp. Sta. Cir. 701:1-16. 4. Doneen, L. D. 1942. Some soil-moisture conditions in relation to growth and nutrition of the sugar beet plant. Ann. Amer. Soc. Sugar Beet Tech., pp. 54-62. 5. Hendrickson, A. H., and F. J. Veihmeyer 1942. Irrigation experiments with pears and apples. California Agr. Exp. Sta. Bui. 667:1-43. 6. Hendrickson, A. H., and F. J. Veihmeyer 1942. Readily available soil moisture and sizes of fruits. Amer. Soc. Hort. Sci. Proc. 40:13-18. 7. Hendrickson, A. H., and F. J. Veihmeyer 1938. Responses of fruit trees to comparatively large amounts of available moisture. Amer. Soc. Hort. Sci. Proc. 35:289-292. 8. Hendrickson, A. H., and F. J. Veihmeyer 1946. Unnecessary irrigation as an added expense in the production of prunes. Amer. Soc. Hort. Sci. Proc. 48:43-47. 9. Knott, J. E., E. M. Andersen, and R. D. Sweet 1939. Problems in the production of iceberg lettuce in New York. New York Agr. Exp. Sta. Bui. 714:1-17. 10. Knott, J. E., and A. A. Tavernetti 1944. Production of head lettuce in California. California Agr. Exp. Sta. Cir. 128:1-51. 11. MacGillivray, J. H., and L. D. Doneen 1942. Soil moisture conditions as related to the irrigation of truck crops on mineral soils. Amer. Soc. Hort. Sci. Proc. 40:483-492. 12. Pryor, D. E. 1944. The big vein disease of lettuce in relation to soil moisture. Jour. Agr. Res. 68(1 ) :l-9. 13. Schwalen, H. C, and M. F. Wharton 1930. Lettuce irrigation studies. Arizona Agr. Exp. Sta. Bui. 133:463-517. 14. Thompson, H. C. 1927. Experimental studies of cultivation of certain vegetable crops. New York Agr. Exp. Sta. Mem. 107:1-73. 15. Thompson, H. C, P. H. Wessels, and H. S. Mills 1931. Cultivation experiments on certain vegetable crops on Long Island. New York Agr. Exp. Sta. Bui. 521:1-14. 16. Veihmeyer, F. J. 1927. Some factors affecting the irrigation requirements of deciduous orchards. Hilgardia 2: 125-291. 17. Veihmeyer, F. J., and A. H. Hendrickson 1943. Essentials of irrigation and cultivation of deciduous orchards. California Agr. Exp. Sta. Cir. 50:1-23. Revised edition. 18. Veihmeyer, F. J., and A. H. Hendrickson 1938. Soil moisture as an indication of root distribution in deciduous orchards. Plant Physiol. 13(1):169-177. 19. Work, R. A., and M. R. Lewis 1936. Relation of soil moisture to pear tree wilting in a heavy clay soil. Jour. Amer. Soc. Agron. 28:124-134. 8£m-3,'49(B1387)