STUDIES ON THE IREIG&TION OF CITRUS GROVES. Edward E Thornae , THE LIBRARY OF THE OF LOS UNIVERSITY CALIFORNIA ANGELES UNIVERSITY OF CALIFORNIA PUBLICATIONS STUDIES ON THE IRRIGATION OF CITRUS GROVES BY EDWARD E. THOMAS BULLETIN No. 341 MARCH, 1922 DIVISION OF SUBTROPICAL HORTICULTURE COLLEGE OF AGRICULTURE BERKELEY, CALIFORNIA UNIVERSITY OF CALIFORNIA PRESS BERKELEY, CALIFORNIA 1922 DAVID P. BARROWS, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS THOMAS FORSYTH HUNT, Dean. EDWARD J. WICKSON, Horticulture (Emeritus). , Director of Resident Instruction. CLARENCE M. HARING, Veterinary Science, Director Agricultural Experiment Station. B. H. CROCHERON, Director of Agricultural Extension. H. J. WEBBER, Citrieulture, Director Citrus Experiment Station. H. E. VAN NORMAN, Dairy Management. WILLIAM A. SETCHELL, Botany. MYER E. JAFFA, Nutrition. RALPH E. SMITH, Plant Pathology. JOHN W. GILMORE, Agronomy. CHARLES F. SHAW, Soil Technology. JOHN W. GREGG, Landscape Gardening and Floriculture. FREDERIC T. BIOLETTI, Viticulture and Fruit Products. WARREN T. CLARKE, Agricultural Extension. ERNEST B. BABCOCK, Genetics. GORDON H. TRUE, Animal Husbandry. WALTER MULFORD, Forestry. JAMES T. BARRETT, Plant Pathology. FRITZ W. WOLL, Animal Nutrition. W. P. KELLEY, Agricultural Chemistry. H. J. QUAYLE, Entomology. ELWOOD MEAD, Rural Institutions. H. 8. REED, Plant Physiology. L. D. BATCHELOR, Orchard Management. J. C. WHITTEN, Pomology. *FRANK ADAMS, Irrigation Investigations. C. L. ROADHOUSE, Dairy Industry. R. L. ADAMS, Farm Management. W. B. HERMS, Entomology and Parasitology. F. L. GRIFFIN, Agricultural Education. JOHN E. DOUGHERTY, Poultry Husbandry. D. R. HOAGLAND, Plant Nutrition. G. H. HART, Veterinary Science. L. J. FLETCHER, Agricultural Engineering. EDWIN C. VOORHIES, Assistant to the Dean. CITRUS EXPERIMENT STATION DIVISION OF AGRICULTURAL, CHEMISTRY W. P. KELLEY A. B. CUMMINS E. E. THOMAS S. M. BKOWN * In cooperation with office of Public Roads and Rural Engineering, U. 8. Department of Agriculture. STUDIES ON THE IRRIGATION OF CITRUS GROVES 1 By EDWAKD E. THOMAS There are many complex and as yet unsolved problems connected with irrigation in semiarid regions. As will be shown later, the method employed in the application of water may be the deciding factor between success and failure of crops. In order to obtain the best results, a careful study of the water requirements of the crops grown and of the best method of applying the water should be made. Similar soils under different atmospheric conditions may require variations in the irrigation practice. The rates of transpiration by the crop and evaporation from the surface of the soil are probably much greater in the interior sections of California than in the coastal regions, because the temperature and aridity of the former are higher than of the latter. These factors may determine, in consider- able measure, the times of application and the amount of water needed. Wide differences occur in the soil conditions in different sections. The method of irrigation best suited to heavy soils may differ from that of light sandy soils, for. the reason that the former type tends to restrict the movement of water, while the latter permits freer percolation and in turn gives up the water more readily. The content of colloidal material may vary in soils which have the same wilting coefficient as determined by the centrifuge method. A soil which contains an appreciable amount of colloidal material will swell or expand upon being wetted. This swelling tends to retard the movement of water and also reduces aeration. A differ- ence in the swelling may be brought about not only by a difference in the amount of colloid present, but also by a difference in the nature of the colloid. Consequently, two soils, although they may have the same water holding capacity as judged by the centrifuge method, may act in an entirely different manner when wetted. Generally speaking, the practice of irrigation in the citrus groves of California has followed certain arbitrary rules, especially in regard to the frequency of application and the amount of water applied. There is very little definite knowledge concerning the economy of the methods in use. i Paper No. 81, University of California Citrus Experiment Station and Grad- uate School of Tropical Agriculture, Riverside, California. 209942 354 UNIVERSITY OF CALIFORNIA KXI'KRIMKNT STATION The fact that crop growth is stimulated by the application of moderate amounts of water, seems to have led some citrus growers into the error of applying excessive amounts. Instances are known where as much as sixty acre- inches per acre have been applied in one irrigation season. In such cases, the naturally dry portion of the year is effectively converted into a wet season, so far as the soil is concerned, since the annual rainfall is only ten to fifteen inches, which falls mainly in the winter months. Excessive irrigation is most prevalent in sections where the water is drawn from readily available supplies, such as rivers. There are instances where water has been applied early in the spring, before the usual irrigation season, with a view to "storing up" moisture in the soil for use during the dry season. Later, whenever irrigation water is available, excessive amounts may again be applied, regardless of the moisture content of the soil at the time. Many citrus groves located on heavy soils have been heavily irrigated as frequently as every ten days to two weeks during the spring and early part of the summer. Citrus trees are more sensitive to an excess of moisture than some other crops. For this reason the supply of soil moisture in citrus groves should be carefully controlled. The methods of distributing the water have not been given ade- quate consideration. At present, the irrigation furrows are often from 500 to 800 feet in length and one citrus grove has been under observation which is irrigated from furrows 1380 feet in length. As will be shown below, a uniform distribution of irrigation water is impossible with the use of furrows of such length. An interesting experiment to test the effect of various moisture conditions on the growth of lemon trees was conducted with a loam soil of the Limoneira Ranch Company, Santa Paula, California, by Fowler and Lipman. 2 By growing young lemon trees in cylinders in which the moisture content was varied, they determined the per- centage of moisture at which the soil must be kept in order to pro- duce the best growth. They found that the growth of the trees was retarded when the moisture content was either above or below the optimum. The optimum, however, appears to be not a fixi-d point. but rather covers a considerable range. Not all of the moisture in a soil can be absorbed by plants. A part of it is unavailable, the amount of which varies with the soil 2 "Optimum Moisture Condition* for Young Lemon Trees on a Loam Soil." By L. W. Fowler and C. B. Lipman, Univ. of California Publication* in Ayri cultural Science*, Vol. 3, No. 2, pp. 25-36. Heptemlxr, 1917. BULLETIN 341] STUDIES ON THE IRRIGATION OF CITRUS GROVES 355 type. The amount of unavailable moisture in a heavy soil is suffi- cient to saturate a light, sandy soil. It is important, therefore, to distinguish between the available and unavailable moisture in a soil. Briggs and Shantz state that, 3 "in connection with the study of the moisture requirements of plants in semiarid regions it is neces- sary to be able to determine quickly the soil-moisture content at the wilting point. This constitutes the datum from which the moisture available for growth can be calculated and without which field de- terminations of soil moisture are of little value." The unavailable water in soils is now commonly determined by the use of the moisture-equivalent centrifuge. The data thus ob- tained are referred to as "wilting coefficients." The wilting co- efficient is supposed to indicate the moisture content of the soil when a plant becomes permanently wilted. By the permanent wilting of the plant, is meant a condition from which it cannot regain its turgor when surrounded by a saturated atmosphere, unless water be added to the soil. As previously intimated, the determination of the so-called wilt- ing coefficient may not always give data by which the water-absorb- ing and water-retaining power of soils derived from entirely dif- ferent sources can be measured accurately. Such data, however, probably afford information of considerable value in comparing soils of a similar type. The present paper is based on experiments which- were conducted for two seasons in certain citrus groves located on a heavy type of soil. The groves studied are located in the Whittier, East Whittier and La Habra sections of Los Angeles and Orange counties. "While the results obtained and the conclusions drawn may not apply equally to other types of soil, it is believed that some of the practical lessons taught by these studies are widely applicable, and sufficiently gen- eral to warrant publication. This investigation was conducted during the irrigation seasons of 1916 and 1917. The soils studied were all of the same general type, having a wilting coefficient of from 9 to 19 per cent, with an average of 14 per cent. Four points have been investigated in this work: (1) the amount of water in the soil at the upper and lower ends of long furrows; (2) the appearance and productiveness of the trees growing at the upper and lower ends of long furrows; (3) the amount of available 3 "The Wilting Coefficient of different plants and its indirect determina- tion. ' ' By Lyman J. Briggs and H. L. Shantz. Bureau of Plant Industry, Bull. No. 230, p. 8, February, 1912. DIVISION OF SUBTROPICAL HORTICULTURE COLLEGE OF AGRICULTURE 356 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION moisture 4 in the soil when the water was applied at intervals of 30, 45 and 60 days; and (4) the effect of deep and shallow cultivation on the water-absorbing power of the soil. Altogether, twenty different groves have been studied. The re- sults obtained with only seven of these, chosen to represent the dis- tricts in question, will be presented at present. 5 The same general course of procedure was followed in each grove. Samples of soil were taken at regular intervals from a given plot and a determination was made of the total moisture content in each foot to a depth of four feet and, in some instances, to a depth of six or eight feet. The wilting coefficient of samples representing each foot of the plots studied was determined by Mr. C. A. Jensen, formerly of the United States Department of Agriculture, to whom acknowledge- ment and thanks are extended. I. Comparison of the amount of water in the soil at the upper and lower ends of long furrows. In the grove chosen to show the effect of the length of furrow on the content of water in the soil, the furrows were 812 feet long with a fall of sixteen feet. Soil-moisture determinations were made on two plots in this grove during the 1917 irrigation season. The wilt- ing coefficient of the soil to a depth of eight feet, is given in table I. TABLE I THE WILTING COEFFICIENT OF PLOTS A AND B, GROVE 1 Plot 1st ft. 2d ft. 3d ft. 4th ft. 5th ft. 6th ft. 7th ft. 8th ft. A 1 16.90% 17.15% 17.30% 16.90% 16.75% 16.75% 18.30% 15.20% B* 16.90% 17.20% 16.90% 18.80% 19.00% 17.60% 16.80% 16.60% 'Situated at the upper end of the irrigation furrows. 'Situated near the lower end of the irrigation furrows. As is common in irrigation practice in certain localities, the owner of this grove irrigates whenever he considers it necessary rather than at any stated interval. During the season of 1917 four irrigations were applied at intervals of thirty days, followed by three additional irrigations at intervals of 25, 37 and 41 days, respectively, 4 The term "available moisture," as use oc r-- c CO OS X CS Q Tt* c& t^* co c* 1 - r CC -, ! C CO < -" CN M C^ 1 1 1 1 1 : x c oc -- c r - > a 1 (V 15 - tN CM O - S 1 CO CO CO ~+ f > ^j 1 N C r? 5 b C i if I CN ) ^ - 1 i- cc os c ?s / i O t Ci Oi C5 0! r cp r>- CN ^ CN CO O M C ) (X IS ) tC X 1 - IX 'N ?! CN W C 1 - 1 1 1 1 1 * M C X C CM O X CO >C "1 ^1 d - 7 ' Oi Cft l~- X OC CO CO C X C ) C 1 s r CC cc 1 - r>. c -" "- 1 o ci - CO O ' CO l^ CO Cf CC i 00 ^H X X X Ot * ) O ,s 1 t> - tf. O *- CM CN 1 1 f 1- o cc - 1 00 CO CO X CC c r ' Tf Cft CN *1< t-> i o o x eo oc > u; > es *4 ~ - C5 ^ -H M O CO t^ (N CO "^ OC V 5 C } ^ >~ / . jg X ^* O5 X OC 1 i i ** < i ic c o *i ~ i- - O CN CN -^ I-H CM rj O J^- C 1 ^ I"*" X Cft CH e* > cc > : i- - i r~ CO CO X CO t~- *T M H i i o CQ eg j <; CH H ^ (NlcX^niCc' CC Si - ; CN 00 PH rt CN S ' ' ' ^^ *o ^^ f^* *o ^* *^" t-^ 00 C^ *^ ^2 ^* ^ es s i - IS > o r~ x cs o c- < ' H " ^ O ^H 1C O X r- o co CN B ^* O t^* l^ Ci u ^ W X t^ C Ed *"! '^ ^ **! eo' r^ co c CN Tf O5 CN ' C Tf 1"" X 5 2 M Si X O Cft 1C 1C * CO low the SJ co x ic >c 2^2^ co cs co o Ji o x oi ci x co r^ x o - CM -r c-. g il moisture Ci c ^T co ^ ""^ O CC 1 ^* 'T O CO C ~- -i co o 55 ft TJ t^- CN c : J 7 i i CO *t* *C CO 1 1 1 I 1 e |. ) a - : = H ? ; - ^3 x _ J= i ^ ; J= J= JS < x ^ - C^ CO f fl ~ 1 - 00 -i CN CO f C CO - oc BULLETIN 341 J STUDIES ON THE IRRIGATION OF CITRUS GROVES 359 II. A comparison of the appearance and productiveness of trees at the upper and lower ends of long furrows. Two plots, K and L, (grove 2) are chosen to illustrate the effects of excessive irrigation on the appearance and productivity of the trees. This grove was irrigated from four distributing lines and plot K was located at the lower end of the furrows in an upper block of trees, while plot L was located just across a distributing pipe line from plot K at the upper end of the furrows in the sec- ond block of trees. Each plot consisted of nine orange trees. Plot K being adjacent to plot L, it is believed that the soil and climatic conditions in the two plots were similar in all respects. The entire grove of sixty-seven acres was given the same kind of cultivation and fertilization, and it was irrigated throughout at thirty-day intervals, beginning the latter part of April. The amount of water delivered to plots K and L was very dif- ferent, as may be appreciated when their location is considered. Plot K was located at the lower end of irrigation furrows 248 feet in length, and plot L was located just across a distributing pipe line from plot K at the upper end of furrows 528 feet long. The entire section in which plot K was situated received an average of 2.43 acre-inches of water per acre at each irrigation/ but as the ex- perimental plot was located at the lower end of the irrigation fur- rows, the amount of water actually delivered to it must have been below the average for the plot. The section in which plot L was located received an average of 4.28 acre-inches at each irrigation. Since plot L was located at the upper end of the irrigation fur- rows, it must have received more water than the average for the section. The difference in the appearance of the trees in different parts of the grove was very noticeable. All of the trees in plot K, the lightly irrigated plot, were in fine condition. They bore an abund- ance of normal green leaves and, without exception, produced a good crop of fruit. The trees in plot L, on the other hand, were in poor condition. Many of the leaves dropped prematurely and a large percentage of those remaining on the tree were yellow. The trees appeared to be suffering from a lack of nourishment and the crop of fruit was light. A careful record of the amount of fruit produced on plots K and L was kept in 1917. It was found that the nine trees in plot K which received the smaller amount of water produced fifty-five boxes of fruit, while those in plot L yielded twenty-four boxes. DIVISION OF SUBTROPICAL HORTICULTURE COLLEGE OF AGRICULTURE 360 rXIVKRSITY OF CALIFORNIA KXPKRIMKNT STATION i i i i H J s. PERCENTAGE OF AVAILABLE MOISTURE IN PLOTS K AND L, GROVE 2* e e o o o 5 OB CO 00 CO 00 00 o M H t- 1 e* CO o N r n o r- o c * ja a - CO CO 05 -H CO 1C M et o o 10 N M 9 "5 O oc q s 7 i X CN Tj *} CN .-H 1 CN CS CO CO CO CO iC O * 00 O CO O cc -r CO CN 1 1 OO t* 1^ M i 1 CN 1 g 1C !> S OC OS SO CO 1 1 O OS t-H 1-4 \ (N ISS 00 iC (N '0 1 1 cs co GO UJ 1 CO 00 t^ oo CS CO Sc^ 00 CO ^t* co CO 1 1 8 Si 1 O (N C- CS CO ^* >5 (0 2 3 1C CO i CO (M N (N ^ 1 1 So CO 1 1 10 o^ ^^ *2! ^ O5 00 ^*^ 5 CC CO O l^ t^ Tf< co CO O5 i-l OO t^- CO co I-H 1 1 CO 1C -( *o c*i co oo i t>- iC " OO CO - CO t^ IM t^ 1-1 O t^- co r** co o 7 i 1C 00 CO 00 CN n o io r- Tf t>- O CO CS OC O d *& ^J* i 1 1C l~- ?1 -^ OO CO iC ^* ^* r- 1 I~- CO OS CO 1^- OO l> O O OS 7 * CO 00 CO O Tf 1C CN .-I CO CO CO ? CC l^ Tjt C CC CC CS l>- t-. CO 1C 1C 1C 'a 1C CD 00 ^ it* ' ^* 01 O^ ^ CO t 1 CO CN " rf OS CO Cl > ^ u: ! CO S u M CO CO CO Tt co OS I-- 00 1-H S3 i JS f 8 o CO c 1C C 1C C S3 l^ co OS 1- OS -H i s 8 CO OS CN go r-< O / / 00 t^ OS C 1 1 i-l CO ** " o M . ^ CN ^ c t>. i 1 JS : _c ' I & * ; . r Qg} H Pen T3 *-*-**- ^- W CO ^ >O CO l^- BULLETIN 341] STUDIES ON THE IRRIGATION OF CITRUS GROVES 361 Thus the trees in the adjoining plots K and L, with the same care and other conditions, with the exception of the amount of water delivered to them, were totally unlike in appearance and pro- duction. (The available moisture in plots K and L, Grove 2, is given in table III.) It seems safe to conclude from the facts above presented, that the trees at the upper end of the long irrigation furrows received too much water. The more or less constantly water-logged condi- tion of the soil in this case must have interfered with its proper aeration and also must have hindered normal root development. III. A comparison of the amount of available moisture in the soil when irrigations were applied at intervals of 30, 45 and 60 days. Whenever possible, irrigation practices should be so arranged that the water may be applied to the soil at the time when needed. The time of application, as well as the amount of water applied, should be controlled by soil moisture tests. While this means of irrigation control may be difficult of execution where the water is obtained from irrigation companies that use the present system of water distribution, it is possible to change the irrigation interval, and this should be done wherever the prevailing system is unduly faulty. It, therefore, becomes important to know whether the in- terval now in use in a given case is best suited to the soil type in question. Most of the groves in the district under consideration are irrigated every thirty days. As a means of studying this question a comparison of the avail- able moisture was made in groves that were irrigated at intervals of 30, 45 and 60 days. Two adjacent groves were studied, one of which, grove 3, was irrigated at intervals of thirty days and the other, grove 4, at in- tervals of sixty days. These two groves were under the same man- agement and, with the exception of the interval between irriga- tions, were given the same cultural treatment. Grove 4 received an average of 5.7 acre-inches of water per acre every sixty days, which provides for a lesser amount of water for the season as a whole, than was applied to grove 3, which received an average of 4.28 acre-inches of water per acre every thirty days. Comparisons of the available moisture between plot C (grove 3) and plot G (grove 4) are shown in table IV. The wilting coefficients of these two soils are as follows : 1st ft. 2d ft. 3d ft. 4th ft. Grove 3, plot C 14.4% 15.2% 14.9% 14.4% Grove 4, plot G 13.6% 14.7% 19.9% 14.5% DIVISION OF SUBTROPICAL HORT!CULUJR , <-. IM/M ii TI inr t'NIVKRSITY OF CALIFORNIA KXPKRIMKNT STATION * c JB C K 2 X cc' H i o o o 5 "3 O a T a n oc 00 00 r (N 1 - '/ '- -J C iC *C f^ cb fi 1 S q 5 cc "l oc co os ec 9 94 00 ^ ci ec <' i C 1C C r ec c ; 1 2 ! c 3j ec - c^' ri (M I- (M f. i - r. / (N ec 1 1 ! ^ -. ^ s 7 iC ?* t^- ^t 00 C5 C^ 1C ^ o c^ ^f r^ J ^f lO CN O o ?i ~ 1 CC CO -< Q "" O ^ C 00 O V4 ' H 1 -r o ^t 1 ! rf Q i-j Tf (N OS 7 ' ^^ _ Oi ^O ^* ** b r- (N o ? I " TABLE ] 2 X H 3 U H r- s - CU t ~ t. 1 1 ec h" t^ iC C: Ci O C 1 1 1 1 o 1 H to jv| 1C 1C <-i Q J as oq oq os W C5 t>- CO ' p j^ CO 00 CO QD - iC ; t>. r^ 2 ^ ; O CO C 7 7 i i "l 1 1 M 5 M ~ i ~. _ - - C OS O OC f CC O CO t- C C<1 CC t^; t^ c i J t^; 3 t| CO - CC ' * ' _ 113 I 1 | - | %$ 2 CN os ec M ic r>- cs i^ I 1 ' 7 i 1- ec -f q i-| " O OS ec 3- ~* ^t* ec cc O r ; o cc r>^ t>^ x :- M 1 1 1 i e "* a "* : g ; B A a - a - -'. a 233 3JS v - i _ _, JS Q ^Se? $ BULLETIN 341] STUDIES ON THE IRRIGATION OF CITRUS GROVES 363 The data show that plot G, grove 4, to which the water was ap- plied at sixty-day irrigation intervals, contained as much available moisture as plot C, grove 3, which received a greater amount of water during the season, but applied at thirty-day intervals. A comparison of the trees in the two groves showed that, on the whole, the trees in the grove irrigated at sixty-day intervals were in better condition than those in the grove irrigated at thirty-day intervals. Studies were also made on another grove (5). This grove was divided into nine plots, A to I inclusive. During the 1917 irriga- tion season, plots A and E were irrigated every sixty days; B, C, F and G every thirty days; and D, H and I every forty-five days. At each irrigation, plots A, B, E and F received an average of four acre-inches per acre ; plots C and G an average of two acre-inches per acre; plots D and H an average of three acre-inches per acre; and plot I an average of six acre-inches per acre. Thus, plots A, C, D, E, G and H received the same amount of water during the season, while plots B, F and I received twice that amount. In this grove the irrigation furrows were 250 feet in length and the plots were all located between forty and sixty feet from the distributing pipe line. The wilting coefficients in the nine plots of grove 5 are given in table V. A study of these data shows that the soil to a depth of eight feet, is quite uniform in all of the plots. TABLE V THE WILTING COEFFICIENT IN THE PLOTS IN GROVE 5 Plot 1st ft. 2d ft. 3d ft. 4th ft. 5th ft. 6th ft. 7th ft. 8th ft. A 14.06% 13.5% 12.5% 11.0% 12.65% 14.9% 13.8% 14.24% B . 14.9 14.1 14.3 14.1 15.3 16 15 14.1 12 2 C 15.2 14.9 15.0 15.3 15.5 146 13.5 127 D ... 14.1 14.6 13.4 13.3 13.85 13.6 13.75 12.45 E 14.6 15.1 14.9 15.3 15.05 14.5 13.6 13.15 F 13.0 12.6 9.5 12.8 156 155 155 150 G 13.45 13.6 13.85 14.1 14.9 14.5 13.3 128 H ... 13.05 14.0 14.5 15.2 15.0 13.6 12.9 11.6 I... 13.4 14.1 14.7 15.9 12.05 11.8 10.7 10.5 The data in table VI show, that the available moisture was more uniformly distributed, to a depth of eight feet, in the plot which was irrigated at sixty-day intervals than in the plots which were irrigated more frequently. OF SUBTROPICAL HOR1ICUL1UI* OF AGRICULTURE UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 5 csxcii^ccec'Nec 5 3 ; q cq q o x -; i-Ioociocix'xx t^ O Q -- >~ O C CC >:? O CC CC oo i^ 'fO QO IN 00 CC X CC Ci ~* ~ >C CC CC'J-flOC^XXCC - 1 O>CO5C5-fXOC5 J * C> t C W tO 1< X k g 1 co CN _ ^ -^4 ^J (Ji ^J ^^ ^^ 1 1 * , 1 W CM. r* QO oc o as cc co H > ^ -HOI-C5XXC:0 ' * cS* e5 Q T3 00 I s * CO CC *^ *O O^ C*J CO l^* *^ t'* ^O CO ^5 *"^ C 1 ! -a o 00 lONO'^'tt^XcO ^ C i X 1C >C C t~ Ci ^ S t- H >. 00 00 O X O t^ cc b- (M'lCiOXt^XCSX U >> > u 0) 23 ^-v5^t>.^.^OS0 CJ {^ogstojoeooooo - CCO" r f'fC5CC' it* T3 J^ o i^- cc 'C >c cc o i-< < S (NCXXXC5^-i(M *OCOC5COX.-Hi2 B "Q. a S a j* a C3 SH h n M ^5 oC C? C^i ^t^ ^^ ~^ C^l iC *C ^C r Sj C3 t^ CC ^C g S c^ccxcscc?]2 a H g O t^ t>! IQ 00 00 O O 1-H ^-( :3 is f C ^-1 1 1 -~ P o $ i S n CC M^ C 'C 't 1 C >O O tn CJ CO Ci Ci O O o M o to 1 O D ^ 1C X S e . a V r. f 1C C CI M V f* lf^ 5^ 1^ ^rt - * ri "f M O *T 09 5 C CC t^ .or>.ow5 ~ t-H p 'C p -^ X iq OJ > *' t^ ij" O t^ Tt< X O "i tC -* - O C -< t>- O I- CO CO W -p T3 ^SSS^rS i r^ ~^ ^! o -r - O ?1 'O : : : C Ci iO -- : : : r o ?o o j : 15 o to X : 5 -3 to t- : " 1 CC (N W : : j - CC O t 1 * ' J h- 1*- CS i 4 j . if) O U3 : . BULLETIN 341] STUDIES ON THE IRRIGATION OP CITRUS GROVES 365 COCOCOOC5OOI>; C^' *C CD 1C CO t^ t^ T-H T-l rH CO CO I I C<> i-HCOiCiCOOOOOOCi tO CO I> GO O O5 00 ^ CO !> 1C *}<_ 1C . O l>; r-t i I 00 C I> 00 CO CD t^- ''t 1 S -^ a I 9 O EH O fi CO '-C S oo S CO (M 1C 1C l-~ * -* q iC 00 ^ CO U3 O 00 O CO e* 'd O I >C 00 O ^H IQ CO CD OO O5 2 CO CD i I OO o CO CO (N CD s SOO i I t^ co 10 c O O i-H I 1 i-H I 1 t- I- 1C O 1C O i I ^ 5 1 1C t^. CO C! H -= Qu; rH H ! *o 1 t) _c V 1 CO Q s AH 00 1 to 00 5 00 O (O : CO 00 00 00 00 O -1 oo X s. - 00 CO O . o ri O 10 CO CO indicates sc CM 1 CO lus sign 'S ::::: u n ^H -r M n : : : : : H * U5 CO t~ 00 ,.v.SION OF SUBTROPICAL HORTICULlUi:! COLLEGE OF AGRICULTURE 366 I'NIVERSITY OF CALIFORNIA KXI'KRJMKNT STATION The data also indicate that all of the plots probably received more water than was essential for the best development of the trees, since the available moisture in all of the plots was comparatively high throughout the season. IV. A comparison of the effect of deep and shallow cultivation on the water-absorbing power of the soil. Studies have also been made on the water-absorbing power of heavy soil as influenced by deep and shallow cultivation. The groves studied (6 and 7) are adjacent and the soil is of a heavy type. Each of these groves was irrigated at sixty-day intervals in 1916 and 1917. Grove 6, however, was irrigated from two pipe lines, the upper fur- rows having a length of 339 feet and the lower ones 300 feet, while grove 7 is provided with but one pipe line at the upper end of the grove and the furrows are 714 feet in length. After the soil in grove 6 had become sufficiently dry near the surface to prevent puddling, it was harrowed lightly. Later, when the soil had dried out more deeply, a cultivation to a depth of seven to nine inches was given the grove. Usually this latter cultivation was not made until twenty-five or thirty days after an irrigation." The treatment of the soil in grove 7 was very different from that just described. In this case the soil was cultivated to a depth of three to five inches at a time when portions of it were so wet that puddling resulted. Later two or three "cultivations of a similar nature were given before the next irrigation. The soil in grove 7 did not absorb water as well as the soil in grove 6. -This was clearly shown during the winters of 1916-17 and 1917-18, when a portion of the water from the rains ran off and was lost, while the soil in grove 6 absorbed all of the rain water. Stable manure had been applied to both groves, but on account of the difference in cultivation, it was more thoroughly incorpor- ated with the soil in the grove that was tilled the more deeply. This fact probably influenced the evaporation of the soil moisture and also tended to increase the water-absorbing power of the soil. How- ever, by far the greatest difference in the treatment of these soils was in the manner and depth of cultivation. It is necessary to delay the cultivation of this soil much longer than with lighter types. Thin is due to the fact that being a heavy soil it absorbs much more water than a lighter type, and more time is required for evaporation and percolation to reduce the moisture to a point where the soil can be thoroughly cultivated without injury from puddling. BULLETIN 341] STUDIES ON THE IRRIGATION OF CITRUS GROVES 367 A comparison of the available moisture (table VII) shows that on May 5, before the first irrigation, the soil in plots A and B, grove 6, which was cultivated more judiciously, contained greater amounts of available moisture than that in plots C and D, grove 7. This difference became greater as the season advanced, until we find that in plot D, grove 7, which received the shallow cultivation, the moisture in the third and fourth feet of soil was below the wilt- ing coefficient continuously after July 5, and the moisture in the first and second feet was below the wilting coefficient a portion of the time. The trees on this plot suffered from a lack of moisture, and the fruit, as well as the leaves of the trees, became wilted. On the other hand, all of the trees in grove 6 were in fine condition and yielded a large amount of good fruit. The great difference in the physical condition and the moisture content of the soil of these two groves clearly illustrates the need of great care in the cultivation of a heavy type of soil. The soil should not be stirred while it is wet, but should be allowed to remain undis- turbed until sufficiently dry to permit thorough and deep cultiva- tion. CONCLUSIONS. (1) The results of this investigation indicate that irrigation fur- rows which exceed 250 or 300 feet in length are undesirable. When furrows of a greater length are used, the soil near the upper end of the furrows must inevitably receive an excess of water. The re- sult is that it becomes "water-logged." A portion of the soluble plant food will be leached and carried below the root zone by the water. (2) Citrus trees growing on heavy soil may become stunted when excessively irrigated; the leaves turn more or less yellow and many of them fall prematurely. The yield of fruit is also greatly impaired and in some instances the trees may become unprofitable. In many groves it would be advisable to install a greater number of distrib- uting pipe lines in order to shorten the irrigation runs. (3) In order to secure the best results from the application of irrigation water, it should be applied at the time when it is needed, as gauged by the moisture content of the soil. Whenever this pro- cedure is impracticable, the irrigation water should be applied at intervals best suited to the soil type, as determined by experience or definite experiments. 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