UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY 4, CALIFORNIA SUGGESTIONS ON IRRIGATING COMMERCIAL TRUCK CROPS L. D. DONEEN' and JOHN H. MacGILLIVRAY" Soil moisture is the most important single factor affecting the yields of California truck crops. According to experimental evidence, no benefit is obtained by irrigation while there is available moisture in a major part of the root zone. In the field, irrigation is usually started before the available supply becomes exhausted; otherwise growth will be retarded and the yields reduced. For a few days after a heavy rain or irriga- tion, the water moves downward. When this move- ment stops, the soil is said to be at "field ca- pacity," which may be taken as the upper limit of moisture readily available to plants. The plant roots gradually reduce the moisture until there is none available in the root zone; then, unless water is added immediately, plant growth will be retarded. This lower limit of moisture is some- times indicated by wilting, and is known as the "permanent wilting percentage" of a soil. The following discussion should be helpful in determining when water is needed and how the ir- rigations may be scheduled. The Water-holding Characteristics of the Soils The amount of water necessary to penetrate to a depth of 1 foot when the soil is dry (that is, when all available water has been exhausted) will vary with the soil type. The approximate amounts of water (as measured in depth of water) required for different soil types follow: sandy soils, 3/4 to 1 inch; loam soils, 1 to 1-3/4 inches; and clay soils, 1-3/4 to 2-1/2 inches. A knowledge of the number of inches of water applied can help in estimating the depth of penetration. Figure 1 illustrates wetting of the different soil types with a definite quantity of water. Effect of Soil Structure on Water Penetration and Depth of Rooting Water or plant roots will not readily penetrate a hardpan or any dense, compact layer. Only enough water should be added to wet the soil to the hardpan; otherwise, a water table will form above this compact layer. If the impervious layers are close to the soil surface, water should be added in smaller quantities, but often -"■Assistant Irrigation Agronomist in the Ex- periment Station. 2 Associate Professor of Truck Crops, and Olericuiturist in the Experiment Station. enough to maintain sufficient moisture in this limited volume of soil for adequate plant growth. With gravelly subsoils, heavy irrigations will cause plant food to leach from the surface layers downward; and as very poor root development is usually found in the gravel layers, this plant food will be lost. [1] Fig. 1. --Effect of irrigation methods on penetration of water in different types of soil. Note that if the soil were loam or sand it would be soaked to a greater depth than if it were clay. A, Since a single sprinkler gives uneven distribution, a 50 per cent overlap is essential. The average depth of water applied in A was 4 inches. B, Single furrow between flat-planted crops, such as canning tomatoes with rows 6 feet apart. Because water moves primarily downward and not laterally, often only one third to one half of the soil is irrigated by the use of single narrow furrows. C, Bed irrigation, as used on row crops such as carrots. The depth of water applied was 6 inches. A water table within 6 feet or less of the soil surface will influence irrigation practice. Under these conditions, overirrigation may cause a total loss of the crop. Although plants may obtain some water from the soil just above the water table, UNIVERSITY OF CALIFORNIA LIBRARY COLLEGE OF AGRICULTURE DAVIS the amount is usually insufficient for maximum growth and yields. Under these conditions, only enough water is added to wet the dry soil above the water table. Any excess will raise the water table. As a result, many crops (notably tomatoes) may be "drowned out" through lack of aeration. Amount and Frequency of Irrigation Heavy infrequent irrigations whenever possible are preferred to light frequent ones. Irrigate as much of the space between rows as is feasible so that a large soil volume in the root zone will be wetted, thus reducing the frequency of irriga- tions and saving labor. With rows spaced far apart (for example, canning tomatoes or water- melons) , irrigation by single furrows wets only a small volume of soil as illustrated in figure 1,B, whereas a wider furrow or two furrows be- tween rows will wet a larger volume. Indications of Lack of Soil Moisture On average to deep soils, many plants do not evidence wilting by drooping leaves; they simply cease to grow, if most of the roots have ex- hausted the available soil moisture. Plants hav- ing insufficient water differ in appearance. Corn leaves, for example, curl as illustrated in figure 7; radishes, peppers, and spinach wilt; the older leaves at the base of cucumber, canta- loupe, and tomato plants may change slightly in color, usually becoming a darker green, often almost bluish or grayish, instead of the light green of new growth. Most of these conditions result from checked growth and will probably re- duce the yield. In such cases, irrigation water should be applied immediately. The applications of irrigation water to shallow-, medium-, and deep-rooted crops are shown in figures 2, 3, and 4, respectively. With some soils and crops, one may obtain a guide to the frequency and amount of applications by using, on a small area in a field, half as much water as normally applied, for comparison. How to Determine When Irrigation is Necessary The rate of plant growth is a good index to the need for water. A lack of adequate irriga- tion is indicated by a slowing down of the growth. Certain suggestions may be helpful in recognizing this condition. Examine the soil for moisture in the root zone. Many soils change color between the field capacity and the permanent wilting percentage; usually the soil is darker at the field, capacity and becomes lighter colored as the moisture content approaches the permanent wilting percentage. Examination of the soil in the root zone, at both the field ca- pacity and the permanent wilting percentage, will be helpful in gauging the time of irrigation. Watch for temporary wilting of shallow-rooted crops in the hot afternoons. Note the change in color of the foliage, and the amount or lack of young shoot growth. These are important and easily detected symptoms of a shortage of available soil moisture. For accuracy, take plant or fruit measurements at regular intervals. These, however, require ^ additional time and equipment. The results of these measurements can be plotted as in figure 5. For a particular soil and crop, such measurements after two or three years' experience are a guide for spacing irrigations in future years. Factors Affecting the Frequency of Irrigation The quantity of water required to wet the dif- ferent soil types in the root zone is illustrated Fig. 2 . --Shallow-rooted crops, such as sweet corn (Oregon Evergreen), show marked reduction in size of plant and yield on the nonirrigated plot A as compared with the irrigated plot, B. There was some rolling of leaves on the dry plot (see fig. 7)- Both lots were grown on loam soil at Davis. by figure 1. Sandy soils require frequent irri- gations as compared to clays. Depth, lateral extent, and thoroughness of rooting of the plant (volume of soil from which water is used) are shown in table 1 and figure 6. Shallow-rooted crops must be irrigated oftener than deep-rooted ones. The latter require not more than one irrigation each month for the heavier-type soils, if the wetting penetrates 4 to 6 feet as shown in figure 6 for tomatoes. On [2] Fig. 3 • --Medium-deep-rooted crops such as snap beans (Full Measure) showed marked difference in growth between the nonirrigated plot, A, and the irrigated plot, B. Beans were obtained on both plots, but the yield was greatly reduced under the dry treatment (though without wilting). Both'' lots were grown on clay loam soil at Davis. medium- and deep-rooted crops, frequent, light irrigations only waste water and labor without in- creasing the yields. Evaporation conditions of the air affect the loss of water from plant parts, especially leaves, and also from the soil surface. Most of the water applied to the top 4 to 6 inches of soil is lost by evaporation and is not used. Plants obtain water through their roots from the soil. Most of this water, in turn, passes from the leaves to the air as vapor. Temperature has a material effect : plants lose more water in the hot interior valleys than in the cooler coastal areas. Older plants with large leaf surface use more water than small, young ones. Young lettuce, potatoes, corn, and perhaps others do not root so completely throughout the soil and so do not ob- tain all the usable water; they must, accordingly, be irrigated oftener than their rooting depths would indicate. An example of this condition for corn is illustrated in figure 7. Moisture is evaporated from the top 4 to 6 inches of a soil after each irrigation. There is therefore a much greater loss where frequent, light irrigations are applied instead of infrequent heavy ones. Determination of Water Penetration After an irrigation, determine the water pene- tration both laterally and downward from the cen- ter of the irrigation furrow by use of a shovel, post-hole auger, screw soil auger, soil tube, or pipe probe. Increased resistance on the tube or probe will often indicate a region of dry soil. The probe is not successful in gravelly or strati- fied soil containing extremely sandy layers. Note any change in the soil color. Fig. 4. --Little difference in growth or yield is noted on a deep-rooted crop, such as Connecticut Field pumpkins. Plot A (nonirrigated) produced 22^ tons per acre; plot B (irrigated), 26* tons. Both lots were grown on clay loam soil at Davis. [3; Variation of Lengths of Irrigation Runs with Soil Type Adjust the length of runs to get even penetra- tion. Water penetrates primarily downward (fig. 1,B), not laterally, except on organic (peat or muck) or stratified soils, or on dense subsoils. Desirable slopes to use with different types are as follows: sandy soil, 3 to 12 inches per 100 feet; loam soils, 2 to 7 inches; clays, 1 to 3 inches. Sandy soil should have shorter rows than clay soils, and the length of row may be limited to from 100 to 200 feet. For clays, however, the rows may be 1,000 feet or more in length and still permit fairly uniform moisture penetration. Observation of Plants to Determine Safe Intervals between Irrigations Study the crop for each soil type and locality in order to gauge the interval between irrigations. Observe rate of growth, wilting, color, and other indications. s /s i ! 18 V \^£ 2. -st^^fc^ — I/ / it 8 f WATERMELONS 4 J_t-1. I ""--^ IS 22 29 9 20 29 8 21 JUNE JULY AUG. /o /j 16 19 ^^ es ?8 3/ 3 e 9 12 is At/va j / September Fig. 5. --Effect of irrigation treatment on height of sweet corn (shallow-rooted) and on diameter of watermelons (deep-rooted). The sweet corn showed symptoms of water shortage as early as the last of May. A, No irrigation; B, frequent irrigation--8 applications, the first on May 20; C, less frequent--3 applications, the first on May 29; D, inf requent--2 applications, the first on July 10. The irrigations for water- melons were the same as for the corn except for dates of application. The curves show that water- melons were little affected by amount of water applied or the frequency of application. Both were on clay loam soil at Davis. Fig. 6. --Water removed from the soil by the tomato plant, a deep-rooted crop, as the root system grows downward. The clear areas in each rectangle indicate the proportion of available water removed; the hatched areas signify water still available for plant use. This plot was not irrigated, and soil moisture was used to a depth of 12 feet by October 23, representing a total depth of 21 inches of water. A slowing down of growth occurred about August 8 when most of the soil moisture had been used in the surface 6 feet of soil. This plot gave a yield of about $0 per cent when compared to the irrigated plots. [4] Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/suggestionsonirr18done Irrigation of Planted Seed Keep the seedbed moist to give rapid germina- tion, thus permitting early cultivation for weed control. Some crops, however, are not seriously- affected during germination by soil moistures near the permanent wilting percentage for a short period — for example, cabbage, turnips, radishes, squash, sweet corn, watermelons, tomatoes, canta- loupe, peppers, cucumbers, onions, and spinach. Soil moisture at planting time should not, how- ever, drop too low for carrots, snap beans, and New Zealand spinach, whose germination is poor just above the permanent wilting percentage. On the other hand, the per cent germination for let- tuce, lima beans, peas, and beets drops rather markedly unless the moisture is maintained at a high level. For celery, good germination will be obtained only when the soil is kept wet. Seeds germinate more rapidly in a soil when the moisture is near field capacity than when it is just above the permanent wilting percentage. Fig. 7- --Rolling of leaves, a typical in- dication of insufficient soil moisture with sweet corn. TABLE 1 Depth of Rooting of Truck Crops and Total Amounts of Irrigation Water Suggested for Commercial Production* Shallow-rooted Depth of Moderately deep-rooted Depth Deep-rooted Depth of (down to 2 feet) water (down to 4 feet) of wate: (down to 6 feet) water inches 12 Beans, pole inches Artichokes inches Brussels sprouts 15 12 Cabbage 12 Beans , snap , spring 12 Asparagus 20 Cauliflower 12 Beans, snap, fall 18 Cantaloupes, inland Celery 30 Beets 18 valleys 18 Lettuce, winter 6 Carrots, coastal area 18 Cantaloupes , Imperial Lettuce , summer and fall 18 Carrots, Imperial Valley 24 Valley 24 Lettuce, Imperial Valley 18 Chard 18 Lima beans 12 Onions, intermediate 15 Cucumber 15 Parsnips 20 Onions, late 24 Eggplant 18 Pumpkins 18 Potatoes, early 30 Peas, winter 6 Squash, winter 18 Potatoes, late 20 Peas , fall 18 Sweet potatoes 18 Radish 12 Peas, Imperial Valley 18 Tomatoes, inland valleys 24 Spinach 9 Peppers 18 Tomatoes , coastal areas 12 Sprouting broccoli 12 Squash , summer 18 Watermelons 15 Sweet corn 18 Turnips 15 *Onions are very shallow-rooted. With this crop it is necessary to maintain available water in the surface 6 to 10 inches for maximum yields. Potatoes, lettuce, and corn develop poor root systems. A few roots penetrate deeply, but they are not numerous enough to permeate the soil thoroughly at greater depths. It is necessary to keep avail- able soil moisture in the surface foot of soil, especially for the first part of the growing period. Growers believe that frequent irrigations give best yields with celery. Tomatoes will root 6 feet or more in deep soils before irrigation is necessary. Apply heavy irri- gations before harvesting, and wet the soil to 6 feet. With canning tomatoes, in most soils no irri- gation will be necessary during the harvesting period if the soil has been thoroughly wetted just be- fore harvest. Cantaloupes will root to nearly 6 feet by maturity. There may be reduction in set of late fruit when moisture is almost depleted in the surface 3 feet. The size of melons is not materially affected by soil moisture. [5] 5m-9,'43(7686)