factors affecting soil moisture plant growth relations(hagan

82 SYMPOSIUM II

FACTORS AFFECTING SOIL MOISTURE - PLANT GROWTH RELATIONS1

ROBERT M. HAGAN -

Department of Irrigation, University of California. Davis, Ca!if., U.S.A.

SUMMARY The gross effects of deficient and of excessive soil moisture on plant growth are well known, but controversy has existed for many years around the question whether the so-called “'available moisture” is equally available for plant growth or available only with such increasing difficulty that plant growth functions are retarded before the wilting point is reached. Existing viewpoints are illustrated by schematic diagrams and the probable effects of plant, soil, weather, and some other miscellaneous factors on. soil rnoisture-plant growth relations, discussed. Considered are the theory that plants can obtain water with equal facility between field capacity and permanent wilting percentage; the notion that growth diminishes progressively as soil moisture falls below field capacity; and the theory that plant functioning is directly related to the tenacity with which water is held by soil. Also considered are views that physiological activity may be related to time integrated soil moisture stress or to maximum soil moisture stress." Various measurable aspects of plant 'growth' (elongation, increase in fresh or dry weight and vegetative versus reproductive development) do not respond in the same manner to increasing moisture stress. Appreciable shifts may occur m the relative abundance of certain chemical constituents within plants. Indicated are possibilities of so controlling soil moisture stress over the growing season as to favour production of that plant organ or constituent for which the crop is grown. Greater emphasis on such considerations would lessen confusion now existent in our understanding of plant-soil-water relations and thus in irrigation recommendations. Rooting characteristics of well established perennials and the expanding root systems of annual crops are discussed. Where plants have few widely spaced roots, soil moisture determinations may give quite a false picture of moisture conditions at root surfaces. Such crops will respond to irrigations, although measured soil moisture stress is quite low. Problems of relating plant growth to soil moisture stress conditions varying with both time and soil depth are considered. The question is raised whether any type of integrated or average soil! moisture stress value, which includes some low stress values from portions of the root zone containing few roots, can represent the true stress experienced by plants. On the other hand, deep roots absorbing water against low soil moisture stresses mask the effects of relatively high soil moisture stresses occurring over parts of root systems. Some problems of studying plant responses to soi5 moisture conditions in small containers are mentioned. Soil factors affecting density or depth are reviewed and several ways weather factors influence soil, -moisture-plant-growth relations summarized. When the sizable number of plant, soil, weather, and miscellaneous factors influencing the relation of soil moisture to yield of some specific plant organ or constituent, are considered, it is not surprising that conflicting results have been obtained even in irrigation experiments involving some given crop. This discussion suggests the probable impossibility of finding any one relation between crop yields and moisture depletion or soil moisture stress, at least as measured with our present methods. Present information, though meagre in many respects, allows some fairly accurate predictions whether. 1 Paper read by J. R. FURR.

Reprinted from the Report of the XIVth International Horticultural Congress, The Netherlands. 1955

Published by H. Veenman & Zonen, Wageningen (Holland)

R. M. HAGAN 83

growth or yield in given situations is likely to be unaffected by depletion of nearly all available water or is likely to be increased by irrigation at low soil moisture stresses. Table IV lists those conditions lowering the probability that yields will be increased by irrigation at relatively low soil moisture stresses, and Table V conditions raising the probability that yields will be increased by avoiding relatively high soil moisture stresses. Mention is made of some economic and other factors which

under practical conditions may of irrigations.

The gross effects of deficient and of excessive soil moisture on plant growth are, of course, well known. With irrigation and, where necessary with drainage, the farmer has the opportunity to exercise greater control over soil moisture than he does over any of the other soil physical factors. A lively controversy has existed or many years around the question, whether the so called available moisture is equally available for plant growth, or available only with such increasing difficulty that plant growth functions are retarded before the wilting point is reached. These viewpoints have been summarized in recent review articles by VEIHMEYER and HENDRICKSON [12] and by RICHARDS and WADLEIGH [5]. This difference of opinion is more than academic, for our answer to the farmer's very practical question, “when should I irrigate”, depends upon our understanding of soil moisture-plant growth relations. It is not particularly helpful for the present purpose to recite the evidence that supports these two viewpoints. Instead, our efforts should be directed towards an understanding of some factors which have contributed to this controversy. The now voluminous literature on plant-soil-water relations includes many reports of irrigation experiments on given crops from which seemingly diametrically opposed conclusions have been drawn. This situation is most confusing, especially to those of you who must attempt to apply the results of research to the practical problem of crop production. The important task now is to attempt to analyze carefully the conditions prevailing in the various studies which have been reported, with the hope of learning why competent investigators have been led to such divergent conclusions. I shall attempt to illustrate by schematic diagrams the existing viewpoints and to suggest the probable effects of plant, soil, water, and some other miscellaneous factors, on soil-moisture-plant-growth relations. In attempting this, I am well aware that much of what follows involves some speculation and may someday be shown to have been quite erroneous. In assuming this risk, I take some comfort from a remark, attributed to FRANCIS BACON, that “truth arises more easily from error than from confusion”. The diagrams presents may contains some errors, but I hope they may serve to lessen the confusion now prevalent.

PLANTGROWTH RESPONSES WITHIN ONE IRRIGATION CYCLE Growth rate not diminished over available range Let us consider first, plant growth responses as the available soil moisture is depleted within one irrigation cycle. VEIHMEYER and HENDRICKSON's theory, that plants can obtain a supply of water with equal facility between field capacity (FC) and the permanent wilting percentage (PWP), can be illustrated by figure 1. This represents their view (VEIHMEYER and HENDRICKSON, [11, 13]) that rate of growth is not diminished

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over the available range or, in other words, that no measurable increases in rate of growth are obtained by irrigating until the soil moisture falls to near the permanent wilting percentage. The question mark indicates the uncertainty in the vicinity of the permanent wilting percentage.

Growth rate progressively diminished as soil moisture content falls below FC

On the other hand, it has been maintained that plants respond favourably to relatively high soil moisture conditions, and the statement is sometimes made that plant growth diminishes progressively as the soil moisture content falls below field capacity and ceases at the permanent wilting percentage. Such a picture, figure 2, of soil-moisture- plant-growth relations is not uncommon and often creeps into textbooks and popular articles, although, as Í understand it, there is little support for this idea among research workers in this field. This may be called the “more water, more growth” idea. 'The foregoing notion arises in part from numerous experiments which have suggested that plant growth is related to the tenacity with which the water is held by the soil. Growth rate as a function of soil moisture stress The 'availability' of soil moisture is now frequently described in terms of soil moisture tension which is dependent upon surface forces, and in terms of total soil moisture stress (SMS), which includes all surface and osmotic forces arising from the presence of solutes in soil solution. The soil moisture-component-moisture-stress curves of figure 3 illustrate the moisture retention characteristics of four non-saline soils of different textures (sands to clays). In saline soils, it is considered important to include the osmotic effects. Moisture content-soil moisture stress relations for a given soil, to which increasing amounts of soluble salt had been added, are given in figure 4. The theory that plant growth is a function of soil moisture stress can be expressed

FIG. 1. Schematic representation of VEIHMEYER and HENDRICKSON'S theory of “equal availability”.

R. M. HAGAN 85

O 25 50 75 100

AVAILABLE MOISTURE DEPLETION, PER CENT

O 25 50 75 100 AVAILABLE MOISTURE DEPLETION, PER CENT

AVAILABLE MOISTURE DEPLETION. PER CENT

Fig. 2. Representation of view that plant growth is directly related to soil moisture content within this available range.

Fig. 3. Idealized moisture retention curves for a non-saline sandy soil (curve 1), loam soil (curve 2), and two clay soils (curves 3 and 4)

Fig. 4. Relations between soil moisture depletion and (total) soil moisture stress for Panoche loam to which had been added sufficient salt to give and osmotic pressure of the soil solution at approximately the field capacity of nearly O atmospheres(curve 1), 2 atmospheres (curve 2), and 4 atmospheres (curve 3). (Redrawn from WADLEIGH, 1946)

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100


diagrammatically as in figure 5. If such a relation exists, little retardation in growth on the non-saline sandy soil would be expected until nearly all the available water had been depleted (curve 1), but on the non-saline clay soil some slowing of growth should occur after about 50 per cent. depletion (curve 2). On the saline soil (curve 3), a somewhat reduced growth rate would be expected at moisture contents near field capacity, and it would decline appreciably even in the upper half of the available range.

CUMULATIVEGROWTH OVER SEVERAL IRRIGATION CYCLES So far consideration has been confined to growth rates within one irrigation cycle. If moisture is equally available for plant growth over the available range, then total growth over a period of time will be independent of the level of moisture depletion permitted before irrigations. This relation is indicated by curve 1 of figure 6. If, on

FIG. 5. Representation of theory that plant growth is a function of soil moisture tension or total soil moisture stress. For non-saline sandy (curve 1) and clay (curve 2)soils and for saline soil (curve 3)

FIG. 6. Curve 1 represents total growth over a series of cycles for irrigations at given levels of moisture depletion as predicted by the theory of equal availability. Curve 2 gives total growth as predicted by the theory that growth isa function of time integrated soil moisture stress. Curve 3 indicates total growth on the basis that brief periods of 100 high moisture stress may have an exaggerated effect on plant growth

R. M. HAGAN 87

the other hand. growth rates are related to soil moisture stress, then total growth over a period during which. the stress varies may be related to some average stress condition. WADLEIGH [18] proposed that cumulative growth be related to a moisture stress function integrated (averaged) over time. Total growth over the time interval of one irrigation cycle or of several cycles calculated on this basis is represented by curve 2 of this figure. Its shape will depend on that of the soil moisture stress-moisture content curve. On the other hand, some field studies have suggested that yields are related to maximum stress prevailing prior to irrigation. WADLEIGH and other workers [1, 6] recognize that an integrated moisture stress may not adequately ex- press the physiological effect of the extreme variation in moisture stress over the irrigation cycle and suggested that high moisture stress values, though present íbi' only a brief time interval, might have an exaggerated effect on plant response. This possibility is depicted by curve 3. Further work will be needed to determine the importance of high stress periods and the relative effects on growth of moisture stress and specific ion toxicities in saline systems. Before leaving this figure, it should be pointed out that much greater decreases in yield would occur, if all the available moisture is depleted in the major portion of the root zone during one or more of the irrigation cycles. Doubtless the serious reductions in yield, often attributed to treatments calling for depletion of nearly all the available water, have been caused by failure to recognize that actually all the available water had been exhausted over much of the root system. The occurrence of wilting may not always be a reliable indicator of such moisture ex- haustion, for under field conditions some crops do not wilt until long after growth has been checked. OTHER FACTORS INDEPENDENTLY AFFECTING PLANT GROWTH Considerable experimental support can be found for each of the generalized soil moisture - plant growth relations represented in figure 6. This situation indicates that moisture-growth relations must be greatly influenced by the interplay of other factors which independently affect plant growth. The possible effects of some plant, soil, weather, and other miscellaneous factors, are discussed in the following sections.

Plant Factors Insufficient attention to the specification of what is meant by plant growth has often complicated the interpretation of the results of irrigation experiments. We easily recognize several different manifestations of growth, such as elongation of plant organs, increase in fresh or dry weight, and vegetative versus reproductive development. These common-place processes are resultants of intricate combinations of many physiological processes which are probably not all equally affected by increasing soil moisture stress and an accompanying change in the internal water balance of cells and tissues. Thus it is not surprising that various measurable aspects of growth do not respond in the same manner to moisture stress. Greater emphasis on this point would aid materially in lessening the confusion now existent in our understanding of plant-soil-water relations and thus of irrigation needs.

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Available Moisture Depletion per cent

FIG. 7. Effect of moisture depletion on rate of petiole elongation during dark and light periods for ladino clover grown in containers in a controlled environment room (hagan, et al, 1951)

FIG. 8. Effect of moisture depletion on green weight yields, dry weight yields, and dry matter percentage of forage from

ladino clover grown in containers (HAGAN, et al, 1951).

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A number of workers have shown that certain aspects of growth are more affected by moisture stress than others. Data from recent studies on ladino clover (HAGAN et al [3]) illustrate this point. In figure 7 the rate of petiole elongation for clover grown in containers is shown to decrease markedly with rising soil moisture stress and during the light periods to approach zero before wilting appears. In another container ex- periment, figure 8, green weight production declined with increasing moisture stress, while dry matter production was largely unaffected until at least 75 per cent of the total available moisture had been depleted. This suggests that, although cellular elongation may be retarded, the rates of photosynthesis and respiration must be little affected over a considerable range of moisture stress. The net rate of carbon dioxide exchange for clover, grown in containers under continuous light for two drying cycles, are given in figure 9. This suggests that the rate of photosynthesis was not diminished appreciably until about the time wilting appeared. The respiration rate was little affected over this stress range. These results are in general agreement with the findings of others that photosynthesis and respiration are relatively insensitive to

Fig. 9. Effect of moisture depletion on photosynthesis in ladinoclover grown in containers under continuous light and at constant temperature and humidity. (Adapted from UPCHURCH et al, 1956)

FIG. 10. Differences in the effects of soil moisture stress on several aspects of growth: Curve (1) photosynthesis or respiration, curve (2) dry weight yield, and curve (3) elongation or fresh weight yield

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moisture stress (for a discussion see: RICHARDS and WADLEIGH [5]). Differences in the effects of soil moisture stress on several aspects of growth for one plant, ladino clover, are summarized schematically in figure 10. As plants are subjected to increasing moisture stress, appreciable shifts in the relative abundance of a variety of chemical constituents may occur in some plants, see figure 11. The rubber content of guayule has been shown to rise under increased stress (HUNTER and KELLEY [4]). The percentage of sugar in cane and beets can also be raised by moisture stress (WIERSMA [15]). In tobacco, increasing stress is reported to have lowered the sugar content and raised the percentage of nicotine and nitrogen in the cured leaves. (VAN BAVEL [10]). Thus, the economic value of a crop may be influenced appreciably by moisture stress, particularly during the period of maturation. Differences in the response of various growth processes to moisture stress conditions point the way to the possibilities of so controlling the soil moisture stress over the growing season as to favour the production of that constituent or plant organ for which the crop is grown. The effects of given soil moisture stress conditions on the yields of field corn have been reported to depend upon the stage of growth. Corn appears to be particularly sensitive to moisture stress during the tasseling period.


FIG. 11. Rubber content of guayule as affected by irrigation within the available range and by de-

pletion of available moisture on Hesperia sandy loam. In treatment A the soil moisture was maintained near field capacity at all times (29 irrigations), in treatment B an average moisture depletion of 25 % was permitted in 12-24 inch depth (18 irrigations), and in treatment C an average moisture depletion of 67% was permitted in the same depth of soil (11 irrigations). Treatment D received one pre-irrigation in early April which wetted the soil to a depth of over 8 feet. By harvest on August 8, nearly all the available water in treatment D was depleted from the 8-fooí depth. Treatment E received no irrigations and by August was at or below the PWP throughout the sampling depth. (Adapted from HUNTER and KELLEY, 1946)

R. M. HAGAN 91

Vegetative vigour is not necessarily associated with a comparable degree of produc- tivity. This is well illustrated by studies on guayule, figure 12, which show that increased moisture stress retards the vegetative growth of the shrub, but leads to a higher production of rubber.

Another plant factor of extreme importance in determining the relation between measurable soil moisture stress and plant growth is the nature of the root system. Different interpretations of root development and of moisture conditions within the soil penetrated by roots contribute to the existence of contradictory views on plant- soil-water relations. Under favourable soil and growing conditions, most perennial crops develop well-branched root systems which thoroughly permeate the soil depth characteristic of the plant. Below this depth, the spatial density of absorbing roots diminishes until so few remain that moisture extraction cannot be de- tected. This depth distribution of roots is illustrated by the drawing in figure 13 and leads to a moisture depletion pattern of the type shown here for bermuda grass (figure 14). This illustration calls attention to several features of a root system of importance in analyzing water relations and raises ques- tions about what is meant by the loosely used term “root zone”. It is desirable to distinguish what may be called the 'complete extraction zone' from the 'partial extraction zone' or 'transition zone'. The complete extraction zone may be defined as the volume or, in the case of closely spaced plants,

FIG. 13. Fibrous root system of grass showing decrease in root density

with depth (Adapted from SPRAGUE, 1940)

FIG. 12. Difference in effects of moisture depletion on dry weight yield of shrub and on yield of rubber per acre. (Adapted from HUNTER and KELLEY, 1946)

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the depth from which all the available water has been removed when the crop ceases growth. The soil within this zone is thoroughly permeated with fine roots. Here the complete extraction zone would extend to a depth of about 2 feet. The transition zone contains a varying concentration of roots usually decreasing with distance from the plant and here extends to a depth of at least 6 feet. It should be remembered that in isolated plants, especially when young or planted on a wide spacing, the root zone is three-dimensional so that the transition zone may form a considerable volume around the complete extraction zone. The existence of this transition zone often seriously complicates the interpretation of moisture-growth experiments.

Analysis of moisture conditions within the expanding root system of an annual crop is even more complicated. In the seedling state, only a tap root or a few branched roots penetrate the soil. Some annuals rapidly develop a well-branched root system which permeates an ever-enlarging soil volume. At the same time, some roots grow out ahead into a correspondingly enlarging transition zone. Thus, if the soil has been previously wet to field capacity through a considerable depth, these growing roots continuously come into contact with additional supplies of available water at low tensions. If the roots are well-branched and grow rapidly enough, they may contact new supplies of readily available water with sufficient rapidity to meet the needs of transpiration. A crop such as watermelon on the deep alluvial soil at Davis does not respond to irrigation, although a relatively high soil moisture stress may develop within an ever-increasing soil volume. Other annuals send out a few widely spaced roots which leave large volumes of un- explored soil between roots, particularly in the early stages of growth. Figure, 15 shows the skeletal root system of a young sweet corn plant. Soil moisture samples or even

FIG. 14. Differences in moisture depletionunder bermuda-grass reflecting decreasing numbers of absorbing roots with greater depth. (Unpublished studies by HAGAN and MADISON, Univ. of Calif., Davis)

R. M. HAGAN 93

FIG. 15. Skeleton root system of young sweet com growing in pervious deep soil. (Unpublished

studies by DONEEN, Univ. of Calif., Davis)

moisture indicating devices may give quite a false picture of moisture conditions at the root surface. Crops with sparse-roots will respond to irrigations although the measured soil moisture stress may be quite low. The sparser the roots, the greater the likelihood that growth will be retarded by delaying irrigation (figure 16). Very similar varieties of beans have shown deferent responses to irrigations which are related to differences in their root development.1 Thus, the fraction of the available range which can be utilized before growth is checked will vary with root density. Were it possible to sample only the soil adjacent to absorbing roots, we might find that the actual moisture stress conditions were consider- ably higher than indicated by our present methods. Until methods are developed to measure the moisture stress experienced by plants, it apparently will be necessary in the case of sparsely-rooted crops to establish some rather arbitrary moisture depletion limits, which unfortunately will depend on stage of growth, growing conditions, and on such other factors as soil and weather. It is not uncommon to find reports that crop yields declined, unless irrigated at some given soil "moisture stress. Often it is difficult to learn at what depth the stress was measured and seldom is its relation to rooting depth and spatial density indicated. Where spatial density diminishes rapidly with depth, the soil moisture stress corresponding to any plant response will vary markedly over a short depth interval.

1 Unpublished studies at the University of California, Davis, by D. W. HENDERSON and L. D. DONEEN.

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The suggestion has been offered (RICHARDS & WADLEIGH [5]) that roots in the transition zone absorbing water against low soil moisture stresses may mask the effects of relatively high soil moisture stresses occurring over only a part of the root system. Thus, it may be argued that whenever the transition zone extends through a considerable volume of soil, depletion of most of the available moisture from the complete extraction zone may have relatively little influence on growth. This idea is represented schematically in the diagram of figure 17. Whether the crop will respond to increasing stress within its complete extraction zone, or will be unaffected, will depend on whether the roots in the transition zone can supply water fast enough under the prevailing conditions to maintain the necessary water balance in the plant. The presence of an unknown fraction of the total absorbing roots in the transition zone and the continued elongation of the roots into moist


25 50 75

FIG. 17. Schematic representation of the relation between the portion of the roots subjected to stress and rate of growth

FIG 16. Schematic representation of the effect of spatial density of roots on the relation of growth to the apparent depletion of available moisture

25 50 75 100

100

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soil, particularly in annual crops, makes it very difficult to evaluate the actual soil moisture stress to which the plant is subjected. Growth responses to increasing soil moisture stress in container experiments may be much larger than experienced in field experiments set up under as nearly comparable conditions as possible. This is illustrated in figure 18 by the relative yields of ladino clover for comparable container and field experiments. The smaller decrease in yield produced by increasing soil moisture stress within the complete extraction zone in the field experiments may be attributable to those roots in the transition zone which are in contact with moisture at relatively low stress. For these and several other reasons, container experiments may not be reliable indicators of crop response to soil moisture stress under field conditions. Soil Factors Soil properties and conditions listed in table I have a decided effect on soil moisture - plant growth relations.

Any soil factor which affects root density or depth can be expected to influence the response of the crop to irrigation. Mechanical impedance, slow water penetration and poor internal drainage, and deficient aeration, are frequently responsible for sparse and shallow roots.

TABLE I. Soil factors affecting soil moisture-plant growth relations

SOIL STRUCTURE, TEXTURE AND DEPTH Mechanical impedance Infiltration rate Internal drainage rate Aeration Moisture retention characteristics (soil moisture versus soil moisture stress relations) Unsaturated hydraulic conductivity

WATER TABLE

SALINITY Soil moisture stress Toxicity Soil structure

PLANT DISEASES AND NEMATODES

TEMPERATURE

FERTILITY

Soil structure, texture, and depth determine the total capacity of the soil for storing available water for plant growth. The total available moisture capacity within the root zone and the moisture-release characteristics of the soil are both important factors determining the rate of change in soil moisture tension or stress. Deep-rooted crops on deep soils usually show smaller responses to irrigations than shallower rooted crops on the same soil. Crops growing on a soil in which 75 to 85 per cent. of

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the available water is released at tensions below one atmosphere may be expected to show a smaller response to irrigations at a given moisture depletion level than the same crops growing on a soil in which less than 50 per cent. is released at such low tensions. Great emphasis has been placed on the 'energetics' of moisture availability in recent years, but doubtless the 'dynamics' must also be considered. The rate at which water can move to the absorbing root surface may play an important role in plant-soil-water relations.

A stable water table in the lower portion of the normal root zone of a crop may supply considerable portion of the water absorbed by the roots and make the plants less responsive to moisture changes in the soil above the capillary fringe. On the other hand, a fluctuating water table may increase crop responses to early irrigation by restricting live roots to a shallow depth. High water tables in semi-arid regions usually lead to the accumulation of salts. As mentioned previously, salinity may affect soil moisture-plant growth relations by decreasing moisture availability through increased soil moisture stress, by interfering with root growth and absorption through toxicity reactions, and by contributing to poor soil structure which, in turn, influences infiltration, drainage, aeration, and root growth.

Soil-borne plant diseases and nematodes, by reducing root surface, may cause crops to respond favourably to irrigations at seemingly very low moisture stress levels. Soil temperature also affects the rate of root growth and their distribution with depth (HAGAN [2]).

The fertility status of the soil and possibly the depth distribution of some essential element may also determine the growth response of crops to irrigations at various moisture depletion levels. These data (Table II) illustrate the effect of nitrogen applications on the response of corn to two moisture tension levels. Without nitrogen

FIG. 18. Relative green weight yields for ladino clover under corresponding irrigation treatments in containers and in field plots. (Adapted from HAGAN et al, 1951)

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fertilizer, the additional irrigations required to maintain low tensions lowered yields presumably as a result of leaching out some nitrogen. The difference in yield between the relatively wet and dry treatments increased with greater amounts of fertilizer. In soils where the available supply of some essential element is confined to the top-soil, drying out of the upper portion of the root zone may seriously retard plant growth, even though the plant may still be adequately supplied with water. In a current ex- perítnent1 with sugar beets growing on a phosphate fertilized soil, leaf analyses show a phosphate deficiency whenever the surface soil dries out, although deeper roots are obtaining sufficient water to maintain adequate plant turgor. In such cases yield increases produced by more frequent irrigations are not chargeable directly to an inade- quacy of moisture supply. Fertility responses have complicated the interpretation of many soil moisture versus plant growth experiments.

TABLE II. Corn yield as affected by nitrogen and soil moisture

tensión*

Yield of corn (bushels/acre) Nitrogen applied (Ib/acre)

Low SMT

High SMT

Difference low-high

0 62 70 -8 40 95 88 7

120 130 105 25 240 142 111 31

* Irrigated when soil moisture tension (SMT) at 9 inch depth in row reaches 0.8 atm. (Iow SMT) and 4.1 atm. (high SMT). (Adapted from SINGLETON et al. [7]).

Weather Factors Weather factors can influence soil moisture-plant growth relations in several ways, as summarized in table III. Time permits little more than a mention of these factors. Weather conditions, particularly light and temperature, may so influence the growth characteristics of the shoot and root as to affect soil-moisture-growth relations. Current work by DONEEN2 suggests that late-planted sugar beets, which must develop roots during hot dry weather, fail to develop as dense or deep a root system as early seeded beets. The length of the crop season before autumn rains or frost may at least partially determine whether harvestable yields will be affected by imposing different soil moisture stress levels during the growing period.

Meteorological factors-light, temperature, humidity, and wind-control the rate of water loss by transpiration and evaporation. Where water use rates are high, crops will deplete the available soil moisture more rapidly, and growth may be more affected, by increasing soil moisture stress. Plant growth is probably dependent upon plant turgor or plant moisture stress, whose relation to soil moisture stress for

1 Unpublished studies at the University of California, Davis, by HILLS et al, 1954. 2 Unpublished studies at the University of California, Davis, 1954. 7

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different rates of transpiration needs to be explored. It can be reasoned that an increased rate of transpiration would lower the plant turgor corresponding to any given soil moisture stress and would have the effect of causing growth to diminish at higher moisture levels. This notion is illustrated schematically in figure 19. Much more work on this point is needed.

TABLE III. Weather factors affecting soil moisture — plant growth relations

GROWING CONDITIONS Light Temperature Length of growing season

EVAPORATING CONDITIONS Light Temperature Humidity Wind

Miscellaneous Factors Problems associated with insect control or harvesting may at times influence the apparent effects of soil moisture conditions on crop yields. A study of forage and seed production by ladino clover provides an interesting example. As shown in figure 20, increased dryness reduced forage yields but increased the harvestable yield of seed. The total seed actually produced by the clover also diminished with increased stress, but the higher humidity associated with the wettest treatment caused such a serious pre-harvest loss of newly produced seed as to reduce the harvestable yields at the end of season below those on the drier treatments. Some type of harvesting problem may affect the results of soil moisture-plant growth experiments more frequently than is realized. .

FIG. 19. Schematic representation of the possible effect of transpiration rate on soil moisture-plant growth relations

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FIG. 20. Difference in effects of moisture depletion on harvestable yields of vegetative and reproductive phases of growth in ladino clover. (Adapted from HAGAN et al, 1951)

Mention has been made of a sizable number of soil, plant, weather, "and other miscellaneous factors, which may influence the effects of various moisture depletion levels on plant growth or yield of some specific organ or constituent. When viewed against this background, it is not at all surprising that conflicting results have been obtained even in irrigation experiments involving some given crop. It also seems that this discussion emphasizes the probable impossibility of finding any one relation between crop yields and soil moisture depletion or soil moisture stress, at least as measured with our present methods. The problem is further complicated by the plant itself. There is some evidence that growth is related to the turgor of plant tissues, but turgor ís not a function of soil moisture stress alone. Rather it is dependent upon the relative rates of water absorption and transpiration and upon the time interval over which any differences in rates may have continued. Until more is learned about the effects of turgor on various physiological processes and about other factors which may influence soil moisture-plant growth relations, our approach to problems of soil moisture control must continue to involve considerable empiricism.

Present information, meagre though it is in many respects, may allow us to make some fairly accurate predictions whether growth or yield in given situations is likely to be unaffected by depletion of nearly all the available water as measured by our present methods, or is likely to be increased by irrigation at lower soil moisture stresses. At the very real risk of some error, two check lists have been prepared in the following two tables. Listed in the first (Table IV) are conditions which may lower the probability that yields will be increased by irrigations at relatively low soil moisture stresses. The

100 SYMPOSIUM II other (Table V) lists conditions which raise the probability that yields will be increased by avoiding relatively complete depletion of the available soil moisture. These are merely lists, and it is not implied that all conditions must be present. Relative weight- ing of each of these conditions is not considered. If a given situation is described by some entries from both tables, prediction will be much more difficult.

TABLE IV. Conditions which lower the probability that crop yields are increased by irrigations at low soil moisture stress

PLANT Deep, dense, fast-growing roots Xerophytic characteristics Dry weight yields of reproductive organ desired Harvest for content of sugar, oil, etc.

SOIL Deep soil; good structure Good infiltration, internal drainage, aeration Large fraction of available water held at low tensions Non-saline Fertility level low; nutrients distributed in profile Constant water table in reach of roots

WEATHER Planted well ahead of hot dry weather Major growth period before hot dry weather Low evaporation rates

TABLE V. Conditions which raise the probability that crop yields are increased by irrigations at low soil moisture stress

PLANT

Shallow, sparse, slow-growing roots

Fresh weight yield of vegetative organ desired

Quality dependent upon size of vegetative organ

SOIL Shallow soil; poor structure impeding root growth Slow infiltration and intemal drainage; poor aeration Root disease, nematodes present Small fraction of available water held at low tensions Saline soils or water Fertility level high; nutrients concentrated in top-soil Very high soil temperatures, with shallow rooted crops

WEATHER Planting at beginning of hot dry weather Major growth period during hot dry weather High evaporation rates

Up to this point, we have been concerned only with the physiological response of plants to soil moisture. No mention has been made of economic and other

R. M. HAGAN 101

factors which under practical conditions may dictate the timing of irrigations. It is often questionable whether the increased yields, sometimes obtainable under relatively frequent irrigations, will pay for the added cost of water and labour. Even those, opposed to the concept of equal availability as a scientific theory, are in general agreement that the following practical considerations all suggest the desirability of a sparing use of irrigation water: maximum use of limited water supplies, water and nutrient losses caused by deep percolation, danger of developing a drainage problem through over-irrigation, problems of soil structure, quality of the marketable product, and problems of plant disease and longevity in perennial crops, which are often aggravated by frequent irrigation. Although these considerations are of real importance in determining farm irrigation practices, their relative importance differs from place to place and even from year to year. Much confusion would be avoided in preparing irrigation recommendations, if the relative importance attached to soil moisture-plant growth responses and to other considerations was clearly indicated.

One other practical problem should be mentioned. The 3 curves of an earlier figure (figure 6) represent the considerable differences in yield to be predicted on the basis of existing theories. One point, however, should be mentioned. In order to ensure a continuous supply of available soil moisture and to allow for unforeseen delays in irrigation or unusually dry weather, the irrigation farmer generally cannot plan to allow nearly complete depletion of the soil moisture; but, to allow for a margin of safety, he should plan to irrigate while some available moisture still remains. The fraction of the total available moisture range which can be utilized with safety depends on a number of factors including crop rooting characteristics, the soil and the irrigation system. In figure 21, let us consider the effects of allowing a safety margin of 15 per cent. If such an allowance is made to meet the practical problems of irrigation under farming conditions, then a considerable portion of the differences predicted by these several theories on plant-soil-water relations tends to disappear. However, to raise

FIG. 21. Irrigation before complete moisture depletion is generally necessary under actual irrigation conditions in order to ensure a continuous supply of available moisture. This practice also avoids the portion of the available range where the largest differences in growth response would be predicted by the different theories on plant-soil-water relations

102 SYMPOSIUM II

irrigation efficiency and to increase crop production, it is highly important that vigorous programs of research be directed towards increasing our knowledge of soil moisture- plant growth relations.

REFERENCES [1] BERNSTEIN, L., and PEARSON, G. A., 1954. The influence of integrated moisture stress achieved

by varying the osmotic pressure of culture solutions on the growth of tomato and pepper plants. Soil Sci. 77, 355-368.

[2] HAGAN, R. M., 1952. Soil temperature and plant growth. In: Agronomy II. Soil physical conditions and plant growth, B. T. Shaw, editor. Academic Press, New York, pp. 367-447.

[3] ———, PETERSON, M. L., UPCHURCH, R. P., and JONES L. G., 1951. Relationships of soil. moisture stress to different aspects of growth in ladino clover. (Manuscript submitted to Proc. Amer. Soil Sci. Soc.).

[4] HUNTER, A. S., and KELLEY, O. J., 1946. The growth and rubber content of guayule as affected by variations in soil moisture stresses. J. Amer. Soc. Agron. 38, 118-134.

[5] RICHARDS, L. A., and WADLEIGH, C. H., 1952. Soil water and plant growth. In: Agronomy U. Soil physical conditions and plant growth, B. T. Shaw, editor. Academic Press, Inc., New York. pp.73-251.

[6] ROBINS, J. S., and DOMINGO, C. E., 1953. Soma effects of severe soil moisture deficits at specific growth stages of corn. Agron. J., 45, 618-621.

[7] SINGLETON, H. P., et al., 1950. Soil, water and crop management investigations in the Columbia Basin Project. Washington Agr. Exp. Sta. Bul. 520, 29-30.

[8] SPRAGUE, HOWARD B., 1949. Better lawns for homes and parks. McGraw-Hill, Inc., New York. [9] UPCHURCH, R. P., PETERSON, M. L., and HAGAN, R. M. The effect of soil-moisture content on

the rate of photosynthesis and respiration in ladino clover (Trifolium repens L.) Plant Phys. 30, 297-303.

[10] VAN BAVEL C. H. M., 1953. Chemical composition of tobacco leaves as affected by soil moisture conditions. Agron. J. 45, 611-614.

[11] VEIHMEYER, F. J., and HENDRICKSON, A. H., 1950(a). Essentials of irrigation and cultivation of orchards. Univ. of Calif. Agr. Ext. Service Circ. 50, 23 pp.

[12] ——— and ———, 1950(b). Soil moisture in relation to plant growth. Ann. Rev. Plant Physiol. 1,285-304.

[13] ——— and ————, 1952. The effects of soil moisture on deciduous fruit trees. Report 13th Int. Hort. Congress, 1, 306-319.

[14] WADLEIGH, C. H., 1946. The integrated soil moisture stress upon a root system in a large container of saline soil. Soil Sci. 61, 225-238.

[15] WIERSMA, D., 1955. Soil moisture conditions and sugar accumulation in the sugar beet. Ph. D. Thesis, University of California.

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