soil plant water

7/28/2019 Soil Plant Water

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WATER-SOIL-PLANTContinued from preceding page

the stress varies may be related to someaverage stress condition-curve 2-where the shape of the curve will dependon the soil moisture stress-moisturecontent curve for that particular soil.On the other hand, some field studies sug-gest that yields are related to maximum

00L5 so 75 100AVAILABLE MOISTURE MPLETION, PER CENT

stress prevailing prior to irrigation. Inother words, high moisture stress values,though present for only a brief time in-terval, may have exaggerated effect onplant responses-curve 3. Considerableexperimental support can be found foreach of these generalized relations. Thissituation indicates that moisture-growthrelationships must be greatly influencedby the interplay of other factors whichindependently affect plant growth.P l a n t Factors

Several different .aspects of plantgrowth-such as elongation of plant or-gans, increase in fresh or dry weight,and vegetative versus reproductive de-velopment-are easily recognized. Thesecommonplace processes are resultants ofintricate combinat ions of many physio-logical processes which are probably notall equally affected by increasing soilmoisture stress and an accompanyingchange in the internal balance of cellsand tissues. Thus it is not surprisingthat various measurable aspects ofgrowth do not respond in the same man-ner to moisture stress.Data from studies on ladino clover il-lustrate this point. Some plant functionssuch as photosynthesis and respiration

AVAILABLE MOISTURE DEPLETION. PER CENT

are relatively insensitive to moisturestress-curve 1. Dry weight productionmay be more sensitive-curve 2. Freshweight yield and elongation of plant or-gans appear to be still more sensitive tomoisture stress-curve 3.As plants are subjected to increasingmoisture stress, appreciable shifts in therelative abundance of a variety of chemi-cal constituents may occur in someplants. The percentage of sugar in caneand beets is raised by moisture stress.In tobacco increasing stress is reportedto lower the sugar content and increasethe percentage of nicotine and nitrogenin cured leaves. Thus the economic valueof a crop may be influenced appreciablyby moisture stress particularly during theperiod of maturation. Differences notedin the response of various growth proc-esses to moisture stress point the way tothe possibilities of so controlling the soilmoisture stress through the growing sea-son as to favor the production of thatconstituent or plant organ for which theplant is grown.The effects of given soil moisture stressconditions on crops are often dependentupon the state of plant growth. Corn ap-pears to be particularly sensitive to mois-ture stress during the tasseling period.Vegetative vigor is not necessarily asso-ciated with a comparab le degree of pro-ductivity, as in the case of cotton whereyields may be relatively higher on smallplants than on tall or rank plants.Another plant factor of extreme im-portance in determining the relation be-tween measurable soil moisture stressand plant growth is the nature of theroot system. Different interpretations ofroot development and of moisture condi-tions within the soil penetrated by rootscontribute to the existence of contradic-tory views on water-soil-plant relations.Under favorable soil and growing condi-tions, most perennial crops develop well-branched root systems which thoroughlypermeate the soil to a depth character-istic of the plant. Below this depth, thespatial density of absorbing roots dimin-ishes until so few remain that moistureextraction can not be detected.Moisture conditions within the ex-panding root system of an annual cropare even more complicated. In the seed-ling state, only a taproot or a fewbranched roots penetr.ate the soil. Someannuals rapidly develop a well-branchedroot system which permeates an ever-enlarging soil volume. Thus if the soilhas been previously wet to field capacitythrough a considerable depth, thesegrowing roots continuously come intocontact with additional supplies of avail-able water at low tensions. If the rootsare well-branched and grow rapidlyenough, they may contact new suppliesof readily available water with sufficientrapidity to replace the water lost from

the leaves by transpiration. Because ofrapid root growth, a crop such as water-melon on a deep alluvial soil may notrespond to irrigation even though a rela-tively high soil moisture stress may de-velop within an ever-increasing soilvolume. Other annuals send out a fewwidely spaced roots which leave largevolumes of unexplored soil between rootsparticularly in the early stages of growth.Under such conditions soil moisture sam-ples or even moisture indicating devicesmay give quite a false picture of mois-ture conditions at the root surface. Cropswith sparse roots will respond to irriga-tions although the measured soil mois-ture stress may be quite low. As indicatedin the following diagram, the sparser theroots the greater the likelihood thatgrowth will be retarded by delaying irri-gation. Very s imila r varieties of beanshave shown different responses to irriga-tions, which are related to differences intheir root development. Thus the frac-tion of the available moisture rangewhich can be utilized before growth ischecked will vary with root density.

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Until methods are developed to nieas-ure the moisture stress experienced byplants, it apparently will be necessaryin the case of sparse-rooted crops to es-tablish some rather arbitrary moisturedepletion limits which unfortunately willdepend on the stage of growth, growingconditions, and upon such other factorsas soil and weather.Where some roots extend beyond thebulk of the root system and absorb wateragainst low soil moisture stresses, theymay mask the effects of relatively highsoil moisture stresses occurring over theremaining part of the root system. Whenmost of the roots are confined to a givenvolume of soil and few roots extend outinto relatively moist soil, the crop maybe expected to be relatively sensitive todepletion of the available moisture. How-ever, when a considerable number ofroots extend into moist soil, depletionof the available moisture from the majorpart of the root zone may have relativelylittle influence on growth as indicatedschematically at the top of the next vol-umn. The presence of an unknown frac-tion of the total absorbing roots extend-ing out into relatively moist soil makes

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AVAILABLf . MGiSTURE DEPCETWJN, PER CENT

it very difficult to evaluate the actual soilmoisture stress to which the plant is sub-jected. Plants confined to containers orshallow soils may be expected to be quitesensitive to depletion of the available soilmoisture.Soil Factors

Any soil factor which affects rootdensity or depth can be expected to in-fluence the response of the crop to irri-gation. Mechanical impedance, slowwater penetration and poor internaldrainage, and deficient aeration fre-quently are responsible for sparse andshallow roots. Soil structure, texture, anddepth determine the total capacity of thesoil for storing available water for plantgrowth. The total available moisturecapacity within the root zone and themoisture-release characteristics of thesoil are both important factors determin-ing the rate of change in soil moisturetension or stress. Deep-rooted crops ondeep soils usually show smaller responsesto irrigations than shallower-rootedcrops on the same soil. Crops growingon a soil in which 75%-8574 of theavailable water is released at tensionsbelow one atmosphere may be expectedto show a smaller response to irrigationsat a given moisture depletion level thanthe same crops growing on a soil inwhich less than 5074b is released at suchlow tensions. The rate at which watercan move to the absorbing root surfacemay play an important part in water-soil-plant relations.A stable water table in the lower por-tion of the normal root zone of a cropmay supply a considerable portion ofthe water absorbed by the roots andmake the plants less responsive to mois-ture changes in the soil above the capil-lary fringe. On the other hand, a fluctu-ating water table may increase cropresponses to early irrigation by restrict-ing live roots to a shallow depth. Salinitymay affect soil moisture-plant growthrelations by decreasing moisture avail-ability through increased soil moisturestress, by interfering with root growthand absorption through toxicity reac-tions, and by contributing to poor soilstructure, which in turn influences infil-

tration, drainage, aeration, and rootgrowth. Soil -borne plant diseases andnematodes, by reducing root surface,may cause crops to respond favorablyto irrigations at seemingly very lowmoisture stress levels. Soil temperaturealso affects the rate of root growth androot distribution with depth.The fertility sta tus of the soil and pos-sibly the depth distribution of some es-sential element may also determine thegrowth response of crops to irrigationsat various moisture depletion levels. Atlow nitrogen levels, infrequent irriga-tions may produce as high or perhapseven higher yields than more frequentirrigations which cause some loss oflimited nitrogen by leaching. However,when ample nitrogen is applied, thissame crop may respond very favorablyto the more frequent irrigation schedule.In soils where the available supply ofsome essential element is confined to thetop soil, the drying out of the upper por-tion of the root zone may seriously re tardplant growth even though the plant maystill be adequately supplied with waterfrom a less fertile subsoil. Fertility re-sponses have complicated the interpreta-tion of many soil moisture versus plantgrowth experiments.Weather Factors

Weather conditions-particular1 y lightand temperature-may so influence thegrowth characteristics of the shoot androot as to affect soil moisture-growth re-lations. Late-planted sugar beets whichmust develop roots during hot dryweather may fail to develop as dense ordeep a root system as early seeded beets.Such beets are much more sensitive todepletion of available moisture than arethe deep rooted beets planted early inthe year. The length of the crop seasonbefore fall rains or frost may at kastpartially determine whether harvestableyields will be affected by imposing dif-ferent soil moisture stress levels duringthe growing period.Meteorological factors-light, temper-ature, humidity and wind-control therate of water loss by transpiration fromplant leaves and evaporation from thesoil surface. Plant growth is probablyI-W

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dependent upon plant turgor, whose rela-tion to soil moisture stress for differentrates of transpiration needs to be ex-plored. It can be reasoned that an in-creased rate of transpiration would lowerthe plant turgor corresponding to anygiven soil moisture stress. This wouldhave the effect of causing growth todiminish at higher moisture levels asillustrated below. Much more work onthis point is needed.Miscellaneous Factors

Problems associated with insect con-trol or harvesting may at times influenccthe apparent effects of soil moisture con-ditions on crop yields. The followingstudy of forage and seed production byladino clover provides an interestingexample.

Increased dryness reduced forageyields but increased the harvestable yieldof seed. The total seed actually producedby the clover also diminished with i n -creased stress, but the higher humidityassociated with the wettest treatmentcaused such a serious preharvest loss ofnewly produced seed as to reduce theharvestable yields at the end of the sea-son below those on the drier treatments.Some types of harvesting problems mayaffect the results of soil moisture-plantgrowth experiments more frequentlythan is realized.Mention has been made of a sizablenumber of soil, plant, weather, and othermiscellaneous factors which may influ-ence the effects of various moisture de-pletion levels on plant growth o r yield ofsome specific organ or constituent. Whenviewed against this backedground, it isnot at all surprising that conflicting re-sults have been obtained even in irriga-tion experiments involving some givencrop. This discussion emphasizes theprobable impossibility of finding anyone generally applicable relation betweencrop yields and soil moisture depletionor soil moisture stress, at least as meas-ured by our present methods.Present information, meager thoughit is in many respects, may allow us tomake some fairly accurate predictions

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whether growth or yield in given situa-tions is likely to be unaffected by deple-tion of nearly all the available water asmeasured by present methods or is likelyto be increased by irrigation at lowersoil moisture stresses. The following twocheck lists may be helpful in anticipatingthe response of crops when given condi-tions prevail. It is not implied that allconditions must be present, nor is therelative weighting of each condition con-sidered. If a given situation is describedby some entries from both tables, predic-tion of response to ir rigat ions will bemuch more difficult.It is often questionable whether theincreased yields sometimes obtainablewith relatively frequent irrigations will

pay for the added cost of water andlabor. The following practical considera-tions all suggest the desirability of usingirrigation water sparingly: maximiumuse of limited water supply, water andnutrient losses caused by deep percola-tion, danger of developing a drainageproblem through overirrigation, main-tenance of favorable soil tilth and, insome cases, obtaining high quality ofthe marketable product. Frequent irriga-tions often aggravate the problems ofplant diseases, insects and longevity inperennial crops. Although these consider-ations are of real impor tance in deter-mining farm irrigation practices, theirrelative importance differs from place toplace and even from year to year.To assure a continuous supply of .avail-able soil moisture and to allow for un-foreseen delays in irrigation or unusually

Some Conditions Which Wi ll Increase or Deerease the Pr obab ility That Crop Yields W il l BeReduced by Allo wing Neorly Complete Depletion of Available Soil MoistureIncreased Probabilit y Decreased Probability

Plant Shallow, sparse, slo w-gr owi ng roots Deep, dense, fast-gr owi ng roots(Relatively frequent irrigatlon aesirable) (Relatively infrequent irrigation possible)

Dry weight yields of r eproduc tive organHarvest for content of sugar, oil, etc.Deep soil; good structure

Fresh weight yield of vegetative organQuality dependent upon size of vegetativeShallow soil; poor structure impedin g rootSlow infiltration and intern al drainage;Root disease, nematod es presentSmoll fraction of availabl e water held at Nonsalinelow soil mo isture stress levelsSaline soils or waterFertility level high; nu trients concentratedPlanted at beginning of hot d ry seasonMaior growth period during hot dry seasonHigh evaporation rates season

desired desiredorgan

grow th Good infiltration, inter nal drainage,poor aeration

Soilaerationlow soil m oisture stress levelsLorge fraction of avoilable water held at

Fertility lev el low; nutr ients distribu tedConstant water table in reach of rootsPlanted well ahead of hot dry seasonMajor growth period before hot dryLow evaporation rates

in prof ilein topsoil

Weather

dry weather, the irrigation farmer gen-erally can not allow nearly completeavailable soil moisture depletion. To al-lo w for a margin of safety, he shouldplan to irrigate while some availablemoisture s till remains. The fraction ofthe total available moisture range whichcan be utilized with safety depends on anumber of factors including crop rootingcharacteristics, the soil and the irrigationsystem. If, as illustrated below, a safetymargin of 157h is made to meet the prac-L

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tical problems of irrigation under farm-ing conditions, then a considerable por-tion of the differences predicted by theseveral current theories on water-soil-plant relations tends to disappear. How-ever, to raise irrigation efficiency and toincrease crop production, vigorous pro-grams of research must be continued.

Ro b er t M . Hagan is Associate Prolessor o fIrr igation, Universi ty a/ Calilornia , Davis .The s tudies on sugar beets were conductedb y L. D. Doneen, and those on beans by L. D .Doneen, Professor o j Irr igation. and D . W . Hen-derson, Assis tant Prolessor o i Irrigation, U n i -versi ty 01 California, Davis.

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the root zone and the salt injury to thetrees disappeared.The type and condition of the soil de-termine to some extent the hazards of saltaccumulation from salines in the irriga-tion water. On open and well drainedsoils where deep percolation of water iseasily accomplished the effective salinitycan be much higher than on poorlydrained soil where there is a high watertable. Many soil conditions-stratifiedsoils, clay lenses, some clay and adobesoils, dense or compact subsoils, heavyclay subsoils-may seriously reduce deeppercolation of the irrigation water in areasonable time. These soil conditionsmay prevent sufficient leaching to removethe salines from an irrigation water hav-ing an appreciable amount of salts.Another important consideration injudging quality of water is the quantityof water that penetrates below the rootzone. In low rainfall areas, as the San

Joaquin Valley and the Imperial Valleysome leaching may be desirable, but be-cause 20%-70P/o of the water appliedmay penetrate below the rooting depth ofplants, most surface soils are adequatelyleached of excess salts. It is nearly impos-sible to adequately irrigate any sizablearea for maximum production withoutsome deep percolation of the water. Withcareful control of the water there will besufficient leaching to maintain a lowsalinity in the root zone for most plants.Sodium content or percentage of thetotal salts is very important. If the percent sodium is low good friable soil struc-ture is maintained and soil will takewater readily. However, if the sodiumpercentage i s high the soil will disperse-the structure destroyed-and the rate ofwater infiltration will be reduced. In ex-treme conditions of dispersal, the soilwill be effectively sealed against the pen-etration of water so it remains on the sur-face until it evaporates or is removed bysurface drainage. Some quality of waterstandards have indicated that when 6076

or more of the salts are sodium, troublecan be expected from soil dispersal andreduced water penetration. Recent inves-tigations indicated this it not necessarilya simple percentage relationship, but therole sodium plays has not been entirelyinvestigated. For example, with the pre-cipit ation of the lime salts in the soil-calcium and magnesium carbonate-thesodium percentage of the soil solutionwill increase over that of the originalirrigation water. This increase in sodiumpercentage may be sufficient in somecases to cause a dispersal of the soil andreduce the rate of water infiltration. Thismay be particularly serious where theirrigation water contains more bicarbo-nate ion than calcium and magnesium.This type of water occurs from wells inan area of about 18,000 acres north ofBakersfield in Kern County. These watersare low in total salt but the principal saltis sodium bicarbonate. To successfullyuse these waters for irrigation, they mustbe amended by the addition of gypsum.

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soil plant water

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