Water Movement in Soilsby DR. WALTER H. GARDNERProfessor Emeritus, Washington State University

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A LIQUID or vapor, water isnearly always moving in the soil.It moves downward after rain

or irrigation. Itmoves upward to evapo-rate from the soil surface. It movestowards and into plant roots, and even-tually into the atmosphere throughtranspiration. And during the night,when transpiration is greatly reduced,water moves from moist soil betweenroots into soil adjacent to absorbingroots that has dried during the previousday.

Horizontal movement also is impor-tant, as, for example, when water movesfrom an aeration hole. Water movementcan be in any direction, depending onconditions.

Water flows through the open poresbetween soil particles. In an ordinarysilt loam, for example, half the soilvolume is pore space. Water and airshare this pore space. For most plantsit must be possible for air from the rootzone to exchange with air from thesurface. Air from the root zone is ladenwith carbon dioxide, as a result ofmetabolism in the roots.

Pores in different soils vary in sizeand number. Silty and clayey soils

Dr. Walter H. Gardner

generally have smaller but many morepores than sandy soils. Because of thenumber of pores, silty and clayey soilsfilled with water contain more totalwater than sandy soil with all its poresfilled.

Some of the water in soils with finepores is held so tightly the plant can'tabsorb it. Even so, the amount in these

soils isgreater than the amount availableto the plant in soils with large pores.

Two major forces move liquid waterthrough the soil pores; these forces aregravity and adhesion. The movement ofwater is entirely different under thesetwo conditions. To understand the dif-ferences, let me first tell you about sur-face tension of liquid water.

You have seen raindrops or dropsfrom a dripping tap, and you probablynoticed they are roughly spherical, witha positive radius of curvature. They areheld in this shape by a force called sur-face tension, which acts at the air-waterinterface in a somewhat similar manneras a rubber balloon, opposing a positivepressure inside of the droplet. Now,much of the water you see - water froma tap, water in a lake or stream, or waterin the cup you drink from - is underpositive pressure. This is how mostpeople think of water. Water under posi-tive pressure moves in response to thepressure of a column of water or bygravitational forces.

Now, let me discuss another class ofwater you ordinarily think of under theterm moisture. You are equally familiarwith this water, inasmuch as it is the

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When water reaches the clay,the very fine pores of this layerresist waterflow. Althoughwater does pass through theclay, its penetration is so slowthat water tables often buildup above the clay. Somehardpans act similarly.

moisture in, for example, a dish-dryingtowel, material of your shirt when youperspire, and the soil when it is notsaturated. It is the water that is said tobe absorbed by a porous material, andit is water that exists with a negativecurvature in the air-water interface asyou would observe it under a high-powered microscope. This water isunder negative pressure, contrasted tothe water of the raindrop, where the air-water interface is positive and the

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pressure is posItIve. Water in porousmaterials under negative pressure mustbe pulled along by attractive forces thatexist between water and the walls of theporous material associated with it,and forces in a negative air-water inter-face that is always present. The bestexample of capillary water is waterpulled upward into a small tube byadsorptive and cohesive forces. Theabsorptive property of blotting paperis a good illustration. Adhesion - to-

gether with cohesion, which causes watermolecules to hang together - makeswater move on particle surfaces andthrough the finer pores.

The differences in the positive andnegative forces that move water in thetwo cases make huge and often dramaticdifferences in phenomena that involvewater. Most phenomena involving watermovement under positive pressure takeplace in pipes and in streams and ditches.Considerable water is usually moved in

Any change in soil porosityencountered by a wetting front

affects water movement. Inthese photographs, a layer of

coarse soil aggregates actsmuch like a layer of sand, with

one important difference:water can move through the

interior of the aggregatesthemselves. But the relatively

small number of contactsbetween the aggregates limits

the amount of water thatactually moves through thislayer. Only when the soil is

nearly saturated does thewater move rapidly through

the soil aggregate layer.Saturation was not reached

in this test.

this condition. By contrast, movementin porous materials under negative pres-sure takes place in thin films, and conse-quently the quantity of water movedwith a similar size of moving force is asmall fraction of that where a positivepressure exists.

Water moves until the forces balance,at which point the curvature of air-water interfaces is the same, except forsome vertical differences that exist be-cause of gravity. If the soil is not uni-

formly homogeneous, the portions ofthe soil that have the smallest poresretain water most strongly.

In stratified soils - soils with various"layers" such as those recommended inthe USGA Green Section Specificationsfor Putting Green Construction - thesize of the pores in the strata affectwater flow. If an advancing wettingfront encounters fine materials, theresistance in the extremely fine poresmay slow the movement. But the water

nevertheless continues to move. If thewetting front encounters coarse materials,water movement stops until the soilbecomes nearly saturated.

Stratified soils also tend to hold morewater for plant use than uniform soils.Since the different layers slow the move-ment of water, more remains in the rootzone. A sandy, droughty soil can thus bemade to hold more water, and yet willdrain rapidly when it is saturated.

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(Top) Here, deep vertical channels are cut in the soil and filled with coarse material. If the channelsremain open to the surface, the largepores in the coarse material take free water from rain orirrigation and transmit it deep into the soil. Then it is absorbed by the soil. If the channels are notopen to the soil surface, vertical mulching does little good. Holes left in the soil by angleworms,rodents, or aerification act like vertical mulch channels. If they remain open to the surface andexposed to free water, they carry water readily.

(Above) Note channel open to the surface rapidly moved water into the soil. Buried channel has noeffect.

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The same amount of water wasapplied to each of three soils.

The clayey soil holds water in asmaller column than loam orsandy soil. This indicates thatclay soils can hold more total

water than loams or sands. Underirrigation, the poor water-trans-mitting properties of such soilsmake them less desirable than

sandy soils.

Dye tracers indicate the directionof water movement in soil. Waterand soluble fertilizers movealmost radially away from thepoint where water was applied.After the wetting fronts join, thedirection of flow changes slightly.Above the water level, the move-ment is upward toward drier soil.Below the free water level, solublematerials move downward. Inaddition, evaporation from thesoil surface causes an upwardmovement of soluble materialsin the soil solution.

These principles of how water movesin soils have been incorporated in theconstruction of USGA Green Sectiongreens. The effect on water penetrationof such practices as a physical soilanalysis, off-site uniform soil mixing,adequate soil depth, a sand and gravellayer, tile lines, mechanical aeration ofthe putting surface, and the importanceof keeping vertical aeration channels

open to the surface through the use ofsand cannot be overemphasized.

The knowledge of these principlesand their application are essential toproper management of turf areas.

EDITOR'S NOTE: This article is basedon direct excerpts of Dr. WalterGardner'stalk and film presentation during the1988 USGA Educational Program in

Houston andfrom an American Societyof Agronomy 1979 reprint, "How WaterMoves In Soil, " by Dr. Gardner.

For details regarding the 27-minute,16mm, color, time-lapse, sound motionpicture film or video cassette, pleasecontact your regional Green Sectionoffice or the Agronomy Club, Depart-ment of Agronomy and Soils, WashingtonState University, Pullman, WA 99164.

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