How Water Moves in the Soil - Five pages of photographs explain soil water movement

Sep 8, 2026

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Photographs and explanation describe how water moves through soil in multiple directions as it responds to rain, irrigation, gravity, adhesion, and cohesion. The text outlines pore structure in silt loam, clayey and sandy soils, distinguishing water held tightly in fine pores from water available to plants. It compares uniform versus stratified soils, showing how wetting fronts slow or stop at fine or coarse layers. Artificial profiles illustrate clay barriers, sand layer drainage, subsoil effects, aggregates, and vertical mulching.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages6

Reading Time15 min

AccessFree to download

How water moves in soil

Uniform or homogeneous soils

Clay layer

Sand layer

Coarse sand or gravel subsoil

Layer of coarse aggregates in fine soil

Vertical mulching

Howwatermovesin the Five pages of photographs reveal the greatdiversity in one of the most commonplace phe-nomena in agriculture BY WALTER H.GARDNER WATER—as a liquid or vapor—is nearlyalways moving in the soil.It moves down-ward following rain or irrigation.It movesupward to evaporate from the soil surface,or into plant roots and eventually into theatmosphere through transpiration.Horizontalmovement also is important as for examplewhen water moves out from an irrigationfurrow.Water movement can be in any direc-tion depending on conditions. Water flows through the open pores be-tween soil particles.In an ordinary silt loam,for example,half the soil volume is porespace.Water and air share this pore space.For most plants it must be possible for airfrom the root zone to exchange with air fromthe surface.Air from the root zone is ladenwith carbon dioxide as a result of metabolismin the roots. Pores in different soils vary in size andnumber.Silty and clayey soils generally havesmaller pores but many more pores thansandy soils.Because of the number of pores,when silty and clayey soils are filled withwater these soils contain more total waterthan sandy soil with all its pores flled. Some of the water in soils with fine poresis held so tightly that it is unavailable.Evenso,the amount that is available in these soils Walter H.Gardner is a soil scientist at WashingtonState University.This article is adapted from a two-part series that appeared in the October and November1962 issues.Because of a steady demand from teachersand others,the issues have become virtual collectors'items.The editors feel that these dramatically illustratedbasic concepts are worth repeating. is greater than the amount available to plantsin soils with large pores. Two major forces move liquid waterthrough the soil pores:gravity and adhesion.Gravity is most important in saturated soils.It causes a downward force on water.Whena soil is near saturation,the large pores arefilled and water moves rapidly through them. When a soil is not saturated,the largerpores are empty and contribute little to flow.In the unsaturated soils in which most cropsgrow,the major force moving water is ad-hesion.Adhesion—together with cohesion,which causes water molecules to hang to-gether—makes water move on particle sur-faces and through the finer pores.These arethe same forces that make water rise in capil-lary tubes and that account for the absorptiveproperties of blotting paper. Water moves until the forces balance,atwhich point water films on soil particles areuniform in thickness throughout any homo-geneous soil except for some vertical differ-ences that exist because of gravity.If the soilis not uniform or homogeneous,the portionsof the soil that have the smallest pores retainwater most strongly. In stratified soil—soil with various "lay-ers”—the size of the pores in the strata affectwater flow.If an advancing wetting frontencounters fine materials,the resistance in theextremely fine pores may slow the movement.But the water nevertheless,continues tomove.If the wetting front encounters coarsematerials,water movement stops until thesoil becomes nearly saturated. Stratified soils also tend to hold morewater for plant use than uniform soils.Since the different layers slow the movement ofwater,more remains in the root zone. The photographs on these pages showsome basic principles of water movementusing artificial soil profiles. # Uniform or homogeneous soils Water was added to the center of this dryhomogeneous soil.Under this unsaturatedcondition the water moves out almost equallyin all directions.Gravity has only a smalleffect as indicated by the slightly greaterdownward wetting.Under saturated condi-tions,or as saturation is approached,gravitybegins to play a much greater role in watermovement. # Clay layer When water reaches the clay,the very finepores of this layer resist water flow.Althoughwater does pass through the clay,its pene-tration is so sl

FAQs

How do gravity and adhesion affect water movement in soil?

Gravity mainly drives downward movement in saturated or near-saturated conditions. In unsaturated soils, adhesion (with cohesion) moves water along particle surfaces and through finer pores.

Why do fine-pored soils hold more total water but sometimes less available water?

Fine and clayey soils contain many small pores, so they store a larger total amount of water. Some water is held tightly in these pores and becomes unavailable to plants.

How do stratified layers change the progress of a wetting front?

A wetting front slows when it encounters fine materials because of resistance in extremely fine pores. It may stop when it meets coarse materials until the fine layer becomes nearly saturated.

Sep 8, 2026

Upload by Patrick