Soil Structure and Moisture Movement
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SOIL STRUCTURE AND MOISTURE MOVEMENT
Horace J. Harper1
The development of a soil structurewhich will increase the rate of moisturemovement into the soil is an important objec-tive, but achievement is difficult when eco-nomic factors favor the production of cropswhich do not protect the surface of the landfrom the destructive effect of torrentialrain. Many experiments have been conductedto measure the percentage of total rainfallwhich is lost by runoff, and variations fromless than one to nearly fifty per cent havebeen obtained on different soils under dif-ferent conditions. Such factors as slope,soil texture, vegetative cover, and rate ofrainfall have an important effect on thequantity of water which is absorbed by dif-ferent soils, however, the engineer has con-sidered slope as the most important factorin making recommendations for the spacing ofterrace ridges on different areas of land.
Although simplicity is an importantessential for the success of any educationalmovement, lack of information concerning theeffect of divergent conditions which retardor increase the rate of absorption of waterby different soils is an important reason whythe recommendation for the location of ter-race ridges has been prepared for all soilson the basis of 'slope and no provision hasbeen made for variations in soil type orchanges in cropping systems which may be usedon similar areas of land.
It is a well-known fact that grassland or forest soil is not affected apprecia-bly by soil erosion when it is first put intocultivation because soil porosity is favora-ble for the rapid movement of rain into thesoil and very little runoff occurs. Auten (l)2has shown that soil in a virgin forest has alower volume weight than similar land whichhas been cultivated for many years. Brad-field (4) and others have emphasized the im-portant effect of grass on the granulationprocess in soil, and suggest that soils con-taining from three to ten per cent of organicmatter and saturated with lime provide favor-able conditions for the development of crumbstructure.
Bouyoucos (3) stated that alternatedrying and wetting soil with water will causegranulation. In this process water is ab-sorbed, the colloids swell and push the soilparticles apart, and the cohesive force of theclay will pull the particles together as thewater is removed. Even when soil is puddledit will granulate due to the effect of freez-ing and thawing or other physical and chemi-cal processes which occur in the soil, butwetting and drying is most universal and mostactive. Baver (g) suggested that irreversiblecolloids are the best cements to develop apermanent aggregation of particles and thatthe calcium ion may cause floccules which maybe bound together into stable aggregates byorganic matter.
Vinokurov (14) states that the dis-persed condition of the soil is not uniformand that less dispersion occurs when theweather is moist and warm. Rhoades (ll) foundthat 80.4 per cent of the fine material inGrundy silt loam was in the form of stable ag-gregates, as compared with 44.4 per cent ofthe fine material in Marion silt loam. TheGrundy soil is high in total organic matter,and the Marion silt loam is low in this ma-terial. Recently Lutz (9) has suggested thatthe stable character of soils may be due tothe cementing effect of iron oxide and foundthat a bentonite clay became non-gelatinouswhen iron was added to the colloid. Stephen-son and Marquardt (13) report that excess cal-cium salts are needed to coagulate the clayand that calcium sulphate had a good effecton four refractory soils which were investi-gated. Soils which had a good structure werelow in base exchange capacity in relation tototal colloidal content and they believe thatthe feasibility of chemical treatments canonly be determined by field trial. Buehrer (5)studied the rate of air movement through asoil at varying stages of moisture content inorder to determine the relation between natu-ral pore space in soil and the degree of hy-dration of the soil particles. Janota (8)found that soils having a pore space of lessthan 40 per cent were impermeable to water.
Oklahoma Agricultural Experiment Station, Stillwater, Oklahoma.2Humbers in parentheses refer to literature cited.
16 SOIL SCIENCE SOCIETY PROCEEDINGS 1937
Sideri (IS) has suggested that humus is ab-sorbed by clay and causes a cementing effect,although selective orientation is requiredfor absorption and some clay is not affectedby humus. Carneo (6) observed that rainwaterbreaks down the binding power of crumb struc-ture and published a quotation from Russellstating that rain falling on soils containing40 per cent or more of fine sand will tend todevelop a hard surface crust. The chemicalnature of the soil affects the breakingstrength of these crusts. Musgrave and Free(10) have studied the effect of using muddywater on the rate of infiltration and believethat erosion on cultivated land may be accel-erated because the presence of suspended ma-terial qloses the soil pores and reduces therate of water movement into the soil.
Since runoff water occurs on slopingland protected by grass or forest althoughthe soil structure is favorable for the ab-sorption of rain, runoff will also occur oncultivated land under similar conditions. Theimportant problem is to create a condition bythe improvement of structure> or the use ofmechanical methods which will result in a max-imum absorption of water in order to reducethe damage from physical losses which occuras a result of soil erosion. Soils contain-ing a high percentage of coarse sand and lim-ited quantities of clay absorb water readilybecause of the large diameter of openings oc-curring between sand grains. This conditioncannot be called a favorable structure be-cause the large pores have not been developedas a result of the favorable arrangement ofthe soil particles, consequently it may notalways be possible to differentiate accurate-ly betwe'en the movement of water in soil asit may be affected by a porosity due to open-ings between sand grains, macroscopic poreswhich have developed as a result of biologi-cal activity or a porosity which has origi-nated as a result of granulation processes orthe aggregation of particles. Normally theopenings occurring between soil granules whichhave developed in soils containing a largequantity of organic matter and lime are large.Water moves readily into these non-capillarypore spaces and such soils have a high infil-tration capacity. When relatively large quan-tities of organic matter are present in asoil, the physical condition is favorable forthe absorption of water regardless of soilgranulation because the organic matter oc-cupies several times as much volume as anequivalent weight of soil. Three per cent oforganic matter in a soil by weight Will equalapproximately 15 per cent of the total massof the soil by volume. This will have a pro-
nounced effect on soil structure as comparedwith similar soils which have lost 40 to 60per cent of the organic matter originallypresent in them as a result of cultivation,although distribution of organic matter insoil is more important than quantity as faras its effect on moisture movement is con-cerned. This condition has not received suf-ficient consideration by soil physicists whohave conducted extensive investigations todetermine the effect of both chemical andphysical factors on moisture movement. A bet-ter picture of changes in the structure ofcropped and virgin soils can be obtained bya microscopic study of thin sections whichshow the natural arrangement of groups ofparticles which are responsible for the de-velopment of macroscopic pore space. This newfield of investigation will help to answermany problems which are recognized at thepresent time and will aid materially in theirsolution.
The flocculating effect of solublematerial which is slowly liberated.when or-ganic matter decays should not be disregard-ed in any study which includes the effect oforganic matter on soil structure. Frequentlymuddy water in reservoirs or ponds will clar-ify rapidly when the water level is raisedand a dense growth of vegetation is submerged.Soluble salts which are liberated from theorganic matter assist in the coagulation ofthe clay, consequently similar effects shouldoccur when organic matter containing largequantities of calcium are added to the soil. .,
Too little is known concerning the ef-fect of root excretions on soil structure.Certain annual legumes such as cowpeas, soy-beans, and Austrian winter peas leave thesoil in a very mellow condition after they-aregrown, and when the land is not protected byvegetation, granules occurring in the surfacesoil are easily moved by sheet erosion. Wheth-er a desirable physical condition develops be-fore or after the roots decay has not been de-termined. A grain sorghum crop produces acondition which is quite different from theeffect observed following the grpwth of cer-tain legumes. An unfavorable physical condi-tion is produced in many soils by grain sorg-hums which is not easily corrected by tillage.This crop produces an abundance of fine rootswhich are low in calcium. When the crop isgrown on soil containing a considerable quan-tity of clay a cloddy condition occurs whenthe land is plowed. Whether the soil ispacked as a result of the slow and uniform re-moval of water during hot summer weather orwhether some other factor is responsible forthe puddled condition of the soil is a probleft
SOIL STRUCTURE AND MOISTURE MOVEMENT 17
hich needs further investigation. Althoughcertain types of organic matter may have adispersing effect rather-than a coagulatingffeot on clay particles, the concentration ofsoluble salts in the soil is an important fac-tor in regulating the rapid movement of waterinto the soil. Since the nitrate nitrogen ina soil is normally higher under legume cropsas compared with a similar area where sorg