Soil Structure and Moisture Movement

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<ul><li><p>SOIL STRUCTURE AND MOISTURE MOVEMENT</p><p>Horace J. Harper1</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>Oklahoma Agricultural Experiment Station, Stillwater, Oklahoma.2Humbers in parentheses refer to literature cited.</p><p>15</p></li><li><p>16 SOIL SCIENCE SOCIETY PROCEEDINGS 1937</p><p>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.</p><p>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&gt; 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-</p><p>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.</p><p>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. .,</p><p>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</p></li><li><p>SOIL STRUCTURE AND MOISTURE MOVEMENT 17</p><p>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-hums or small grains are grown, this may be apossible explanation of the structural changeswhich are observed in soils where these cropsare grown. The effect of soluble salts on themovement of water through soils is well known.Rate of infiltration decreases rapidly as thesoluble salts are removed by the downwardmovement of percolating water. A puddled con-dition will frequently develop in fine-textured soils following a period of excessiverainfall, and thorough tillage is the onlyrapid method which can be used to improve soilstructure. Although aggregation of particlesmay not occur in the presence of soluble saltsthe effective diameter of soil'pores may beincreased by the dehydrating effect of solu-ble salts and this would permit a more rapidmovement of water into the soil profile. Al-though different investigators have shown thatcalcium hydroxide does not cause the develop-ment of stable aggregates which improve condi-tions for the downward movement of water bythe formation of large macroscopic pores be-tween the granules, calcium hydroxide or cal-cium carbonate will greatly improve the rateof filtration of soil suspensions, and thepresence of calcium in a soil should also havea favorable effect on the rate of infiltra-tion of water under field conditions unlessthe effect of the calcium is neutralized bythe dispersing action of potassium or sodiumions in the soil solution.</p><p>Since the actual presence of aggre-gates may have very little effect on moisturemovement in some soils because the intersti-tial spaces may be filled with fine material,data on the volume weight of soils in rela-tion to moisture movement may have very lit-tle value unless correlated with other physi-cal factors such as clay-pore space ratio (7).Boils which have a high volume weight may con-tain, a relatively large percentage of sandand have a low percentage of total pore space,but water will, move rapidly through the largeopenings which are present between the sandgrains. Other soils may also have a high vol-ume weight and contain considerable quanti-ties of silt and clay interspersed betweenlarger particles. The total pore space undersuch conditions is also low, but the openingsbetween the particles are too small for the</p><p>rapid movement of water. Occasionally soilsare found which contain a high percentage ofclay, have a high total pore space and a lowvolume weight, but absorb water very slowly.The swelling coefficient of the soil colloidscannot be disregarded under such conditions.In the illuviated horizon of clay pan soils,no structural characters are evident whenthe soil is wet, but small clods, nut struc-ture, or occasionally short prisms developwhen the clay is dry. Microphotographs ofthe dense clods reveal a non-porous condi-tion, which might be expected from a physi-cal examination. When no cracks develop inthe surface layer of soil occurring over aclay pan during periods of dry weather watercannot move into the subsoil rapidly excepton listed or chiseled land. When cracks ex-tend through the surface horizon runoff israpidly absorbed until the openings areclosed by the expansion of the clay.</p><p>A close examination of the largeropenings which appear in thin sections of un-disturbed soil under a microscope reveals twotypes of pores. One type is round or oval inshape and has smooth sides. These openingshave developed as a result of root develop-ment or insect activity and form a continu-ous channel of relatively uniform diameterinto the subsoil. Water can move readilythrough these openings. Another type of soilpore is very irregular in shape and the out-line is determined by the position of soil ag-gregates. An effect which is similar totillage has occurred as a result of root de-velopment which forces the soil aggregatesapart to create numerous macroscopic open-ings. These irregular-shaped openings arenot continuous, although when present in suf-ficient number they provide irregular chan-nels into which water may move rapidly andenter the deeper horizons of the soil profileif the surface soil has not been packed byovergrazing or a surface crust has not de-veloped as a result of torrential rains. Insome types of parent material which do notrestrict the downward movement of gravita-tional water, pores may be observed which ap-pear to have developed as a result of solu-tion. When impervious layers do not developwithin a soil profile which has been formedon a porous parent material, conditions arefavorable for good internal drainage, and theimportant problem as far as.these soils areconcerned is the absorption of water at theimmediate surface.</p><p>Recent microscopic studies revealthat part of the organic matter in many soilsappears not as a film surrounding the parti-cles but as individual units between soil</p></li><li><p>18 SOIL SCIENCE SOCIETY PROCEEDINGS 1937</p><p>aggregates or sand grains. The effect ofthese larger masses of organic material be-tween mineral aggregates has not been studiedsufficiently to show what effect this type ofdistribution has on moisture movement, but itis reasonable to believe that water will movethrough the pore space in organic matter morerapidly than it wi...</p></li></ul>

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