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<br />three to four times that of alkali soils or dense clay
<br />with poor structure. Range-soil groups with equal
<br />amounts of vegetal covers do not necessarily have
<br />similar water intake rates. Differences are usually
<br />traceable to good or poor structure. The rate of water
<br />intake on soils with good structure is always greater
<br />than that on soils with poor structure with equal
<br />vegetal cover. The subsurface texture and structure
<br />influence the downward movement of water after the
<br />surface layer is saturated. For instance, prismatic and
<br />subangular blocky structure with vertical cleavage in
<br />the subsoil is conductive to water movement, whereas
<br />coarse blocky or angular blocky structure with
<br />greater horizontal than vertical cleavages retards
<br />downward movements of water.
<br />
<br />A major factor among many soil properties
<br />influencing infiltration is the degree that the surface
<br />soil is compacted. Modification of pore sizes and
<br />pore-size distribution occurs in the field. Under wet
<br />conditions, for example, one pass of a tractor has
<br />been known to reduce noncapillary pore space by
<br />half and infiltration rate by 80 percent (Steinbrenner,
<br />1955) and in another instance, two passes with a
<br />tractor reduced infiltration rate from I A to 0,6 in./hr
<br />(Doneen and Henderson, 1953). When a soil is
<br />compacted, its total porosity is decreased, the major
<br />reduction being in noncapillary porosity. The con-
<br />verse is found in tillage when, at least for a short
<br />time, the soil is opened up and large pores are
<br />provided.
<br />
<br />Duley and Kelley (1939) tested intake rates on
<br />different soil types, such as clay loams, silty clay
<br />loams, silty loams, and sandy loams, TheIr subsoil
<br />varies from deep, uniform sandy materials and uni-
<br />form silty subsoils to heavy clay and clay pan
<br />subsoils, The amount of water absorbed by these
<br />different soils is given in Table I, The results shown
<br />are for bare cultivated soils on a 4 percent slope for a
<br />1,5 hour period of water application, After this water
<br />had been applied, the plots were allowed to stand
<br />
<br />until the following day when a second application of
<br />water was made. The total intake of water and the
<br />final infiltration rates on cultivated bare land show
<br />much less variations among the different soil types
<br />than anticipated. In spite of the fact that the soils
<br />tested varied greatly in texture of the surface soil and
<br />in profile characteristics, the amount of water taken
<br />into the soil in a given time was strikingly similar for
<br />all soils, and the infiltration rates were finally reduced
<br />almost to a common level. The difference between
<br />the first day tests and second day tests is probably
<br />due mainly to antecedent moisture content.
<br />
<br />Meeuwig (1970) has found that water retention
<br />(catchment at the surface plus infiltration) decreases
<br />with increasing bulk density. However, this relation is
<br />strongly affected by the coarseness of the soiL When
<br />a large proportion of the soil is composed of particles
<br />and aggregates larger than 0.5 mm, the effect of bulk
<br />density is minimal, but when the soil is fine and
<br />poorly aggregated, water retention dec.reases sharply
<br />as bulk density increases.
<br />
<br />Many experiments have shown that pore sizes
<br />and pore-size distribution are greatly affected by the
<br />content of soil organic matter because both the sizes
<br />of soil aggregates and their stability in water are
<br />related to the amount of soil organic matter. Organic
<br />matter is conducive to the formation of relatively
<br />large stable aggregates, This is not only true in silts
<br />and clays, but also in most soils containing colloidal
<br />material. The addition of organic matter or its
<br />removal, as by intensive cultivation and oxidation,
<br />changes the prevailing permeability, However, these
<br />effects are more pronounced in some silts and clays
<br />than in sands,
<br />
<br />The influence of soil texture and structure on
<br />infIltration, as described through noncapillary
<br />porosity, clay content, and organic-matter content,
<br />was also shown in a statistical study of 68 soils (Free
<br />et aL, 1940), in which noncapillary porosity of the
<br />
<br />Table I, Effect of soil types on total intake of water and inf'dtration rate on bare cultivated soil with a slope of
<br />4 percent (after Duley and Kelley, 1939),
<br />
<br />Soil Type
<br />
<br />Total intake of water
<br />in 90 minu tes (inches)
<br />1st day 2nd day
<br />
<br />InfIltration rate
<br />at end of 90
<br />minutes (inches/hour)
<br />1 st day 2nd day
<br />
<br />Pawnee clay loam
<br />Lancaster sandy loam
<br />Knox silt loam
<br />Butler sil t loam
<br />Dickinson sandy loam
<br />Marshal silt loam (eroded phase)
<br />Marshal silt loam (heavy subsoil)
<br />Butler silt clay loam
<br />
<br />1.20
<br />L71
<br />1.05
<br />1.32
<br />1.37
<br />1.08
<br />2A3
<br />1.24
<br />
<br />033
<br />032
<br />0.21
<br />0.25
<br />0,24
<br />0.21
<br />0.21
<br />0,16
<br />
<br />0,76
<br />0,61
<br />0.58
<br />0,57
<br />OA8
<br />0.43
<br />038
<br />037
<br />
<br />0.50
<br />0.68
<br />038
<br />038
<br />OAO
<br />OA2
<br />0.28
<br />030
<br />
<br />5
<br />
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