The solution of the differential equation describing the flow of moisture in unsaturated porous media is difficult because of the dependence of the permeability on moisture content. In the case of horizontal flow this equation can be written in a form similar to the non‐linear diffusion equation. The diffusivity in this equation is dependent upon the moisture content of the medium. Boltzmann has developed a treatment of the non‐linear diffusion equation which allows one to calculate the diffusivity‐moisture content function from a moisture content distribution curve. This treatment assumes that the moisture content is a function of a variable dependent on distance and the square root of the time. Columns of sand at a constant moisture content throughout their length were prepared. Water was applied at one end of the columns and allowed to move into them for a measured period of time. The distribution of moisture content in the columns was then determined and calculations of the diffusivity‐moisture content function were made. Data from one column gave moisture diffusivity values representing the entire moisture range of that column. The results indicate that there may be a maximum in the diffusivity‐moisture content function at a moisture content less than saturation.

No abstract

No abstract

In the Piedmont of the Southern Appalachian region, soil degradation is most often expressed by crop water deficit that limits crop yield in the warm season. To evaluate the nature of variability on these cropped lands, soybean [Glycine max (L.) Merr.] yield and associated soil characteristics were measured across the range of surface soil conditions in 40 farm fields. Factor analysis of the data permitted identification of variables responsible for most of the yield variability. Carbon in the surface soil was identified as the manageable soil variable that could significantly influence crop water availability and curtail soil erosion. Subsequently, selected crop cultures that supplied a range in quality and quantity of crop biomass to the soil surface were applied on three soil erosion classes for 5 yr. Large increases in rainfall infiltration and reduced soil erodibility were associated with no‐till planting of grain sorghum [Sorghum bicolor (L.) Moench] into crimson clover (Trifolium incarnatum L. ‘Tibbee’) in comparison to conventional tillage of grain sorghum and soybean. The maintenance of a decomposing mulch by crop residue additions of about 12 Mg ha−1 yr−1 generated high soil C levels in the 0‐ to 15‐mm depth and a high water stability of aggregates in the 0‐ to 80‐mm depth in comparison to incorporated crop residues. In the 6th yr, grain yield of conventionally tilled soybean was 30 to 100% greater on the previously no‐till crop culture than on the conventionally tilled. The restoration and maintenance of soil productivity commensurate with inherent site resources was associated with maintenance of a decomposing mulch on the soil surface derived from an appropriate quantity and quality of crop residue produced in situ.

No abstract