WINTER-ASSOCIATED CHANGES IN DRY-SOIL AGGREGATION AS INFLUENCED BY MANAGEMENT

Changes in surface soil structure during winter can increase soil erod ibility. This study was conducted to examine winter-associated changes in dry soil aggregation as influenced by crop, residue, and tillage s ystem. These changes were studied at says, KS, on a Harney silt loam ( fine, montmorillonitic, mesic Typic Argiustoll) in a winter wheat (Tri ticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]-fallow rotation. Three tillage systems were used - clean tillage with residue buried, stubble-mulch tillage, and zero tillage. Residue and crop cov er present during the winter was wheat residue, sorghum residue, and w inter wheat. Measurements of dry aggregate stability, aggregate-size d istribution as geometric mean diameter, and aggregate density were mad e before and after the winters of 1988-1989 and 1989-1990. Little diff erence occurred between clean and stubble mulch tillage systems, which were usually different from the zero-tillage system. Greater changes in aggregation occurred during the 1989-1990 winter when precipitation was greater. Residue maintained higher surface water contents, decrea sed freeze-thaw cycling and drying by sublimination, and decreased flu ctuations in water content. Aggregates from plots with low residue cov er decreased in stability more than aggregates from high residue treat ments. Generally, differences in aggregation between tillage systems w ere maintained during the drier winter and minimized during the wetter winter. Soil aggregates were smaller, less dense, and less stable on the zero-tillage plots in March 1989. Therefore, insufficient residue production for wind erosion control in a zero-tillage system could lea d to more erodible conditions than in a conventional tillage system.