Overwinter changes in physical properties of no-tillage soil

Soil physical properties often improve during winter, even without tillage. Based on observations, improvement is greater on ridged than on relatively flat soil, and it appears related to soil freezing and thawing. The objective of this study was to determine changes in bulk density, penetration resistance, aggregate-size distribution, organic-matter concentration, and water retention that occur during winter in a soil having low or high ridges formed by tillage. The study was conducted from 1985 to 1988 on Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll) at Bushland, TX. The 1985-1986 winter was coldest and had the least late-period precipitation (19 mm). For the 0.04- to 0.07-m depth increment during this winter, bulk density (Db) decreased 0.10 Mg m-3; penetration resistance (PR) decreased 0.14 and 0.20 kPa at -4.89 and -19.6 kPa matric potentials, respectively; mean weight diameter (MWD) of water-stable aggregates increased 0.5 mm; organic matter (OM) decreased 0.7 g kg-1; and water retention at potentials from -2.44 to -19.6 kPa decreased 0.019 to 0.069 m3 m-3 all significant at P = 0.05. Changes were slight during the 1986-1987 winter. During 1987-1988, Db increased 0.07 Mg m-3, PR increased 0.06 kPa (at -19.6 kPa matric potential), OM decreased 0.4 g kg-1, MWD of water-stable aggregates decreased 0.7 mm, MWD of dry aggregates increased 3.3 mm, and most changes in water retention were not significant. The 1987-1988 changes were opposite those for 1985-1986, apparently because of less intense freezing and more late-period precipitation (44 mm). Late precipitation possibly reconsolidated soil that may have been loosened previously by freezing. In most cases, results for low- and high-ridge treatments were similar. Freezing can improve soil physical properties, but improvements may be negated by late-winter or early-spring precipitation.