TEMPORAL VARIATION IN AGGREGATE STABILITY ON CONVENTIONAL AND ALTERNATIVE FARMS

Low-input alternative farm management practices have been sh to decrease erosion rates and increase soil organic matter contents relative to conventional management practices in eastern Washington state. Little is known about the effect of such alternative practices o aggregate stability. Temporal variations in aggregate stability we measured at three slope positions on soils from adjacent farms managed for many years using conventional vs. alternative tillage and crop rotations. The conventional farm was first cultivated in 1908, and uses a winter wheat (Triticum aestivum L.)-spring pea (Pisum sativum L.) rotation with summer fallow every sixth year. The alternative farm was first cultivated in 1909, and uses a winter wheat-spring pea-Austrain winter peas [P. sativa ssp. arvense (L.) Poiret]-spring wheat-spring pea-summer fallow rotation with an alfalfa (Medicago sativa L.)-wheatgrass [Elymus trachycaulus (Link) Gould ex Shinn.) mixture every 19th and 20th year. Surface soils (0-15 cm) were collected at foot-, back-, and topslope positions from both farms during October 1987, March 1988, and June 1988. The 0.5- to 1.0-mm sieved aggregate-size fraction from each farm, landscape position, and sampling time was analyzed for aggregate stability using the high-energy moisture characteristic method. Aggregate stability on both farms decreased significantly from October to March in response to precipitation and cycles of winter freezing and thawing. Significant increases in stability occurred from March to June on both farms. These increases were interpreted in terms of recovery of cohesion by aggregates following their disruption during the winter. Differences in stability resulting from management practices on the two farms were not significant, even though organic C contents on the alternative farm were significantly higher than those on the conventional farm.