A METHOD FOR MEASURING AGGREGATE WATER STABILITY OF DRYLAND SOILS WITH RELEVANCE TO SURFACE SEAL DEVELOPMENT

This paper examines the potential for a test of aggregate stability-or more correctly, of the water-stable size distributions resulting from wetting-to be correlated with surface seal development and infiltration of rainfall under field conditions. Aspects of measurement of water stable aggregation considered include wetting method, sample depth, area, antecedent water content, and the use of whole soil rather than restricted dry aggregate size ranges, and the water stable size fractions likely to restrict water entry and form surface seals. From consideration of pore sizes likely to prevent direct entry of raindrops into the soil, it is concluded that surface seal development is likely to be related to the proportion of particles < 100 mu m. For the northern cereal growing areas of eastern Australia, essential features of a method for studying aggregate stability of dryland soils are: (a) wetting of air-dry soil by simulated rain with kinetic energy close to 29 J m(2) mm(-1) and intensities of 85-100 mm h(-1) for at least 20 min; (b) the wetting applied to field or laboratory plots of 'whole soil' rather than selected dry aggregate sizes, with plots at least 0.36 m(2) in area, and laboratory plots being approximately 100 mm deep; (c) samples of the compacted surface layer under high energy rain (to a depth of 33 mm) taken with a spatula; (d) size distribution measured by sieving in tap water using a modified Yoder wet sieving apparatus, a nest of sieves with minimum aperture of 0.125 mm, and material passing through the finest sieve collected and weighed; no distinction being made between aggregates and primary particles. Subsequent papers will deal with validation of the proposed test and its use to consider effects of tillage management on water stability.