EFFECT OF SOIL MACROPOROSITY AND AGGREGATE SIZE ON SEED-SOIL CONTACT

The influence of soil structure on the degree of seed-soil contact wit hin a seedbed is poorly understood. This paper presents a simple analo gue of seed-soil contact which allows the examination of the influence of macroporosity and relative aggregate size on the degree of contact within a bed of deformable spheres, A method is described in which a rigid disc or sphere representing a seed is placed within a bed of def ormable spheres of uniform size representing soil aggregates. The stru cture is then compressed uniaxially to a given macroporosity. Contact areas were measured by a technique involving the use of paint, dismant ling of the sample, and image analysis. Results show that degree of co ntact increases as macroporosity decreases, Greatest levels of contact are achieved where rigid and deformable spheres are of similar size. This result appears to be a consequence of maximum stress concentratio n occurring at this size ratio, Contact points were unevenly distribut ed over the surface of the rigid sphere. The applicability of these fi ndings is considered.