TILLAGE MANAGEMENT CHANGES SIZE-DISTRIBUTION OF AGGREGATES AND MACRO-STRUCTURE OF SOILS USED FOR IRRIGATED ROW-CROPS

The Tatura system for the preparation of seed-beds for irrigated annual row-crops is described, where the soil is tilled when wet and friable and so requires few passes with implements to become suitable for crops, and where seeds are sown into wet soil. In soil prepared by the Tatura system, the percentage of aggregates < 0.5 mm diameter (as measured by dry-sieving) in the seed-bed was about half that found in commercially prepared seed-beds which were tilled up to 50 times when dry. With the Tatura system, the wetter the soil (up to 22% water content) when tilled, or the more passes (up to 4) of the implement at a water content of 22%, the less dust (< 0.5 mm diameter) and/or fewer clods (> 20 mm diameter) were formed. The macro-structure of the surface layer of soil tilled at different water contents by the Tatura system was also quantified statistically by the method of wax-impregnation. The macro-structures were compared at the 10 mm, 20 mm, 40 mm, 60 mm and 80 mm depths in beds of soil prepared for irrigated annual row-crops by a system which has been described previously. Within each treatment (21.7%; 19.0%; 11.6% water content at 0-100 mm depth at tillage), the linear porosity and mean pore-size each tended to decrease with depth to 40 mm, with no further change or slight decrease to 80 mm depth. In all treatments, the mean aggregate-size tended to increase with depth from 10 mm depth to 80 mm depth. The sizes of pores and aggregates varied across each bed and possibly depended on the position of tines within the bed at each pass at tillage. Water content at tillage led to small differences in structure of the beds of soil. Soil tilled at a water content slightly above the Casagrande Plastic Limit generally had slightly larger pores and aggregates than soil tilled at lower water contents.