RESPONSE OF GRAIN CORN TO SUBSOILING AND CHEMICAL WETTING OF A COMPACTED CLAY SUBSOIL

The effect of annual compactive loads of 10 and 18 Mg per axle on soil structure and crop yields were compared with effects of loads limited to 5 Mg per axle. Chemical conditioning and subsoiling were also applied to determine their alleviating effects. Significant topsoil and subsoil compaction resulted from both loading levels. Maximum dry bulk densities of up to 1.79 Mg per m-3 were observed in the subsoil at depths between 0.2 and 0.3 m. Compactive loading significantly reduced subsoil pore sizes greater than 30 mum. Crop yield reductions ranged from 18 to 27% under optimal weather conditions and were as high as 55-86% under adverse weather conditions. Chemical conditioning had no significant effect on soil bulk densities but was associated with improved crop yield in a year with above normal precipitation. Changes observed in water retention were attributable to the effects of the conditioner on water desorption characteristics. Inter-row subsoiling with narrow-tined and winged subsoilers was effective in loosening the compacted profiles, but improvements in crop response varied according to rainfall regime.