REDUCED GROWTH AND YIELD OF WHEAT WITH CONSERVATION CROPPING .2. SOILBIOLOGICAL FACTORS LIMIT GROWTH UNDER DIRECT DRILLING

Wheat was grown in intact cores of soil removed from a field experimen t in which seedlings had grown more slowly in direct-drilled soil than in cultivated soil. Experiments were conducted in controlled environm ents to resolve (1) whether shallower sowing of direct-drilled crops c aused slower growth, (2) whether the soil factors causing the slower g rowth were physical or biological and (3) if biological, whether Rhizo ctonia solani was the major pathogen. The conditions of the experiment s removed possible constraints of water and nutrient supply but otherw ise simulated the environment of wheat seedlings in southern Australia . Shallower sowing led to faster emergence and increased seedling grow th, but irrespective of sowing depth, direct-drilled plants grew more slowly than plants in cultivated soil. Shoot growth of direct-drilled plants was 25-65% less than that of cultivated plants. These growth re ductions were largely overcome by sterilizing the soil with gamma radi ation or by fumigation with methyl bromide, indicating that biological factors were primarily responsible. Rhizoctonia tvas implicated as th e cause of the reduced shoot growth when infection was severe (>3 on a 0-5 scale). There was no correlation between infection severity and s hoot growth at moderate levels (<3) and significant reductions in shoo t growth occurred in the absence of Rhizoctonia. These reductions were evident on the first leaf suggesting a direct influence on shoot grow th rather than one mediated through inadequate uptake of water or nutr ients. Infection of the germinating seed by Pythium spp. or the effect s of phytohormones produced by inhibitory bacteria on the roots are po ssible causes of reduced shoot growth of direct-drilled seedlings. The dominant role of biological factors in determining plant response to soil management indicates the need for further studies to identify the organisms responsible, to determine the mechanism by which they influ ence shoot growth, and the effect of management on their populations a nd activity.