Nitrogen needs of wheat. 1. Grain yield in relation to soil nitrogen and other factors
A four year study of the relations between wheat yields and various edaphic, cultural and climatic factors in the wheat belt of southern New South Wales indicated that seasonal rainfall was the major source of yield variation. A regression equation involving pre-sowing (January to April) and growing season (May to September) rain, soil nitrate nitrogen concentration (0-30 cm), average annual rainfall, sowing time and weed density proved to be the most useful function for predicting yield (R2 = 0.61 3). Equations relating expected grain yields to soil nitrate nitrogen concentrations were derived for a combination of average annual rainfall and pre-sowing and growing season rainfall conditions. These show that, under optimum rainfall conditions, soil nitrate levels would restrict yields on most farms, wherever nitrate concentrations in the surface 30 cm fell below 20 p.p.m.