Nitrogen fertilizer and wheat in a semi-arid environment. 3. Soil and cultural factors affecting response

The response of Gabo wheat to applied nitrogen at 52 sites in the wheat growing areas of South Australia during 1956-61 was examined in relation to soil and cultural factors, as separate groups and together with climatic factors, by a stepwise multiple regression analysis using a computer. The 10 dependent variables were the linear and quadratic coefficients obtained by fitting orthogonal polynomials to response curves of various parameters (grain yield, grain + straw yield, harvest index, grain nitrogen percentage, and grain nitrogen yield) to applied nitrogen at each of 52 sites. The independent variables were 14 soil properties, such as total nitrogen content and initial nitrate status, and 6 cultural characteristics, including date of sowing and period of cultivation. In addition, 23 climatic variables were also included in analysis considering all independent variables. Of the soil variables the most potent was initial nitrate content of the 0-6 inch horizon. Date of sowing was the most potent cultural variable. The proportion of variance explained in the final analysis by the variables examined was greatest for grain nitrogen yield (73.0 per cent) and grain + straw yield (72.1 per cent). The value for grain yield was 48.9 per cent. Differences between nitrogen and phosphorus response in a semi-arid environment and the theoretical and practical implications of these differences are discussed in relation to the predictive value of soil analyses.