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Abstract Results from a long term study of wheat experiments on Rothamsted farm were examined by multiple regression analysis to estimate the size of the contributions made by four weather factors to the year‐to‐year variance of grain protein. Whether considered singly or in either of two chosen models, air temperature during the early part of the grain‐filling period had a positive effect, incident radiation during the early or middle part a smaller negative effect. Both models accounted for 25% of the total variance or 60% of the year‐to‐year variance. After fitting either model winter leaching and maximum potential soil‐moisture deficit were not significant. Closely matching results were obtained by including 15 nearby farms. Some soil factors were investigated in a similar manner. A soil N index accounted for 30% of the total variance, with grain N% of 1.95 corresponding to Index 0 and 2.23 to the average of 1 and 2. In models combining the best weather and best soil variables (N Index and wetness class), the variance accounted for was 46%, with all effects significant. There were significant interactions of radiation, winter leaching and soil wetness class with the response to fertiliser nitrogen. As the yield data are less well based than those for grain N% their relationships were examined only in parallel to the main study.
Benzian et al. (Thu,) studied this question.