The significance of crop canopies for N nutrition

Abstract

A rationale is described in which the diminishing response in crop yield to increasing amounts of applied nitrogen (N) is explained by taking (i) growth during the yield forming period to be directly proportional to radiation intercepted, (ii) radiation intercepted to be related to canopy size through Beer's Law, (iii) canopy size to be directly proportional to N uptake through a constant 'canopy N requirement', and (iv) N uptake to be directly proportional to fertilizer N applied. This analysis highlights the importance of canopy size, both as an index of crop N status and of crop productivity and, knowing the value of fertilizer N and crop produce, it allows crops' N requirements to be re-expressed as optimum canopy sizes. Taking observed or published coefficients for each of its four component relationships, the rationale predicts optimum canopy sizes (as Green Area Index (GAI)) for conditions in the UK to be 5.2 for wheat. Thus it is predicted that the profitability of wheat might be improved by producing and maintaining canopies of about 5 during the yield forming period, instead of the larger canopies that are often grown. One example experiment showed how this strategy might be applied with advantage. A series of experiments, from which the example was taken, showed variation in the relative performance of the two approaches but, generally, 'canopy management' was economically better than 'normal' N management. Given that canopies can be easily assessed, it is concluded that their size is a useful diagnostic of crop N requirement