QUANTITATIVE ASPECTS OF NITROGEN NUTRITION IN CROPS

The processes affecting the response of crops to nitrogen fertilizers are reviewed, with special emphasis on quantitative relationships that enable the development of deterministic models. Total dry matter production is described in terms of length of the growing period and average growth rate, and the effect of nitrogen status of the vegetation on both is discussed. Attention is paid to nitrogen influence on stomatal response and crop water use. It is shown that the effect of differential nitrogen nutrition can in some cases be described in terms of light interception, but not always. It is concluded that the present insight in the relevant processes is insufficient to use deterministic models directly for application in fertilizer practices at the farm level, but that further development of such models is an important aid in structuring thinking about the system.