A data base was constructed of the % N and plant d. wts (W) in t ha" 1 of C3 and C4 crops that had been grown with sufficient nitrogen to permit maximum growth rate. The % N of all crops declined sharply with increase in Wbut this decline differed between C3 and C4 crops. When Wwas greater than I t ha" 1 , 86% of the variance in In % N was removed by the model % N = aW' b with b =-05 for all crops, and a = 5-7 % for C3 crops and 4-1 % for C4 crops. The same model gave a good description of data on C3 and C4 crops entirely independent of that used for developing the model. According to this relationship the fractional decline in % N with increase in plant mass was the same for both types of crops, but C4 crops contained about 72% of the nitrogen in C3 crops at equivalent d. wts. As approx. 32% more dry matter was produced per unit of intercepted radiation for C4 and C3 crops, the N uptake (or weight of plant protein produced) per unit of intercepted radiation was approximately the same for both types of crops. A small improvement in the degree of fit to % N = aW~" was obtained by allowing both a and b to vary with the crop. Values of b obtained in this way for tall fescue, lucerne and winter wheat, but not for potato and sorghum, were consistent with Hardwick's 'skin core' hypothesis (Annals of Botany, 1989,60,439-46). The entire data set was, however, consistent with Caloin and Yu's model (Annals of Botany, 1984, 54, 69-76) in which there is a conceptual N pool for photosynthesis and another N pool for the other processes.

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Apparent recoveries of fertilizer N by crops were studied as their treatment in simulation models is a serious problem. Multi-level N fertilizer experiments in which fertilizer was broadcast and incorporated in soil immediately before drilling were carried out on a range of vegetable crops on adjacent sites of the same field. At final harvest, apparent recovery always declined approximately linearly with the increase in fertilizer N even when less was applied than was needed for maximum uptake; this contrasts with the well-known constancy of apparent recovery of Gramineae (winter wheat and grass) over widely different rates. At the seedling stage (of vegetables), when N-uptakes were very small, N-uptake and invariably dry weight, unexpectedly increased with the increase in fertilizer N until very high levels were reached. A single simulation model, with species differences accounted for by variations in the value of only one coefficient, defined, at least qualitatively, all the various phenomena. It is argued that substantial quantities of nitrate may always remain in soil at harvest of some vegetable crops if a sufficient amount of N-fertilizer is applied by the conventional broadcast method to obtain high yields.