Previous studies indicated that the optimum leaf N concentration for rice grain production was higher in plants grown in complete nutrient solution than in those grown in flooded paddy. Factors of physical habitat were not found to be responsible. Of the various possible nutritional differences, S seemed most likely to be involved.
Sand culture experiments with rice (Oryza sativa L. ev. Colusa) showed that the optimum N concentration in the index leaves (second leaf below the panicle, sampled at flower emergence) was a direct linear function of the S concentration, up to the levels giving maximum yield. At each optimum for N the ratio S/N was close to the previously observed ratio of 1/17 for vegetable proteins, which supports the concept of a natural balance for the two elements. However, a balanced condition occurred at deficiency levels of both elements as well as at the respective critical concentrations of N and S.
It is concluded that deficiency of N or S limits protein production and thus limits the utilization of the other element. Thus, a suitable estimate of critical concentration of either element cannot be made unless both elements are supplied in adequate amounts.
On the basis of the observed range of optimum N concentrations, the known chemistry of S in flooded paddy, and the few data available on S concentrations in paddy-grown rice, it is suggested that the variable optimum concentrations of N previously observed may be due to variable S supplies to the plants.
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