NITROGEN UPTAKE CAPACITIES OF MAIZE AND SORGHUM CROPS IN DIFFERENT NITROGEN AND WATER-SUPPLY CONDITIONS

The dynamics of dry-matter accumulation and nitrogen uptake of sorghum and maize crops were compared over two successive years under dry and irrigated conditions. In non-limiting situations for growth, with irr igation and high nitrogen fertilization, maize growth was higher than that of sorghum because the leaf area index of maize developed earlier , leading to a larger quantity of intercepted radiation. The efficienc y of transforming intercepted energy into aerial biomass varied little between the two species. In situations where nitrogen was limiting, t he capacity of sorghum crops to take up nitrogen from the soil was alw ays higher than that of maize. This feature was not linked to the grea ter planting density of the sorghum crop (50 cm between the rows) comp ared with the maize crop (75 cm), but seemed to be a characteristic of the species. A careful study of the evolution of the nitrogen nutriti on level of the different crops throughout their growth showed that so rghum was capable of taking up much larger quantities of nitrogen from the soil than maize before its nitrogen nutrition became limiting. Dr ought reduced nitrogen availability in the soil for both crops. Maize was more sensitive to this restriction in nitrogen nutrition, and in a ddition to the direct effect of a water deficit on dry-matter accumula tion in maize there was also this indirect effect of a nitrogen-nutrit ion deficit. All the results obtained in this series of experiments sh owed that the potential production of sorghum was lower than that of m aize, but that its adaptation to limiting growth conditions, drought a nd limited nitrogen input should enable this species to grow in extens ive cropping conditions. Moreover, the higher N uptake/water consumpti on of sorghum compared to maize indicates that the risk of nitrate lea ching in the winter following a sorghum crop should be lower than afte r maize.