THE EFFECTS OF NITRATE - AMMONIUM RATIOS AND DICYANDIAMIDE ON THE NITROGEN RESPONSE OF ZEA-MAYS L IN A HIGH RAINFALL AREA ON AN ACID SOIL

Hydroponic studies under controlled environmental conditions indicated that maize plants respond better to combinations of nitrate and ammonium nutrition than to either form supplied separately but that this response depended upon the total N concentration. An attempt was made to maintain different nitrate: ammonium ratios and concentrations in the soil by the addition of a nitrification inhibitor. Five nitrate: ammonium ratios at three N application rates were tested with and without dicyandiamide (H2NC(NH)NHCN) on a low-pH, sandy soil for two years. Treatments were applied to field-grown maize in two applications, one at planting and the other at 21 to 30 days after planting. Under favourable climatic conditions for crop growth the optimum nitrate: ammonium ratio for grain yield was between 3:1 and 1:1 over all N rates. Under unfavourable climatic conditions, ratios of 3:1 and 1:1 showed in contrast to all other ratios no grain yield depressions at high N rates. Dicyandiamide did not interact with N rates or ratios, but did increase grain yield over all N treatments under favourable conditions. N ratio interactions with N rates and dicyandiamide were also shown for N concentrations of the leaves at anthesis, for the grain at harvest and for mineral N in different soil layers at anthesis. These results imply that nitrate: ammonium ratios between 3:1 and 1:1 should be recommended at the optimum N rate on a low-pH sandy soil in a high rainfall area for maize production.