AMMONIUM AND NITRATE SOURCE EFFECTS ON FIELD CROPS

Several recent studies indicated that greater biomass accumulation, nitrogen (N) uptake and grain yield by various crops can be obtained with a mixture of fertilizer ammonium (NH4+) and nitrate (NO3-) than with either form alone. However, in most soils, NO3- is the primary form available to crops because applied ammoniacal-N is readily converted to NO3- by microorganisms. Relatively high NH4+/NO3- ratios can be maintained in the field by selection of ammoniacal-N source, nitrification inhibitors and incremental applications of mixtures of ammoniacal-N and a nitrification inhibitor. This paper reviews greenhouse, growth chamber and field results on crop responses to NH4+ and NO3- nutrition.