Increasing cereal yields with higher ammonium/nitrate ratios: Review of potentials and limitations

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Abstract
Cereal yield increases have been achieved in recent solution cultured studies by supplementing nitrate with limited levels of ammonium where secondary effects on factors such as pH and micronutrient availabiltiy in the solution were controlled. In these studies, ammonium rates were controlled within relatively narrow limits of growth stimulation and excess ammonium. In contrast, NFDC greenhouse studies with spring wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] indicate that a wider range of supplemental ammonium rates can be applied with a basal nitrate rate in soil without causing yield reductions, probably because ammonium movement to roots is slower in soils than in solution culture. Also, ammonium/nitrate ratios required to maximize wheat and grain sorghum yields in the greenhouse were higher than ratios typically observed in field soils during periods of primary N uptake, suggesting a potential for yield increases in the field. Limitations for achieving potential cereal yield increases with higher ammonium/nitrate ratios are (i) inadequate characterization of optimum ammonium/nitrate ratios in relation to stage of plant growth, (ii) inadequate technology and practices for simultaneously controlling ammonium/nitrate ratios throughout most of the growing season and providing adequate positional availability of ammonium to roots, and (iii) inadequate determination and exploitation of genetic variability regarding yield response to ammonium/nitrate ratios.