Temporally and Spatially Dependent Nitrogen Management for Diverse Environments

Nitrogen (N) fertilizer is the most expensive input for cereal production worldwide. What is probably most important about nitrogen requirements in cereal crop production is that the demand changes drastically from field to field and from one year to the next. Of all the information that should be communicated to farmers in any locale is that this temporal and spatial dependency influences optimum nitrogen fertilizer rates. Long-term winter wheat (T. aestivum L.) research in the southern Great Plains of the US has shown that the average fertilizer-N application rates would have been correct in the ensuing year only 20% of the time; yet, using the same rate from one year to the next is common practice for wheat, maize (Zea mays L.), and rice (Oryza sativa L.) farmers worldwide. In addition, extensive research has shown that indigenous soil N across the landscape can vary several-fold, resulting in very different N recommendations depending on the location within the field. In this chapter, we describe two alternatives which improve upon current methods for determining fertilizer N rates and have the flexibility to be used in environments ranging from 260-ha fields in the High Plains of the US to 1-ha fields in Sub-Saharan Africa (SSA). A direct measure of N requirement can be made with the ramp calibration strip (RCS), which is superimposed on farmer practices at or near planting. A range of N rates (zero to more than sufficient) is applied mechanically or by hand. The RCS is urgently needed in wheat production because N response varies considerably SUMMARY