Using Potentially Mineralizable Nitrogen for N Fertilization Planning in Wheat.

Soil testing is commonly used in Uruguay for assessing soil fertility and managing fertilizer N in wheat. Previous research developed critical values for the soil nitrate test at planting and tillering stages. For example, 22 ppm N-NO₃ at planting indicates that N supply from the soil likely will be adequate to achieve high yields. However, the soil nitrate test is frequently subject to considerable within-season variations. Therefore, estimating required nitrogen rates based upon this test has not proved to be acceptably accurate. In addition, yield averages in commercial wheat crops increased substantially in the last 10 years due to improved production technology, while the variability of cropping history of Uruguay´s wheat fields increased as well. The objective was to develop a method to (a) account for effects due to previous history and management, and (b) quantitatively estimate total N supply from the soil. Total N uptake of wheat was evaluated in unfertilized plots of 29 experiments between 2001 and 2007. Potentially Mineralizable Nitrogen (PMN) was tested at planting, and soil nitrates at planting and at the two-tiller stage. We hypothesized that total N uptake by wheat can be estimated more accurately with the PMN test than with the soil nitrate test. As a result, we adjusted and validated a model for predicting total uptake of soil N based on the PMN test. Based on these findings, we propose using the PMN test and expected yield as input data, for quantitatively estimating the optimal fertilizer N rate at the beginning of the growing season.