Evaluation of Variable Rate Technology as a Management Tool for Nitrate Fertilization of Wheat in the Argentineans Humid Pampas.

The introduction of variable rate application technology and grain yield monitors has greatly increased farmer interest in varying input rates to match crop productivity. The use of yield monitors allows producers the opportunity to evaluate the effects of varying input rates on grain yield and ultimately profit.  Specific recommendations for variable rate nitrogen (VRN) fertilization in wheat are required to realize the potential environmental and economic benefits of this technology. A one-year study was conducted in a farm fields (45 ha.) that were subdivided into 42 blocks of 150 x 100 m. Each block was divided into three plots and three N fertilizer rates were randomized within these blocks (0, 50, and 100 kg N ha^-1). Nitrogen was measured at each plot at 60 cm depth. The depth to a petro-calcic horizon (DPH) was measured at a grid of 30 m.  Apparent soil electrical conductivity at 30 cm depth (ECa) observations were collected in the winter of 2008 using a Veris® 3100 in rows of 20 m. The field was planted with wheat in July 2008 and harvest in December 2008. Yield monitor data for each field were accessed from the farmer who has collaborated with INTA in this research from 2003 to 2008. Data were interpolated using kriging via the Surfer software.  These interpolated files, where each observation represented a 100 m² area were sorted by ranks of the average normalized yield from the period 2002/08 and the mean value calculated.  Those average values were analyzed by correlation studies. Normalized yield means were negatively correlated with ECa (R² = 0.867) and positively correlated with the depth DPH (R² = 0.866). The values of DPH were grouped into three depth ranges (60 to 100, 100 to 120, and 120 to 140 cm).  An analysis of variance was performed to check the response to N application in each of these ranges. In the range of DPH from 60 to 100 cm were significant differences among the three N rates, instead in the range of DPH from 100 to 120 and 120 to 140cm was only found significant differences between doses and the check.