Phosphorus-based variable rate manure application in wheat and barley

Manure is used as a source of nutrients and organic matter for agricultural lands, but its unbalanced use has unfortunately made nitrogen (N) and phosphorous (P) leakage to the environment quite common in Western Europe. One way to optimize the use of manure is variable rate (VR) application, wherein within field spatial variability of soil nutrient provision is accounted for. This work evaluated the economic and environmental benefits of a P-based VR manure application and compared it to an N-based uniform rate (UR) application. An on-line visible and near-infrared spectroscopy sensor was used to collect on-line spectral data from soils in two fields of 7.9 and 5.1 ha with barley and wheat, respectively, in Belgium. A calibration model to predict extractable P (P) in soil was developed by partial least squares regression. Based on the locally predicted P level, the P-fertilizer application rate was calculated. According to P2O5 content in manure, manure application rates were set based on the level of soil P measured with the on-line soil sensor. Based on application maps, a strip experiment was conducted to compare the efficiency of VR and UR treatments. The results showed that P-based VR manure application resulted in almost the same yield as the UR treatment; however, it saved the environment by reducing N by 44 kg/ha and 29 kg/ha, and P2O5 by 18 kg/ha and 12 kg/ha, when applied in the two fields. Since farmers received compensation from the manure providers for allowing the manure to be applied in their fields, the P-based VR manure application using less manure has resulted in a smaller gross margin than the N-based UR method, by which more manure was applied. However, if farmers had to pay for this application (as is the case in some European countries), a P-based VR manure application would result in positive gross margins. It is encouraged to adopt VR manure applications in arable crop production, as both economic and environmental benefits are feasible.