Comparisons of Uniform and Variable Rate Nitrogen and Phosphorus Fertilizer Applications for Grain Sorghum

Variable rate fertilizer application has the potential to improve fertilizer use efficiency, increase economic returns, and reduce environmental impacts. This study was designed to examine differences in yield and economic returns betweenuniform and variable rate fertilizer applications. During the 1997 and 1998 growing seasons, a variable rate applicator, capable of varying two liquid fertilizers simultaneously, was used to evaluate three fertility strategies: conventional uniformN, uniform N and P, and variable rate N and P. The three treatments were assigned in six blocks within three 14ha grainsorghum fields (two blocks in each field) in a randomized complete block design. Thirtysix soil samples were taken in astaggered systematic grid from each field, and levels of soil nutrients were determined. Application rate maps for the variablerate N and P treatment were generated based on a fixed yield goal and sitespecific soil N and P levels across the experimentalplots, while application rates for the uniform N and P treatment were calculated from the same yield goal and average soilN and P levels for all three fields. Yield monitor data indicated that the variable rate treatment resulted in significantly higheryields than the uniform N and P treatment for both years (400 kg/ha higher in 1997 and 338 kg/ha higher in 1998). Moreover, coefficients of variation of yield monitor data for the variable rate treatment were smaller than those for the two uniform ratetreatments. A simple economic analysis showed that the variable rate treatment had positive relative economic returns overthe uniform N and P treatment (27/ha in 1997 and 23/ha in 1998). However, if additional costs for soil sampling, equipment, and data analysis associated with variable rate application were considered, these returns would be much lower or evennegative. These results showed that variable rate fertilization can increase yield, reduce yield variability, and improveeconomic returns. More experiments are needed to evaluate the longterm agronomic, economic, and environmental viabilityof variable rate technology in the Rio Grande Valley of south Texas