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 between 
uniform 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 uniform 
N, uniform N and P, and variable rate N and P. The three treatments were assigned in six blocks within three 14–ha grain 
sorghum fields (two blocks in each field) in a randomized complete block design. Thirty–six soil samples were taken in a 
staggered systematic grid from each field, and levels of soil nutrients were determined. Application rate maps for the variable 
rate N and P treatment were generated based on a fixed yield goal and site–specific soil N and P levels across the experimental 
plots, while application rates for the uniform N and P treatment were calculated from the same yield goal and average soil 
N and P levels for all three fields. Yield monitor data indicated that the variable rate treatment resulted in significantly higher 
yields 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 rate 
treatments. A simple economic analysis showed that the variable rate treatment had positive relative economic returns over 
the 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 even 
negative. These results showed that variable rate fertilization can increase yield, reduce yield variability, and improve 
economic returns. More experiments are needed to evaluate the long–term agronomic, economic, and environmental viability 
of variable rate technology in the Rio Grande Valley of south Texas