Field Validation of N-ST*R: A Soil-Based Nitrogen Test for Silt Loam Soils.

Researchers at the University of Arkansas have successfully correlated and calibrated a soil-based N test for rice (N-ST*R) using 27 site-years of data collected from N rate trials on experiment stations and producer fields. A series of small-plot to field-scale experiments are being conducted to validate the N-ST*R calibration.  Field validation studies included five N rate treatments and four replications. Nitrogen rate treatments included a check (0 kg N ha^-1), standard recommendation for silt loam soils in Arkansas (168 kg N ha^-1) and prescription N rates for each field based on the N-ST*R value and the three calibration curves used to achieve 90, 95 and 100% relative grain yield. Using N-ST*R for the 95% relative grain yield goal resulted in N rate recommendations ranging from 22 to 235 kg N ha^-1. Yields were compared for the 90, 95 and 100% relative grain yield treatments to the standard recommendation  to evaluate the ability of N-ST*R to predict site-specific N rates that would maximize yield. Maximal yields were obtained using the 95 and 100% relative grain yield curves. In many cases the yield obtained using the N rate from the 90% relative grain yield calibration curve was not significantly different from the maximal yield for a given location. Success of the N-ST*R program will lead to further validation studies and implementation of strip verification trials in producer fields to gain more data on the ability of N-ST*R to prescribe site-specific N rates and provide a field-scale demonstration of this exciting new management tool.