Nitrogen Soil Testing for Rice Grown On Clayey Soils: Development of N-STAR Correlation and Fertilizer Calibration Curves.

The efficient management of nitrogen (N) fertilizer sources continues to be an elusive goal sought after by researchers in an attempt to develop sustainable recommendations for agricultural production. Two chemical soil test methods that quantify plant available-N utilize diffusion (DIF) or direct steam distillation (DSD) of alkaline hydrolyzable-N (AH-N). The DIF and DSD soil test methods have been used to distinguish between fields of high and low native N fertility and DSD has recently been developed for use as a site-specific N soil test for rice grown on silt loam soils in Arkansas. The objective of this study was to evaluate DIF and DSD as potential N soil test methods for rice grown on clayey soils.

Grain yield and growth of rice (Oryza sativa) in response to incremental N fertilization was evaluated using 11 site-years of small-plot field trials on clayey soils in Arkansas. The strength of the grain yield correlation curves for AH-N determined by DIF (R² = 0.77) and DSD (R² = 0.79) indicated that the greatest predictive ability occurred when clayey soils were sampled to a 30 cm depth. The results of small-plot field trials in Arkansas stimulated interest in evaluating 6 site-years of field trials in Mississippi to assess the 30 cm sampling depth protocol established in Arkansas. The N fertilizer calibration curve developed for Arkansas and Mississippi clayey soils for DIF (R² = 0.87; p < 0.0001) and DSD (R² = 0.88; p = 0.0001) indicated that AH-N concentration at the 0 to 30 cm depth can adequately determine the N rate needed to achieve 95% relative grain yield.