Development of a Soil-Based Nitrogen Test for Clay Soils.

Successful correlation and calibration of soil hydrolyzable-N to rice grain yield and fertilizer N to achieve 95% relative grain yield, respectively, on silt loam soils has led to research on clay soils.  Fertilizer N rate studies have been conducted on several clay soils at experiment stations and producer fields across Arkansas. Grain yield was used for correlation and calibration of hydrolyzable N as determined by the Illinois Soil Nitrogen Test (ISNT) and Direct Steam Distillation (DSD). Percent relative grain yield and N fertilizer rate to achieve 95% relative grain yield was regressed against the mean ISNT and DSD hydrolyzable-N values for the 0 kg N/ha rate plots at each location. Currently, 14 site-years have been used to develop soil-based N tests for rice with significant relationships between the two soil tests and percent relative grain yield and N rate to achieve 95% relative grain yield.  Results show a good correlation between percent relative grain yield and ISNT and DSD at the 0-30cm depth.  The coefficients of determination increased for percent relative grain yield and N rate to give 95% relative grain yield as depth increased until 30 cm, but then dropped significantly at the 0-45 and 0-60 cm depths.  Soils with a wider range of native N than what the current 14 site-years provide will be needed to develop a robust calibration curve.