Assessment of Soil-Based Tests for Site-Specific Nitrogen Fertilizer Management of Rice Grown on Clay Soil.

A routine soil test method capable of accurately and reliably quantifying nitrogen (N) mineralized during the growing season would be of benefit to agricultural row crop production. While the use of site-specific N management (i.e. N-ST*R) has shown promise for rice grown on silt loam soils throughout Arkansas, a similar test specific for rice grown on clay soils has yet to be identified. Therefore, the research objectives for this project were: (1) evaluate the ability of Illinois Soil Nitrogen Test (ISNT) or Nitrogen Soil Test for Rice (N-ST*R) to quantify alkaline hydrolyzable-N (AH-N) available for rice uptake; (2) correlate the quantity of AH-N, as determined using ISNT or N-ST*R, to TN uptake, grain yield and percent relative grain yield for plots receiving no added fertilizer N (0 kg N ha^-1); and (3) to develop a calibration curve capable of predicting the N fertilizer rate needed to maximize yield for rice grown on clay soil in Arkansas. Nitrogen rate trials were conducted at eleven site-years on research station and producer fields using N fertilizer rates ranging from 0 to 236 kg N ha^-1. At each site the soil was sampled to a 60 cm depth in 15 cm increments. Soil sample analysis has shown that AH-N ranges from 38 to 156 mg N kg^-1 as measured by ISNT and from 52 to 175 mg N kg^-1 as measured by N-ST*R across sample depths. Correlation results have shown that the coefficients of determination range from 0.47 to 0.77 using ISNT and from 0.60 to 0.70 using N-ST*R. Preliminary calibrations have shown that coefficients of determination reach a maximum (R² = 0.84) using the 0-30cm depth for ISNT and reach a maximum (R² = 0.70) using the 0-45cm depth for N-ST*R.