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The immediate profitability and long-term sustainability of domestic crop production have been threatened by increasing N fertilizer costs. Currently, there is no soil test which can accurately predict the N fertilizer needs of direct-seeded, delayed-flood rice (Oryza sativa L.) produced on silt loam soils in Arkansas. Fertilizer N recommendations could be improved substantially with a calibrated soil N test, while also lowering potential environmental impacts. Twenty-five N response trials were conducted between 2004 and 2008 to correlate alkaline-hydrolyzable N (AH-N), as quantified by the Illinois Soil N Test (ISNT) and direct steam distillation (DSD), with rice response parameters such as total N (TN) uptake, check plot grain yield, and percentage of relative grain yield (RGY) and calibrate AH-N to predict the fertilizer N rate required to achieve 95% RGY. Relationships with the selected parameters were evaluated for both methods over a series of soil depth increments that included 0 to 15, 15 to 30, 30 to 45, 45 to 60, 0 to 30, 0 to 45, and 0 to 60 cm using linear regression models. Alkaline hydrolyzable-N was significantly and positively correlated with all rice response parameters except check plot grain yield and percentage of RGY using AH-N at the 45- to 60-cm depth. Coefficients of determination were greatest for percentage of RGY at the 0- to 30- and 0- to 45-cm depth for the ISNT (r2 = 0.57) and DSD (r2 = 0.73), respectively. Calibration of the fertilizer N rate to achieve 95% RGY resulted in similar trends as the correlation of rice response parameters, but with higher r2 values. Alkaline hydrolyzable-N explained 68 and 89% of the RGY variability in calibration for the ISNT using the 0- to 30-cm depth and the DSD using the 0- to 45-cm depth, respectively. These successful calibrations can be attributed to the N dynamics that exist in direct-seeded, delayed-flood rice production systems and identification of the proper sampling depth.
Roberts et al. (Sun,) studied this question.