Soil Depth Coupled with Soil Nitrogen and Carbon can Improve Fertilization of Rice in Arkansas

Abstract

Success of alkaline hydrolyzable nitrogen (AHN) to predict N fertilizer needs when soils were sampled over the effective rooting depth of the crop rekindled an interest in more traditional soil procedures including soil total nitrogen (STN) and total carbon (STC). The benefits of a N soil test that can accurately predict N fertilizer needs are not solely about optimizing economic or agronomic returns, but include making environmentally sound N fertilization decisions. This paper presents the results of a study assessing the ability of STN and STC to predict the N fertilizer needs of direct‐seeded, delayed‐flood rice (Oryza sativa L.) produced on silt loam soils in Arkansas. Twenty‐five N response trials were conducted from 2004 to 2008 to correlate STN and STC with rice response parameters such as total nitrogen uptake (TNU), check plot grain yield, and percent relative grain yield (RGY) and calibrate STN and STC 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 using linear regression models. Soil TN was significantly and positively correlated with all rice response parameters except check plot grain yield at the 15‐ to 30‐ and 45‐ to 60‐cm depths. Coefficients of determination were greatest for percent RGY at the 0‐ to 45‐ cm depth for STN (r2 = 0.62) and STC (r2 = 0.33). Calibration of the fertilizer N rate to achieve 95% RGY mimicked correlation of rice response parameters with soil depth, but with higher r2 values. The highest coefficients of determination were observed in the 0‐ to 45‐cm depth where STN explained 74% and STC explained 54% of the variability in the fertilizer N rate required to achieve 95% RGY. These relationships indicate the importance of proper sampling depth for successful correlation and calibration of soil analytical methods.