Using Soil EC to Identify Soil Sampling Zones for Nitrogen Management

Elevated levels of soil nitrate-nitrogen have been documented in Texas and across the Cotton Belt, which can have significant economic and environmental consequences. Cotton and rotational grain crops can recover residual soil nitrogen from profile depths up to 4 ft. However, significant spatial variability of soil profile nitrate-nitrogen may exist within fields creating challenges for sampling and accurate crediting for residual nitrogen.  A multi-year study was initiated in 2012 to evaluate management zones developed using spatial measurement of soil electrical conductivity (EC) for deep profile nitrogen sampling.  Five fertilizer nitrogen rates were imposed in replicates within each management zone for corn production during 2014 and cotton in 2015. A strong quadratic relationship (r² = 0.62) was observed between soil EC and soil clay content, creating management zones with contrasting soil textures. Soil texture is likely to influence spatial distribution of residual nitrogen. In 2013, total profile soil nitrate nitrogen content did not vary across zones. Yet, distribution of nitrate nitrogen within the soil profile did differ (p < 0.05) by zone. During 2014, profile nitrogen content (2 ft depth) was 1.5-fold greater in zones with greater clay content compared to the zone with lowest clay content. Management zones with greater clay content and residual nitrogen reduced optimum fertilizer rates for corn production 36-54 lb/a. Study results reveal the potential for spatial management of profile nitrogen levels using EC-based zones.  The results for the 2015 cotton crop will be reported during the presentation.