Developing Management Zones for Deep Profile Nitrogen Sampling.

Elevated levels of soil nitrate-nitrogen have been documented in Texas soils, which can have significant economic and environmental consequences. Crops can recover residual soil nitrogen from profile depths up to 1.2 meters. However, significant spatial variability of soil profile nitrate-nitrogen may exist within fields creating challenges for sampling and 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 (0 to 225 kg N/ha) were imposed in replicates within each management zone for corn production during 2014. 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. During 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 (0.60 m deep) 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 40-60 kg/ha. Study results reveal the potential for spatial management of profile nitrogen levels using EC-based zones.