State‐Space Description of Field Heterogeneity

Field heterogeneity in soil texture and site elevation (SE) may affect crop water and N use. A 2‐yr (1998–1999) study was conducted on the Texas High Plains to determine the interdependence between management (irrigation and N fertilization), soil heterogeneity (texture, soil water, and NO 3 ‐N) and topography, and their impact on cotton ( Gossypium hirsutum L.) lint yield and N uptake. Treatments were irrigation at 50 and 75% estimated cotton evapotranspiration (ET) and N rates at 0, 90, and 135 kg ha −1 Soil water content (SWC), lint yield, N uptake, and N fertilizer recovery, measured as a function of management and space, were higher on low positions. Mixed model analysis showed that irrigation was significant on SWC, lint yield, and N uptake ( P < 0.05) each year. The N treatment had no effect on lint yield or N uptake in 1998 because of high soil residual NO 3 ‐N, and the model residual was significant for all measured variables ( P < 0.0001). Sand, SWC, lint yield, and N uptake were negatively correlated with SE ( r ≥ −0.64). In 1998, lint yield, SWC, clay, sand, and SE were cross correlated within 60 to 80 m. Multivariate state‐space analysis showed that lint yield at position i was weighted on lint yield, SWC, clay, sand, and SE at previous position i − 1. It is concluded that 75% ET and N rate at 90 kg ha −1 would be the basis to consider variable water and N rates related to field conditions, and the state‐space model quantified spatial interdependence between irrigation, fertilization, and field heterogeneity.