Spatial variability of soil properties and weed populations in the Mississippi Delta

Simulation models and precision agriculture practices may require more deta il and certainty about soil spatial variability than provided by soil surve ys. This study described soil and weed spatial variability in 50-ha subarea s of two sites included in the Mississippi Delta Management Systems Evaluat ion Areas project, Objectives were (i) to describe the spatial variability of soil properties and (ii) to determine relationships between spatially va riable weed populations and soil properties, Surface soil samples were coll ected at nodes of 60-m square grids prior to planting cotton(Gossypium hirs utum L,) in 1996, Field-moist soil was analyzed for microbial activity. Air -dried soil was used to determine soil organic C, pH, and texture. Fluometu ron and either clomazone, metolachlor, or noflurazon were banded over the c rop row at planting, Weed counts were taken 6 wk after herbicide applicatio n. The spatial variability of soil properties and weed populations was desc ribed using geostatistics. Soil microbiological activity exhibited limited spatial dependence, but pH, organic C, and texture semi-variograms were wel l-described with spherical models. Although short-range (< 60 m) variabilit y was often high, the range of spatial dependence typically exceeded 120 m, Total weeds were spatially dependent both years; however, weeds susceptibl e to control by herbicide were not. Weed densities were significantly great er (P < 0.05) in areas that had higher organic C and finer texture. Areas o f low organic C and coarse soil often had no weeds. Thus, more uniform weed control might be achieved by varying preemergence herbicide application ra te. Acceptable weed control might be achieved with lower herbicide applicat ion rates in certain areas.