Tillage Effects on Soil Properties and Spatial Variability in Two Mississippi Delta Watersheds

This study evaluated changes in soil properties several years after implementation of conservation measures. Two approximately 50-ha fields within two Mississippi Delta oxbow lake watersheds (Deep Hollow and Beasley) were laid out in 60-m grids. Soil from a tilled cotton field in Deep Hollow watershed was sampled at each node in 1996 and again in 2000 after 4 years of reduced tillage cotton (Gossypium hirsutum) and winter wheat (Triticum aestivum L.) cover crops; soil also was collected from conventional tillage cotton in Beasley Lake watershed in 1996 and again in 2006 from the same grid nodes after 4 years of reduced tillage cotton and 5 years of reduced tillage soybeans. Organic matter levels in the soil surface were higher in both watersheds after conservation tillage was implemented, likely caused by increased plant residue accumulation and limited soil mixing. Higher soil P levels in both watersheds under conservation management were attributed to less distribution in soil because of reduced tillage, Lower NO3-N in Deep Hollow in 2000 suggested N immobilization in the soil surface. Nitrogen was not analyzed for Beasley soil samples. Soil pH values were also higher in the later samplings for both watersheds, but because lime was applied in the interim, it is difficult to ascribe an effect from tillage. Potassium, calcium, and magnesium were higher in 2000 after the 4-year reduced tillage practice in the Deep Hollow watershed. However, potassium, calcium, and magnesium were lower in 2006 than in 1996 at the Beasley Lake watershed because no fertilizer was applied to the field since 2001. Regardless of tillage system, relatively high P levels in soils from both watersheds are indicative of high native soil P levels in the Mississippi Delta soils. Similar to other studies, water-soluble P was positively correlated with Mehlich III P. Spatial relationships of P examined using kriging showed that P data were spatially dependent, and their spatial dependence was impacted by tillage practices.