Spatial Variation of Plant-Available Phosphorus in Pastures with Contrasting Management

Land application of animal manure, at rates based on soil nutrient content or crop requirements, optimizes nutrient recycling and minimizes offsite environmental impacts. The objective of this research was to characterize the spatial variation of plant-available P and other soil properties (C, N, and pH) in two pastures having contrasting grazing and poultry litter management. One site (Cellar Ridge) was a lightly grazed 6-ha tall fescue (Festuca arundinacea Schreb.) pasture with limited poultry litter application and the other (Haxton) was a 9.5-ha tall fescue pasture with annual poultry litter application and intensive rotational grazing for 10 yr. Soil cores (0–0.15 m) were collected on a 30-m grid at both sites and analyzed for plant-available P (Mehlich-3 extract), total C and N (combustion method), and pH (1:1 water/0.01 M CaCl2). Cellar Ridge had significantly less Mehlich-3 extractable P (32 vs. 341 mg kg-1), more acid pH (5.25 vs. 5.73), and significantly greater C (23.3 vs. 16.3 g C kg-1) and N (1.76 vs. 1.54 g N kg-1). Spatial dependence over approximately 1 to 3 lag distances with a consistent orientation (across ridge) was observed for all parameters at Cellar Ridge. No spatial dependence was observed for Mehlich-3 P, C, N, or pH at the Haxton site (all parameters exhibiting nugget effect). Ten years of poultry litter application likely eliminated spatial structure for these properties. Further research is needed to determine whether additional costs associated with grid sampling and variable rate litter application can be justified.