Spatial variability of soil test phosphorus in manure-amended soils on three dairy farms in North Central Texas

Dairies in Erath County, Texas have been subject to strict monitoring for phosphorus (P) management due to contamination of the North Bosque River by a number of agricultural and nonagricultural sources. Soil conditions and management practices vary greatly among dairies, which make a difference in soil test P (STP) levels and runoff potential, and thus spatial variability of STP was studied. Surface soil samples were collected from three dairies and were measured for STP, water soluble P (WSP), CaCO₃ equivalent, organic carbon, clay content, and pH. Mehlich 3-ICP STP values ranged from 19 mg kg ^-1 (ppm) to 1,814 mg kg^-1, and WSP ranged from 2 mg kg^-1 to 317 mg kg^-1. Phosphorus levels varied with soil types and management practices. Geo-statistical analysis showed greater spatial variability of STP and WSP compared to other soil properties like CaCO ₃, clay, organic carbon, and pH.The STP and WSP extraction behaved differently on the soil samples from the three dairies studied so that areas with the highest STP levels did not show the highest WSP and vice versa. Based on this study's results, it is recommended that Phosphorus Index calculations, regulatory monitoring, production, or conservation practices be based on WSP as Mehlich 3 extractant (a strong acid) extracts phosphorus that is not plant available or readily released to leaching or runoff.