The purpose of this research was to relate runoff dissolved (DP) and total P (TP) losses to site-specific characteristics from CAFOs and compare them to their corresponding risk assessment using the Texas Phosphorus Index (PI). Initial studies showed that soil test P (STP) methods used in Texas were highly reproducible regardless of manure source or application rate. However, NH4OAc-EDTA extraction efficiency was increased as soil conditions became less acidic resulting in STP three times greater than those of Mehlich III for the same soil. Surface application of dairy manure to high pH soils were positively correlated to STP at depths to 15 cm. First order linear relationships between STP values and DP concentrations in runoff were statistically significant for extraction methods and sampling depths but were different for soils under neutral to calcareous conditions. Attempts to reproduce this on fields that received periodic applications of manure or effluent failed, although there was a highly significant relationship between STP and runoff DP for different soils under acidic conditions ( pH<6.5). Analyses of NH4OAc-EDTA extractable soil elements showed Mg was significantly correlated to DP concentration across various management and soils. Use of the Texas PI reflected vegetation type closely; grass-covered sites averaged the lowest risk rating, while tilled sites had the highest. However, overall relationship was poor when RUSLE 2 erosion rates were used due to experimental design limitations. Using actual rates for plots and inclusion of extractable Mg improved correlations of PI rating to DP and TP losses, with r2 ranging from 0.60 to 0.87.
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