Measuring and predicting the phosphorus sorption capacity of manure-amended soils

An accurate measurement of the phosphorus (P) sorption capacity of soils is essential to understanding and managing P losses from manure-amended soils. Total P sorption capacity (PSC) was measured in Wellsboro (coarse-loamy, mixed, active, mesic Typic Fragiudept) and Oquaga (loamy-skeletal, mixed, superactive, mesic Typic Dystrudept) channery silt loam soils with varying histories of dairy (Bos taurus) and poultry (Gallus domesticus) manure addition. Batch isotherm analyses were conducted, from which Langmuir sorption maxima were estimated using two methods. Sorption maxima were used to determine the total PSC. Soils ranged in total P from 1080 to 2580 μg g^-1 for dairy fields and 1654 to 11684 μg g^-1 for poultry fields. The total PSC was poorly related to the total P levels in unamended soils (r² = 0.42) but increased nonlinearly with total P in poultry and dairy soils (r² = 0.85). Total PSC was well correlated to a single-point P sorption index in all soils (r² = 0.74). The most accurate predictions of PSC were obtained using a model based on molar amounts of Mehlich-3 Fe, Al, and Ca (r² = 0.83). When this model was used to calculate the degree of P saturation (DPS), the comparison of DPS with 0.01 M CaCl₂ P showed that at DPS greater than 30%, 0.01 M CaCl₂ P increased rapidly. We conclude that Mehlich-3 Fe, Al, and Ca data can be used to predict PSC and DPS across soils with widely varying manure management histories.