Estimating Available Soil Phosphorus Increases after Phosphorus Additions in Mollisols

Accurate critical levels must be accompanied by predictive models on the amount of P required to increase P availability to a target value to obtain reliable P recommendations. We estimated, based on information on soil properties, the increase in soil available P after the addition of a unit of P (b coefficient) in an area of homogeneous although geographically distant soils. All 71 selected soils were noncalcareous, belonging to the Mollisol order and located in the Pampean Region (Argentina). Samples (0-20 cm) were incubated for 45 d after the addition of five doses of P. Soils were characterized for parameters related to soil P availability: pH, particle size distribution, organic C, total P, initial soil available P, and two P retention indices differing in the interaction period between the added P and the soil matrix (1 or 18 h). Obtained b values ranged from 0.27 to 0.74. Soils located at the northern area of the study region had higher b values (mean = 0.58) than southern ones (mean = 0.42). The best multiple regression model for estimating the b coefficient (R2 = 0.70) included initial soil P, a binary variable that accounts for the location of the soil, and the P retention index with the shorter interaction period. The strong effect of the geographic position of the soil on the estimation of the b coefficient led us to hypothesize about a possible role of the mineralogic characteristics of the parent material on P dynamics. Our results indicate that single independent variable models may not be enough to predict the b coefficient in homogeneous soils.