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.
Decreasing P buffer capacity of the soil surface in no‐tilled soils has been attributed to enrichment of both P and OM (OM). As soil surface enrichment with P and OM usually occurs simultaneously in no‐tilled soils, it is not clearly established whether the increase of soil OM affects the capacity of soils to retain P. A study was conducted to assess whether the variations in soil OM (total and particulate) at soil surface affects P retention capacity. Thirty five soils from the Rolling Pampa (17 tilled and 18 no‐tilled) were selected. All soils had medium to low levels of available P (PBray1 < 20 mg kg−1). Soil samples were taken from the top 5 cm, and several soil characteristics determined: available and total P, P sorption index, OM, particulate OM (POM) ( >53 μm), particle size distribution, and pH. Tilled and no‐tilled soils did not differ in soil texture, pH, total and available P content. On the contrary, no‐tilled soils had more OM (+14%) and POM (+56%) than tilled soils. The capacity of soils to retain added P was not different in both groups of soils. Phosphorus sorption index was not related to soil content of total OM or POM. Variation in P sorption index was only related to soil clay content (r2 = 0.44). This study provides evidence contradicting the long held assumption that the reported decrease in P sorption at the surface of no‐tilled soils was caused in part by OM enrichment.
Core Ideas Predictive models of phosphorus sorption were developed for a group of Mollisols. Alox was the best predictor of short‐term phosphorus sorption indices. Feox was the best predictor for those indices that involves longer interaction periods. Phosphorus needs to maximize yield avoiding environmental risks can be estimated with our data. The role of different soil properties on defining the P sorption capacity was investigated in a group of Mollisols of the Pampean Region (Argentina). Two‐single point and three‐multiple point P sorption indices were evaluated. Both simple and multiple regression models indicated that ammonium oxalate extractable Al (Alox) and ammonium oxalate extractable Fe (Feox) were the main properties defining P sorption. Ammonium oxalate extractable Al prevailed as the best predictor of short‐term P sorption indices and Feox as the best predictor for those indices that involves longer interaction periods. After Alox and Feox, the best correlations were obtained for the sum of Ca and Mg. The higher P retention of soils located in the Southern part of the Region was associated with the parental material rather than with anthropogenic effects. Obtained information can strengthen P fertilization programs by providing a methodology to estimate fertilizer P requirements to optimize crop yields while reducing environmental risks.
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