EVALUATION OF PHOSPHORUS RETENTION INDICATORS IN AGRICULTURAL, LIVESTOCK AND FORESTRY SYSTEMS

Abstract

High concentrations of extractable P and favorable conditions for transport increase runoff, erosion or leaching of phosphorus (P) in the soil. The objectives of this study were to evaluate the relationship between P sorption indices (ISP, P75 absorbed) and degrees of P saturation (GSPP75 and GSPISP), and to determine how these are related to the content of total organic carbon (TOC) and pH in the soil. Samples were taken at 0-5 cm depth from soils under different production systems: crop rotations, pastures, acacia (Acacia melanoxylon) and eucalyptus (Eucalyptus globulus) plantations, and natural grasslands.. The statistical analysis showed differences between ISP and P75, and both indices were significantly affected by the production system. The highest values of ISP and P75 reached 322 y 506.1 mg P kg-1, respectively, in the acacia forest. The lowest value of P75 was also recorded in the acacia plantation, and averaged 301.5 mg P kg-1 in barley in the soil with crop rotation with soybean and natural grassland. ISP averaged 186.4 mg kg-1 in plots with crop rotation, pastures several years, and natural grassland. The GSPs differed between production systems, presenting the highest values in the natural grasslands, and the lowest values in the acacia and eucalyptus plantations. GSPP75 recorded values that ranged between 2.8 and 15.5%, while GSPISP ranged between 4.6 and 24.3%. GSPP75 never reach the value of 20% indicated in the literature as the risk of soil P loss. No effect of pH was observed on P sorption parameters. ISP and P75 were positively correlated with TOC content.