Phosphorus buffer capacity of soils with medium clay and high organic matter content

ABSTRACT Determining soil phosphorus (P) buffer capacity (PBC) is crucial for establishing optimal P fertilizer rates in corrective fertilization practices. However, in subtropical regions, changes in soil chemical and physical properties, such as soil organic matter (SOM) accumulation and texture modifications, can significantly impact PBC values, altering soil adsorption capacity. Consequently, P rates recommended by generalized guidelines for soil correction in these regions may be insufficient to achieve the critical P level in the soil. This study aimed to define PBC, develop an equation for PBC estimation, and establish P rates for corrective fertilization in soils with high SOM and medium clay content under subtropical climate conditions in Brazil compared with CQFS-RS/SC (2016) recommendation. The study used 29 soils collected from the Serra Gaúcha region, Rio Grande do Sul state, Brazil. Soil samples were collected from native forest areas in the 0.00-0.20 m layer. After drying and sieving, the samples were subjected to incubation with phosphate rates for 30 days. Ten treatments, ranging from 0 to 300 % of the recommended P 2 O 5 rate to increase P levels to high sufficiency levels were applied, with three replicates. After the incubation period, Mehlich-1 P (P_M1) levels were determined. The mean PBC value was 26.3 kg ha -1 P 2 O 5 , representing the rate required to increase P_M1 levels by 1.0 mg dm -3 in the soil. Terrain altitude was negatively correlated with PBC, while P_M1 showed a positive correlation with SOM content. An equation for estimating PBC was proposed [PBC (kg ha -1 P 2 O 5 ) = 49.75 - 0.063A + 0.692Clay - 1.869P_M1]. The current corrective fertilization recommendations for the southern region of Brazil underestimate the P 2 O 5 rates for very low and low classes in Serra Gaúcha soils by an average of 1.5 times. Therefore, we recommend adjusting phosphate fertilizer rates, monitoring soil P content, and maintaining SOM levels.