Comparison of methods for extracting available phosphorus from soils of the semi-arid¹

Comparação de métodos de extração de fósforo disponível em solos do semiárido

ABSTRACT

Mehlich-1 is the most used method in Brazil for extracting available phosphorus (P) from the soil. Given the heterogeneity of soils in the semi-arid region of the Northeast, and the need for an analytical protocol for P extraction, the Mehlich-1, Olsen and ion-exchange resin methods were compared in samples of ten representative soils taken from the semi-arid region between the valley of the River Piranhas-Açu (Rio Grande do Norte) and the valley of the River Jaguaribe (Ceará), where the soil pH ranges from 4.1 to 8.0. The soil samples received four doses of P based on the phosphate buffering capacity (PBC) of the soils, and were incubated for 30 days. The treatments were divided into randomised blocks, in a 10×4 factorial arrangement of ten soils and four doses of phosphorus, with three replications. Each experimental unit comprised one plastic pot containing 3.0 dm³ of soil, where two maize plants were grown for 31 days in a greenhouse. The P extracted by each method was correlated with the P accumulated in the aerial part of the plants. The mixed-resin and Olsen methods showed the best correlation with the P accumulated in the plants, irrespective of the pH or PBC of the soil. Neutralisation of the acidity of Mehlich-1 was significant in alkaline soils with high levels of CaCO₃ equivalent. Mehlich-1 was suitable for strongly acidic soils with a low PBC, however, the use of this extractor in pH neutral to alkaline soils with a high Ca²⁺ content can overestimate P availability when the soils are rich in P-Ca, or underestimate the availability in soils rich in CaCO₃.

Keywords:
Alkaline soils; Mehlich-1; Olsen; Ion-exchange resin