Phosphorus extraction by Mehlich 1, Mehlich 3 and Anion Exchange Resin in soils with different clay contents

ABSTRACT Phosphorus extractors (P) Mehlich 1 (M-1), Mehlich 3 (M-3) and Anion Exchange Resin (RTA) show sensitivity to soil clay content. Also, the use of equations for conversion of available P extracted by M-3 into equivalent values of M-1 may not be adequate for a set of contrasting soils. The objective of this work is to evaluate the influence of clay content on the extraction of P by M-1, M-3 and RTA, as well as to evaluate the effectiveness of using the equation proposed by Local Soil Fertility Committee to convert M-3 contents to equivalent values of M-1. It was selected 301 farmers’ soil samples from the soil test laboratory at Santa Catarina State University, distributed in four classes of clay: 0-20, 21-40, 41-60 and > 60% clay. The samples were prepared and the available P was extracted by M-1, M-3 and RTA. M-3 and RTA extracted 12 and 11% more P than M-1, M-1 extracted less P than M-3 and RTA in sandy soils and extracted more P than others in clayey soils. The equation suggested by Local Soil Fertility Committee for converting M-3 values to equivalents in M-1 underestimated P values by up to 24%. Therefore, it is proposed to use the equation P-M1eq (mg dm-3) = (P-M3 / (1.38-0.01 * Arg)). The evaluated methods present sensitivity to soil clay content, and the classification of P availability should be performed by texture bands.