Most soils in semi-arid Northeastern Brazil are considered phosphorus deficient. The great diversity of orders they belong to, however, indicates that they are not similar in relation to the concentrations of their different P forms. The distribution of phosphorus forms in 69 soils of the most common soil orders in the semi-arid region was evaluated in samples from the 0-20 and 20-40 cm soil layers. Phosphorus was determined by Mehlich-1 and by a sequential fractionation, through extractions with resin, NaHCO3, NaOH and H2SO4, followed by peroxide/sulfuric digestion. Total P (the sum of all fractions) varied from 52 to 1625 mg kg-1. Most soils were in a range from 100 to 200 mg kg-1 in the 0-20 cm layer, with a large variability between and within soil orders. Large variability was also observed for all P fractions. In general, Fluvic Neossols, Vertisols, Luvisols and Cambisols had the highest total P values; Argisols, Litholic Neosols and Latosols, intermediate values; and Regolitic Neosols, Planosols and Quartzarenic Neosols, the lowest values. The largest proportions of total P were in the residual fraction and the smallest in the labile fractions (resin-P and NaHCO3-Pi), which together represented 7-12 % of total P. Mehlich-1 extracted low proportions of P from Vertissols (1 %) and high proportion from Latosols (24 %), while the opposite was true for bicarbonate extraction (11 and 5 %, respectively). Resin extracted low proportions from all soil orders (1 to 6 %).
P fractionation; labile P; organic P; inorganic P; residual P
