Phosphorus fractionation in a sandy soil under organic agriculture in Northeastern Brazil

This work aimed to evaluate the impacts of the organic fertilization and the use of green manure (GrM) on the different soil P fractions in areas under organic agriculture in the municipality of Ubajara, state of Ceará, Brazil. The work was carried out in an organically cultivated farm situated in the Ubajara municipality, state of Ceará, Brazil. Soil samples of a sandy soil were collected at the 0- to 5-, 5- to 10-, 10- to 20-, 20- to 40- and 40- to 50-cm layers in areas of Caribbean cherry with (+) and without (−) previous incorporation of GrM in the rows (+GrMr and −GrMr) and between-rows (+GrMb-row and −GrMb-row). Area under natural vegetation (NV) was also sampled and considered as reference. P fractionation was performed by using a sequential extraction scheme adapted from Hedley. The cultivated systems presented higher P contents than NV, mainly in the most labile P pool (res-Pi + NaHCO3-P), indicating that organic management favoured greater P cycling of this fraction. In the cultivated areas labile P pool accounted for 20, 17, 21 and 27% of sum of soil total P in the −GrMr, −GrMb-row, +GrMr and +GrMb-row, respectively. There was an increment of 86% in the organic P pool in the organically cultivated areas in relation to the NV. The organic management promoted an increase on the available-P forms by avoiding and/or decreasing the rate of transformation of the Pi in more stable P forms, enhancing the P cycling. The use of green manure promoted increase on the most labile P pool (Resin-P+NaHCO3-P) and seems to contribute to the P incorporation into biological cycle. On the other hand, the risks of P losses in the organically managed areas should be considered and suggest a scientific remark to farmers for a better management of organic farming in sandy soils.