Availability of phosphorus in a Brazilian oxisol cultivated with eucalyptus after nine years as influenced by phosphorus-fertilizer source, rate, andplacement

This work evaluated the effect of different placement and rates of two phos phorus (P) fertilizers on P-availability by three methods of extraction, ni ne years after application to a Brazilian Oxisol cultivated with Eucalyptus camaldulensis. The treatments were applied to 24x18 m plots and 96 seedlin gs of E. camaldulensis were planted (3.0x1.5 m) in each plot. Single superp hosphate (SSP) and rock phosphate (RP) were tested in three rates (100, 200 , and 400 kg ha(-1) of P2O5). Each fertilizer was either (1) surface-applie d ih bands (0.6 m either side of the rows of trees) and incorporated before planting or (2) incorporated into furrows (0.2 m deep in the tree rows) be fore planting. As additional treatments, the combination of RP (96 kg ha(-1 ) of P2O5 applied in broadcast, or bands, or in furrows) + SSP (54 kg ha(-1 ) of P2O5 localized in the planting hole before planting) were tested. Twel ve soil subsamples from two layers (0-15 and 25-40 cm) were taken from each plot (from the planting rows or between the planting rows) and were analyz ed for pH in water (1:2.5), available P by Mehlich-1, Bray-1 and anionic re sin, exchangeable Ca, and Al by 1 mol L-1 Kcl. For both methods of fertiliz ers placement, the highest values of available P were observed in the surfa ce soil and in the planting row, and were strongly related to fertilizer ra te. Samples taken between the planting rows did not exhibit treatment effec ts on available P. The higher values of available P obtained with Mehlich-1 and the lower eucalyptus plant uptake efficiency of fertilizer-P from band ed RP confirms the fact that this extractant can overestimate the availabil ity of P in soils receiving RP. The use of anion exchange resin in this sit uation to estimate available P is supported. The results obtained with the localized application of RP indicate root system activity (P and Ca uptake and acidification of rhizosphere) as a factor in increasing fertilizer diss olution rates.