Nairam Felix de Barros

Availability of phosphorus in a Brazilian Oxisol cultivated with eucalyptus after nine years as influenced by phosphorus‐fertilizer source, rate, and placement

Abstract This work evaluated the effect of different placement and rates of two phosphorus (P) fertilizers on P‐availability by three methods of extraction, nine years after application to a Brazilian Oxisol cultivated with Eucalyptus camaldulensis. The treatments were applied to 24x18 m plots and 96 seedlings of E. camaldulensis were planted (3.0x1.5 m) in each plot. Single superphosphate (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‐applied in 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) before 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. Twelve 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 analyzed for pH in water (1:2.5), available P by Mehlich‐1, Bray‐1 and anionic resin, exchangeable Ca, and Al by 1 mol L‐1 Kcl. For both methods of fertilizers placement, the highest values of available P were observed in the surface soil and in the planting row, and were strongly related to fertilizer rate. Samples taken between the planting rows did not exhibit treatment effects on available P. The higher values of available P obtained with Mehlich‐1 and the lower eucalyptus plant uptake efficiency of fertilizer‐P from banded RP confirms the fact that this extractant can overestimate the availability of P in soils receiving RP. The use of anion exchange resin in this situation 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 dissolution rates.

Effects of nitrate, phosphate and sulfate combinations on growth and kinetics of phosphate and sulfate uptake by eucalypt seedlings

This trial was carried out to study the effect of nitrate, phosphate and sulfate on growth and kinetics of P and S uptake by Eucalyptus grandis seedlings. The seedlings had their taproot pruned and were placed in sets of geminated plastic pots, with the root system split and growing in solutions containing the three nutrients, or with one of them isolated in one of the geminated pots. Seedling shoot dry weight when the solution in both pots contained the three nutrients was higher than that in the other treatments, which did not differ. Absence of NO3 − in the nutrient solution significantly reduced root dry weight. When both pots contained the three nutrients, the Km-SO4 2− was two to three times lower than that of the other four treatments. Influx of SO4 2− and H2PO4 − increased when NO3 − was supplied separately from SO4 2− or H2PO4 −.