PHOSPHORUS DIFFUSIVE FLUX AS AFFECTED BY PHOSPHATE SOURCE AND INCUBATION-TIME

Phosphorus supply to eucalypts is deficient in many Brazilian soils an d must be augmented with fertilizers to promote economic growth of pla ntations. Diffusion is generally considered to be the major mechanism of P fertilizer movement in these types of soils; therefore, we studie d the effect of P fertilizer source and time of incubation of fertiliz er with soil on the P diffusive flux in four Oxisols, Five soluble P s ources [Ca(H2PO4)(2), NH4H2PO4, (NH4)(2)HPO4, Na2HPO4. 2H(2)O, and K2H PO4] were applied to the soils at a rate of 500 g P m(-3) soil. Micros cope slides were covered with anion-exchange paper (filter paper impre gnated with Fe hydroxide) and laid between two soil layers in a diffus ion chamber, Fifteen days later, P was extracted from the exchange pap er and analyzed. Five hundred grams P m-3 soil as the Ca source were a lso applied to soil samples and incubated for 30 d, Phosphorus diffusi ve flux was measured as described above. The Ca source of P reduced P diffusive flux by as much as 77% compared with the other sources. This reduction was attributed to the effect the P source had on soil pH, S andy soils showed less difference between P sources than did clayey so ils because the P applied to the sandy soils was in excess of their ma ximum sorptive capacity. Incubation for 30 d reduced P diffusive flux by up to 81%, Different P sources affect the availability of P in thes e soils by influencing mobility, Choosing the best source can increase diffusive nux by up to 242% under these soil conditions.