CHANGES IN SOIL SOLUTION COMPOSITION AND ALUMINUM SPECIATION UNDER LEGUME-BASED PASTURES IN RESPONSE TO LONG-TERM PHOSPHATE FERTILIZER APPLICATIONS

This study describes some of the effects of 8 years of annual applicat ion of 6 types of phosphatic fertilisers on the chemical composition a nd aluminium (Al) speciation in soil solution extracted from a soil un der pasture. Soil samples at 2 depths, 0-30 and 30-75 mm, were collect ed at the end of 8 years. Soil solutions were extracted by centrifugin g at 12 000 RCF and analysed for Al, Na, K, Ca, Mg, F, NO3, Cl, and SO 4, as well as pH and ionic strength. Soil and soil solution pH were si gnificantly increased at both depths by application of North Carolina phosphate rock (NCPR) compared with the control. In contrast, diammoni um phosphate (DAP) significantly decreased the soil and solution pH. S ingle superphosphate (SSP) did not have any significant effect on soil or solution pH compared with the untreated control. The surface soil (0-30 mm) solution pH was on average 0.6 of a unit higher than the sub surface soil (30-75 mm) solution pH. Total monomeric Al concentration [Al], measured by the pyrocatecol violet (PCV; 4 min) method, ranged f rom 1.5 to 4.8 mu M in the surface soil and 2.5 to 12.2 mu M in the su bsurface soil. The DAP and higher rates of SSP application resulted in a large increase in total and inorganic monomeric [Al] in the soil so lution extracted from the subsurface soil. Total soluble [F] ranged fr om 2.7 to 23.5 mu M and 3.2 to 25.6 mu M in the surface and subsurface soils, respectively, and was significantly increased by the applicati on of NCPR and by higher rates of SSP. The predominant forms of inorga nic monomeric Al present in the soil solution were estimated to be the non-phytotoxic Al-F complexes, AlF2+, and AlF2+. There was a marked d ecrease in toxic Al species (Al3+, Al(OH)(2+), Al(OH)(2)(+)) in soil s olution following NCPR and SSP application. This was due primarily to complexation of Al with F derived from these fertilisers forming non-t oxic AI-F complexes. The results suggest that the long-term applicatio n of reactive phosphate rock such as NCPR may contribute to ameliorati on of soil acidity and Al toxicity under legume-based pastures. In con trast ammonium-containing phosphate fertilisers such as DAP probably d ecrease soil pH and increase the formation of toxic Al species in the soil solution.