Changes in soil solution composition and aluminium speciation under legume based pastures in response to long term phosphate fertiliser applications

Abstract

This study describes some of the effects of 8 years of annual application of 6 types of phosphatic fertilisers on the chemical composition and aluminium (Al) speciation in soil solution extracted from a soil under pasture. Soil samples at 2 depths, 0-30 and 30-75 mm, were collected at the end of 8 years. Soil solutions were extracted by centrifuging at 12 000 RCF and analysed for Al, Na, K, Ca, Mg, F, NO3, Cl, and SO4, as well as pH and ionic strength. Soil and soil solution pH were significantly increased at both depths by application of North Carolina phosphate rock (NCPR) compared with the control. In contrast, diammonium phosphate (DAP) significantly decreased the soil and solution pH. Single 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 subsurface soil (30-75 mm) solution pH. Total monomeric Al concentration [Al], measured by the pyrocatecol violet (PCV; 4 min) method, ranged from 1.5 to 4.8 µM in the surface soil and 2.5 to 12.2 µM in the subsurface soil. The DAP and higher rates of SSP application resulted in a large increase in total and inorganic monomeric [Al] in the soil solution extracted from the subsurface soil. Total soluble [F] ranged from 2.7 to 23.5 µM and 3.2 to 25.6 µM in the surface and subsurface soils, respectively, and was significantly increased by the application of NCPR and by higher rates of SSP. The predominant forms of inorganic monomeric Al present in the soil solution were estimated to be the non-phytotoxic Al-F complexes, AlF2+, and AlF2+. There was a marked decrease in toxic Al species (Al3+, Al(OH)2+, Al(OH)2+) in soil solution following NCPR and SSP application. This was due primarily to complexation of Al with F derived from these fertilisers forming non-toxic AI-F complexes. The results suggest that the long-term application of reactive phosphate rock such as NCPR may contribute to amelioration of soil acidity and Al toxicity under legume-based pastures. In contrast ammonium-containing phosphate fertilisers such as DAP probably decrease soil pH and increase the formation of toxic Al species in the soil solution.