ALUMINUM-ORGANIC MATTER-FLUORIDE INTERACTIONS DURING SOIL DEVELOPMENTIN OXIDIZED MINE WASTE

A three year field trial assessing the suitability of mine waste rock as a surrogate subsoil with different depths of topsoil showed that pa sture yield was significantly lower in the unmodified waste rock (UWR) treatments compared to waste rock modified with lime and potassic sup erphosphate (MWR) during the first two years but there was no differen ce in yield in the third year between the waste rock treatments. The r educed yield in UWR during the first two years was considered to be du e to phytotoxic levels of aluminium (Al) in the UWR soil solution. Soi l samples from the waste rock interface (A) and waste rock at depth gr eater than or equal to 300 mm (B) collected at the end of the three ye ar trial were analysed for different forms of soil Al, total soluble f luoride (F), dissolved organic carbon (DOG) and soil pH to determine w hether organic matter and F leachate from the topsoil reduced Al toxic ity at the waste rock interface (A) during soil development over 3 yea rs, Results showed that after 3 years, 0.02 M CaCl2 extractable Al (Al -Ca) and 1 M KCI extractable exchangeable + soluble Al (Al-K) were eff ectively reduced in the MWR interface (A) by lime and fertiliser appli cations but they remained at very high phytotoxic levels (Al-Ca = 17-2 1 mu g g(-1), Al-K = 261-339 mu g g(-1)) in the UWR interface (A) irre spective of topsoil depth treatment. The corresponding organic bound A l (Al-OM) ranged from 200 to 214 mu g g(-1) and DOC was in the range 1 69-203 mu g g(-1) in both UWR and MWR interfaces (A). These values wer e a significant two-fold higher than the values at depth greater than or equal to 300 mm (B) which were considered to be the original levels of Al-OM and DOC in UWR. The increase in Al-OM and DOC by the third y ear is probably due to progressive leaching of organic ligands produce d from the decomposition of organic matter in the topsoil subject to a n annual rainfall of 2500 mm. This evidence for the reduction in Al to xicity by organic matter is further supported by significant dry root density (DRD) in the UWR interface (A). This indicated that organic ma tter may have complexed toxic Al in UWR and alleviated Al toxicity in the third year of the field trial. Total soluble F levels were similar between interface (A) and waste rock depth (B) of the UWR, suggesting that F levels have not changed during the three years of the trial an d therefore there was no evidence of F complexing with phytotoxic Al i n the soil solution in the presence of organic ligands.