Aluminium interaction with organic matter and fluoride during soil development in oxidised mine waste : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science, Massey University

Abstract

Significant differences in plant growth response between unmodified waste rock (UWR) and modified (treated with lime and phosphate fertilizers) waste rock (MWR) with topsoil treatments during the first two years of a field trial conducted by Gregg & Stewart (1986; 1987) indicated that Al toxicity was affecting root growth in the UWR. By the end of three years of field trial there was no significant difference in dry matter yield between the MWR and UWR with topsoil treatments indicating possible amelioration of Al toxicity (Gregg et al., 1990). The objective of this study was to measure different forms of Al and F in the soil and waste rock at the end of the third year of field trial and to determine the mechanism of amelioration of Al toxicity in these soils.

Results of this study revealed that there was a significant 2-fold increase in organic-Al (Al-OM) at the UWR interface (B) compared with the waste rock at depth (C) by the end of three year period of soil development. Organic matter decomposition in the topsoil during three years and movement of the soluble organic ligands to UWR interface (B) resulted in conversion of phytotoxic soil solution Al into non-toxic Al-OM forms by complexation with organic ligands in leachates accumulated in the UWR interface.

Total soluble F was consistently high in the UWR profile with a mean value ranging from 1.0 to 1.9 µ,g g·1 . Liming has reduced the F level to 0.3-0.4 µ,g g·1 in the MWR interface (B) and 0.4-0.5 µ,g g·1 in the topsoil (A). The waste rock at depth (C) had mean F values in the range 1.2-1.9 µ,g g·1 . High levels of F in the waste rocks could have also ameliorated Al toxicity in the UWR by formation of non-phytotoxic soluble AlF complexes. This was considered to be another possible reason for no significant differences in the pasture yields between UWR and MWR plots at the end of the third year of trial.

The mean soil pH(H20 ) for the UWR was 5.1 and liming has raised it by about 1 unit in the MWR. The corresponding pH(0.01 M CaCl2) values were 0.4-0.8 units less than the pH in water. The 0.02 M CaC12-extractable solution Al (Al-Ca) and 1 M KCl­- extractable exchangeable+ solution Al (Al-K) have remained high in UWR soil solution irrespective of different depths of topsoil treatments. The Al-Ca and Al-K ranged from 16.7 to 20.5 µ,g g·1 and 261 to 339 µ,g g·1 respectively in the UWR interface (B). The Al-Ca and Al-Kin waste rock at depth (C) remained at 16.8-22.9 µ,g g·1 and 238-369 µ,g g·1 respectively. Modifications have lowered both Al-Ca and Al-K to 0.8-2.7 µ,g g·1 and 35-66 µ,g g·1 respectively in the MWR interface (B). Topsoil Al-Ca and Al-K were not significantly different between UWR and MWR and the values for these two forms of Al ranged from 0.6 to 6.0 µ,g g·1 and 32-87 µ,g g·1 respectively at the end of the field trial. Normal plant growth response in the UWR in spite of high levels of Al-Ca and Al-K indicated that 0.02 M CaC12 and 1 M KCl extractants are not good indicators of phytotoxicity as reported in literature because these extractants may be extracting a substantial proportion of non-phytotoxic species of Al.