Forms of extractable aluminium in Canadian acid soils and their relations to plant growth

Abstract

Five forms of extractable soil aluminium (Al) were quantified and their effects on plant dry matter production studied. Thirty surface and five subsurface soils ranging in pH from 3.5 to 5.4 were sequentially extracted with 1 M KCl, 0.1 M CuCl₂ and 1 M NH₄OAc (pH 4.0) to remove exchangeable, organic bound and sorbed polynuclear hydroxy-Al respectively. Acidic (0.2 M, pH 3) ammonium oxalate was used to extract amorphous Al, and 0.33 M (pH 7.3) sodium citrate interlayer Al. Amorphous Al (average 53.7% of total extractable Al) < interlayer Al (19.3%) < sorbed polynuclear hydroxy-Al (16.7%) < organic bound Al (5.3%) ≥ exchangeable Al (5.0%). However, for soils with pH > 4 (this included most of the subsoils used), exchangeable Al < organic bound Al. Lime requirement to pH 6, as determined by the Shoemaker-Mc Lean-Pratt buffer method, was positively correlated with exchangeable and organic bound Al. Plant dry matter was negatively correlated with exchangeable Al and sorbed polynuclear hydroxy-Al in that order. Although organic bound Al influenced the lime requirement, it was not toxic to plants. An equilibrium relationship between some of the Al pools is suggested by significant correlations.