Relationships between soil solution aluminium and extractable aluminium in some moderately acid New Zealand soils

Abstract

This work investigates the relationship between soil solution aluminium (Al) and extractable Al in some New Zealand soils giving high extractable Al levels, yet with pH(H₂O) values ≥ 5.2. Total Al in 1 M KCl extracts ranged from 0.8 to 11.6 cmol(+)/kg, and in corresponding 0.02 M CaCl2 extracts from 0.002 to 0.39 cmol(+)/kg. Soil solutions had low total Al concentrations, ranging from < 0.5 to 12.5 µM, with < 10% of the Al in the monomeric Al form as determined by the chromeazurol S colorimetric method. There was a poor correlation between Al in soil solution and that extracted by either 1 M KCl or 0.02 M CaCl₂.

The measured monomeric Al concentrations in the soil solutions did not exceed levels corresponding to Al toxicity threshold activities set at 10 or 2 µM, related to a range of pasture plant tolerances, whether based on the activity of Al³⁺ species alone, or on the sum of the individual activities of Al³⁺, Al(OH)²⁺ and Al(OH)²⁺ species. The high 1 M KCl-extractable and 0.02 M CaCl₂-extractable Al values provided a misleading indication of potential Al toxicity status, probably due to the generation of artificially high extracted Al concentrations from these particular types of soils.