RELATIONSHIPS BETWEEN SOIL SOLUTION ALUMINUM AND EXTRACTABLE ALUMINUMIN SOME MODERATELY ACID NEW-ZEALAND SOILS

This work investigates the relationship between soil solution aluminiu m (Al) and extractable Al in some New Zealand soils giving high extrac table Al levels, yet with pH(H2O) values greater than or equal to 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 1 2.5 mu M, with <10% of the Al in the monomeric Al form as determined b y 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 solu tions did not exceed levels corresponding to Al toxicity threshold act ivities set at 10 or 2 mu M, related to a range of pasture plant toler ances, 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 statu s, probably due to the generation of artificially high extracted Al co ncentrations from these particular types of soils.