A comparison of soil tests to predict the growth and nodulation of subterranean clover in aluminium-toxic topsoils

Total Al concentration or pH in 1:5 10 mM CaCl₂ extracts and exchangeable Al in 100 mM BaCl₂ extracts cannot always distinguish between Al-toxic and Al-nontoxic topsoils. Our objectives were to compare the abilities of different measures of Al and pH in various extracts to predict the effects of acidity on growth and nodulation of subterranean clover. In a glasshouse experiment, Trifolium subterraneum L. cv. Mt Barker was grown in acidic soils from 3 sites in the Western Australian wheatbelt with different histories of phosphate fertilizer application. The pH was adjusted to give a range of 3.8-7 in the centrifuged soil solution (SS). Total (Al-tot), reactive Al (8-hydroxyquinolineextractable, Al-HQ) and pH were measured in SS and 1:5 extracts of KCl, CaCl₂ and LaCl₃. Another method of estimating reactive Al (Al which reacts with Chelex-100) was also measured in SS only. Other measurements included exchangeable Al and H, Ca in SS, and p in SS and the CaCl₂ extracts. Both plant growth and early nodulation decreased with increasing acidity. Plant growth in the acidified and unlimed treatments of all soils was best described by Al-HQ in SS, KCl or CaCl₂ (r² = 0.68-0.70). Multiple regression of relative yield against Al or pH with the concentration of P in SS increased the percentage variation explained by 10% and 30%, respectively. Early nodulation was well correlated (r² = 0.67-0.91) with pH or exch. H, Al-tot or exch. Al and Al-HQ. No improvement in the correlation was gained by including P using multiple regression. At constant ionic strength, increasing the valence of the extracting cation decreased the ability of soil tests to distinguish phytotoxic Al.