A bioassay procedure is described for diagnosing aluminium toxicity in soils using short term root growth in extracted soil solution. Soil solution is extracted from moist soil which has been incubated at “field capacity”; for 4 days. Soil solution extracted is divided into two portions, each of which is treated with CaCl2and H3BO3to ensure that neither Ca nor B is limiting root growth. One portion is adjusted to pH 5.5 (pH adjusted treatment) with saturated Ca(OH)2solution. Aliquots (11 ml) of each portion are separately dispensed into each of five polypropylene tubes. Seedlings (in our experimentGlycine maxcv. Forrest) of uniform root length are inserted into each tube (one per tube) and grown for 48 h. The increase in root length during the 48 h growth period (root elongation) in the unadjusted solutions is expressed as a percentage of that in the pH adjusted solutions to derive relative root elongation (RRE) ‐ an index of aluminium toxicity.