Effects of aluminium and calcium in the soil solution of acid soils on root elongation of Glycine max cv. Forrest

Abstract

In the course of three experiments, soybean (Glycerine max (L.) Merr.) cv. Forrest was grown in 21 soils (four surface soils and 17 subsoils) amended with liming materials (CaCO3 and Mg CO3) and soluble Ca salts (CaSO4.2H20 and CaCl2.2H2O). In most soils, the soluble salts increased concentrations and activities of Al species in solution to levels that restricted root growth, and MgCO3, induced a Ca limitation to root growth. Root lengths after three days were related to so11 and soil solution attributes.Suitable diagnostic indices for the prediction of Ca limitations to root growth were either Ca saturation of the effective cation exchange capacity or Ca activity ratio of the soil solution, which was defined as the ratio of the activity of Ca to the sum of the activities of Ca, Mg, Na, and K. Values corresponding to 90% relative root length (RRL) of soybean were 0.05 for the Ca activity ratio and 11% for Ca saturation. Calcium activity and Ca concentration in the soil solution and exchangeable Ca were less useful for this purpose.Soil Al saturation was not a good predictor of Al toxicity, but soil solution measurements were. The activities of Al3+ and AlOH2+ gave the best associations with RRL, and values corresponding to 90% RRL were 4 8M and 0.5 8M respectively. The results suggested that Al(OH)3� , Al(OH)2+, and AlSO4+, were not toxic species. Soil solution pH and soil pH measured in water were more sensitive indicators of root growth than soil pH measured in 0.01 M CaCl2.Using a Ca activity ratio of 0.05 and an Al3+ activity of 4 8M as diagnostic indices, none of the 20 soils in two experiments were toxic in Al, while 13 (all subsoils) were deficient in Ca. Thus the first limitation on root growth was Ca deficiency and not Al toxicity, in spite of high Al saturations and relatively low pH in these soils. However, Al toxicity could be induced by increasing the ionic strengths of soil solutions.