Soil Aluminum Effects on the Growth and Aluminum Concentration of Sorghum

Aluminum toxicity, especially in the subsoil, is a major limiting factor in crop production on acid soils. A greenhouse experiment with sorghum [ Sorghum bicolor (L.) Moench)] was conducted on topsoils and subsoils of four widely different acid southeastern soils [Congaree (Typic Udifluvent), Pacolet (Typic Hapludult), Norfolk (Typic Paleudult), Pelham (Arenic Paleaquult)] to determine relationships between plant growth and levels of Al and other major cations in the soil. Plants were grown for 6 wk after which plant height, weight, and root weight were recorded. Plant tissue was analyzed for Al, Mn, Ca, K, and P concentrations. Soils were analyzed for pH, 1 M KCl‐exchangeable Al, Al saturation of the CEC, and 0.01 M CaCl 2 ‐extractable Al. Major soil solution ions were also determined and the activity of Al species was calculated using GEOCHEM. Reductions in plant growth were ascribed to Al toxicity and not to Mn toxicity or Ca deficiency. In general the correlations between plant growth and soil Al levels were significant for topsoils [ r = −0.53* to −0.91** ( P = 0.05 and 0.01, respectively)] but nonsignificant for subsoils. Topsoil data and subsoil data were markedly different; subsoil Al values were low compared to those for topsoil (exchangeable Al 49 vs. 58 mg kg −1 ), but the plants grew better in topsoils (51 vs. 29 cm). The best predictors of plant height and plant Al were soil solution Al 3+ activity ( r = −0.91**, 0.83**) and Al saturation of the exchange complex ( r = −0.89**, 0.82**), but CaCl 2 ,‐extractable Al was also correlated well ( r = −0.78**, 0.94**).