Aluminum toxicity and high bulk density: Role in limiting shoot and root growth of selected aluminum indicator plants and eastern gamagrass in an acid soil

Shallow rooting and susceptibility to drought are believed to be caused, at least in part, by strongly acidic (pH <5.5, 1:1 soil-water), aluminum (Al)-toxic subsoils. However, this hypothesis has not been clearly confirmed under field conditions. The Al toxicity hypothesis was tested on a map unit of Matawan-Hammonton loam (0-2% slope) on unlimed and limed field plots (pH range 5.1 to 5.8) at Beltsville, MD, during 1994 to 1998. Aluminum-tolerant and sensitive pairs of barley (Hordeum vulgare L.), wheat [Triticum aestivum (L.)], snap bean (Phaseolus vulgaris L.), and soybean [Glycine max (L.) Merr.] cultivars were used as indicator plants. Eastern gamagrass [Tripsacum dactyloides (L.) L.], cultivar 'Pete', reported to tolerate both chemical and physical stress factors in soils, was grown for comparison. Shoots of Al-sensitive 'Romano' snap beans showed a significant response to liming of the 0-15 cm surface layer, but those of Al-tolerant 'Dade' did not, indicating that Al toxicity was a growth limiting factor in this acid soil at pH 5.1. Lime response of the Al-tolerant and sensitive cultivars of barley, wheat, and soybean were in the same direction but not significant at the 5% level. Aluminum-tolerant and sensitive cultivars did not differ in abilities to root in the 15-30 cm soil depth. Only 9 to 25% of total roots were in this layer, and 75 to 91% were in the 0-15 cm zone. No roots were found in the 30-45 cm zone which had a pH of 4.9. Soil bulk density values of 1.44 and 1.50 g cm(-3) in the 15-30 and 30-45 cm zones, respectively, indicated that mechanical impedance was a primary root barrier. Results indicated that restricted shoot growth and shallow rooting of the Al-indicator plants studied in this acid soil were due to a combination of Al toxicity and high soil bulk density. Confounding of the two factors may have masked the expected response of indicator plants to Al. These two growth restricting factors likely occur in many, if not most acid, problem subsoils. Studies are needed to separate these factors and to develop plant genotypes that have tolerance to multiple abiotic stresses. Unlike the Al indicator cultivars, eastern gamagrass showed high tolerance to acid, compact soils in the field and did not respond to lime applications (pH 5.1-5.8).