Soil solution Al as a measure of al toxicity to alfalfa in acid soils

Abstract

The root system of alfalfa (Medicago sativa L.), known for extreme sensitivity to Al toxicity, has been observed to penetrate acid subsoil horizons high in exchangeable Al. In our previous studies we noted that root penetration into acid subsurface horizons occurred where lime had been surface applied under both no‐till and conventional tillage management systems. In order to more fully understand this seeming contradiction, the present study was undertaken. It was hypothesized that exchangeable Al may not be an adequate indicator of the activity of Al in solution and consequently the potential toxicity to the plant may be over estimated. To test this hypothesis, concentrations and activities of Al, Ca, and other major macro and micro elements and pH were determined in an Ernest silt loam (fine‐loamy, mixed, mesic Aquic Fragiudult) soil solution and compared with soil exchangeable levels. Soil samples were taken from the surface (0–5 cm) and 1 m depth in soils that had received 13 Mg/ha surface dolomitic lime, 13 Mg/ha gypsum, or no amendments. Aluminum activities were as low as 0 and 0.05 uM in soil solution from the surface and 1 m depth, respectively, for surface limed soils. Calcium activities in soil solution from the surface 5 cm ranged from a high of 4500 to a low of 670 uM in soils which, received gypsum and no amendment, respectively. Calcium activities in soil solution from the subsurface (1 m) ranged from 690 to 1200 uM for the unlimed and limed soils respectively. Despite the high exchangeable Al levels present in the Ernest silt loam soil (>50 mg/kg), relatively low Al activities were present in soil solution, which may explain why alfalfa does not show characteristic Al toxicities when grown on these soils.