The root system of alfalfa (Medicago sativaL.), 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.