Subsoil A1 can restrict deep root growth which, under dryland conditions, can cause lower yields during periods of water deficit. Soils that have high levels of subsoil Al must be identified so that appropriate management strategies, such as selection of deep rooting, Al tolerant cultivars or chemical amelioration of the subsoil, can be implemented. Thirteen soils representing three landscape positions from the Southern Mississippi Valley were sampled to 75 cm at 15-cm depth increments. Short-term bioassays were conducted with wheat (Triticum aestivumL.), cotton (Gossypium hirsutumL.), and soybean (Glycine max(L.) Merr.) at each depth increment to identify soils with potentially phytotoxic concentrations of A1. Subsoil pH values generally decreased with depth to a low of 4.1 at the 45- to 60-cm depth increment. Values of 1NKCl-extractable Al were as high as 1010 mg kg−1in the 60- to 75-cm depth increment, whereas exchangeable Ca concentrations dropped as low as 207 mg kg−1. Concentrations of 1NKCl-extractable A1 in soils of the Loessial Plains (326 mg kg−1) were nearly twice those measured in soils from the Loessial Hills (180 mg kg−1) and more than three times greater than soils from the Bottomlands and Terraces (99 mg kg−1). Aluminum (KCl-extractable) concentrations that resulted in a 30% reduction in root length (a threshold value considered to reduce crop yields) varied among crop species and ranged from 2 mg kg−1to more than 667 mg kg−1. Subsoil Al is a problem for a significant percentage of hectarage in the Southern Mississippi Valley and should be taken into consideration, especially under dryland farming.