Phytotoxic concentrations of subsoil aluminum as influenced by soils and landscape position

Subsoil Al can restrict deep root growth which, under dryland conditions, c an cause lower yields during periods of water deficit. Soils that have high levels of subsoil Al must be identified so that appropriate management str ategies, such as selection of deep rooting, Al tolerant cultivars or chemic al amelioration of the subsoil, can be implemented. Thirteen soils represen ting three landscape positions from the Southern Mississippi Valley were sa mpled to 75 cm at 15-cm depth increments. Short-term bioassays were conduct ed with wheat (Triticum aestivum L.), cotton (Gossypium hirsutum L,), and s oybean (Glycine max (L.) Merr,) at each depth increment to identify soils w ith potentially phytotoxic concentrations of Al, Subsoil pH values generall y decreased with depth to a low of 4.1 at the 45- to 60-cm depth increment, Values of 1N KCl-extractable Al were as high as 1010 mg kg(-1) in the 60- to 75-cm depth increment, whereas exchangeable Ca concentrations dropped as low as 207 mg kg(-1), Concentrations of IN KCl-extractable Al in soils of the Loessial Plains (326 mg kg(-1)) were nearly twice those measured in soi ls from the Loessial Hills (180 mg kg(-1)) and more than three times greate r than soils from the Bottomlands and Terraces (99 mg kg(-1)). Aluminum (KC l-extractable) concentrations that resulted in a 30% reduction in root leng th (a threshold value considered to reduce crop yields) varied among crop s pecies and ranged from 2 mg kg(-1) to more than 667 mg kg(-1): Subsoil Al i s a problem for a significant percentage of hectarage in the Southern Missi ssippi Valley and should be taken into consideration, especially under dryl and farming.