Aluminum‐sensitive ‘Monon’ wheat (Triticum aestivum, spp vulgare (Vill. Host) Mac Key and ‘Kearney barley (Hordeum vulgare) (L. emend. Lam) varieties had higher root cation exchange capacities and induced lower pH levels in nutrient solutions than did Al‐tolerant ‘Atlas 66’ wheat and ‘Dayton’ barley varieties. Sensitive varieties contained higher concentrations of Al (and usually P) in their roots and lower concentrations of Ca in their tops than tolerant varieties, when grown in solutions containing Al. Furthermore, roots of Al‐sensitive varieties made less growth and contained higher concentrations of Al than those of tolerant varieties, even when the two were grown in the same container of vigorously aerated nutrient solution. This suggests either that zones of differential pH still exist around the roots of different varieties or that sensitive varieties absorb more Al at the same pH or both. Whether the lower Al tolerance of certain varieties is simply the result of greater accumulation of Al by roots, greater sensitivity to the same concentration of Al already absorbed, or both, cannot be determined from the present study. Differential Al tolerance of varieties was not closely related to differences in the Al or P contents of plant tops.

Synopsis Varieties of wheat and of barley differed widely in their tolerance to acid soils containing high levels of KCl‐extractable Al. With few exceptions, varieties developed in the eastern United States were more tolerant than those developed in the Plains and Western States. Wheat varieties from Brazil were exceptionally tolerant. The results indicate that certain varieties have been selected for properties that are closely associated with their abilities to tolerate Al in acid soils.

Differential aluminum tolerance of ‘Perry’ and ‘Chief’ soybean varieties, determined previously from growth on acid Bladen soil, was confirmed in nutrient solutions containing Al as the known growth‐limiting factor. Differences in Al tolerance between the two varieties were steadily increased as the Ca level of the nutrient solution was reduced from 50 to 8 to 2 ppm. Aluminum toxicity in soybeans was associated with decreases in concentrations of Ca in the tops and roots of both varieties, but this effect of Al was much more pronounced in the Al‐sensitive Chief variety than in the more tolerant Perry. Greater Al sensitivity of the Chief variety was associated with greater susceptibility to a petiole collapse symptom. This symptom was related to a lower Ca concentration in the leaves and petioles, and specifically, related to a lower Ca concentration in the small petiole zone actually showing the collapse. Soil and solution studies indicated that the Ca deficiency observed in acid Bladen soil was Al induced. Aluminum appears to interfere to different degrees in the uptake and use of Ca by these two soybean varieties. The petiole collapse appeared to be a secondary effect of Al injury, and the presence of the symptom was not required for yield reduction by Al. The fact that soybean varieties differ in Al tolerance suggests that plant breeders may be able to develop varieties that can root more effectively in acid, Al‐toxic subsoils. Varieties differing in Al tolerance also provide valuable tools for fundamental studies on the physiological nature of Al toxicity in plants. The petiole collapse symptom associated with Al sensitivity may be useful to plant breeders in screening genetic populations of soybeans for Al tolerance.

Thirty cultivars and experimental strains of winter barley (Hordeum vulgare) were grown on acid, aluminum‐toxic soils treated with two rates of lime in greenhouse tests and three rates in field tests. The objectives were to determine if greenhouse tests could be used for certain phases of breeding for Al‐tolerant barley varieties and to find possible new sources of Al‐tolerant germ plasm. Dry‐weight yields of top and root growth were determined after 7 weeks in the greenhouse. Grain yields were determined from field plots. Correlation coefficients between root and top yields in the greenhouse were 0.93 for the low lime rate and 0.71 for the high rate. Root and top yields at the low lime rate in the greenhouse were highly correlated with grain yield in the field with no lime added (r = 0.77 and 0.66, respectively) but there was no significant correlation between the same measurements at the high lime levels. Results of these studies indicate that plant populations of barley can be initially screened for Al tolerance in pots of Al‐toxic soil in a greenhouse. Al tolerance is apparently genetically controlled. Experimental lines derived from backcrossing to A1‐tolerant parents were also tolerant. Selection for A1 tolerance in a mixed population on acid, A1‐toxic soil in the field was demonstrated.