Indicator Plants

Baker (328) proposed that there are three types of plant responses to increasing heavy metal contents in soil: (a) accumulators, where heavy metals are concentrated in above-ground plant parts; (b) indicators, where internal concentrations reflect external levels; and (c) excluders, where metal concentrations in shoots are low and constant over a wide range of soil concentrations up to a critical soil level above which unrestricted transport occurs. It might be expected that aluminum accumulators would be good indicator plant species; however, this relationship has not been found to be true. Truman et al. (14) reported that only a weak linear relationship was found between foliage aluminum concentration of Pinus spp. and exchangeable aluminum in soil. Even in controlled nutrient solution culture, foliar aluminum levels of red spruce varied almost fivefold at a similar solution of aluminum concentration (78).

An alternate method of determining the status of soil aluminum is to grow pairs of aluminumtolerant and sensitive genotypes of some common crops, such as barley or snapbean, then observe their differential responses. For example, shoots of the aluminum-sensitive 'Romano' snapbean showed a significant response to liming of an acid (pH 5.1) soil from Beltsville, Maryland, but those of the aluminum-tolerant 'Dade' did not; this dry weight difference indicated that aluminum toxicity was the main factor limiting growth (329). Sanchez (300) reported that there was a high degree of tolerance to acid (mostly Al) soil in many varieties of upland rice and cowpea. Such knowledge would be very useful in identifying and managing aluminum-toxic soils.