Traditionally, the use of indicator plant species or assemblages to detect the possible presence of metal-rich deposits. It is based on the limits-of-tolerance principle, i.e. it assumes that only specialized species can withstand metal-contaminated soils. In practice, plant response may be confusingly more complex (e.g. plants may respond to the low availability of essential nutrients rather than to a high concentration of toxic minerals) which makes such indicators unreliable. In modern use the concept includes the collection and chemical analysis of plant materials or soil layers, especially humus, in which metal ions may accumulate. It is a supplementary rather than a primary prospecting method.