The lime-silicate question

Hikers passing through nature easily recognize that the vegetation on limestone (calcareous soils) and silicate (acidic soils) is very different. This is not so much the case with trees but with herbs, particularly in grasslands. These differences in the vegetation on both soil types, referred to as the lime silicate question, were recognized as early as the 18th century. Even so, fairly little is known about why this occurs. The current paucity of information exists mainly because the determinants that govern the formation of plant communities on either lime or acid soils mainly reside below-ground, in the root-surface area, and these complex below-ground interactions are almost impossible to explore through experimental approaches and direct measurements. Element availability or toxicity and interactions with microorganisms are both equally important. The calcium cation, for example, is not only an essential growth component but can be severely toxic to plants in unbalanced concentrations. Plants also vary in their ability to acquire iron and phosphorus. Aluminium toxicity affects plant growth, particularly in acid soils, and acid-tolerant plants have acquired the capability to cope with high levels of Al in soils. Mainly saprophytic fungi serve to degrade litter in acid soils and liberate nutrients for plant growth. In calcareous soils, bacteria and small animals, such as earthworms and their inhabitant bacteria, are the main decomposers of litter and contribute significantly to the fertility of the soils. Arbuscular mycorrhizal fungi (AMF) have a particularly significant role in calcareous soils, where they are believed to determine the competitiveness of plants in lime meadows. Physical factors also impact the different composition of plant communities in acid and calcareous soils. (C) 2015 Elsevier Ltd. All rights reserved.