1 (pedol.) The degree of acidity or alkalinity of a soil, expressed as a value on the pH scale. See also acid soil; alkaline soil.
2 (ecol.) In plant succession, the ability of an individual plant species or vegetation community to modify the physical environment and so favour further successional development. For example, the presence of the first colonizing plants in a hydrosere will tend to reduce water movement, favouring accelerated silting and so paving the way for plants typical of shallower water to arrive as new colonizers.