ABSTRACT
Soil reaction is the degree of soil acidity or alkalinity which is caused by a particular chemical, mineralogical and biological environment. Soil reaction affects element availability and toxicity, microbial activity, and root growth. Alkaline soil infertility tends to be associated with calcareous soils where both pH and presence of CaCO3 affect nutrient availability. Iron is the most common nutrient element deficiency of plants growing on alkaline soil and higher plants have devised two "strategies" to increase the availability of Fe in soils. The mineral weathering sequence concept is well established and is useful in relating differences in soil pH and action exchange relationships between several of the soil orders. Molybdenum anions are strongly adsorbed on pH-dependent clays and Mo tends to become less available and more deficient in acid soils. Although it is generally considered that a pH less than 4 is needed to affect plant growth directly, the infection process of some rhizoid strains is acid sensitive.
