Soil pH extremes.

Most soils cultivated for crop production fall within the pH range of pH 6-8, where nutrient availability to the plant is optimal. Acid soils (pH<5.5) and alkaline soils (pH>8), however, fall outside this optimal pH range and pose challenges for the plant such as low nutrient availability, ion toxicities and nutrient imbalances. The characteristics of acid and alkaline soils are described. Among the alkaline soils one needs to differentiate between calcareous (pH>7.5) and sodic (ESP>15) soils, as they present a different set of challenges. Most nutrients are not equally available to plants across the pH spectrum. Several mineral nutrients are severely affected in these non-optimal pH soils, particularly Ca, K, P and Fe. The reactions of plants to these nutrient elements under extreme soil pH conditions are discussed in detail, with emphasis on plant growth, morphological, physiological and membrane transport processes. Finally, a special case is presented of the recently discovered complex interactions between salinity, boron-toxicity and pH in plants.