Crops also vary greatly in their tolerance to the various components of acidity. As a result, the cause of soil acidity damage to crops is often rather complex. The following discussion and Table 1 describe the effect of soil acidity on crops at various pH levels. The effects of acidity on crops can be divided into two general categories - direct effects and indirect effects. Direct effects Acid soils often contain soluble forms of aluminum and manganese.
Aluminum toxicity restricts root growth and phosphorous uptake.
Manganese toxicity causes black necrotic spots or streaks on leaves of cereals and chlorosis on leaf margins and cupping of leaves of canola and legumes. Aluminum and manganese toxicity often reduce the yield of crops grown on acid soils. Soil acidity also has a direct effect on the survival and growth of rhizobium bacteria which fix nitrogen in association with legumes. The rhizobium bacteria associated with alfalfa and sweet-clover are especially sensitive to acidity. Indirect effects The application of lime to acid soils can affect biological, chemical, and physical properties of the soils.