A large proportion of crop plants can tolerate slight to moderate acidity, a few can tolerate slightly alkaline soils, but very few are able to tolerate moderately alkaline soil conditions. There are some plants that require strongly acid soils, but all plants of agricultural value fail to grow and die in strongly alkaline soils. The preference of plants for soil pH seems to be linked with the type of buffering system present in the cell sap.
Buffering the Soil Reaction:
Buffering refers to resistance to a change in pH. If 1 ml HCl (of 0.01 N) is added to one litre of pure distilled water of pH 7.0, the resulting solution would have a ph. of about 5.0. If on the other hand, this same amount of acid is added to a litre of soil suspension the resulting change in pH would be very small.
There is, a distinct resistance to a change in pH. This power to resist a change in pH is called buffer action. A buffer solution is one which contains reserve acidity and alkalinity and does not change pH with small additions of acids or alkalies.
The colloidal complex thus acts as a powerful buffer in the soil and does not allow rapid and sudden changes in soil reaction. Buffering depends upon the amount of colloidal material present in soil. Clay soils and soils rich in organic matter are more highly buffered than sandy soils.
Buffer capacity of the soil varies with its cation exchange capacity (C.E.C.), the greater the C.E.C. the greater will be its buffer capacity. Thus, heavier the texture and the greater the organic matter content of a soil, the greater the amount of acid or alkaline material required to change its pH.
Importance of Buffering to Agriculture:
Change in soil reaction (pH) have a direct influence on the plants and it also affects the availability of plant nutrients. Deficiency of certain plant nutrients and excess of others in toxic amounts would seriously upset the nutritional balance in the soil. Buffering prevents sudden changes and fluctuations in soil pH, so it regulates the availability of nutrients and also checks direct toxic effect to plants.
Buffer tends to assure reasonable stability in the soil pH and prevents drastic fluctuations that might be detrimental to microorganism and plants. Buffer also influences the amount of chemicals, such as lime application in acid soil.
Soil Reaction and Plant Growth:
Most of the plants can grow on soils with a wide range of pH. However, plants have preference for a specific range of soil pH (Table 7.3). Soil reaction (pH) and plant growth are interrelated. It increases or decreases the availability of nutrient elements. For example, in acid soil (low pH) the concentration of Al+++,Fe+++ or Mn++ increases to the toxic level. High pH (alkaline soil) generally lowers availability of all nutrient elements except molybdenum and boron.