The unproductiveness of acid and alkali soils is very often due to the lack of available plant nutrients. In highly acid soils (low pH), the availability of some of the nutrients such as aluminum, iron, manganese etc., is increased to a point to become toxic to the plant. At the same time the supplies of available calcium, nitrogen, phosphorus etc., are reduced to starvation level (become unavailable). The same is the case at high pH (alkaline conditions), plant growth suffers due to the unavailability of nutrients like nitrogen, phosphorus and some minor elements (e.g., iron, manganese, boron etc). Another indirect effect occurs through the activity of microorganisms. Most microorganisms function at their best within a pH range 6.0 to 7.5. If soil reaction is changed beyond this range, the microorganisms become functionless. Consequently the supply of some of the essential plant nutrients like nitrogen is considerably reduced.
1. Nitrogen: Plant absorbs most of their nitrogen in the form of nitrate of which availability depends on the activity of nitrifying bacteria. The micro- organisms responsible for nitrification are most active when the pH is between 6.5 and 7.5. They are adversely affected if the pH falls below 5.5 and rises above 9.0. Nitrogen fixing bacteria (like Azotobactor) also fail to function below pH 6.0. The decomposition of organic matter which is the primary source of nitrogen is also slowed down under acidic condition.
2. Phosphorus: Its availability is at its highest when the reaction is between 6.5 and 7.5. When the reaction is above or below this range, availability is reduced. In the strongly acidic soil (pH 5.0 or less), iron, aluminum, manganese and other bases are present in a soluble state and in more quantity. The phosphates of these elements are formed and become unavailable.
3. Potassium: The availability of potassium does not influence by soil reaction to any great extent. In acid soil potassium is lost through leaching. The unavailability of K is due to the conversion of exchangeable to non-exchangeable potassium in alkaline soil. Particularly if the alkalinity is due to CaCO3 (brought about by over liming in acid soil), the solubility of soil potassium is depressed.
4. Calcium and magnesium: Acid soils (base unsaturated) are poor in / available calcium and magnesium. In alkaline soil (pH not exceeding 8.5) the availability of Ca and Mg nutrients are always high. When the pH is above 8.5, the availability of these nutrients again decreases.
5 Iron, aluminum and manganese: When the pH is low the solubility of “iron, aluminum and manganese compounds are increased. and hence they are readily available in acid soils. At the pH range 5.5 to 7.0, iron and manganese are present in the soluble ferrous (Fe++) and manganous (Mn++) forms. At pH below 5.5 the solubility of these compounds considerably increased with the result that they have a toxic influence on plant growth. Under neutral and alkaline conditions, iron and manganese are usually present in ferric (Fe3+ ) and manganese (Mn++++) states. Hence in soils with pH 7.5 and above, they become unavailable and sometimes produce deficiency diseases like chlorosis in plants.
6. Sulphur: The availability of sulphur is not affected by soil reaction as sulphur compounds are soluble. in low pH range. However, it is more soluble in acid soil and lost in leaching. Acid conditions, which retard the decomposition of organic matter, therefore, retard the release of available sulphur. The availability of sulphur present in organic matter depends upon the decomposition of organic matter.
7. Micronutrients: In general, the availability of boron, copper and zinc is reduced in alkaline soils and that of molybdenum in acid soils. The availability of these nutrients progressively decreases as the soil pH increases. Their availability also decreases under highly acid condition when the pH is below 5.0. Zinc availability in alkaline soils from insoluble zinc salts (calcium zincates) is reduced. Zinc and copper are adsorbed on the clay colloids and not easily displaced and hence not available for plant growth. The availability of molybdenum is reduced under acid soils. It is more available in neutral and alkaline soils.
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