After reading this article you will learn about the influence of soil reaction on availability of nutrients:- 1. Nitrogen 2. Phosphorus 3. Potassium 4. Calcium and Magnesium 5. Iron, Aluminium and Manganese 6. Sulphur 7. Micronutrients.
Plant absorbs most of their nitrogen in the form of nitrate whose availability depends on the activity of nitrifying bacteria. The microorganisms 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 greater than 9.0 (Fig. 7.3).
Nitrogen-fixing bacteria (like Azatobactor) also fails to function below pH 6.0. The decomposition of organic matter which is the primary source of nitrogen also slows down under acidic condition.
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, aluminium, manganese and other bases are present in a soluble state and in more quantity.
The phosphate ions react with these bases (iron, aluminium etc.) and insoluble phosphates of these elements are formed and become unavailable.
The phosphates react with hydrated oxides of iron and aluminium and form insoluble hydroxy-phosphates of iron and aluminium. Unavailability of phosphorus is called phosphorus- fixation. Fixation of phosphate takes place even when the soil is alkaline (high pH). Phosphate ion combines with calcium ion and calcium (or magnesium) carbonates and, form insoluble calcium (or magnesium) phosphate.
The reaction is as follows:
The availability of phosphorus at different pH is linked with the ionic form in which it is present in soil solution. The monovalent H2 PO4– ions predominate in highly acid (at pH 4.0-5.0) solutions. With decreasing acidity, the divalent HPO4—ions begin to appear. In alkaline soil the trivalent PO4— ions are present in extremely small quantity. At pH 9.0 and above, PO4— is available to plants.
The ionic form has a large influence on the availability of phosphorus to plants. The H2PO4 and HPO4 ions are considered to be more available than PO4 ions. Thus, plants depend for their phosphorus requirement on these two ionic forms to a much greater extent than PO4 ions; H2PO4– at neutrality and below (acid range) and HPO4– at neutrality and above (alkaline range).
In the pH range 6.0 to 7.0, phosphate-fixation is very slight. Consequently at the pH range phosphorus availability is highest. At this pH range only 20-30% phosphorus is actually assimilated by the growing plant, if soluble phosphorus is added in the soil as fertilizer. It is believed that the presence of calcium hinders the absorption of phosphate by plant.
In acid soil (low pH), phosphorus becomes available by anion exchange. Part of the phosphate which has reacted with iron and aluminium compounds is subject to replacement by other anions, such as the hydroxyl ion (OH-). Such replacement is called anion exchange.
It is the reverse reaction of phosphate fixation is acid soil given above:
One anion (OH) has been exchanged for another (H2PO4). Phosphorus (H2PO4-) become available after liming in the acid soil.
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 (or is brought about by over liming in acid soil), the solubility of soil potassium is depressed (results in non-availability).
4. Calcium and Magnesium:
Acid soils (base unsaturated) are poor in available calcium and magnesium.
In alkaline soil (pH not exceeding 8.5) availability of Ca and Mg nutrients is always high. When the pH is above 8.5, the availability of these nutrients again decreases.
5. Iron, Aluminium and Manganese:
When the pH is low the solubility of iron, aluminium and manganese compounds is 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 is considerably increased with the result that they have a toxic influence on plant growth (Fig 7.3).
Under neutral and alkaline conditions, iron and manganese are usually present in ferric (Fe+++) and magnaic (Mn++++) states. Hence, the soils with pH 7.5 and above, they become unavailable and sometimes produce deficiency disease like chlorosis in plants.
The availability of sulphur is not affected by soil reaction as sulphur compounds are soluble in the whole 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.
In general, the availability of boron, copper and zinc is reduced in alkaline soils and that of molybdenum in acid soils. The availability of boron, copper and zinc 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 zincate) which reduces its availability, zinc and copper adsorbed on the clay colloids and are 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.