Soil reaction varies due to following factors
1. Nature of soil colloids: The colloidal particles of the soil influence soil reaction to a very greatest extent. When hydrogen (H+) ion forms the predominant adsorbed cations on clay colloids, the soil reaction becomes acid.
2. Soil solution: The soil solution carries a number of salts dissolved in capillary water. The cations of the salts intermingle with those of the diffuse double layer of the clay particle and increase the concentration. The concentration of cations in bulk of the solution is more or less (or nearly) the same as that near the particle surfaces. For an unsaturated soil (Clay), the more compact the layer the greater is the number of hydrogen ions dissociating into the solution. This increases the acidity of the soil solution or lowers its pH. Under field conditions, the concentration of salts varies with the moisture content of the soil. The more dilute the solution, the higher the pH value. Hence the pH tends to drop as the soil gets progressively dry. Soil reaction is also influenced by the presence of CO2 in soil air. As the CO2 concentration increases, the soil pH falls and increases the availability of the nutrients. Under field conditions, plant roots and micro-organism liberate enough CO2 , which results in lowering the pH appreciably. This principle of increasing the concentration of CO2 in soil air is also used in the reclamation of alkali soils.
3. Climate: Rainfall plays important role in determining the reaction of soil. In general, soils formed in regions of high rainfall are acidic (low pH value), while those formed in regions of low rainfall are alkaline (high pH value).
4. Soil management: Cultural operations in general tend to increase soil acidity. They make an acid soil more acidic, and an alkaline soil less alkaline. As a result of constant cultivation, basic elements are lost from the soil through leaching and crop removal. This leads to change the soil reaction to the acid side.
5. Parent materials: Soils developed from parent material of basic rocks generally have higher pH than those formed from acid rocks (e.g. granite). The influence of parent material is not very important as it is completely masked by the climatic conditions under which the soil is developed.
6. Precipitation: As water from rainfall passes through the soil, basic nutrients such as calcium (Ca) and magnesium (Mg) are leached. They are replaced by acidic elements including Al, H and manganese (Mn). Therefore, soils formed under high rainfall conditions are more acid than those formed under arid conditions.
7. Decomposition of organic matter: Soil organic matter is continuously being decomposed by micro-organisms into organic acids, carbon dioxide (CO2) and water, forming carbonic acid. Carbonic acid, in turn, reacts with the Ca and Mg carbonates in the soil to form more soluble bicarbonates, which are leached away, leaving the soil more acid.
8. Native vegetation: Soils often become more acid when crops are harvested because of removal of bases. Type of crop determines the relative amounts of removal. For example, legumes generally contain higher levels of bases than do grasses. Calcium and Mg contents also vary according to the portion (s) of the plant harvested. Many legumes release H ions into their rhizosphere when actively fixing atmospheric N2. The acidity generated can vary from 0.2 to 0.7 pH units per mole of fixed N.
9. Soil depth: Except in low rainfall areas, acidity generally increases with depth, so the loss of topsoil by erosion can lead to a more acid pH in the plough layer. The reason is that more subsoil is included in the plow layer as topsoil is lost. There are areas, however, where subsoil pH is higher than that of the topsoil.
10. Nitrogen fertilization: Nitrogen from fertilizer, organic matter, and manure and legume N fixation produces acidity. Nitrogen fertilization speeds up the rate at which acidity develops. At lower N rates, acidification rate is slow, but is accelerated as N fertilizer rates increase.
11. Flooding: The overall effect of submergence is an increase of pH in acid soils and a decrease in basic soils. Regardless of their original pH values, most soils reach pH of 6.5 to 7.2 within one month after flooding and remain at the level until dried. Consequently, liming is of little value in flooded rice production. Further, it can induce deficiencies of micronutrients such as zinc (Zn).
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