Fundamentals of Soil Science (1+1)

Factors Affecting Soil pH

Factors affecting soil pH:
Percent base saturation:

• A low percent base saturation means acidity, whereas a percent base saturation of 50-90 will result in to neutrality or alkanity.

Nature of soil colloids:

• The colloidal particles of the soil influence soil reaction to a very greatest extent.
• Different types of colloids vary in their pH at the same percent base saturation . This is due to the difference in the ability of different colloids to release H⁺ ions to the soil solution. For example at the same percent base saturation the smectite has much lower pH than kaolinite.
• When hydrogen (H⁺) ion forms the predominant adsorbed cations on clay colloids, the soil reaction becomes acid.

Soil solution:

• 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.

Adsorbed basic cations:

• The comparative quantity of exchangeable Ca, Mg, Na and K adsorbed on the colloids will determine the pH. The dominance of Na⁺ will raise pH much higher than other basic cations.

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).

Native Vegetation:

• Soils become more acidic when develops under conifer ecosystem
• 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.

Nitrogen fertilization:

• Nitrogen from fertilizer, organic matter, 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.

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.