Steps on How Soil Become Acidic

The following ways are how soil becomes acidic;

Decomposition of Soil Organic Matter (SOM)

A recent report on the effect of organic matter on soil acidity has not been consistent. Decomposition of SOM has been known to lead to the introduction of organic and inorganic acids with concomitant increases in soil acidity.

However, the studies of Agbin and Adeoye (1991) showed that cassava peel (CP) had a positive effect on the pH of acid soil which had low buffer capacity. On the other hand, when (NH₄)₂SO₄ fertilizer at 120 kg/ha was imposed on the CP treatment, the pH was depressed.

The same studies showed that rice husk depressed soil pH. Studies with other residues by the same workers revealed that the significant pH improvement obtained with lighter soils was not observed in heavier ones.

Incubation studies using glucose–C (Agbede, 1984) revealed that Glucose–C significantly lowered soil pH (r = 0.963). Differences in the effect of glucose-C and soil pH due to soil types were also observed.

 The pH of Insch soil (light textured) decreased with glucose application; whereas, in Tillycorthie and Craden Bay soils (medium and heavy textured) added glucose tended to increase pH during the incubation period.

Carbon Dioxide, CO₂

The release of carbon dioxide, and CO2 to the soil atmosphere by (a) respiration of plant roots and (b) microbes could result in the production of a weak acid.

The production of acid from CO₂ depends greatly on the solubility of CO₂ in water, which in turn also depends on (a) partial pressure of CO₂ in the soil and (b) soil temperature. The partial pressure of CO₂ is related to the presence of CO₂ and other gases in the soil.

In neutral or alkaline soils containing large quantities of carbonates or bicarbonates and in equilibrium with CO₂ at the pressure of normal above-ground air, the pH is 3.5. A decrease of CO₂ partial pressure in such soil atmosphere to 0.02 atmosphere, gives a pH drop to about 7.5.

Read Also: Soil Reaction: Meaning, Acidity, and Liming

Oxidation of Ammonium Compound

Oxidation of ammonium compound in the soil produces hydrogen ions, H⁺, which lower soil pH. For example, NH⁺₄ could be released into the soil by the decomposition of SOM and by the addition of ammoniacal fertilizers.

Oxidation of Sulphur

When elemental sulphur is finely ground and mixed with soil it is oxidized to sulfate by sulphur-oxidizing bacteria in the soil.

The overall result of sulphur oxidation is the production of sulphuric acid thus making sulfur an effective agent for increasing soil acidity.

Leaching of Cations

Soil may become acidic through the leaching of cationic elements in the soil most especially potassium (K+), Calcium (Ca2+), magnesium (Mg2+), and sodium (Na+). Leaching affects soil reaction indirectly by impairing the ionic balance when leached cations are more than anions from the soil surface.

To re-establish the balance H+ is released from the exchange complex into soil solution to balance the net positive charges left behind by leached cations. Leaching is more pronounced in areas of high rainfall; hence, there is a negative correlation between rainfall and soil acidity.

However, this trend may not apply to every agroecological zone, especially in well-forested areas with effective nutrient recycling systems. In the fallow farming system in rainforest zones, trees and other deep-rooted plants remove cations from the subsoil by nutrient uptake and bring them back to the soil surface in the form of litter (organic matter).

Removal of Cations by Harvest

As crops are being harvested and removed away from the field base cations are simultaneously being removed away from the soil. For example, a maize grain yield of 5.0 tons/ha leads to the removal of 75kg, 25kg, and 20kg of potassium, calcium, and magnesium respectively from the maize farm.

This harvest may occur over a number of years giving a net result of the soil becoming acidic. As the cation is removed, hydrogen ion, H⁺, is released from the exchange complex to balance the charge as explained for the leaching of cations.

The concentration of Aluminum in the Soil

Each aluminum ion, A1³⁺, released from the exchange site gives three hydrogen ions, 3H⁺. There is the general belief that H⁺ and A1³⁺ exist in equilibrium in acid soils. It should also be noted that aluminum does not just occur in form of A1³⁺ but in a complex form that hydrolyses to release H⁺.

Use of other Acid forming Fertilizers

An acid-forming fertilizer is one capable of increasing the acidity of the soil which is derived mainly from the oxidation of salts or ordinary elements by soil bacteria.

Sulphur is in effect one of the most effective agents for increasing soil acidity. The most common form of bacterial species participating in S- oxidation are the obligate autotrophic aerobes such as Thiobacillus thiooxidans, T. thioparus, and T. Copraliticus.

In areas with little or inadequate liming programs such as in humid tropical regions the continued use of acid-forming fertilizers leads to increase soil acidification. However, on alkaline soils, acid-forming mixed fertilizers like ammonium sulphate or elemental sulphur may be used to depress the soil pH.

Read Also: Soil Temperature and Plant Growth