Understanding Soil Acidity

Soil acidity refers to the measurement of hydrogen (H) and aluminum (Al) cations (positively charged ions) present in soils. When the levels of hydrogen or aluminum become excessive, causing the soil to become overly acidic, it leads to the saturation of the soil’s negatively charged cation exchange capacity (CEC) with the positively charged hydrogen and aluminum ions. As a consequence, the essential nutrients required for plant growth are displaced. The presence of acidic soils poses challenges to production as it restricts the availability of vital plant nutrients.

When the soil pH drops below 5.5 (pH being the indicator of soil acidity where lower pH values correspond to higher acidity), there is a concentration of aluminum that can impede or halt root development. Consequently, plants are unable to absorb water and nutrients, resulting in stunted growth and displaying symptoms of nutrient deficiencies.

Factors Causing Soil Acidity

1. Rainfall and Leaching

Under heavy rainfall, soils often become acidic. This is primarily because rainwater, having a pH of approximately 5.7, undergoes a reaction with atmospheric CO2, forming carbonic acid. When this acidic rainwater percolates through soil pores, it washes away basic cations from the soil in the form of bicarbonates. As a result, the proportion of AI3+ and H+ cations relative to other cations in the soil increases. Additionally, the respiration of roots and the decomposition of organic matter by microorganisms release CO2, which further elevates the concentration of carbonic acid (H2CO3) and contributes to leaching.

2. Nitrogen Fertilizers

Certain fertilizers, such as ammonium (NH4+) fertilizers, undergo a process called nitrification, which converts them into nitrate (NO3−). During this transformation, H+ ions are released, leading to soil acidification. The application of larger quantities of ammoniacal nitrogen fertilizer results in greater soil acidity.

3. Parent Material

Differences in the chemical composition of parent materials contribute to varying rates of soil acidification over time. Soils derived from granite materials are more prone to acidity compared to soils developed from calcareous shale or limestone.

4. Decay of Organic Matter

The decomposition of organic matter generates H+ ions, which contribute to soil acidity. CO2 produced by decaying organic matter reacts with water in the soil, forming a weak acid known as carbonic acid. Although several organic acids are produced during organic matter decay, they are also relatively weak acids.

5. Crop Production

Soil acidity development is influenced by crop harvesting since crops absorb lime-like elements as cations for their nutritional needs. When crops are harvested and their yield is removed from the field, the basic materials that counteract acidity generated by other processes are lost. Consequently, soil acidity increases. Increasing crop yields result in the removal of larger quantities of basic materials. Grain contains fewer basic materials compared to leaves or stems. Hence, continuous wheat pasture, as well as high-yielding forages like bermudagrass or alfalfa, can accelerate soil acidity development compared to other crops.

How is soil acidity corrected?

HUMIPOWER®, a soil conditioner, is utilized as an amendment for acidic soils, particularly when combined with acidifying fertilizers at a ratio of 1 kg per 50 kg bag of basal fertilizer. Its effectiveness extends beyond acidity correction as it also aids in nutrient uptake.

HUMIPOWER® is a water-soluble soil amendment and blend available in granular form, containing potassium, Iron EDDHA, humates, and fulvic acid. It finds application in agriculture, horticulture, field crops, landscaping lawns, pot plants, and gardening. It serves as both a soil amendment and fertilizer blend, providing plants with readily available organic matter that is convenient to use and cost-effective.

To rectify soil acidity, another approach is to lime the soil by adding basic materials to neutralize the existing acid. Agricultural limestone is the most commonly employed liming material due to its affordability and ease of management. It possesses low water solubility, making it convenient for handling. As lime dissolves in the soil, calcium (Ca) migrates to the soil particle surfaces, displacing acidity. The acidic components react with carbonate (CO3) to form carbon dioxide (CO2) and water (H2O). Consequently, the soil becomes less acidic, exhibiting a higher pH. Lime also supplies essential nutrients, enhances the availability of other vital nutrients, and prevents the toxicity of elements like magnesium and aluminum to plants.

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