Mada za sehemu hiiSoil ChemistryMada 5
Soil pH is the degree measure of the acidic and basic nature of the soil. The knowledge of soil pH helps us to understand soil acidity or alkalinity.
Importance of soil pH
- Soil pH influences the activities of soil organisms.
- Soil pH affects the availability of mineral nutrients in the soil.
- Soil pH indicates the types and amount of amendments or corrections needed for a particular soil prior to cultivation.
Examples: Soil can be:
- Acidic soil
- Neutral soil
- Basic soil
Note: Acidic soils supply H⁺, and basic soils supply OH⁻. A soil with a high concentration of hydrogen ions becomes acidic, while a high concentration of hydroxyl ions (OH⁻) makes it basic.
Acidic soils
This type of soil has a dominant or excessive concentration of hydrogen ions. Acidic soils have a pH less than 7.
Basic soils
In basic soils, the concentration of hydroxyl ions (OH⁻) is dominant or excessive. These soils have a pH greater than 7.
Neutral soils
Neutral soils have a pH of 7, which occurs when neutralization takes place.
Soil acidity is the condition where the concentration of hydrogen ions (H⁺) in the soil becomes dominant.
Types of soil acidity
- Active acidity: This occurs when the soil pH drops below 1. It is very dangerous for soil health and can lead to plant death.
- Potential acidity: This occurs when the soil pH is greater than 1.
The pH of a soil is calculated as:
Example: If H⁺ = 4, then:
Causes of soil acidity
- Rainfall: Produces dilute carbonic acid () or dilute nitric acid.
- Application of fertilizers: Some fertilizers, such as ammonium sulfate, are acidic in nature.
- Soil living organisms: Organisms produce carbon dioxide gas, which combines with soil water to form carbonic acid.
- Decomposition: Releases carbon and nitrogen, which combine with oxygen to form acids.
Sources of hydrogen ions in the soil
- Presence of metals: For example, aluminum reacts with water to produce free hydrogen ions:
- Irrigation: Water dissociates to form H⁺ and OH⁻.
- Acidic compounds in the soil: For example, dissociates as:
Sources of hydroxyl ions in the soil
- Alkaline metals: Sodium (Na), potassium (K), calcium (Ca), etc., react with water to form alkaline solutions, supplying OH⁻.
- Irrigation: Excessive water promotes hydroxyl ion production, making the soil basic.
- Presence of basic components: Basic compounds in the soil contribute to OH⁻.
Definition: Liming is the addition of lime to the soil to neutralize acidity.
Reaction: Acid + Base → Salt + Water (Neutralization)
Examples of lime
- Limestone ()
- Quick lime (CaO)
- Magnesium oxide (MgO)
- Calcium hydroxide ()
- Magnesium hydroxide ()
Classification of liming materials
- Carbonates: e.g.,
- Hydroxides: e.g.,
- Oxides: e.g., CaO
Advantages of liming
- Neutralizes soil acidity.
- Increases soil fertility by improving soil structure and pH balance.
- Soil living organisms
- Soil dead organic matter
- Soil minerals
- Soil water
- Soil air
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