Unit 2: Chemical Properties of Soil

1. Nutrient Status

• Definition: Nutrient status refers to the soil’s fertility and its ability to provide essential nutrients for plant growth.

• Macronutrients (needed in large amounts):
  • Primary: Nitrogen (N), Phosphorus (P), Potassium (K)

  • Secondary: Calcium (Ca), Magnesium (Mg), Sulfur (S)

• Micronutrients (needed in small amounts):
  • Boron, Copper, Iron, Chloride, Manganese, Zinc

• Effect on Fertility: More nutrients result in higher fertility and better crop production. Fewer nutrients lead to poor soil fertility.

2. Soil pH

• Definition: Soil pH determines the acidity or alkalinity of the soil, also known as soil reaction. It is based on the concentration of hydrogen ions (H⁺) and hydroxyl ions (OH⁻) in the soil.

• Influence on Plant Growth: Soil pH affects the availability of nutrients to plants. Acidic soils (pH < 7) can hinder the availability of certain nutrients, while alkaline soils (pH > 7) can limit the availability of others.

• Factors that Increase Soil Acidity:
  • Carbon dioxide from microorganisms

  • Breakdown of organic matter

  • Excessive fertilizer application

• Factors that Cause Soil Alkalinity:
  • Application of lime

  • Use of fertilizers such as calcium ammonium nitrate (CAN)

• pH Scale: The pH scale ranges from 0 to 14:
  • pH < 7: Acidic

  • pH = 7: Neutral

  • pH > 7: Alkaline

3. Salinity

• Definition: Salinity refers to the salt content in the soil. High salt concentrations can lead to soil degradation and poor crop yields.

• Causes of Salt Build-up:
  • Irrigation

  • Parent rock material

  • Use of certain fertilizers

  • Compost and manure

• Impact on Crops: Excessive salts reduce water uptake by plants, leading to dehydration, stunted growth, and potentially the death of crops.

4. Cation Exchange Capacity (CEC)

• Definition: Cation Exchange Capacity is the soil’s ability to retain, attract, and exchange positively charged ions (cations). Cations include:
  • Calcium (Ca²⁺), Magnesium (Mg²⁺), Potassium (K⁺), Ammonium (NH₄⁺), and Iron (Fe²⁺)

• Importance: Soils with a high CEC can store more nutrients, making them more fertile. Clay and organic matter have higher CEC, while sandy soils have lower CEC due to the absence of organic matter.

• Effect on Soil Fertility: High CEC allows soils to retain more nutrients, ensuring that plants have a constant supply. Soils with low CEC leach nutrients more easily, leading to reduced fertility.

Factors Affecting Soil pH

1. Parent Material: Soils from limestone are more basic, while soils from quartz-rich sandstone are more acidic.

2. Fertilizers: Some fertilizers release hydrogen ions, which can lower soil pH.

3. Leaching: Heavy rainfall can leach basic ions, reducing soil pH.

4. Vegetation: Some plants, like conifers, produce organic acids that lower soil pH.

Ways to Modify Soil pH

• To Raise pH (Increase Alkalinity):
  • Apply agricultural lime (calcium carbonate)

  • Use certain fertilizers (e.g., ammonium sulfate) to gradually lower acidity

• To Lower pH (Increase Acidity):
  • Apply organic manure or certain acidic fertilizers

Factors Affecting Nutrient Status of Soil

1. Parent Material: Soils derived from rocks with few nutrients result in low fertility.

2. Soil pH: Soil pH affects the solubility and availability of nutrients.

3. Leaching: Loss of essential nutrients from the soil, especially in areas with heavy rainfall.

4. Soil Erosion: Erosion removes topsoil and nutrients, reducing fertility.

5. Crop Removal: Harvesting crops or clearing land depletes the soil of nutrients.

6. Nutrient Uptake by Plants: As plants take up nutrients, soil nutrient levels decrease.

7. Cultivation Methods: Practices like shifting cultivation can reduce soil fertility due to the burning of vegetation, which causes nutrient loss.

Effects of Chemical Properties on Crop Production

1. Nutrient Status: A decrease in nutrient status leads to poor soil fertility and low crop yields, while high nutrient status supports healthy crop growth.

2. Soil pH: Affects the availability of nutrients, microbial activity, and plant growth. Acidic soils may increase the levels of toxic elements like iron (Fe), aluminum (Al), and manganese (Mn), while other nutrients become less available.

3. Salinity: High salinity affects water uptake, causing dehydration and stunted plant growth. It also reduces nitrogen uptake and can lower crop quality.

4. Cation Exchange Capacity: High CEC enhances soil fertility by retaining and supplying more plant nutrients, leading to better crop growth. Sandy soils, with lower CEC, are less fertile and may not support healthy crops.