Soil fraction refers to the mineral components of soil separated by size. Specifically, it commonly refers to the proportions of sand, silt, and clay that make up the mineral part of the soil.
Understanding Soil Fractions: Sand, Silt, and Clay
The composition of soil, particularly the proportions of these three size fractions, significantly impacts its properties and suitability for different uses. A balanced mix is typically ideal, as soils comprised predominantly of a single fraction often present challenges.
- Sand: Sand particles are the largest of the three fractions. They contribute to good drainage and aeration but have low water and nutrient retention. A sandy soil will feel gritty.
- Silt: Silt particles are intermediate in size between sand and clay. They offer a balance of drainage and water retention and contribute to soil fertility. Silt feels smooth.
- Clay: Clay particles are the smallest. They have excellent water and nutrient retention but can lead to poor drainage and compaction. Clay feels sticky when wet and hard when dry.
Why Soil Fractions Matter
The relative proportions of sand, silt, and clay influence many critical soil properties, including:
- Water-holding capacity: Clayey soils hold more water than sandy soils.
- Drainage: Sandy soils drain more quickly than clayey soils.
- Aeration: Sandy soils are better aerated than clayey soils.
- Nutrient retention: Clay particles have a high surface area, enabling them to retain more nutrients.
- Workability: The ease with which soil can be tilled or cultivated.
Ideal Soil Composition
The ideal soil composition depends on the intended use. For example, agricultural soils often benefit from a loamy texture, which is a balanced mixture of sand, silt, and clay. This balance provides adequate drainage, water retention, aeration, and nutrient availability for plant growth. A soil texture triangle is often used to visually determine the soil type based on percentage of sand, silt and clay.
Addressing Imbalances in Soil Fractions
If a soil is predominantly sand, it can be improved by adding organic matter or clay to increase its water-holding capacity and nutrient retention. If a soil is predominantly clay, it can be improved by adding sand or organic matter to improve drainage and aeration.
