Differences Between Soils Rich in Sand, Silt, or Clay
Particle Size and Composition
- Soils contain mineral particles of three size classes.
- Sand is the largest particle, typically 0.05 to 2 mm in diameter.
- Silt is intermediate in size and feels smooth or flour-like.
- Clay is the smallest particle, often less than 0.002 mm in diameter.
- Sand and silt are mainly made of quartz, a mineral of silicon and oxygen.
- Clay is formed from complex silicate minerals containing elements such as aluminium, magnesium or iron.
- The small size of clay particles creates very large surface areas, making them chemically reactive.
- Sand contains very few charged sites, so interacts weakly with nutrients.
- Clay contains many negatively charged sites, giving it strong attraction to nutrient cations.
Cation Exchange Capacity (CEC)
Definition
Cation-exchange capacity (CEC)
Cation-exchange capacity (CEC) is the ability of a soil to hold positively charged ions due to negatively charged particles like clays and humus.
- CEC is the ability of soil to hold positively charged ions such as calcium, magnesium and potassium.
- Sand and silt have low CEC because quartz has few negative charges.
- Clay and humus have high CEC, meaning they hold nutrients and prevent them from leaching.
- Soil colloids such as clay and humus form a clay–humus complex, the main site of chemical exchange.
- Plants absorb nutrients when root hairs release hydrogen ions that replace nutrient cations attached to clay particles.
- Soils with high CEC are generally more fertile, provided aeration allows plants to access nutrients.
- Sandy soils lose nutrients easily because cations are weakly attached and wash away in rain.
- Clay soils retain nutrients but are often difficult to cultivate because of poor aeration and waterlogging.
| Property | Sand | Silt | Clay |
|---|---|---|---|
| Particle Size | Largest (0.05–2.00 mm) | Medium (0.002–0.05 mm) | Smallest (<0.002 mm) |
| Texture | Gritty | Smooth | Sticky |
| Water Retention | Low | Moderate | High |
| Drainage | Excellent | Moderate | Poor |
| Cation-Exchange Capacity | Low | Low | High |
| Nutrient Availability | Low | Moderate | High |
| Workability | Easy to work | Moderate | Difficult |
Fertility Differences Among Soil Types
Sandy soils
- Very low nutrient retention
- Fast drainage and poor water holding
- Require organic matter to improve CEC
Silty soils
- Moderate fertility and water holding
- More nutrient retention than sand
Clay soils
- High nutrient retention due to high CEC
- Slow drainage and tendency to become waterlogged
- Can become compacted and limit root growth
- Think of sand like a basket with big holes, silt like a colander with medium holes and clay like a cloth bag with tiny pores.
- The smaller the pores, the more water and nutrients stay inside.
Determining Soil Properties
1. Soil Texture: Percent Sand, Silt and Clay
- Soil texture controls water retention, aeration and nutrient supply.
- Measured by feel tests, mechanical sieving or sedimentation.
- Classified using the soil texture triangle into categories such as loam, sandy clay, or silty clay loam.
- Texture strongly influences crop suitability, infiltration rate and erosion risk.
2. Organic Matter Content
- Organic matter improves water retention, especially in sandy soils.
- Improves drainage in clay soils by creating stable aggregates.
- Increases microbial activity, enhancing decomposition and nutrient cycling.
- Acts as a pH buffer, stabilising acidity and alkalinity.
- Boosts fertility by contributing negative charges that increase CEC.
3. Soil Moisture Content
- Soils hold varying amounts of water, influenced by rainfall, evaporation and soil structure.
- Clay retains water, often remaining cold and wet for long periods.
- Sand drains quickly, becoming dry soon after rainfall.
