How soil forms and what it contains
Soil forms when weathering breaks rock into small particles and organisms add humus (decayed organic matter). Soil is a mixture of mineral particles (sand, silt and clay), organic matter, water, air and living organisms. A soil profile shows layers called horizons.
| Particle | Size | Properties |
|---|---|---|
| Sand | 0.05–2 mm | Large gaps: drains quickly, holds little water or nutrients |
| Silt | 0.002–0.05 mm | Medium |
| Clay | Less than 0.002 mm | Holds water and nutrients, but drains slowly and may become waterlogged |
Soil pH and nutrient availability
Most crops grow best between pH 6 and 7. In acid soil (pH below 5.5) aluminium and manganese become soluble and toxic and phosphorus is locked up; in alkaline soil iron and some trace elements become unavailable. Soils become acidic through rain, leaching, and the use of ammonium fertilisers. Liming (adding calcium carbonate or calcium hydroxide) raises the pH: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂.
Cation exchange capacity and soil nutrients
Clay and humus have negative surfaces that hold positive ions (Ca²⁺, Mg²⁺, K⁺, NH₄⁺) and exchange them with the soil water. The cation exchange capacity (CEC) measures how many cations a soil can hold. A soil with a high CEC (clay, humus) keeps nutrients better against leaching than sandy soil.
| Nutrient | Needed for | Sign of deficiency | Source |
|---|---|---|---|
| Nitrogen (N) | Leaves and protein | Pale yellow older leaves, stunted growth | Urea, ammonium nitrate, manure, legumes |
| Phosphorus (P) | Roots, flowers, seeds | Purple leaves, poor roots | Superphosphate, bone meal |
| Potassium (K) | Water balance, fruits | Brown leaf edges | Potassium chloride, wood ash |
| Calcium (Ca) | Cell walls | Blossom-end rot in tomatoes | Lime, gypsum |
| Magnesium (Mg) | Chlorophyll | Yellow between the veins | Dolomite |
An NPK fertiliser is labelled with the percentages of nitrogen (N), phosphorus pentoxide (P₂O₅) and potassium oxide (K₂O). Choose the bag mass and the label.
Managing soil
| Practice | Effect |
|---|---|
| Adding compost and manure | Raises humus, CEC and water retention |
| Crop rotation with legumes | Bacteria in the roots fix nitrogen |
| Liming | Raises pH |
| Mulching and terracing | Reduces erosion and keeps moisture |
| Avoiding over-fertilising | Prevents nitrate pollution of water and soil acidification |
Key terms
- Humus
- Dark, decayed organic matter in soil.
- Cation exchange capacity
- The amount of positive ions a soil can hold on its surfaces.
- Liming
- Adding lime to neutralise soil acidity.
- Leaching
- The washing of soluble nutrients down through soil by water.
Summary
- Soil is minerals, organic matter, water, air and organisms arranged in horizons.
- Most crops need a pH of 6 to 7; liming raises the pH.
- Clay and humus hold cations (CEC) and reduce leaching.
- Good soil management uses organic matter, crop rotation and careful fertiliser use.