5.1G Soil properties (HL) Notes

Differences Between Soils Rich in Sand, Silt, or Clay

Particle Size and Composition

1. Soils contain mineral particles of three size classes.
2. Sand is the largest particle, typically 0.05 to 2 mm in diameter.
3. Silt is intermediate in size and feels smooth or flour-like.
4. Clay is the smallest particle, often less than 0.002 mm in diameter.
5. Sand and silt are mainly made of quartz, a mineral of silicon and oxygen.
6. Clay is formed from complex silicate minerals containing elements such as aluminium, magnesium or iron.
7. The small size of clay particles creates very large surface areas, making them chemically reactive.
8. Sand contains very few charged sites, so interacts weakly with nutrients.
9. Clay contains many negatively charged sites, giving it strong attraction to nutrient cations.

Cation Exchange Capacity (CEC)

1. CEC is the ability of soil to hold positively charged ions such as calcium, magnesium and potassium.
2. Sand and silt have low CEC because quartz has few negative charges.
3. Clay and humus have high CEC, meaning they hold nutrients and prevent them from leaching.
4. Soil colloids such as clay and humus form a clay–humus complex, the main site of chemical exchange.
5. Plants absorb nutrients when root hairs release hydrogen ions that replace nutrient cations attached to clay particles.
6. Soils with high CEC are generally more fertile, provided aeration allows plants to access nutrients.
7. Sandy soils lose nutrients easily because cations are weakly attached and wash away in rain.
8. Clay soils retain nutrients but are often difficult to cultivate because of poor aeration and waterlogging.

Fertility Differences Among Soil Types

Sandy soils

1. Very low nutrient retention
2. Fast drainage and poor water holding
3. Require organic matter to improve CEC

Silty soils

1. Moderate fertility and water holding
2. More nutrient retention than sand

Clay soils

1. High nutrient retention due to high CEC
2. Slow drainage and tendency to become waterlogged
3. Can become compacted and limit root growth

Determining Soil Properties

1. Soil Texture: Percent Sand, Silt and Clay

1. Soil texture controls water retention, aeration and nutrient supply.
2. Measured by feel tests, mechanical sieving or sedimentation.
3. Classified using the soil texture triangle into categories such as loam, sandy clay, or silty clay loam.
4. Texture strongly influences crop suitability, infiltration rate and erosion risk.

2. Organic Matter Content

1. Organic matter improves water retention, especially in sandy soils.
2. Improves drainage in clay soils by creating stable aggregates.
3. Increases microbial activity, enhancing decomposition and nutrient cycling.
4. Acts as a pH buffer, stabilising acidity and alkalinity.
5. Boosts fertility by contributing negative charges that increase CEC.

3. Soil Moisture Content

1. Soils hold varying amounts of water, influenced by rainfall, evaporation and soil structure.
2. Clay retains water, often remaining cold and wet for long periods.
3. Sand drains quickly, becoming dry soon after rainfall.
4. Organic matter helps retain moisture, reducing rapid evaporation.
5. Waterlogging reduces oxygen, slowing respiration and decomposition.

4. Infiltration Rate

1. Indicates how quickly water enters soil from the surface.
2. High in sandy soils due to large pore spaces.
3. Low in clay soils, which can lead to runoff and erosion.
4. Compaction reduces infiltration, harming crop growth and increasing flooding risk.
5. Measured using a metal ring and timed water drop tests.

5. Bulk Density

1. Bulk density is mass of dry soil per unit volume.
2. Low bulk density indicates high porosity, allowing root penetration and aeration.
3. High bulk density suggests compaction, hindering root growth.
4. High organic matter lowers bulk density, improving soil health.

6. Soil Colour

1. Black soils indicate high humus content.
2. Red soils contain well-aerated iron compounds.
3. Yellow soils contain hydrated iron compounds.
4. Grey or blue soils indicate poor drainage, common in gleyed soils.
5. White crusts signify salinization, often from evaporation in dry regions.
6. Colour measured using the Munsell colour chart for standardization.

7. Soil pH

1. pH influences nutrient availability, decomposition rates and microbial activity.
2. Acidic soils may release toxic aluminium ions, harming plant roots.
3. Weathering, rainfall and fertilizers alter pH.
4. Limestone raises pH, reducing soil acidity.
5. Different nutrients dissolve more readily at specific pH levels.

Carbon Release from Soils

1. Soils store huge amounts of carbon in organic matter and humus
2. Aerobic decomposition releases carbon dioxide
3. Anaerobic decomposition releases methane
4. Wetlands, peatlands, and rice paddies are major methane sources due to low oxygen
5. Carbon release accelerates under warmer temperatures, increasing greenhouse gas emissions

Human Activities Increasing Carbon Release

1. Ploughing exposes soil to oxygen, accelerating decomposition and releasing CO₂
2. Draining wetlands introduces oxygen, causing stored carbon to oxidize
3. Intensive farming increases microbial activity and CO₂ emissions
4. First year of cultivation causes the highest carbon loss, especially in former grasslands
5. Heavy machinery adds additional CO₂ through fuel combustion

Tipping Points and Positive Feedback

1. Warming causes faster decomposition, which releases more greenhouse gases
2. More greenhouse gases cause further warming
3. This forms a positive feedback loop
4. Carbon-rich soils in tundra and boreal biomes may release methane rapidly if temperatures rise
5. Methane clathrates, huge stores of methane in ice-like structures, may destabilize with warming