Soil Composition and Properties (HL) (DP IB Environmental Systems & Societies (ESS))

Soil Composition and Properties

• Soil is a mixture of mineral particles, organic matter, water, and air

  • Mineral particles include sand, silt, and clay

    • These vary in size and chemical properties

  • Organic matter comes from decomposed plants and animals

    • This improves soil fertility

  • Water and air occupy spaces between soil particles

    • This is essential for plant growth and microbial activity

Differences between sand, silt and clay

• Sand:

  • Made up of the largest soil particles

    • Sand particles are less than 2 mm in diameter

    • Derived mainly from quartz

  • Has low cation-exchange capacity (CEC)

    • This means it does not hold onto nutrients well

  • Drains quickly due to large particle size

    • This limits its water retention

• Silt:

  • Composed of medium-sized particles

    • Silt particles are less than 0.02 mm

    • Also often derived from quartz

  • Also has a relatively low CEC (so holds fewer nutrients)

  • Feels smooth to the touch

  • Retains more water than sand but drains better than clay

• Clay:

  • Contains the smallest particles

    • Clay particles are less than 0.002 mm

  • Has a high CEC

    • This allows it to hold onto and release nutrients like calcium, magnesium, and potassium

  • Has a fine, sticky texture

  • Drains poorly but retains water well

Soil structure

• Soil structure refers to the arrangement or shape of soil particles

  • Soil structure has a direct impact on primary productivity

• Clay soils have a high potential for nutrient exchange due to their large surface area relative to volume

  • However, they tend to become waterlogged and are often described as 'cold' or 'heavy'

  • In periods of drought, clay soils can shrink, leading to structural damage (e.g. cracking)

• Sandy soils have excellent drainage capabilities

  • They are commonly referred to as 'light' soils

• Silt soils can be easily compacted if ploughed when wet

  • This can negatively affect soil structure and plant growth

• Loam soils are a balanced combination of sand, silt, and clay

  • They are often considered the most favourable for cultivation

  • They are easy to work with, drain well, retain moisture and nutrients, and provide good aeration

  • As a result, loam soils have the highest potential for primary productivity (plant growth)

Soil Properties Summary Table

Soil texture triangles

• A soil texture triangle is a graphical tool used to classify soil types based on their relative proportions of sand, silt, and clay

  • The three side of the triangle represent the proportions of the different soil particle sizes: sand, silt, and clay

  • The soil texture triangle allows for easy visualisation and classification of soil types based on their particle size distribution

  • By locating the percentage of sand, silt, and clay on the triangle, it is possible to determine the textural class of a soil sample (e.g. sandy loam, silty clay, etc.)

Key soil properties

• Texture:

  • Soil texture describes the proportions of sand, silt, and clay

  • This information can help to predict how soil behaves in terms of water and nutrient retention

• Organic matter:

  • Organic matter improves soil structure, water-holding capacity, and nutrient availability

  • Healthy soils contain organic matter from decomposing plants and animals, vital for supporting plant growth

• Water content and infiltration:

  • Soil water content measures how much water the soil can hold, which depends on texture and organic matter

  • Infiltration rate measures how quickly water enters the soil, with sandy soils allowing faster infiltration than clay-rich soils

• Bulk density:

  • Bulk density measures the mass of soil per unit volume

    • This indicates the level of compaction

  • Lower bulk density is ideal for plant growth as it suggests good soil structure and easier root penetration

• Soil colour:

  • Soil colour provides clues about its composition and conditions

    • Dark soils are often rich in organic matter, indicating high fertility

    • Red or yellow soils contain iron oxides, indicating good aeration

    • Greyish-blue colours indicate waterlogging

• pH:

  • Soil pH measures how acidic or alkaline the soil is, affecting nutrient availability

    • Most plants grow best in neutral to slightly acidic soils (pH 6–7)

    • Acidic soils (below pH 6) limit nutrient availability

    • Alkaline soils (above pH 7) often leading to nutrient imbalances