For SL and HL: Soil contains inorganic components, mainly mineral particles, water and air, and organic components, including living organisms, dead organic matter and humus. Together, these components make soil a dynamic system that supports terrestrial ecosystems.
The Components of Soil
The mineral component originates from the weathering of parent material such as rock. Mineral particles are classified by size as sand, silt and clay; their relative proportions strongly influence soil drainage, aeration, nutrient retention and productivity.
The organic component comes from organisms and their remains. Decomposers such as bacteria and fungi break down dead material, releasing mineral nutrients and producing humus, a dark, relatively stable form of decomposed organic matter.
| Component | Examples and significance |
|---|---|
| Inorganic mineral matter | Sand, silt and clay; supplies mineral nutrients and determines much of the soil's texture, drainage and water-holding capacity. |
| Soil water | Occupies pore spaces and carries dissolved mineral nutrients that plant roots can absorb. |
| Soil air | Occupies pore spaces not filled by water and supplies oxygen for root and decomposer respiration. |
| Living organic matter | Plant roots, bacteria, fungi and soil animals; contributes to decomposition, nutrient cycling and soil structure. |
| Dead organic matter | Leaf litter, dead roots and animal remains; provides material and energy for decomposers. |
| Humus | Stable, decomposed organic matter; improves nutrient retention, water-holding capacity and soil structure. |
The balance varies among soils. For example, fertile Chernozem soils contain abundant humus, whereas sandy soils generally contain a larger proportion of coarse mineral particles and retain less water and fewer nutrients.
A common misconception is that soil is simply weathered rock. Soil also contains organic matter, water, air and living organisms, all interacting through transfers and transformations.
Exam Technique
For an IB ESS Topic 5.1 question, classify the components accurately and explain at least one function of each. If asked to draw a soil system, use boxes for storages and arrows for flows such as litter fall, decomposition, leaching and mineral uptake.