IB Environmental systems and societies 5.1A Soil System Notes

Soil Is a Dynamic System of Storages and Flows

1. Soil has five components: mineral particles, organic matter, water, air, and living organisms.
2. Their proportions shift with climate, parent rock, vegetation, land use, and time.
3. Because these components interact, soil behaves as a self-regulating system that responds to change.

The Five Components of Soil

Mineral Particles

1. Come from the weathering of parent rock and set the soil's texture (sand, silt, and clay).
2. They strongly influence fertility, pH, structure, and water-holding capacity.
3. Sand drains fast but holds few nutrients, while clay has a high surface area that grips water and nutrients.

Soil texture and its effect on productivity are covered in detail in 5.1C.

Organic Matter

1. Includes living organisms, fresh plant and animal residues, and humus (well-decomposed material).
2. Humus binds particles into stable aggregates that improve soil structure.
3. It raises nutrient-holding capacity, water retention, and aeration, and feeds soil organisms.

Water and Air

1. Soil water arrives from precipitation, irrigation, and capillary rise from groundwater.
2. Water fills pores and drives chemical reactions, nutrient transport, and weathering.
3. Air fills the pores not taken by water and supplies the oxygen roots and decomposers need.
4. Waterlogged soils lose their air, forcing anaerobic processes like denitrification that lower fertility.

Living Organisms

1. Range from bacteria and fungi to earthworms, nematodes, mites, and plant roots.
2. They decompose organic matter, cycle nutrients, and build humus and structure.
3. Earthworms raise infiltration and aeration, while mycorrhizal fungi improve phosphorus uptake.

Modelling Soil: Inputs, Outputs, Storages, Transfers, and Transformations

Inputs (matter entering the system)

1. Organic matter
   1. Leaf litter, dead plants and animals, and root exudates.
2. Parent material
   1. Minerals released from weathered bedrock.
3. Water and gases
   1. Precipitation and runoff, plus oxygen, nitrogen, and carbon dioxide from the atmosphere.
4. Human inputs
   1. Compost and manure, and inorganic fertilisers that can also cause leaching and acidification.

Outputs (matter leaving the system)

1. Leaching: water carries dissolved nutrients down and out of the root zone.
2. Gaseous losses: carbon dioxide from respiration, methane from waterlogged soils, and nitrous oxide from denitrification.
3. Erosion: wind and water strip particles away, removing nutrient-rich topsoil first.
4. Crop removal: harvesting takes nutrients stored in plants out of the system.

Storages

1. Soil stores organic matter, mineral particles, water, air, and nutrients.
2. The size of each store sets the soil's texture, structure, fertility, porosity, and water-holding capacity.

Transfers (material moved without changing form)

1. Infiltration and percolation move water down into and through the soil.
2. Biological mixing by earthworms and termites blends the layers.
3. Translocation carries minerals between horizons in moving water.

Transformations (material changed chemically or biologically)

1. Decomposition breaks organic matter into simpler compounds.
2. Humification turns decomposed matter into stable humus.
3. Mineralization releases inorganic nutrient ions that plants can absorb.
4. Weathering breaks parent rock into minerals and clay.
5. Nitrogen transformations (fixation, nitrification, denitrification) convert nitrogen between forms.

Soil Forms Slowly, So It Is Effectively Non-Renewable

1. A mature soil with distinct layers takes hundreds to thousands of years to form.
2. Because it cannot be rebuilt on human timescales, soil is treated as a non-renewable resource.

The soil profile and its horizons are covered in 5.1E.