In the soil system, inputs include dead organic matter, inorganic minerals, water and energy; outputs include eroded or leached minerals, decomposed organic matter, water and heat. Storages include organic matter, inorganic mineral particles, soil water, soil air and living organisms. This is core SL and HL content in 5.1 Soil.
A soil is an open system because both matter and energy cross its boundaries. Its components form storages connected by flows, which may be transfers or transformations.
| System category | Main examples | Explanation |
|---|---|---|
| Inputs | Leaf litter, dead organisms, animal waste, weathered mineral material, precipitation and solar energy | Matter or energy entering the soil system |
| Storages | Humus and other organic matter, mineral particles, soil water, soil air and living organisms | Matter held within the soil at a particular time |
| Outputs | Minerals removed by leaching or erosion, organic matter lost through decomposition or erosion, water lost by evaporation or drainage, and heat | Matter or energy leaving the soil system |
A transfer changes the location of matter without changing its chemical form. Examples include infiltration of water into soil, percolation between layers and movement of minerals by leaching.
A transformation changes the state, chemical nature or energy of material. For example, decomposers convert dead organic matter into humus and inorganic nutrients. Cellular respiration also transforms chemical energy, with some energy eventually leaving the system as heat.
Human activity can alter every part of this system. On China's Loess Plateau, vegetation loss and cultivation increased soil erosion, removing mineral and organic storages. Terracing and revegetation reduced outputs from erosion and helped rebuild soil storages, although management decisions also affected farmers' land use and livelihoods.
Exam technique: For a systems diagram, draw storages as rectangular boxes and flows as labelled arrows showing direction. A common misconception is to label decomposition as a storage; it is a transformation, while organic matter is the storage. If asked to distinguish transfers from transformations, refer explicitly to whether matter changes only location or also changes form.