5.1.1—Soil as dynamic system

A soil system needs a stated boundary so stores, inputs, outputs and internal flows can be tracked.

Choose the depth and area first. Then label water, organic matter, minerals and organisms as stores; rainfall, litter and fertilizer as inputs; leaching, erosion and gases as outputs.

For a farm plot, fertilizer enters, nitrate leaches downward, carbon is stored in humus and CO₂ leaves through respiration.

Inputs and outputs change when depth, area or time window changes.

A system diagram without a boundary cannot show whether matter is entering, leaving or merely moving internally.