5.1 Soil

Soil is far more than the dirt beneath our feet. It is a living, self-regulating system that interacts continuously with the atmosphere, hydrosphere, biosphere, and lithosphere. Those interactions make it dynamic: its composition shifts in response to climate, organisms, water movement, and time.

Every soil is built from five components: mineral particles, organic matter, water, air, and living organisms. A well-structured soil is roughly half solid material and half pore space, with that pore space shared between water and air in proportions that shift constantly.

The exact balance of these five components varies with climate, parent rock, vegetation, land use, and age. That variability is why soils across the world look, feel, and behave so differently from one another, and why soil is described as a resource for life rather than an inert substrate.

Understanding what soil is made of is the starting point for understanding how it functions.

Mineral particles originate from the weathering of parent rock. Their size determines texture. Sand particles are the largest and create wide pore spaces that drain quickly but hold few nutrients. Silt particles are intermediate, retaining moisture more effectively. Clay particles are the smallest, with a very large surface area relative to their volume, which gives them the capacity to hold both water and nutrients.

Organic matter takes three forms: living organisms, fresh plant and animal residues, and humus.

Humus improves soil structure, increases water retention and nutrient-holding capacity, and provides the main energy source for soil organisms.

Soil water arrives through precipitation, irrigation, and capillary rise. It fills pore spaces, enables chemical reactions, and carries nutrients to plant roots. The distinction between gravitational water (drains rapidly), capillary water (available to plants), and hygroscopic water (bound too tightly for plant uptake) is important for understanding plant productivity.

Soil air occupies the pore spaces not filled by water. Well-aerated soils support aerobic respiration by roots and decomposers. Waterlogged soils restrict oxygen, promoting anaerobic processes such as denitrification, which reduces soil fertility.

Living organisms include bacteria, fungi, earthworms, nematodes, mites, and plant roots. They decompose organic matter, cycle nutrients, build soil structure, and form humus. Earthworms, for instance, increase infiltration and aeration through burrowing, while mycorrhizal fungi extend the effective surface area of plant roots and enhance phosphorus uptake.

A systems approach to soil means identifying what enters, what leaves, what is stored, and what moves within. Starting with inputs gives a sense of what sustains the soil.

Dead organic matter inputs include plant litter such as leaves, twigs, seeds, and bark; dead animal biomass; and manure. Roots also release organic compounds (root exudates) that feed soil microorganisms directly. Decomposition of these materials releases nitrogen, phosphorus, potassium, and carbon into the soil.

Inorganic mineral inputs come from several sources:

• Weathering of the underlying parent rock, which releases mineral ions and clay particles
• Wind-blown and waterborne deposition, which carry sediment from other ecosystems (for example, Saharan dust depositing minerals over the Amazon basin)
• Precipitation carrying dissolved minerals such as nitrates, sulfates, and bicarbonates
• Diffusion of atmospheric gases, including oxygen and nitrogen, into pore spaces
• Solar energy, which heats the soil and drives biological and chemical activity

In managed soil systems, many inputs are anthropogenic:

• Compost and manure add organic carbon and improve structure
• Inorganic fertilizers supply concentrated nitrogen, phosphorus, and potassium, though excess application can cause leaching and soil acidification
• Irrigation provides water but can introduce salinization in arid regions where evaporation concentrates dissolved salts
• Agrochemicals such as pesticides and herbicides can reduce biodiversity and harm soil microorganisms