What Is Soil? Rock, Air, Water and an Enormous Quantity of Life

Soil is not dirt and it is not simply small rock. The components and their proportions define everything about how it behaves:

• •Mineral particles, classified by size into sand, silt and clay, whose proportions give a soil its texture. Sand drains fast and holds few nutrients, clay holds water and nutrients tightly and drains badly, and a loam with a mixture of all three is what agriculture wants
• •Organic matter, typically two to ten percent by weight, consisting of decaying plant and animal material and the stable humus it becomes, which holds water, supplies nutrients and binds particles together
• •Pore space, around half the volume, filled with air and water in competition, since water displaces air and waterlogged soil suffocates roots
• •Living organisms, including bacteria, fungi, protozoa, nematodes, mites, springtails, earthworms and plant roots, which are not residents in the soil so much as constituents of it
• •Structure, meaning how the particles are aggregated into crumbs, which determines drainage, aeration and root penetration and is destroyed by compaction and by working wet ground

Soil scientists summarise formation with five factors: parent material, climate, organisms, topography and time. Bare rock is broken down physically by freezing water, temperature cycling and root pressure, and chemically by water, dissolved carbon dioxide and acids produced by lichens and roots. Plants colonise the resulting mineral fragments, add organic matter when they die, and support the decomposer community that turns it into humus. Over centuries, distinct horizontal layers develop, visible in any roadside cutting as a profile: an organic litter layer at the surface, a dark topsoil rich in organic matter, a subsoil in which clay and minerals leached from above accumulate, and weathered parent material below. Climate determines which processes dominate, so tropical soils are deeply weathered and often nutrient-poor because heavy rain has leached them, while cool grassland soils accumulate deep organic layers because decomposition is slow relative to plant growth, which is why the world's most productive cereal regions sit on former prairie and steppe.

The biological activity is what distinguishes soil from ground rock. Decomposers break organic matter down and release the nutrients locked in it, which is the only reason nitrogen, phosphorus and the rest cycle rather than accumulating in dead material. Nitrogen-fixing bacteria, some free-living and some in nodules on the roots of legumes, convert atmospheric nitrogen into forms plants can use. Mycorrhizal fungi extend the effective root system of most plants enormously, trading minerals for sugars. Earthworms move large quantities of material, mixing layers, creating channels for water and air, and passing soil through a gut that improves its structure, which Darwin measured and wrote a book about in his last years. Predatory nematodes and protozoa release nutrients by eating bacteria. The whole system is a nutrient economy in which almost nothing is available to a plant until something else has processed it, which is why adding mineral fertiliser to biologically dead soil works less well than the chemistry suggests.

Erosion removes soil far faster than it forms, and the mechanisms are well understood. Bare ground is the immediate cause: raindrops striking exposed soil break the aggregates and the fragments wash away, while vegetation and residue intercept the impact. Ploughing loosens soil and leaves it exposed between crops, wind removes dry fine particles, and overgrazing removes the cover that holds it. The American Dust Bowl of the 1930s, in which ploughing of semi-arid grassland met a multi-year drought and removed topsoil across an enormous area, is the standard example and produced the first soil conservation service. Compaction by heavy machinery destroys pore space and structure. Salinisation ruins irrigated land in dry climates. Loss of organic matter, from continuous cultivation without return of residue, degrades structure and water holding, and the slow depletion is easy to miss until a threshold is crossed. Estimates of global soil loss vary widely and the direction is not disputed, and since formation takes hundreds of years per centimetre, the accounting is unfavourable.

The measures that work are consistent across climates and mostly involve keeping the ground covered and disturbing it less. No-till and reduced-till farming plant directly into the residue of the previous crop, which preserves structure and organic matter at the cost of more herbicide in most systems. Cover crops occupy the ground between cash crops, holding soil with roots, feeding the biology and adding organic matter. Crop rotation including legumes restores nitrogen and interrupts pest cycles. Contour ploughing and terracing slow water on slopes. Windbreaks reduce wind erosion. Returning organic matter, as compost, manure or residue, is the single measure that addresses the most problems at once. The broader recognition is recent: soil was treated as an inert medium for most of the twentieth century and is now understood as an ecosystem, with soil carbon a significant part of climate accounting, since soils hold more carbon than the atmosphere and vegetation combined and can be a source or a sink depending entirely on how they are managed.

Soil is about half pore space and half solid, with the solid part mostly weathered rock plus a few percent organic matter that does a disproportionate share of the work, and it is constituted by its organisms rather than merely containing them. It forms over centuries from parent rock under the influence of climate, life, slope and time, developing distinct layers. It is lost to erosion when left bare, to compaction, to salinisation and to the slow depletion of organic matter, and it is held by keeping the ground covered, disturbing it less and returning organic material.