Soil is a mixture of four main ingredients: minerals, organic matter, air, and water. In a typical sample, minerals make up about 45% of the volume, water and air each account for roughly 25%, and organic matter fills the remaining 5%. That ratio shifts constantly depending on rainfall, temperature, and what’s growing in it, but those proportions give you a reliable baseline for understanding what’s beneath your feet.
The Mineral Fraction
Nearly half of any scoop of soil consists of tiny rock fragments, the product of centuries of weathering. These mineral particles come in three size classes. Sand is the coarsest, ranging from 0.05 to 2.0 millimeters in diameter. Silt particles are much finer, between 0.002 and 0.05 millimeters. Clay is the smallest at less than 0.002 millimeters, so fine you can’t see individual grains even under a magnifying glass.
The relative mix of sand, silt, and clay determines a soil’s texture, which in turn controls nearly everything about how that soil behaves. Sandy soils drain fast and warm up quickly in spring but don’t hold nutrients well. Clay-heavy soils retain moisture and nutrients but can become waterlogged and compacted. Silt falls in between. Most productive garden and farm soils are loams, a balanced blend of all three particle sizes.
The surface of each mineral particle carries a negative electrical charge. This matters because essential plant nutrients like calcium, magnesium, and potassium carry a positive charge, which means they’re attracted to and held on particle surfaces the way a magnet holds iron filings. Clay particles, because of their enormous combined surface area, are especially effective at this. A soil’s ability to hold onto these nutrients is called its cation exchange capacity. Soils with a high cation exchange capacity act like a nutrient bank, storing minerals and releasing them slowly to plant roots. Sandy soils, with their low surface area, let nutrients wash away more easily.
Organic Matter: The 5% That Punches Above Its Weight
Organic matter is everything in soil that was once alive: fallen leaves, dead roots, insect remains, animal waste, and the bodies of microorganisms themselves. As these materials break down, they pass through stages. Fresh residues decompose first into simpler compounds, then eventually transform into humus, a dark brown or black substance so thoroughly decomposed that you can no longer identify its original source. Humus is the most stable form of organic matter in soil, breaking down at a rate of only 2 to 5% per year.
Humus does several things at once. It holds water like a sponge, improving drought resistance. It binds to mineral particles, creating the crumbly, granular structure that lets roots penetrate easily and water drain at a healthy rate. And like clay, humus particles carry surface charges that attract and hold nutrients. Gram for gram, humus holds far more nutrients than mineral particles alone. Humus also contains 3 to 6% nitrogen, a critical plant nutrient, which it releases slowly as microorganisms continue to nibble away at it.
Globally, soils store an enormous amount of carbon in their organic matter. Recent estimates put the figure at roughly 2,800 gigatons in the top meter alone, far more than the carbon floating in the atmosphere. That makes soil one of the planet’s largest carbon reservoirs.
Air and Water Fill the Gaps
Between the solid particles of mineral and organic matter lies a network of tiny pore spaces. These pores are filled with varying proportions of air and water, and the balance between the two shifts constantly. After a rainstorm, pores fill with water. As that water drains or evaporates, air moves back in. In an ideal soil, roughly half the total volume is pore space, split more or less evenly between air and water.
Soil Air
Soil air has a similar composition to the atmosphere above ground, mostly nitrogen and oxygen. The key difference is that roots and microorganisms consume oxygen and release carbon dioxide as they respire. In poorly drained or compacted soils, oxygen levels can drop significantly while carbon dioxide builds up. This is why waterlogged soil kills most plant roots: they literally suffocate.
Soil Water
Not all water in soil behaves the same way. Gravitational water is the loosely held water that drains downward through large pores after rain. It passes through too quickly for most plants to use. Capillary water clings to particle surfaces and fills smaller pores through the same force that pulls water up a narrow straw. This is the water plants actually drink, held tightly enough to resist gravity but loosely enough for roots to extract. Hygroscopic water forms an ultra-thin film bonded directly to particle surfaces. It’s held so tightly that only oven-drying at 105°C can remove it, making it completely unavailable to plants.
Sandy soils have large pores that drain quickly, leaving little capillary water behind. Clay soils hold enormous amounts of water, but some of it is gripped so tightly between tiny particles that roots can’t pull it free. This is why clay soil can feel damp yet still leave plants wilting.
Billions of Living Organisms
Soil is not just a chemical mixture. It’s an ecosystem. A single gram of healthy topsoil can harbor up to 10 billion microorganisms spanning thousands of species. Bacteria are the most abundant, but the community also includes fungi, protozoa, nematodes, and larger creatures like earthworms and insects.
These organisms drive the nutrient cycle. Bacteria and fungi decompose dead plant and animal material, converting complex organic molecules into forms that plant roots can absorb. Certain bacteria convert atmospheric nitrogen into a plant-usable form, a process no other type of organism can perform. Fungal networks thread through the soil like underground highways, shuttling water and nutrients between plants in exchange for sugars. Earthworms physically mix organic matter into deeper layers and create channels that improve drainage and aeration. Without this living community, soil would be little more than crushed rock.
How pH Shapes Everything Else
Soil pH measures how acidic or alkaline the soil is, on a scale from 0 to 14. Most productive soils fall between 6.0 and 7.5. Within that range, the major plant nutrients remain chemically available, meaning they dissolve in soil water where roots can reach them. Outside that window, nutrients start locking up. Phosphorus, for example, becomes most available right around pH 6.6 to 7.3 and increasingly unavailable as soil turns either very acidic or very alkaline.
Extreme pH also suppresses microbial activity. The billions of organisms that cycle nutrients and build soil structure slow down or die off when acidity or alkalinity swings too far. This creates a cascading effect: fewer microbes means slower decomposition, less nutrient cycling, and declining soil health over time.
Layers From Surface to Bedrock
Soil isn’t uniform from top to bottom. If you could slice a clean vertical cross-section, you’d see distinct layers called horizons, each with its own color, texture, and composition.
- O horizon: A thin surface layer of leaf litter, twigs, and other decomposing organic material. Not all soils have one, but forest floors typically do.
- A horizon (topsoil): The dark, nutrient-rich layer where organic matter mixes with minerals. Most plant roots and soil organisms concentrate here.
- E horizon: A pale, leached layer found in some soils where water has washed minerals and organic matter downward. Common under coniferous forests.
- B horizon (subsoil): A denser layer where minerals, clay, and iron compounds accumulate after washing down from above. Often reddish or yellowish in color.
- C horizon: Partially weathered rock fragments with little biological activity. This is the parent material from which the soil above gradually formed.
- R horizon: Solid bedrock. Not technically soil at all, but the foundation everything else rests on.
The thickness and character of each horizon varies dramatically by location. Prairie soils can have A horizons several feet deep, rich with centuries of grass roots and organic matter. Desert soils may have almost no O or A horizon at all. Understanding which horizons are present tells you a great deal about a soil’s history, its fertility, and how well it will support whatever you’re trying to grow in it.