Soil is a mixture of four main components: mineral particles, organic matter, water, and air. In a typical loam soil, minerals make up about 45% of the total volume, organic matter about 5%, water about 25%, and air about 25%. Those proportions shift constantly depending on rainfall, compaction, plant roots, and the type of soil you’re looking at, but this 45-5-25-25 split is the standard baseline for understanding what soil actually is.
The Four Main Components
The mineral portion is the largest share of any soil. These are tiny fragments of rock broken down over thousands of years by weather, water, and chemical reactions. The three most abundant elements in this mineral fraction are oxygen, silicon, and aluminum, with aluminum alone making up about 8% of the Earth’s crust by mass. Most soil minerals exist as silicate compounds, essentially combinations of silicon and oxygen bonded with metals like aluminum, iron, calcium, and potassium.
Organic matter is the smallest fraction by volume, typically around 5%, but it has an outsized effect on soil health. It includes everything from freshly fallen leaves to ancient, fully decomposed material called humus. Ohio State University Extension breaks organic matter into three useful categories: the “living” (microorganisms actively eating and reproducing), the “dead” (fresh plant and animal residue that microbes are currently breaking down), and the “very dead” (humus that can persist for thousands of years). About 65% of soil organic matter is in that stable humus form. The remaining 35% is the active fraction, cycling nutrients in real time.
Water and air split the remaining pore space roughly evenly in well-drained soil. After a heavy rain, water fills most of the pores and air gets squeezed out. During a dry spell, the reverse happens. This balance matters enormously: soil microbes do their best work when pore spaces are about 60% filled with water. Push that above 70% and the soil becomes oxygen-starved, which triggers nutrient loss through a process where bacteria strip nitrogen from the soil and release it as gas.
Mineral Particles: Sand, Silt, and Clay
Not all mineral particles are the same size, and the size differences define your soil’s texture, drainage, and fertility. The USDA classifies mineral particles into three groups:
- Sand: 0.05 to 2.0 mm in diameter. You can see and feel individual grains. Sand drains fast and holds very few nutrients.
- Silt: 0.002 to 0.05 mm. Silt feels smooth, like flour. It holds water better than sand but can compact easily.
- Clay: smaller than 0.002 mm. Clay particles are so fine they’re invisible to the naked eye. They hold water tightly and have a massive surface area for nutrient storage.
The ratio of sand, silt, and clay determines a soil’s texture class. A loam, often considered ideal for growing plants, contains a balanced mix of all three. Sandy soils drain quickly but dry out fast. Clay-heavy soils retain moisture and nutrients but can become waterlogged and hard to work with. Silt loams fall somewhere in the middle.
Clay’s nutrient-holding ability comes from its electrical charge. Clay particles carry a negative surface charge that attracts and holds positively charged nutrients like calcium, magnesium, and potassium. This holding power, measured as cation exchange capacity, varies dramatically by soil type. Pure sand holds almost nothing (1 to 5 units), a loam holds 5 to 15, and heavy clay soils hold 30 or more. Organic matter is even more powerful, with a holding capacity of 200 to 400 units, which is why adding compost to sandy soil makes such a noticeable difference in plant growth.
Organic Matter and Soil Biology
A single gram of healthy topsoil contains between 100,000 and 100 million bacteria, along with millions of actinomycetes (bacteria that decompose tough organic compounds like cellulose) and hundreds of thousands of fungal colonies. These organisms are the engine that drives nutrient cycling. Without them, dead leaves and roots would simply pile up, and plants would have no access to the nitrogen, phosphorus, and sulfur locked inside organic material.
When plant residue lands on or in the soil, microbes go to work. Of every 100 grams of dead plant material, 60 to 80 grams get converted to carbon dioxide and released into the atmosphere. The remaining 20 to 40 grams become a mix of new microbial bodies (3 to 8 grams), partially decomposed compounds (3 to 8 grams), and stable humus (10 to 30 grams). That humus is the long-term savings account of the soil. It improves structure, holds water, and slowly releases nutrients over decades or centuries.
Soil Layers From Surface to Bedrock
If you dug a deep hole in undisturbed ground, you’d see distinct horizontal bands called horizons. Each one formed through different processes over time, and together they tell the story of how that soil developed.
The O horizon sits at the very top and is made of at least 20% organic matter by mass. It’s the layer of leaf litter, decomposing wood, and other plant debris you’d find on a forest floor. It ranges from freshly fallen material on top to nearly unrecognizable, fully decomposed matter at the bottom. This layer is typically dark brown or black.
Below that is the A horizon, often called topsoil. It’s a mineral layer, but richer in organic matter than the layers beneath it, giving it a darker color. Over time, water moving downward through this layer carries dissolved minerals and fine clay particles deeper into the profile, leaving the A horizon coarser in texture than what lies below.
Some soils have an E horizon beneath the A, a pale, washed-out layer where clays and iron compounds have been stripped away by downward-moving water. It’s lighter in color than both the A above and the B below, and is most common in forested soils with high rainfall.
The B horizon is where much of that transported material ends up. Clays, iron, salts, and other compounds accumulate here, often giving this layer a reddish or yellowish tint. It’s denser and stickier than the layers above. Many plant roots reach into the B horizon, but it’s generally less biologically active than topsoil.
At the bottom sits the C horizon, which is essentially unaltered parent material. This might be glacial sediment, weathered rock fragments, or lake deposits. Very little biological or chemical change happens here. Below the C horizon is solid bedrock, which isn’t considered soil at all.
How pH Shapes Nutrient Availability
Soil chemistry isn’t just about what nutrients are present. It’s about whether plants can actually access them, and pH is the single biggest factor controlling that access. Most plants grow best in soil with a pH between 6.0 and 7.2, a slightly acidic to neutral range where nitrogen, phosphorus, potassium, and most micronutrients are all readily available.
When pH climbs above 8.0, iron, zinc, manganese, and phosphorus become chemically locked up in forms that plant roots can’t absorb, even if there’s plenty in the soil. This is a common problem in arid and limestone-rich regions. On the acidic end, soils below pH 5.5 can release aluminum and manganese at levels toxic to many crops, while calcium and magnesium become scarce. The chemistry of soil is always a balancing act, and pH is the fulcrum.
Why Composition Varies So Much
The 45-5-25-25 breakdown is a useful starting point, but real soils deviate from it constantly. Desert soils may contain less than 1% organic matter. Wetland soils, called histosols, can be more than 50% organic material. A compacted athletic field might have almost no air space, while a freshly tilled garden bed could be 30% or more air by volume.
Five factors drive these differences: the parent rock the soil formed from, the climate (temperature and rainfall), the organisms living in and on it, the landscape’s slope and drainage, and how long soil-forming processes have been at work. A volcanic soil in Hawaii and a glacial soil in Minnesota started from completely different parent materials, weathered under different climates, and support different biological communities. The result is two soils that look, feel, and behave nothing alike, even though both are still a mix of the same four basic components.