Soil is a three-phase system composed of solids, liquids, and gases, serving as the foundation for nearly all terrestrial ecosystems and plant life. A functional soil is built upon a balanced mixture of four primary components: mineral content, organic material, soil water, and soil air. Understanding the nature and role of each component is the first step in appreciating the intricate environment that supports plant growth and numerous biological processes.
Mineral Content
Mineral matter constitutes the largest fraction of soil volume, making up about 45 percent. This solid scaffolding originates from the slow physical and chemical breakdown, or weathering, of bedrock and parent material. The specific composition of these minerals depends heavily on the original rock source and the degree of weathering it has undergone.
The physical nature of this mineral fraction is defined by the size of its particles, which are separated into three main classes: sand, silt, and clay. Sand particles are the largest, ranging from 0.05 to 2.0 millimeters in diameter, and are often made of primary minerals like quartz. Silt particles are intermediate in size, spanning from 0.002 to 0.05 millimeters, and feel smooth like flour.
Clay particles are the smallest, measuring less than 0.002 millimeters, and are often secondary minerals formed from the intense weathering of primary minerals. The relative proportion of these three sizes determines the soil’s texture, which significantly impacts properties like water movement and aeration. Clay’s minute size gives it a vast surface area, which is important for the soil’s chemical interactions, such as holding onto nutrient ions.
Organic Material
Organic material, though the smallest component by volume at around 5 percent, is disproportionately important for soil health. This fraction includes both the living soil biota, such as microbes, insects, and plant roots, and the non-living material derived from dead and decomposing organisms. The decomposition process is driven by microorganisms that transform raw plant and animal residues into more stable organic compounds.
Humus is a dark, amorphous material that resists further rapid decay. Humus is a reservoir for essential plant nutrients, particularly nitrogen and phosphorus, which are released slowly as the material continues to cycle. This stable organic matter also acts like a sponge, helping to bind mineral particles into aggregates, which improves the soil’s overall physical structure.
Humus gives the soil a high capacity for cation exchange, allowing it to temporarily store positively charged nutrient ions and prevent them from being washed away. Humus can hold a substantial amount of water—up to 80 to 90 percent of its own weight—which helps to buffer the soil against drought conditions. Maintaining a healthy level of organic material is therefore central to sustaining soil fertility and ecosystem function.
Soil Water
Soil water fills about 25 percent of the total soil volume. This water is not pure, but rather a “soil solution” that contains dissolved minerals, salts, and nutrients. This solution acts as the primary transport mechanism, carrying essential nutrients from the soil matrix into the plant roots.
Soil moisture thresholds describe the availability of this water to plants. Field capacity is the maximum amount of water that a soil can hold against the force of gravity after excess water has drained away. At this point, the water content is considered ideal for plant growth.
The permanent wilting point is the moisture content at which the remaining water is held so tightly by soil particles that plant roots cannot extract it fast enough to prevent permanent wilting. The difference between field capacity and the permanent wilting point represents the total amount of water available for plant uptake.
Soil Air
Soil air occupies the remaining 25 percent of the soil volume. The amount of air in the soil is inversely related to the water content. This gaseous phase facilitates the necessary exchange of gases between the soil and the atmosphere.
Soil air has a composition distinctly different from the air above ground due to biological activity. Respiration by plant roots and soil microbes consumes oxygen and releases carbon dioxide. Consequently, soil air has a higher concentration of carbon dioxide and a lower concentration of oxygen compared to the atmosphere.
Oxygen is required for root respiration. If the pore spaces become waterlogged for too long, the lack of oxygen can stress plants and hinder the activity of beneficial aerobic microbes. Gas exchange largely occurs through diffusion, driven by the partial pressure gradient of gases between the soil and the atmosphere, ensuring a continuous supply of oxygen and removal of carbon dioxide.