Soil

Soil is a three-phase system: solid particles (minerals and organic matter), liquid (soil water), and gas (soil air). Mineral particles are classified by size—sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (<0.002 mm)—and their relative proportions determine soil texture, which influences water retention, aeration, and workability. Soil structure refers to the arrangement of these particles into aggregates, or peds, which create pore spaces that hold water and allow gas exchange. The organic component, called humus, is the stable end-product of decomposition and enhances nutrient-holding capacity and soil fertility. Soil pH, typically ranging from 3.5 to 10, controls nutrient availability and microbial activity. The soil profile, a vertical section from the surface to parent material, is divided into horizons (O, A, E, B, C, R) that reflect the cumulative effects of leaching, accumulation, and biological mixing.

Pedogenesis, the formation of soil, is driven by five factors: parent material, climate, organisms, topography, and time. Climate, particularly temperature and precipitation, is the most influential factor, controlling weathering rates and organic matter decomposition. Organisms, from bacteria to earthworms to plant roots, mix the soil, cycle nutrients, and contribute organic matter. Topography affects drainage and erosion, while time determines the degree of horizon development. Soils are classified using systems such as the U.S. Soil Taxonomy, which groups soils into 12 orders (e.g., Mollisols, Alfisols, Oxisols), and the World Reference Base for Soil Resources (WRB), which uses 32 reference groups. These classifications help scientists and land managers predict soil behavior and suitability for different uses. The global distribution of soils follows climate and vegetation patterns, with, for example, Oxisols dominating tropical regions and Gelisols occurring in permafrost areas.¹

Soil is the foundation of terrestrial ecosystems, providing anchorage, water, and nutrients for plants, which in turn support food webs. It is estimated that a single teaspoon of soil contains billions of microorganisms, including bacteria, fungi, protozoa, and nematodes, which drive nutrient cycling, decompose organic matter, and suppress plant diseases. Soil organic matter, particularly in the form of humus, improves soil structure and water-holding capacity, and it is a major carbon sink—the top metre of soil stores more carbon than the atmosphere and vegetation combined. In agriculture, soil fertility is managed through practices such as crop rotation, cover cropping, and the addition of organic or synthetic fertilizers. However, unsustainable practices—overgrazing, deforestation, intensive tillage, and excessive chemical use—lead to soil degradation, including erosion, salinization, compaction, and loss of organic matter. Soil degradation threatens global food security and contributes to climate change through the release of stored carbon.²

Beyond its familiar roles, soil has surprising and niche dimensions. For instance, soil is a source of antibiotics: the drug streptomycin was discovered in the soil bacterium Streptomyces griseus in 1943, and soil microbes continue to be screened for novel antimicrobial compounds. Soil also plays a role in the global climate system through the production and consumption of greenhouse gases—methane is oxidized by soil bacteria, while nitrous oxide is emitted from denitrification. In archaeology, soil layers (stratigraphy) are used to date human settlements and understand past environments. Soil is also used in forensic science, where soil samples can link a suspect to a crime scene based on mineral and biological signatures. Additionally, the concept of "soil security" has emerged as a framework linking soil to food, water, energy, climate, and biodiversity security. Finally, soil is a cultural resource: the word "humus" is related to "human," reflecting the ancient belief that humans are made of soil, a theme echoed in many creation myths.³

Pedogenesis

The process of soil formation, involving the weathering of parent material and the accumulation of organic matter.

Humus

The stable, dark-colored organic matter in soil, formed by the decomposition of plant and animal residues.

Soil texture

The relative proportions of sand, silt, and clay particles in a soil, which affect its physical properties.

Soil horizon

A distinct layer of soil, roughly parallel to the surface, with characteristics resulting from soil-forming processes.

Oxisol

A soil order in U.S. Soil Taxonomy, characterized by intense weathering and low fertility, common in tropical regions.