Soil
Soil is a complex ecosystem supporting life and regulating Earth's cycles.
Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. It is a complex three-state system of solids, liquids, and gases, and is a product of several factors including climate, relief, organisms, and parent materials interacting over time. Soil is regarded by ecologists as an ecosystem due to its complexity and strong internal connectedness.
The solid portion of soil consists of minerals and organic matter, forming the soil matrix, while the porous phase holds both gases (the soil atmosphere) and a liquid phase containing water and dissolved substances in ionic or molecular form (the soil solution). Most soils have a dry bulk density between 1.1 and 1.6 g/cm³, though the actual particle density is much higher, ranging from 2.6 to 2.7 g/cm³. Geologically, little soil on Earth is older than the Pleistocene, and none predates the Cenozoic, though fossilized soils have been preserved from as far back as the Archean.
Collectively, Earth’s soil is termed the pedosphere, which interfaces with the lithosphere, hydrosphere, atmosphere, and biosphere. Soil performs four critical functions: as a medium for plant growth; for water storage, supply, and purification; as a modifier of the atmosphere; and as a habitat for soil organisms. These functions in turn modify the soil itself. Soil science divides into edaphology, which studies soil’s influence on living things, and pedology, which focuses on soil formation, morphology, and classification. In engineering, soil is part of the broader regolith, which includes loose material above bedrock on Earth and other celestial bodies.
Soil provides ecosystem services as an engineering medium, a recycling system for nutrients and organic wastes, a regulator of water quality, and a habitat supporting a prominent part of Earth’s genetic diversity. A single gram can contain billions of organisms from thousands of species, mostly microbial and largely unexplored, with a mean prokaryotic density of roughly 10⁸ organisms per gram—far exceeding the ocean’s 10⁷ per milliliter. Soil organic carbon is partly returned to the atmosphere via respiration by heterotrophic organisms, but a substantial portion remains as soil organic matter; tillage typically increases respiration, depleting this matter. Soil pores provide aerat
- composition
- ~50% solids (45% mineral, 5% organic matter) and 50% voids (half water, half gas)
- dry_bulk_density
- 1.1 to 1.6 g/cm³
- particle_density
- 2.6 to 2.7 g/cm³
- prokaryotic_density
- ~10⁸ organisms per gram
- age_limit
- Little older than Pleistocene; none older than Cenozoic
- functions
- Medium for plant growth, water storage/purification, atmospheric modifier, habitat for soil organisms
Lore & Background
Soil is a complex, three-state system composed of solids, liquids, and gases. The solid portion, or soil matrix, is a mixture of minerals and organic matter, while the pore space holds both the soil atmosphere (gases) and the soil solution (water containing dissolved organic and inorganic substances in ionic or molecular form). This material develops over time through the interplay of climate, relief (elevation, orientation, and slope), organisms, and parent materials, undergoing continuous physical, chemical, and biological processes such as weathering and erosion. Most soils have a dry bulk density between 1.1 and 1.6 g/cm³, though the actual particle density of the solid components is much higher, ranging from 2.6 to 2.7 g/cm³. Geologically, very little of Earth’s soil is older than the Pleistocene epoch, and none predates the Cenozoic, although fossilized soils (paleosols) have been preserved from as far back as the Archean. The collective global body of soil is termed the pedosphere, which interfaces with the lithosphere, hydrosphere, atmosphere, and biosphere. Soil serves as a medium for plant growth, a water storage and purification system, a modifier of the atmosphere, and a habitat for a vast array of organisms. It contains a prominent part of Earth’s genetic diversity; a single gram can hold billions of organisms from thousands of species, mostly microbial and largely unexplored, with a mean prokaryotic density of roughly 10⁸ organisms per gram—an order of magnitude greater than that found in seawater. Soil also acts as a critical carbon reservoir, recycling nutrients and organic wastes, and can degrade contaminants through natural attenuation.
Reader's Guide
Soil is a major component of the Earth's ecosystem, providing critical ecosystem services such as nutrient recycling, water purification, and atmospheric modification. It acts as an important carbon reservoir, potentially reactive to human disturbance and climate change. Soil contains a prominent part of Earth's genetic diversity, with a gram of soil holding billions of organisms. Its functions include supporting plant growth, storing and purifying water, modifying the atmosphere, and serving as habitat. Soil science has two branches: edaphology (influence on living things) and pedology (formation and classification). In engineering, soil is part of the broader regolith concept.
Did You Know?
- A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely unexplored.
- Soil has a mean prokaryotic density of roughly 10⁸ organisms per gram, while the ocean has no more than 10⁷ per milliliter.
- Soils can effectively remove impurities, kill disease agents, and degrade contaminants, a property called natural attenuation.
- Mature soil profiles typically include three basic master horizons: A, B, and C.
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