Soil is a complex mixture of minerals, organic matter, water, air, and living organisms that forms at the Earth's surface through the weathering of rocks and the decomposition of organic materials over time. It serves as a crucial natural resource that supports plant growth, sustains terrestrial ecosystems, regulates water and nutrient cycles, and provides habitats for various organisms.

The difference between soil and dirt lies in their composition and context. Soil refers to the natural, nutrient-rich material that supports plant life and sustains ecosystems, whereas dirt typically denotes loose, unstructured material lacking in organic matter and nutrients, often considered unwanted or unsightly.

Soil performs several essential functions, including:

• Providing a medium for plant growth by anchoring roots, supplying nutrients, and regulating water availability.
• Supporting diverse biological communities, including microorganisms, insects, and small animals, that contribute to nutrient cycling, decomposition, and soil health.
• Regulating water infiltration, storage, and drainage, influencing local hydrology, groundwater recharge, and flood mitigation.
• Acting as a carbon sink, storing, and cycling organic carbon, thereby mitigating climate change.
•  Filtering, buffering, and detoxifying pollutants, improving water quality, and reducing environmental contamination.

Soils differ from place to place due to variations in factors such as parent material, climate, topography, biological activity, and time. These factors influence soil formation processes, including weathering, erosion, deposition, and organic matter accumulation, leading to the development of distinct soil properties, profiles, and classifications. These differences are important because they influence soil properties such as texture, structure, fertility, and drainage, which in turn affect plant growth, ecosystem functioning, water availability, and agricultural productivity. While there is no single "best" soil composition, loam—a balanced mixture of sand, silt, and clay—is often considered ideal for agriculture due to its favorable physical properties and nutrient-holding capacity.

Scientists test the “healthiness” of soils analyzing soil horizons. Soil horizons refer to the distinct layers or zones within a soil profile that develop over time through the process of soil formation. These horizons are characterized by differences in color, texture, structure, organic matter content, and mineral composition. The most important soil horizon for plant growth is Horizon A, also known as topsoil, which contains the highest concentration of organic matter, nutrients, and beneficial microorganisms necessary for supporting plant roots and sustaining crop growth.