A single gram of soil can hold billions of organisms from thousands of species
Scoop up a gram of ordinary soil and you are holding billions of living things, mostly microbes, belonging to thousands of species that science has barely explored. Soil packs roughly ten times more prokaryotes per gram than seawater does per millilitre, which is why ecologists treat it as an ecosystem rather than mere dirt.
Soil is a three-phase system. A typical sample is about half solid, 45 percent mineral and 5 percent organic matter, and half empty pore space shared roughly equally between water and air. The solids stay constant in the short run, but water and air trade places: when rain fills the pores, gas is pushed out. Heavy machinery squeezes those pores shut, starving roots and organisms of air and water. Some definitions even separate soil from dirt, keeping the latter word for soil that has been moved.
It forms from parent rock shaped over time by climate, terrain and living things, and it is younger than you might think. Hardly any living soil on Earth goes back beyond the Pleistocene, and nothing beyond the Cenozoic, though fossilised soils survive from as early as the Archean. Given time, a soil separates into horizons, usually labelled A, B and C, with life concentrated near the surface and chemistry dominating deeper down. Scientists study it in two branches: pedology, covering formation and classification, and edaphology, covering its effects on living things.
Its services are enormous. Soil stores and cleans water, breaks down contaminants in a process called natural attenuation, recycles dead matter into nutrients, and anchors plants while moderating their temperature. Clay and humus hold nutrients in place and buffer the soil solution as conditions swing, and more heavily weathered soils tend to be more acidic, which changes what plants can absorb. Soils generally take up oxygen and methane and give off carbon dioxide and nitrous oxide.
That makes soil a major carbon reservoir and one of the parts of the carbon cycle most sensitive to disturbance. Ploughing speeds up soil respiration and drains stored organic matter. Some scientists predicted that warming would make soils release more carbon dioxide as microbes grow busier, amplifying climate change, though newer findings on how soil carbon turns over have cast doubt on that forecast.
Source: Soil
