“Soil is a great water purifier," says Gary Pierzynski,. "There are three ways that soil cleans water: physical, chemical and biological.†Here are some facts from Pierzynski, a soil science professor at Kansas State University.
Physical: the particles of soil act like a filter. The ideal soil has a distribution of pore sizes so it can filter water, while also allowing the water to pass through the soil.
Chemical: soil particles have a negative charge! That means that positive ions like calcium, magnesium and potassium can all be removed from passing water and retained by the soil. This is good, because each of these ions is a nutrient for plants! Some contaminants can be removed by this mechanism as well.
Biological: soil is the largest bioreactor on the planet. Within the soil, bacteria and fungi transform and decompose certain chemicals. Soil microbes can change organic forms of nitrogen into ammonium ions—and even into nitrogen gas. They can also decompose some organic pollutants!
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