2015 has been named the “International Year of Soil” (IYS) and the Soil Science Society of America (SSSA) has developed activities to support educational outreach, assigning a theme for each month of the year. April’s theme is “Soils Clean and Capture Water”. Rainwater usually falls with little to no impurities, once it reaches the planet’s surface the precipitation can either infiltrate or wash away.
In rural areas the open, green spaces provide more opportunities for water to infiltrate into soil, as it naturally should. Water that washes over the ground can pick up many different contaminants, like fertilizers, animal waste and other trash, eventually all of this will get deposited into the watershed. Careful and attentive management of soils and landscapes can effectively minimize watershed contamination.
In urban areas water has far fewer chances at infiltration. Typically, stormwater will be be diverted and carried by infrastructure which aids in keeping streets, buildings and homes from flooding, but it does not filter water prior to discharge. Hard stormwater infrastructure could be enhanced with green roofs and rain gardens, both of which utilize soil and it’s inherent, natural filtration to remove pollutants from water.
Soil cleans water in three ways:
Physically: pollutants and contaminants adhere to soil particles, the texture of the soil makes a difference, finer soils make better filters. Ideal soil filters have a heterogeneous distribution of particle soils in order to filter water while still allowing it to pass.
Chemically: Soil’s negative charge attracts positively charged ions from passing water, such as calcium, magnesium and potassium are removed from water and retained within soil.
Biologically: Bacterial and fungal soil inhabitants decompose and transform contaminants; the nitrogen cycle is especially important for both removal of excess nitrogen and conversion into usable forms for plants. Organic pollutants are decomposed into carbon dioxide and water.
The net outcome of soil filtration of water is that once stormwater reaches its eventual discharge pond, stream, river or lake it is far less polluted. This is an example of an ecosystem service: a natural process performed by the environment which improves our lives. Soil is the largest water filter, we need to find more ways for it do this job naturally for the benefit of the planet and population.