Here’s a nice article describing how soil, especially healthy soil (that has a good balance of living material in it – like plants and microbes), helps retain water, prevent sediment run-off, and trap other potential pollutants. It focuses on some results shown through a simple simulator that shows how rainfall (or a sprinkler, in this case) ends up running off or through different kinds of soils:
. . . the simulator’s open frame holds five trays of soil, each slightly larger than a casserole dish. The trays are tilted downward at a four-degree angle and filled with rectangles of soil cut from living fields:
-Compacted dirt from a construction site
-Conventional tilled soil
-Bermuda grass pasture
-No-till soil from a wheat field with remnants of canola and grain sorghum, two crops that are grown in rotation with the wheat, along with corn and soybeans
Farmers that employ no-till practices do not break up and turn over the soil between plantings. They inject new seeds directly into the harvested land, without disturbing the leftovers from last year’s crop. The no-till sample in the tray had roughly four percent organic matter, Scott said. The tilled soil: less than one percent.
Organic matter stores carbon and attracts earthworms, which help create cavities between the soil particles. Bumping up the organic matter by one percentage point increases the soil’s water storage capacity by 25,000 gallons per acre, Scott claimed, citing research from Kansas State University.
The compacted dirt and the tilled soil had the most (and dirtiest) runoff, with the forested sample and the no-till sample retaining the most water. Direct runoff (like you get with compacted soils, or soil with little vegetation) means pollutants, including fertilizers and various biocides applied but not absorbed, will end up traveling away from where they originated, and can more easily end up in streams or lakes.
Sediment runoff is a problem all on its own, as fish that are adapted to cleaner waters will have a harder time breathing in muddier waters; too much sediment can also kill the eggs of some fish (salmon, for one). One of the places where this is a direct problem are streams near forests that have been logged, since there will be logging roads, and water can’t be absorbed by the compacted surface of the roads, a huge quantity of which are dirt or gravel. Whenever an area of National Forest is opened, or reopened, to logging, roads that may have been decommissioned, and are slowly returning to a vegetated state, get turned back into active roads, along with any new roads built . . . and there are already a staggering amount of roads through areas that look wild and untouched.