Yuma Ag and You: The role of soil as a cleaning agent

Bobbi Stevenson-McDermott                   April 10, 2016 publication

What a wonderful article about the water treatment plants in Yuma, keeping our tap water clean and delicious. We are fortunate as well to have plentiful supplies of this precious resource, never enough to waste, but enough to allow our rural lifestyle of small acreage development and beautiful urban settings with trees, flowers and grass.

The relative amounts of sand, silt, clay and organic matter influence how fast water moves through the soil and how well water is cleaned. The longer it takes for water to flow through the soil, the more it has time to interact with the soil and the cleaner the water becomes. Water moves slowly through clayey soils because the spaces between the individual clay particles are very small. Clay particles and organic maters act as magnets that attract some chemicals that keep them from moving through the soil. Water flows faster through sandy soils because of the large spaces between sand grains. The shorter time the water has to interact with the soil particles combined with the smaller surface area results in water that is not as clean as the water that flows through the clayey soil.

The safe handling of wastes generated by society has largely focused on the soil. Treatment of human sanitary waste often relies on soil because it filters out some of the material, while microorganisms break down organic portions into less dangerous compounds. The common home septic drain field is an example.

One way for sewage treatment plants to handle their end products is to spread them on soil. The spreading of human waste on soil has a long history in many societies. In response to federal clean water and air laws, the Environmental Protection Agency estimates that about half the sewage sludge produced in the US is spread on the land. Land application is a form of recycling by closing a nutrient loop. Farm fertilizers are absorbed by crops, the crops are eaten by people, and some of the nutrients are flushed into the sewage system. If the sewage is then spread on farm fields, nutrients return to the land and the nutrient loop is closed. In recognition of the value of this material, it is generally now called biosolids: the primarily organic solid product yielded by municipal waste water treatment plants that can be beneficially recycled.

Fortunately for modern society, organisms inhabit the soil that can break down chemical products and refuse deposited in the soil. Research is identifying strains of microorganisms that are quite effective in digesting chemical wastes in soil, making possible the biological cleanup of contaminated soils.

Farms avoid a buildup of agricultural chemicals in the soil mainly because of microorganisms. While some chemicals leave the soil by leaching or by evaporation, biological decomposition is the most important means of removing chemicals. Some microbes have adapted to soil chemicals so well that that a herbicide or soil insecticide fails altogether because the organisms break them down so fast.

Soil microbes, by generating, absorbing or degrading such greenhouse gases as carbon dioxide, nitrous oxide and methane also interface with global climate.

Fungi found in the soil primarily break down plant litter and release nutrients for reuse. Actinomycetes, also called mold bacteria, produce chemicals that stop of other microorganisms. Many useful antibiotics of modern medicine come from soil actinomycetes.