Fertile soil is one of the most essential elements for life on Earth. Its destruction was responsible for the decline of the Mesopotamian, Persian and Roman civilizations, and in more recent times, contributed to the Great Depression.
A report from the United Nations found that within the lifetimes of many of us, 11 percent of the Earth's vegetated soils have been significantly damaged. Twenty-four billion tons of topsoil are lost worldwide each year, as the human population grows. For each new person to feed, we have 260 tons less topsoil.
To understand what we are losing, let's journey into the good organic soil in our garden. Here is a whole ecosystem in which an incredible number of living organisms use minerals, organic matter, sunlight, air and water to create an environment which nourishes the plants which nourish us.
Green and blue-green algae and some bacteria perform photosynthesis, using the sun's energy to turn carbon dioxide into carbohydrates, adding organic matter to the soil. Other bacteria and algae convert the most plentiful component of the atmosphere, nitrogen, into the form plants use as a major nutrient. Actinomycetes bacteria and many kinds of fungi consume organic matter and create humus, that near magical substance that results from the healthy aerobic, biological decomposition of organic matter. Rhizobia bacteria live in a symbiotic relationship with the roots of legumes such as peas, beans or clover, exchanging nitrogen they extract from the air for carbohydrates produced by the legumes. Still other bacteria break down organic matter, or carry out key steps in the nitrogen cycle.
The bacteria are so numerous that a half teaspoon of good soil can contain billions of them, as well as millions of actinomycetes and fungi, and a hundred thousand algae. There are also tiny animals, thousands of protozoa and rotifers, and nematodes. All this in one gram of soil-about a half teaspoonful.
Insects such as ants, beetles, centipedes, springtails, spiders, sowbugs, mites and millipedes aerate and mix the soil as they feed on organic matter and other living things. Earthworms travel through the soil, aerating it while feeding and leaving behind a trail of their very fertile castings. The web of life is so balanced and interdependent that it is very hard for any disease organism or insect pest to become a problem.
Given enough organic matter, such as plant residues and roots, compost or leaves, and minerals including calcium, this complex ecosystem is continously at work collecting and storing solar energy, feasting on organic matter, cycling minerals, creating humus, and exchanging gases and nutrients with plant roots.
Humus, created by this ecosystem, acts like a sponge to hold water and creates a crumb structure in the soil which allows air and plant roots to penetrate easily. Humus helps a sandy soil hold water and a clay soil drain. It is a reservoir of nutrition.
Plants send their roots through the soil, sometimes many feet down, opening up additional passageways, bringing up fresh minerals from the deeper layers of soil, and creating a zone of extraordinary biological activity, the rhizosphere, around each root. Carbon dioxide and hydrogen given off by the roots increase the acidity nearby and help release nutrients held by the humus and clay particles.
A slightly acid soil, near neutral, will encourage soil organisims, and their processing of organic matter helps maintain that condition. Compaction by machines or feet, and excessive rototilling can be as harmful to soil life as tornadoes or earthquakes are to ours. Chemical fertilizers are harsh materials which tend to make the soil more acid, and destroy organic matter. Toxic pesticides wreck untold havoc on the microscopic life of the soil.
We can't begin to care for the world's soils if we don't each care for the soil in our own yard. In the diversity, ecological complexity, and health of good garden soil, we find an important model for our larger scale relationships to the Earth.
This is Bill Duesing, Living on the Earth
This page and its contents are copyright © 1997 by WSHU-FM, Fairfield, CT, and by Bill Duesing.