Erode rock to mineral dust. Add decaying leaves, and other organic matter. Agitate with worms, insects, mites and other arthropods, nematodes, bacteria, fungi and protozoa. From these components of soil, the miracle ingredient is born: humus — dark, crumbly, spongy, somewhat gelatinous, and sweet-smelling.

Humus converts mineral and organic materials into forms plants can use, releasing nutrients gradually, at a pace plants can take them up. In addition humus stores nutrients, so they remain accessible to plants.

Plants get most of their nutrition through exchange of ions; the process requires a colloid: flat, microscopic particles coated by a liquid. Clay supplies an adequate medium; humus an ideal one (sand particles are too big). Useful ions are held by the colloid, until plant roots absorb them. The liquid that holds together the colloid is not only water, but also viscous products of soil organisms. If humus is deficient, plant nutrients leach through the soil into the water table, where they pollute.

Humus is made by the action of soil organisms; in turn it provides living environment and sustenance for them. The earthworm travels by ingesting the soil in front of it, passing it through its digestive tract, and excreting it; in the process increasing its colloid content. Worms and other soil inhabitants excrete vitamins, enzymes, beneficial microbes, antibiotics, natural plant growth hormones and other biochemical compounds that enable them to live, inhibit pathological organisms, and maintain the humus: a complex interdependence.

Humus also adjusts soil structure. The colloids help stabilize sandy, light soils, decreasing erosion. In addition, the crumb structure of humus provides a variety of pore sizes in soil. Plant roots (as well as other soil inhabitants) need both oxygen and water; soil must retain a certain amount of each. Humus increases the moisture of my sandy soil because water adheres to colloid particles and capillary action holds it in the small pores. At the same time, humus improves drainage in clay, because the large pores improve air and water circulation. One substance amends opposite problems.

The feedback loop of benefits between plants and soil and microorganisms; efficient regulation of electrical, chemical and physical processes within the narrow range favorable to life — these are the kind of actions that led Dr. James Lovelock to propose his "Gaia Hypothesis," in which he considers our planetary ecosphere as a whole ... a sentient organism.

For more information

• Maintaining Soil Humus — detailed information.
• The Mighty Earthworm - Harnessing the Colloid Mill — on the site of Nutri-Tech Solutions, who market a liquid earthworm manure.
  • 23 April 2006: The articles in this section of their site have been "removed for review," the section head says, continuing "Please check back in a few weeks for the updated articles." Unfortunately, there is no date on this notice. But the article was good enough that I will keep checking for a while.
• Organic Matter, Humus, and the Soil Foodweb - part of Sustainable Soil Management, a publication of the National Sustainable Agriculture Information Service. The whole text is online, and also downloadable as a PDF.
• What is Biodynamics? — see the section on soil ecology, under "Fundamentals of Organic Gardening." Lots of useful illustrations here.
• Also see the references on my previous article, "Thankful for Progress toward Healthy Agriculture."
• Gaia: A New Look at Life on Earth, by James Lovelock. The first book in which, in 1979 he described the Gaia Hypothesis. In this 2000 edition, he adds a preface which discusses the debate over the theory.