The organic portion of the soil consists of living and dead plant and animal matter in various stages of growth and decay. This portion makes up only 0.5% to 5% of most native soils, but well-amended garden soil may contain 30% or more organic matter.
As organic matter gradually decomposes in the soil, it forms humus. Humus is a relatively stable, sticky mass of organic chemicals and microbes. It is chemically very complex and its composition varies with the location. In most climates, it lasts for years. If organic matter is added regularly, the humus content of the soil remains fairly stable.
(In very hot climates, however, the humus breaks down quickly and disappears. The humus content of hot soils is usually low, and is difficult to keep high, even with the frequent adding of fresh organic matter.)
Much of the available nitrogen in the soil is bound up in humus, and is released slowly as humus decays to simple compounds that disappear from the soil. This process is called mineralization. Besides nitrates (the simplest form of nitrogen available to plants, and the form in which they absorb most of their nitrogen), the end products of mineralization are carbon dioxide, water, and such mineral ions as calcium, magnesium, and sulfur.
Humus helps the mineral particles stick together in larger aggregates, or crumbs, giving the soil a granulated, crumbly property. Humus bonds with the clay in soil to form these complex aggregates that do much to improve the soil. This property—the way in which soil particles group together to form larger aggregates—is known as soil structure. Soil structure greatly influences the soil workability, or tilth. Being soft and sticky, humus creates crumbs that break and re-form easily, giving soil the consistency of cake crumbs.
The complex pattern of pore spaces in such a soil hold large amounts of both water and air. Because of this ability, and the gradual release of nitrogen and other nutrients by mineralization, soil high on organic matter forms an ideal environment for the growth of plants.
Life in the Soil
The living part of the organic portion of the soil includes some large and obvious creatures, such as earthworms and insects. Plant roots, earthworms, and ants perform a valuable service by creating tunnels for air and water flow through the soil. Earthworms, ants, and other creatures feed on or near the surface and build tunnels in which they live. They deposit their excrement in these tunnels, digging surface organic matter deep into the soil. Their tunnels, and those left by decaying plant roots, persist for months or even years. Undisturbed soil with healthy animal life is riddled with threadlike tunnels from plant and animal activity. These tunnels admit water and air deep into even hard clay soil.
An invisible world of soil bacteria, fungi, and algae is even more crucial. These tiny creatures cooperate to break down organic matter into more and more stable products until it becomes humus. Much of the nitrogen in the soil is held in their bodies as protein until they, in turn, die and are decomposed by other microbes.
Although different organisms are adapted to different soil environments, the majority of them are active in moist soil at temperatures between 60 degrees and 80 degrees. Above 80 degrees, activity drops sharply. Below 60 degrees, activity declines slowly until it comes almost to a halt at the freezing point.