The function of fungi and bacteria is to process and make available food elements too complex for higher plants to absorb. This vital process will be explained later in detail.
Soil organisms are important in another way: as they live and die they absorb, conserve and then gradually release soluble nutrients that might otherwise be lost; thus they increase fertilizer utilization many times over.
SOIL AS A CHEMICAL LABORATORY
Although each phase of soil already mentioned is vital, we cannot lose sight of soil as a chemical-mineral material. We must also remember that all nutrients are absorbed either as relatively simple chemical compounds or in nearly elemental form. Plants are unable to directly use complex proteins, animal wastes or similar products of growth. Decay must first release stored foods in simpler forms, then these are taken up by roots.
For this reason, we must study the chemical side of soils, to understand why conversion of organic matter to simpler products is a vital phase of soil. Chemical, as opposed to organic, fertilizers have become the subject of much controversy. Later on (in Chapter Five) we will explore the relative merits of these fertilizers.
All nutrients used by plants must be in liquid form, since plants have no teeth or specialized organs for grinding solid foods and lack any semblance of a digestive system. For this reason, water is another vital factor in assuring maximum growth in any soil. About 98 per cent of all plant tissue is the product of air and water, with only 2 per cent contributed by soil elements.
In normal soils, plant growth is probably limited more by inadequate water supplies than by lack of any other element. A program of soil management is incomplete if it ignores water and air as vital factors in, and sources of, nutrition.
MANY ROUTES TO BETTER SOIL
In this short resume of a few of the properties of soil, we have discussed several which are vital to plant growth, yet plants in the wild have survived for centuries without any human attention to these properties. Why then bother to change them? Is not natural growth enough to meet our needs?
The answer is a qualified "No." If native soils are rich, well-drained and abundantly watered by rainfall, obviously any crop adapted to the region, whether for food or ornamental purposes, will thrive. Some improvement in plant growth of value to the gardener might result from adjustment of one or more of the soil factors just discussed, but hardly enough to repay any extra effort.