The Dirt On Dirt

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Like any other source of humus, compost improves soil texture and makes the soil easier to work. Compost also adds nutrients and encourages the growth of beneficial soil-borne microorganisms that dissolve those nutrients, so they can be readily absorbed by plant roots. But my compost pile was growing slowly and decomposing even more slowly. I needed lots of humus in a hurry. So I called a local saw mill and had them deliver several truckloads of well rotted sawdust to till into my garden. Voila! I had loam. But it wasn't yet fertile loam.

Plant Nutrients

Clay, sand, silt, and humus all contribute certain nutrients that plants need in order to grow and thrive. Just what kind of nutrients loam contains, and how much of each, depends on three things: the sources of the mineral and organic matter making up the soil, the degree to which the soil has been weathered and eroded, and the amount of nutrients used up by plants previously grown in the soil.

The major plant nutrients nitrogen (N), phosphorus (P), and potassium or "potash" (K)-are the most likely nutrients to be deficient in garden soil. Bagged fertilizers have three numbers on the label (5-10-5, for example), representing the percentage of each nutrient that the fertilizer contains. Chemical fertilizers contain only those elements. Fertilizers derived from natural sources contain, in addition, a variety of micro nutrients, or trace elements, that plants use in lesser amounts. While chemical fertilizers can be manufactured to precise specifications, natural fertilizers vary somewhat in NPK values, depending on the source. Any good garden book lists various sources and their average NPK values. If you buy a bagged natural fertilizer, such as bone meal or blood meal, the label will tell you the NPK value for that particular lot. Compared to natural fertilizers, chemical fertilizers appear to be less expensive. They also dissolve more readily in water, giving plants a quicker boost. But because they dissolve so readily, chemical fertilizers also quickly leach out of the soil. Natural fertilizers, on the other hand, release nutrients over a longer period of time. In the long run, therefore, natural fertilizers are a better buy.

Natural fertilizers are a good deal for other reasons as well. Compared to chemical fertilizers, they don't burn the delicate roots of seedlings, they don't destroy beneficial microorganisms in the soil, and they increase a plant's resistance to disease. Chemical fertilizers do just the opposite, which works out nicely for the manufacturers of chemical products, since they sell more insecticides, fungicides, and other chemical poisons.

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