Organic matter is important to soil fertility, whether a gardener chooses conventional or organic methods of gardening. It is nature's original version of slow-release fertilizer, breaking down over time to provide nourishment to the soil. When organic matter is applied thickly, as in several inches of compost or mulch, it helps to regulate soil temperature and moisture levels as it breaks down. Not only does organic matter nourish the soil, it protects it, as well.
Organic matter includes compost, manure, humus, mulch, plowed-under plants and all other naturally derived soil amendments. Healthy soil incorporates organic matter in order maintain its status as a complex, life-sustaining element. As organic matter decomposes, it becomes part of the soil. As a result of the continual absorption of organic matter, healthy soil is dark brown, crumbly and sweet-smelling.
As organic matter breaks down, it releases nutrients back into the soil. Depending on the soil's pH levels, certain nutrients are made available to the roots of plants growing in that soil. Nitrogen, phosphorus and potassium (also called N-P-K in horticultural literature) are macronutrients that most plants need to survive. In soil with very acidic pH levels (4.5 or below), availability of N-P-K drops off significantly. Micronutrients such as sulfur, calcium, magnesium and iron also become less available, whereas manganese and zinc are more available. Molybdenum, by contrast, becomes more available at neutral and higher pH levels. Organic matter can not only provide nutrients to the soil, but also adjust the soil's pH level. If highly acidic organic matter, such as pine mulch (also called pine straw), is added to the soil, it will make the soil it decomposes into more acidic, as well.
As plants grow, they take up water and nutrition from the soil around them through their roots. Gardeners can and should apply fertilizers, but organic matter is as important, if not more so, than fertilizer. Organic matter supplies nutrients and helps improve soil texture and structure. This makes it easier for the roots of plants to grow healthy and vigorous. Additionally, it attracts beneficial microorganisms and insect life, such as earthworms. All these factors help plants to grow healthy, strong and resistant to diseases.
Application of organic matter to enhance soil fertility is an ongoing process. Organic matter should be applied to soil in active gardens at a rate of one to two inches per season. Mulch applied to soil provides a slow, constant stream of organic matter to the soil as it breaks down.
Overapplication of organic soil amendments such as blood meal, bone meal, green sand and anything that consists mostly of a single nutrient can be detrimental to soil health. An overabundance of any nutrient can burn the tender roots of plants.