Understanding Soil Fertility

Organic matter is a valuable part of soil. It is the final stage of decayed plants and animals and the most effective material for improving soil. When incorporated into soil, compost produces a spongy texture that acts to increase soil water-holding capacity and provides needed pore space, which lets in the air essential to good plant growth. Organic matter prevents tiny particles of clay from cementing themselves into a solid mass. This enables plant roots to move easily throughout the soil. It increases the cation exchange capacity or C.E.C., which is a measurement of a soil's ability to hold nutrients. While organic matter helps soil hold and release more nutrients, it also promotes the growth of microorganisms, which help to condition the soil.  Bottom line: it is good stuff.

Organic matter may be added to soils in the form of manure, compost, peat moss, humus, and mushroom compost. Coarse forms, like sawdust or shredded tree bark, must be composted for one or two years before use. While organic matter provides many plant nutrients, it rarely provides a balanced source of plant nutrients. Plus, decaying straw, leaves, and sawdust can compete with your plants for available nitrogen.

One can have too much of a good thing. Some organic matter, especially manure, is relatively high in nitrogen, phosphorus, and potassium. This can imbalance the nutrient balance ratio of the soil, which can cause problems with your plants. In addition, surplus nutrients, whether from soil amendments or fertilizers, can wash away. Those same nutrients that benefit our plants can cause algal overgrowth followed by oxygen depletion in our waterways. This pollution leads to damaging effects on fish and other aquatic animals. Bottom line: more is not better.

Regular applications of organic matter and complete fertilizer are essential for garden soils used each year. For ornamental plants and turf areas, incorporating adequate organic matter is even more critical since there is only one opportunity to work it into the soil before planting or seeding. Once permanent crops are established, it is difficult to blend in organic matter without injuring plant roots. Only small amounts of organic matter can be top-dressed in this situation.

All crops require a well-balanced supply of the major plant nutrients: nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca). A "complete" fertilizer contains various amounts of the first three elements: nitrogen, phosphorus, and potassium.

Plant nutrient availability directly relates to soil pH. Calcium, magnesium, manganese, copper, zinc, iron, and boron are available to plants when the soil pH is 6.0 to 6.7. Low pH levels also tie up some calcium and phosphorus, reducing their availability. In addition, magnesium and calcium deficiencies may occur in soils with excessive potassium levels. Excessive soil potassium competes for and reduces the uptake of calcium and magnesium by plants. (For more information on soil pH, see Understanding Soil pH.)

Minor elements are needed by plants in very small amounts and are seldom lacking in soil. Most are already present or included in commercial fertilizers. If you suspect a deficiency of minor elements, consult your county extension educator before undertaking any corrective treatments. Some minor elements, especially boron, zinc, and manganese, may be toxic to plants if too much is applied. Bottom line (again): more is not better.

Fertilizer comes in two basic forms: organic, often called natural, and inorganic, often called chemical or synthetic. Organic materials originate from living organisms. They are broken down in the soil by bacteria, into inorganic, water-soluble forms. Inorganic materials are mineral salts that are water-soluble. They do not need bacteria to make them available to plants. Once the nutrient is converted to the available inorganic material, it is equally useful to the plant, whether it originated from organic matter, inorganic fertilizer, or the weathering of soil.

Organic fertilizer in its natural, moist form includes all animal manures and compost made from manure and other plant or animal by-products. Commercial organic fertilizers include dried manures, bone and blood meal, and cottonseed and soybean meals. The nutrients are available more slowly over a longer period and are less likely to be leached from the soil. Organic fertilizers generally cost more than inorganic types and are unavailable to the plants until soil microbes break them down.

Inorganic fertilizers include materials prepared from mineral salts, which contain plant nutrients in combination with other elements. A complete fertilizer, one containing all three major elements, is useful for many soils and crops. What do these numbers on a bag of fertilizer really represent? By law, the label on the fertilizer package must indicate the amount of nitrogen, phosphorus, and potassium in the product, in that order. Each number equals the percentage of the element in the bag. A bag of 5-10-5 contains 5% nitrogen, 10% phosphorus, and 5% potassium. The rest of the bag is filler. Before you think that such low percentages are wasteful, remember that concentrated nutrients will damage or destroy the plants we want to nourish.

Fertilizer formulations suitable for general lawn and garden use are 5-10-5 and 5-10-10. For the most part, fertilizers with a nutrient ratio of 1-2-2 or 1-3-1 for N, P, and K will meet your needs. For planting vegetable and flower transplants, you may wish to use one of the high-ratio, water-soluble fertilizers such as 10-55-10 (1:5:1).

If only one element is deficient in a soil, non-mixed fertilizers will be adequate. The most important of these non-mixed materials are nitrogen and phosphorus. Nitrogen is available in ammonium nitrate, ammonium sulfate, or urea. Phosphorus is available in superphosphates or bone meal. Potassium is available as potassium chloride or potassium sulfate.

Inorganic fertilizers do have advantages. They are relatively inexpensive, and small amounts of them provide necessary nutrients. These soluble nutrients are quickly available to the plants. Because of this, over-fertilization may occur and injure growing plants. A soil test indicates when fertilizer is and is not needed. Over-fertilization is more serious than under-fertilization. Once nutrients get into the soil, it is impossible to remove them.

To make the best use of soil, you should actively participate with soil—test every three to five years and apply lime and fertilizer only as indicated by the soil test report. Keep it natural—use adequate organic matter to improve the soil quality. Put the right plant in the right place—select plants that are suitable for your soil while using disease-resistant varieties of seeds and plants. Prevent competition—avoid over-crowding plants and manage weeds.