Of the more than 115 chemical elements in the periodic table, only 16 are sure to benefit plant life. These are essential nutrients. (There might be others that we don’t know about, but surely they must be needed in very small quantities and are probably never lacking.)
Carbon (C), oxygen (O) and hydrogen (H) are the elements that make up carbon dioxide, oxygen gas and water, all necessary for plant survival and growth. These are always available in the air and soil.
Nitrogen (N), phosphorus (P) and potassium (K) are macronutrients because plants require them in larger quantities than the other essential nutrients. Some soils do not have enough of one or more of these elements in a form that is readily available to plants, so we add them to the soil with fertilizers.
Calcium (Ca), magnesium (Mg) and sulfur (S) are secondary nutrients and are usually available in adequate amounts in most soils. Many general purpose fertilizers contain these elements as well as the macronutrients.
The micronutrients, boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn), are almost always available in sufficient quantities in most soils. Some specialty fertilizers include these.
Commercial fertilizers have their percentages of the three macronutrients: Nitrogen, Phosphorus and Potassium (N-P-K)
If the N-P-K is 10-10-10, it means there’s 10% nitrogen, 10% phosphorus and 10% potassium by weight. If you want to add 1 pound of nitrogen per 100 square feet to your garden, you will need to add 10 pounds of this fertilizer per 100 ft2. Note that you also will be adding 1 pound of phosphorus and 1 pound of potassium. If nitrogen is the only nutrient your soil needs, you’d save money and be just as well off using one labeled 10-0-0. (In fact, some plants can be hurt by too much phosphorus or potassium, so there is no good reason to keep adding more if not necessary.)
Which is better, 10-10-10 or 18-18-18? You’d need less of the 18-18-18. That’s the only difference. (That, and the fact that the arithmetic is harder.)
We add essential nutrients to our soils with organic or synthetic fertilizers
The elements in each are the same and the plants don’t care where they come from! Organic ones act more slowly and last longer than synthetic ones. This is because soil microorganisms must break down the nutrients before they can go up the plant roots. That makes it harder to over-fertilize, and you don’t have to fertilize as often. However, organic fertilizers also release their nutrients more erratically, depending on soil moisture and temperature. The nutrients in organic fertilizers are not concentrate as they are in synthetic fertilizers, and you need very large amounts of organics to deliver the same amount of nutrients as synthetics. Furthermore, organic fertilizers are not “complete” fertilizers and the various types of organic fertilizers contain widely differing amounts of the essential macronutrients.
Some common organic fertilizers and their N-P-K ratios are:
- Dried cow manure (2.0-1.8-2.2), poultry manure (6-4-3) or horse manure (0.7-0.3-0.6)
- Fish meal (7-7-trace)
- Activated sewage sludge (aka, milorganite)(5-3-trace)
- Blood meal (10-1.6-0.8)
- Bone meal (2-30-0)
- Wood ash (0-trace-10)
- Cottonseed meal (7-3-2)
- Typical homemade compost (2-1-2)
Synthetic fertilizers provide better control of nutrient levels and act more quickly than organic fertilizers because they are already soluble in water and in a form that plant roots can absorb. A class of synthetic fertilizer, slow release (or controlled release), have a layer of a substance that dissolves gradually on their granules so that the nutrients are not in the soil all at once. With these (typically more expensive) fertilizers, nutrients are released over a period of three to eight months, up to one year. Common formulations of synthetic fertilizers are 6-6-6, 20-20-20, 5-10-5, 20-10-20 and 12-0-0, with no limit to the possible combinations.
General purpose fertilizers contain equal amounts of nitrogen, phosphorus and potassium (for example, 6-6-6) or slightly higher percentages of nitrogen (such as 10-6-6). These fertilizers supply the nutrients most often needed for most plants in most soils. In most cases, general purpose fertilizers are all you need.
Special purpose fertilizers emphasize one or two of the three macronutrients
During periods of active vegetative growth, plants need lots of nitrogen. A fertilizer, with an N-P-K ratio of 16-6-6, for example, would encourage fast growth without overloading the soil with phosphorus and patssium. This fertilizer would get your lawn off to a good start in spring.
Phosphorus and potassium stimulate root growth as well as flower and fruit production. A fertilizer with higher levels of phosphorus and potassium (for example 6-20-20) would give a welcome boost to tomato production without stimulating unwanted vegetative growth.
Some fertilizers are for specific plants. These have N-P-K ratios specifically designed for the needs of citrus, palms, or acid-loving camellias and azaleas, for instance.
The most important thing to remember about fertilizing is to follow label directions. Have your soil tested by your local extension service and calculate how much nitrogen, phosphorus and/or potassium your soil needs based on the recommendations that come back. For many soils, nitrogen is the only nutrient that is deficient.