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 All commercial fertilizers contain, somewhere on their labels, three numbers separated by hyphens, such as 2-1-1 or 5-5-5 or 14-14-14 or 4-8-3. These are called the fertilizer’s “N-P-K” indicator, named for the chemical symbols for nitrogen (N), phosphorus (P), and potassium (K), the three elements most required in quantity for plant growth and health (there are 14 other nutrients that plants need in very small quantities).
The hyphenated numbers on the fertilizer label represent the percentage of each element that the fertilizer makes available to plants that are fed with it. So a fertilizer labeled 5-5-5 contains 5% available nitrogen, which helps plants create rapid soft growth; 5% available phosphorus, which stimulates flowering and fruiting; and 5% available potassium, which is needed for root and stem growth. Similarly, a 4-8-3 plant food contains 4% available nitrogen, 8% available phosphorus, and 3% available potassium.
Complete fertilizers contain all three major elements (for example, 10-10-10). Simple fertilizers contain only one of the three (such as 10-0-0, which contains only nitrogen). Incomplete fertilizers contain two out of the three (such as 0-10-10, which contains phosphorus and potassium but no nitrogen).
Let's take a closer look at these three vital nutrients.
Nitrogen: Key To Healthy Plant Growth
Nitrogen (N), the first number on the fertilizer label, is vital to rapid plant growth, since it is the basic building block of plant proteins and a major ingredient of chlorophyll, the chemical that makes leaves and other plant parts green and helps plants capture energy from the sun. Most crops rapidly use up available soil nitrogen and require repeated applications of nitrogen fertilizer for best growth.
One notable exception is members of the legume family, such as peas, sweetpeas, beans, and alfalfa: legumes possess specialized root-bacteria that convert nitrogen in the air into a form plants can use, so legumes are often planted as cover crops in poor soils in order to enrich those soils with the nitrogen the plants harvest from the air.
Many concentrated commercial nitrogen fertilizers are derived from nitrates or ammonium (and often both). Nitrate is a mined mineral form of nitrogen that is easily soluble in water and readily available to plants, particularly in the cool soils of spring and autumn. However, too much of it can burn plant roots and pollute groundwater. It is also easily washed away by rains. Ammonium fertilizer is derived from dried stockyard animal blood (also available commercially in powdered form, called "blood meal") or synthesized in the laboratory as IBDU (isobutylidene diurea). Plants can't use the nitrogen in ammonium fertilizer until soil microorganisms break it down into nitrate. Ammonium fertilizer is also available commercially as ammonium sulfate.
For gardeners who prefer avoiding stockyard products or synthetic fertilizers, other good organic sources of nitrogen include plant-derived alfalfa, soy or cottonseed meals, bat or seabird poop (called "guano"), and fish emulsion or meal, usually byproducts of the fishing industry. Such organic nitrogen sources break down very slowly in the soil, and it's very difficult to burn plants with them when they are properly applied.
Too much nitrogen fertilizer can result in weak, leggy plants with few flowers and fruit but lots of lush, tender foliage that’s easily susceptible to pests and diseases. For that reason, we always recommend applying nitrogen fertilizers according to the rates on the fertilizer package.
Phosphorus: The Plant Energizer
 Phosphorus (P), the second number on the label, is often called the “energizer,” since it helps store and transfer the sun’s energy during photosynthesis. It also makes up an important part of a plant’s genetic material, RNA and DNA. Phosphorus stimulates early plant and root growth; improves a plant’s ability to absorb water and other nutrients; stimulates blooming and the development of fruits and seeds, and strengthens plants against environmental stresses.
Most annual plants, which live out their entire lives within one calendar year, require lots of phosphorus, particularly in the beginning of their growing season. So do many vegetables, like lettuce and beans. Older, more established plants need less phosphorus. So-called "bloom fertilizers", such as Carl Pool's BR-60™, are formulated with extra high levels of phosphorus.
Phosphorus is normally derived from mined phosphate-bearing rock. Superphosphate, a highly concentrated form of phosphorus, is created by treating natural phosphate-bearing rock with chemicals. Organic gardeners often prefer untreated, naturally occurring ground phosphate rock, bat guano, or bone meal (a stockyard byproduct) as phosphorus providers.
Too much phosphorus can result in plants that bloom and set seed too young, before they have developed properly, but this is rarely a problem in New Mexico soils, which tend to be low in this element.
Potassium: The Immune System Regulator
 Potassium (K), the third number on the label, is often called the “regulator,” since it is involved with more than 60 different plant enzymes that control all aspects of plant growth. Potassium helps slow down plant diseases; helps plants build the cellulose (plant fibers) needed for stalk and stem strength; aids in photosynthesis; increases root growth; increases the size and quality of flowers, fruits, grains, and vegetables; and improves drought resistance by reducing water loss from leaves, thereby inhibiting plant wilt.New Mexico soils are naturally high in potassium; in fact, more potassium is processed in New Mexico than in any other state in the Union. So plants grown in New Mexican soils rarely need additional potassium. But plants grown in containers filled with potting soil need potassium (along with nitrogen and phosphorus), since it is not naturally present in commercial potting mixes.
How Much and How Often Should I Fertilize?
Some gardeners concerned for the growth and health of their plants may be prone to overfertilizing, on the principle that “if a little bit is good, more will be better.” Don't do it. Plants have evolved to seek out and employ nutrients in highly diluted soil concentrations. So follow the directions on the fertilizer label precisely, particularly when using inorganic fertilizers.
It's also important to fertilize plants regularly, rather than letting them starve for months on end, then dumping a lot of plant food on them all at once. Most outdoor, ground-grown trees, shrubs (including berry bushes), vines, and perennials, respond well to twice yearly fertilization, in spring just around bud-emergence and at midsummer. Avoid fertilizing in the fall — it can stimulate soft growth just in time for it to be blasted by first frosts.
To fertilize outdoor, in-ground plants, pull the mulch away from the plants to be fertilized, spread the fertilizer evenly around the plant, then work it an inch or two into the soil. Replace the mulch and water well.
Alternatively, fertilize outdoor in-ground plants with a water-soluble plant food, either mixed by hand or with the use of a hose-end fertilizer sprayer.
Hint: Many gardeners use water-soluble-fertilizer and report very good results from applying the plant food not only to the roots of the plants, but also to their leaves and stems. This practice, called "foliar feeding" ("foliar" = "pertaining to leaves"), is best done before 9:00 in the morning, when the pores of most plants are open to receiving nutrients.
Foliar Feeding Tips:
• Spray the plants with plain water before foliar feeding.
• Never use this method in the full heat of a peak summer day (especially when temps reach 85ºF or above) because the action of the sun on freshly sprayed foliage can burn the leaves.
Annual vegetables, such as tomatoes, peppers, and squashes, enjoy monthly feeding. Container-grown plants may be fed with every watering, using liquid plant food diluted to half the dosage recommended on the label, or once every week or two at full dosage. Another option is fertilizing with a time-release granulated plant food, such as Osmocote™ which, for three to four months, will release nutrients into the soil each time the plant is watered. This eliminates the bother of mixing fresh plant food. For container grown flowers, fruits, and fruiting vegetables, use 14-14-14 Osmocote™ formula; for leaf vegetables and foliage plants, use Osmocote™ with extra-high-nitrogen.
What About Micronutrients?
Nitrogen, phosphorus, and potassium are by no means the only nutrients plants need for optimum growth and health. Iron, zinc, copper, magnesium, manganese and many others are needed, too, in small but essential quantities. Such nutrients are called trace minerals or micronutrients, and without them, plants can develop trace mineral deficiency diseases. Check fertilizer labels for the presence of micronutrients (the best brands will contain them).
A Word About Iron: Northern New Mexico's red clay soils are naturally high in iron. But they're also highly alkaline, with a pH of over 8.0. At such high alkalinity levels, iron molecules are bound up in the soil and cannot be absorbed by the roots of most of the ornamental and edible plants gardeners favor. For this reason, extra supplements of iron may be necessary to keep plants from developing iron-deficiency symptoms, such as chlorosis (yellowing of the leaves). Ironite™ is probably the most popular iron supplement product, containing iron in a form plants can readily absorb. But be sure to follow the directions on its label exactly. Too much Ironite™ can burn plant roots.
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