Understanding Fertilizers

Fertilizer Time

What you need to know about understanding fertilizers in synthetic vs. organic choices and how they work

Understanding fertilizers at nurseries and garden centers is enough to dazzle most gardeners. Shelves are piled with boxes and bottles, floors covered with bags stacked high. Labels identify fertilizers as rose food, vegetable food, lawn fertilizer or general-purpose fertilizer. In some stores, there will be bins filled with bone meal, blood meal, or hoof-and-horn meal ― all labeled natural fertilizers. Choosing the right products to keep plants healthy can often be quite confusing.

1. Understanding N-P-K ratios

All fertilizers packages will come with information about the nutrients they contain. Prominently featured will be the N-P-K ratio, the percentage the fertilizer by volume of nitrogen (N), phosphorus (P), and potassium (K). A 16-16-16 fertilizer, for example, contains 16% nitrogen, 16% phosphorus, and 16% potassium. A 25-4-2 formulation contains 25% nitrogen, 4% phosphorus, and 2% potassium.

Fertilizers contain at least one of these components; if any is missing, the ratio will show a zero for that nutrient. A 12-0-0 fertilizer contains nitrogen but no phosphorus or potassium. Boxed, bagged, and bottled products display the N-P-K ratio on the label. For fertilizers sold in bulk from self-serve bins, the ratio is noted on the bin; for future reference, take the time to write the information on the bags purchased.

2. Complete vs. incomplete fertilizers

A fertilizer containing all three major nutrients is called a complete fertilizer; a product that supplies only one or two of them is an incomplete fertilizer. Using a complete fertilizer for every garden purpose seems sensible, but in fact it’s not the best choice. If the soil contains sufficient phosphorus and potassium and is deficient only in nitrogen, as is often the case, save money by using an incomplete fertilizer that provides nitrogen alone, ammonium sulfate, for example. In some instances, complete fertilizers can even harm a plant. Exotic, bright-blossomed proteas, for example, will not tolerate excess phosphorus: they “glut” themselves on it and then die.

Inexpensive soil test kits sold at garden centers can give you a general idea of the nutrients available in your garden; for a more detailed evaluation, have a professional analysis to reveal which nutrients may be lacking. Such tests can help choose an appropriate fertilizer.

3. Special-purpose fertilizers

Various general-purpose fertilizers contain either equal amounts of each major nutrient (N-P-K ratio 12-12-12, for example) or a slightly higher percentage of nitrogen than of phosphorus and potassium (such as a 12-8-6 product). Such fertilizers are intended to meet most plants’ general requirements throughout the growing season such as tomatoes.

Special-purpose fertilizers, on the other hand, are formulated for specific needs. They’re aimed at the gardener who wants a particular combination of nitrogen, phosphorus, and potassium for certain plants or garden situations. These fertilizers are of three general types.

One type, used during the period of active plant growth, contains largely nitrogen. Such products, with N-P-K ratios such as 16-6-4, are often used in spring, to encourage lush growth or green up your lawn.

Another type is meant to stimulate root growth, stem vigor, flower and fruit production. Fertilizers of this sort contain little nitrogen and higher levels of phosphorus and potassium; the N-P-K ratio may be 3-20-20. These products are applied at different times and in different ways, depending on what is needed in the garden. To prepare a new planting area, for instance, work a dry granular fertilizer of this sort deeply into the soil to put phosphorus and potassium where roots can absorb them. The nutrients help strengthen the new plants’ developing stems and encourage the growth of a dense network of roots.

To promote flower production and increase the yields of fruit or vegetable crops, apply the same sort of fertilizer to established plants after they’ve completed their first burst of growth. Use either dry granules, scratching them lightly into the soil, or apply a liquid formula with a watering can or a hose-end applicator.

A third group of fertilizers is for use on specific plants with features determined to promote the best performance from a particular plant. These other elements are proven to be valuable to that plant. Such fertilizers are named for the plant they’re intended to nourish. Such as citrus trees and acid-loving plants such as camellia and rhododendron.

Other such plant-specific fertilizers have appeared on nursery shelves claiming to be the best choice for a certain plant or group of plants. Several sorts of tomato food or flower fertilizers are available. The jury is still out on the benefit of many of these products, and often a general-purpose fertilizer will work just as well. The main distinction seems to be the price: the special formulas are usually costlier than general-purpose kinds.

4. Synthetic vs. organic fertilizers

Some fertilizers are manufactured in the laboratory, while others are derived from natural sources. Each has certain advantages.

Synthetic fertilizers: are derived from the chemical sources listed on the product label. They tend to be faster acting than organics and provide nutrients to plants quickly, making them a good choice for aiding plants in severe distress. Synthetic fertilizers are sold both as dry granules to be applied to the soil and as dry or liquid concentrates to be diluted in water before application. In dry form, they’re usually less expensive than their organic counterparts. In some of the dry granular types those known as controlled-release fertilizers, the fertilizer granules are coated with a permeable substance. Each watering releases a bit of fertilizer through the coating and into the soil. Depending on the particular product, the nutrient release may last anywhere from 3 to 8 months.

Some synthetic products are packaged for special purposes; you’ll find spikes and tabs for container plants, for example.

Note that synthetic fertilizers usually do not contain any of the secondary or micronutrients ― but in most cases, these nutrients are already present in the soil. If a test indicates that some are missing, look for a fertilizer that provides them.

Organic fertilizers: organic fertilizers are derived from the remains of living organisms; blood meal, bone meal, cottonseed meal, and fish emulsion are just a few of the many types. Organic fertilizers release their nutrients slowly: rather than dissolving in water, they’re broken down by bacteria in the soil, providing nutrients as they decompose. Because these fertilizers act slowly, it’s almost impossible to kill lawns or plants by applying too much. Overdosing with synthetics, by contrast, can have potentially fatal results. Some manufacturers combine a variety of organic products in one package and offer them for general-purpose or specialized use.

Two commonly used soil amendments ― compost and manure ― have some nutritive value and can be used as part of an organic fertilizing program. The N-P-K ratio of compost varies from 1.5-.5-1 to 3.5-1-2. Chicken manure’s N-P-K ratio ranges from 3-2.5-1.5 to 6-4-3. Steer manure is usually a little less than 1-1-1.

Fertilizers containing seaweed are increasingly popular with many gardeners. Besides providing nutrients in a form immediately available to plants, seaweed contains mannitol, a compound that enhances absorption of nutrients already in the soil, and various hormones that stimulate plant growth. And, the carbohydrates in seaweed break down rapidly, nourishing soil-dwelling bacteria that fix nitrogen and make it available to plant roots.

Mixed with water and sprayed directly on foliage, seaweed-containing fertilizers can make a dramatic effect in a matter of days. Plants green up and begin to produce new growth, and those that are weak stemmed and straggly straighten up and become stronger.


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