The best way to fertilize will depend on what it is that you're planning to fertilize and what is in the fertilizer.
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So to answer the question, “What is the best way to fertilize?”, we really have to address two things:
1. What's in the fertilizer?
2. What is it that we're planning to fertilize?
Starting with “What’s in the fertilizer?”, it’s important to note that any bag of fertilizer that we look at has 3 numbers on it. And those three numbers represent Nitrogren, Phosphorus, and Potassium in that order. It’s always in that order. Those 3.
Nitrogen is responsible mainly for the greening and the overall shoot growth of the plant, whether that’s turf or ornamental plants.
Phosphorus is responsible for the establishment of the plants, whether that be new seed or new sod. In landscape ornamentals, it’s also responsible for blooming.
Potassium is important for the overall cell strength of the plant.
So turfgrass fertilizers, as a general rule, are in a rough ratio of about a 3:1:2. So meaning there’s about 3 times as much Nitrogen as Phosphorus, and about 2 times as much Potassium as Phosphorus.
Now, it’s always important to start with a soil test to really know what your turfgrass needs, but again this would be a very common ratio and it’s really important to note that it’s about the ratio and not necessarily the actual numbers on the bag.
For example, you might see a fertilizer that’s a 24-3-12. You might see one that’s a 24-5-11. But roughly speaking, those are about in that 3:1:2 ratio. A very common fertilizer for turf.
If we were starting a new lawn though or starting sod, again that Phosphorus, that middle number becomes really important. Those seedlings can’t reach the available Phosphorus in the soil.
So that’s where you might see something like a 6-20-10. And again, maybe it’s more 6-20-20. It’s not so much about the exact numbers, but just recognizing that when you see this center number being very high in comparison to the other two, that is mostly likely meant for starting a new lawn or new sod.
Landscape ornamentals on the other hand typically are more balanced. So their ratio would be often times a 1:1:1. Equal parts Nitrogen, Phosphorus, and Potassium. So again, very common to see products like a triple 15 (15-15-15), a triple 9 (9-9-9). And so while those numbers might be slightly different, the ratio is the same. That fertilizer contains equal parts of Nitrogen, Phosphorus, and Potassium.
And so that’s where, and we’ll talk in more detail later, we gotta know not just what these numbers are, but what’s the makeup of each of those. What type of Nitrogen for example? Or do these contain other things like Iron or Manganese or other nutrients in addition to these three that the plants might need.
Sometimes a fertilizer ratio is specified on soil test reports or in fertilizer recommendation sheets. The fertilizer ratio indicates the proportion of nitrogen, phosphate, and potash in the fertilizer product, and is determined by dividing each of the three numbers in the fertilizer grade (listed on the container) by the lowest number in the grade. For example, when each number in a fertilizer grade of 18-6-6 is divided by 6 (the lowest number in the grade), the proportion is determined as: three parts nitrogen to one part phosphate, to one part potash. Thus, the fertilizer has a 3-1-1 ratio.
If your soil test report recommends applying 1.5 lb of nitrogen, 0.5 lb of phosphate, and 0.5 lb of potash per sq ft, you should apply a fertilizer with a ratio of 3-1-1 since you need three times as much nitrogen as phosphate and three times as much nitrogen as potash. The simplest method of determining a ratio is to divide the weights of nitrogen, phosphate, and potash listed on the soil test report by the lowest weight of the three.
To determine a fertilizer ratio for a soil test recommendation of 1.5 lb of nitrogen, 0.5 lb of phosphate, and 0.5 lb of potash, divide the weight of each of the three nutrients (1.5 lb, 0.5 lb, 0.5 lb) listed on the report by the lowest nutrient weight (0.5 lb in this case).
1.5 ÷ 0.5 = 3
0.5 ÷ 0.5 = 1
0.5 ÷ 0.5 = 1
Thus, the fertilizer ratio that best fits this recommendation is 3-1-1. As demonstrated in the first paragraph, you can match the fertilizer ratio for this recommendation with a fertilizer grade by dividing each number in the fertilizer grade by the lowest number in the grade.
Determine ratios for the following fertilizer grades: 21-7-7, 22-6-8, and 18-5-9. Grade = 21-7-7Grade = 22-6-8Grade = 18-5-9 21 ÷ 7 = 3 22 ÷ 6 = 3.7 18 ÷ 5 = 3.6 7 ÷ 7 = 1 6 ÷ 6 = 1.0 5 ÷ 5 = 1.0 7 ÷ 7 = 1 8 ÷ 6 = 1.3 9 ÷ 5 = 1.8 Ratio = 3-1-1 Ratio = 3.7-1-1.3 Ratio = 3.6-1-1.8
As you can see from these three examples, fertilizer grades for certain products do not always match the exact fertilizer ratio you are searching for. Thus, the goal is to find a product with a grade that is as close as possible to the specified ratio.
Learn the basics of fertilizers, and you’ll help your garden grow as never before!
Fertilizer sections at nurseries, garden centers, and supply stores dazzle the gardener. The shelves are piled with boxes and bottles, the floors covered with bags stacked high. Labels identify the package contents as organic compost, rose food or vegetable food, lawn fertilizer or general-purpose fertilizer. In some stores, you’ll find bins filled with bone meal, blood meal, or hoof-and-horn meal―all labeled natural fertilizer. Choosing the right products to keep your plants healthy can often be a bit confusing.
Regardless of its type, any fertilizer you buy will come with information about the nutrients it contains, so you want to understand the N-P-K-ratio.
Prominently featured, the N-P-K-ratio is the percentage the product contains by volume of nitrogen (chemical symbol 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.
All 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, for instance).
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. One tip: Be sure to write the information on the bags you fill before you bring them home.
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 isn’t always the best choice. If the soil contains sufficient phosphorus and potassium and is deficient only in nitrogen (as is often the case), you can 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.
The inexpensive soil test kits sold at garden centers can give you a rough idea of the nutrients available in various parts of your garden. For a more detailed evaluation, you may want to pay for professional analysis, and there are mail-in versions of soil test kits, too. By revealing which nutrients may be lacking, testing can help you choose an appropriate fertilizer.
The various products labeled “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.
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.
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One type, used during the period of active growth, contains largely nitrogen. Such products, with N-P-K ratios such as 16-6-4, are often used in spring, when you want to encourage lush growth or green up your lawn.
Another type is meant to stimulate root growth, stem vigor, and 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, for example. These products are applied at different times and in different ways, depending on what you want to achieve. When you prepare a new planting area, for instance, you’ll work a dry granular fertilizer of this sort deeply into the soil, putting the 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, you apply the same sort of fertilizer to established plants after they’ve completed their first flush of growth. You can use either dry granules, scratching them lightly into the soil, or apply a liquid formula with a watering can or a hose-end applicator.
Courtesy of Urban Farmstead
A third group of fertilizers is designed for use on specific plants. These feature the N-P-K ratios determined to elicit the best performance from the particular plant, as well as other elements proven valuable to that plant. Such fertilizers are named according to the plant they’re intended to nourish. Especially useful are formulas for citrus trees and acid-loving plants such as camellia and rhododendron.
Recently, other such plant-specific fertilizers have appeared on nursery shelves, each claiming to be the best choice for a certain plant or group of plants; you may see several sorts of “tomato food” or “flower fertilizer,” for example. The jury is still out on the benefit of many of these products, and you will often do just as well to use a general-purpose type. The main distinction is often the price: the “special” formulas are usually costlier than general-purpose kinds.
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 available 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.
One of the many benefits of organics is they improve the “structure” of the soil the microorganisms that convert nutrients so plants can access them. Micronutrients are also often present, but they are not in synthetics.
Because these fertilizers act slowly, it’s almost impossible to “burn” or kill lawns or plants by applying too much (overdosing with synthetics, in contrast, can have potentially fatal results). Some manufacturers combine a variety of organic products in one package, then offer them for general-purpose or specialized use. Finally, they aren’t as water-soluble as synthetics, which means they won’t leach into waterways.
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; that of steer manure is usually a little less than 1-1-1.
Fertilizers containing seaweed are gaining favor 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 have dramatic effects 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.
These products are derived from the chemical sources listed on the product label. They’re faster acting than organic kinds and provide nutrients to plants quickly, making them a good choice for aiding plants in severe distress from nutrient deficiencies, but they are more likely to “burn” the plant.
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; with each watering, a bit of fertilizer diffuses through the coating and into the soil. Depending on the particular product, the nutrient release may last anywhere from 3 to 8 months.
Rachel Weill
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 mentioned above―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.