Selecting Fertilizers and Understanding Their Labels

To get the most value out of your lawn fertilizer, an understanding of what your lawn needs and what a particular fertilizer can supply is essential. If you understand what is on the label you can make the right selection, save money and help protect the environment.

There are literally hundreds of fertilizer formulations available, each with different analysis, which means they contain different amounts of nutrients. Nutrients contained in a fertilizer are indicated on the label by the large numbers on the bag. A complete fertilizer is one that contains nitrogen (N), phosphorus (P), and potassium (K). (If you are wondering why the letter K stands for potassium, it comes from the word Kalium which was the name given to this element by its German discoverer.) Depending on your soil nutrient status you may not need all the nutrients in a complete fertilizer. The only way to know is to have your soil tested. Contact your local county extension office for more information on soil testing or to have a soil test kit sent to you.

What You May Find on a Fertilizer Label:
1. The name brand or trademark.
2. Guaranteed chemical analysis which may include % water insoluble nitrogen.
3. Nutrient sources.
4. Manufacturer's name and address.
5. Net weight of the fertilizer in the container.

Two very important features on the label are the guaranteed analysis and the net weight.The guaranteed analysis is always given in a specific order and is given as the percent of nitrogen for the first number, the percent of phosphate for the second number, and percent potash for the third number.

In our example (see Fig.1), there is 30 percent nitrogen, 4 percent phosphate and 4 percent potash. Nitrogen is the first number, phosphate (P₂O₅), is the second number, and potash (K₂O) is the third number. Only nitrogen is given as the actual percentage of the nutrient. The actual amount of phosphorus (phosphate) and potassium (potash) are given as percents of their oxide forms. This is not a problem because soil tests are usually reported in these same oxide forms as are the recommendations for fertilizer application.

Figure 1. Hypothetical Example of Information Presented on a Lawn Fertilizer Bag

EASY GREEN LAWN FERTILIZER 30-4-4 GUARANTEED ANALYSIS:
Total Nitrogen (N) .......................30%
7.5% Water Insoluble Nitrogen (WIN)
22.5% Water Soluble Nitrogen (WSN)
Available Phosphate (P₂ O₅ ) .......4%
Available Potash (K₂ O) ...............4%
Nutrient Sources: Urea, Methylene Ureas,
Ammonium Phosphate, Ammonium Sulphate,
Muriate of Potash.
Easy Green Corporation
Greenlawn, MN 54321
Net Weight 60 lbs.

Getting What You Paid For

Our bag contains 60 pounds of fertilizer. Since we know the percent of nitrogen, phosphate and potash, we can calculate how many pounds of each element using the following formula:

(% nutrient / 100) x total lbs. of fertilizer = lbs. of nutrient in the bag

In our example the calculations would be as follows:

Nitrogen: (30% / 100) x 60 lbs. = 18 lb. nitrogen.Phosphate: (4% / 100) x 60 lbs. = 2.4 lb. phosphate.

Potash: (4% / 100) x 60 lbs. = 2.4 lb. potash.

If you add up the total pounds of nutrients, you may wonder why it does not equal 60. Have they added filler? Not necessarily. The nutrients you are applying may have other molecules (such as chloride or sulfate), attached as part of their naturally occurring form. These additional molecules may be a source of nutrients valuable to your lawn. For example, sulfate will provide sulfur that is beneficial to plants.

In the cases of P and K, this calculation only determines the percentage oxide of these two elements and not the actual percentage of each nutrient. For example, the element P only makes up 44 percent of the P₂O₅ or phosphate molecule and the element K only makes up 83 percent of the K₂O molecule. However, unless a soil test directs you otherwise, the amount of product to apply will be given in terms of the phosphate and potash designations. When occasions arise where the precise amount of P or K need to be determined, multiplying the phosphate percent by .44 and the potash percent by .83 will give the exact amount of P and K respectively.

Types and Forms of Nutrient Sources

Let's look at what else is on the label. The water insoluble nitrogen (WIN) is not available to your turf right away, so it can release the nitrogen to your lawn over a period of months or longer. The 7.5 percent WIN has a low burn potential if accidentally spilled on the lawn. (However, all spills should be cleaned up as quickly as possible to prevent possible injury to the lawn.) In addition to the above advantages, the WIN will stretch out the time between applications and give greater environmental protection since the WIN sources will not leach as rapidly into groundwater. This is especially important on sandy soils.

The water soluble nitrogen (WSN) is available rapidly and gives quick results. The 22.5 percent WSN can "burn" the lawn if spilled or over applied. It can also move into the groundwater if there are heavy rains following the application. Both WIN and WSN sources can be good choices for managing your lawn. However, the higher the percentage of WIN in a fertilizer, the higher the cost as WIN sources are more costly to include than WSN sources.

Another environmental consideration is the phosphorus which may run off into surface water areas and degrade water quality. A lawn needs phosphorus for good health, but if the soil has a sufficient amount, as determined by a soil test, it does not need to be applied every year. There are formulations of fertilizer that contain only nitrogen or only potash if those are the elements you need.

Other Considerations in SelectionThere are other considerations in addition to analysis, cost, and environmental effects when selecting a lawn fertilizer. They include:1. Ease of spreading may depend on granule size or type of spreading equipment.2. Bulkiness of the product, such as some of the organic fertilizers which have low analysis and may require many bags to be applied over a given area.3. Ease of storage. Some types, such as ammonium nitrate, become hard when stored more than one season.4. The size of the fertilizer particles may affect how dusty the fertilizer is and may not provide even coverage with some types of fertilizer spreaders.

5. If you need a greater amount of phosphorus or potassium than that supplied by your complete fertilizer consider applying it as a separate application.

Liquid vs. Dry FertilizersIt is quite common to apply lawn fertilizer dissolved in a liquid rather than as a granular material. It makes little difference to the plants whether the nutrients are applied in liquid or dry form. Plants are able to use nutrients absorbed through the foliage as well as the roots although the majority of nutrient uptake occurs in the roots.Fertilizer-Herbicide CombinationsLawn fertilizers are commonly mixed with herbicides to reduce the labor involved in lawn maintenance. Care should be used in applying these products. Fertilizer applications should be a regular lawn maintenance practice, but herbicides should be used only when specific weed problems occur. For effective weed control, herbicides are used at specific times during the year, sometimes differing for various weeds. The time of year for herbicide effectiveness may not be the optimum time for fertilizer application.

An additional problem with combination products is that fertilizers should be watered-in following application for maximum effectiveness and low burn potential, while many herbicides need to remain on the plant leaves for effective plant control. Consequently, by using fertilizer-herbicide combination products you often compromise the effectiveness of one or both products from a timing or application standpoint, or both. Certainly fertilizer-herbicide combinations should never be used when the herbicide would be ineffective or unnecessary. Always check the specific product label for any post application requirements (e.g., whether it should be watered in or not).

How Much Fertilizer Is Needed for One Application?

The release characteristics of a fertilizer and its burn potential determine the amount that can be applied in a single application. Fertilizers with quick-release sources of nitrogen or potassium can burn the plants if applied at high rates. In addition, applying too much nitrogen in one application is inefficient since the nitrogen not used by the plant can leach through the soil and out of the root zone. Consequently, quick-release forms of nitrogen should always be applied at a rate not exceeding 1 lb. N/1000ft² in any one application. Since slow-release forms of nitrogen are released gradually over a longer period of time, slightly higher rates can be applied. Some fertilizer products will provide suggested application ranges for their product.

The area to be covered by a bag of fertilizer, using a desired nitrogen application rate, can be determined from the information on the bag.weight of bag x N in fertilizer (fractional basis) = area to be covered by fertilizer
desired rate of application in the bagFor example, if you want to apply fertilizer at a rate of 1 lb N/1000 ft² and you have a 20-pound bag of a fertilizer having a grade of 23-3-6 (the fertilizer is 23%N), then

20 lb. x .23 = 4600 ft² covered per bag

1lb N/1000 ft²

The fertilizer should be used to cover 4600 square feet of lawn area. If the lawn area is less than 4600 ft², only a portion of the bag is needed to supply 1 lb N/1000 ft². If your lawn area is 2500 ft², then 11 pounds of fertilizer should be applied.

20 lbs x 2500 ft² = 11 lbs.
4600 ft²/bag

Calibrating Your Spreader to Insure Accurate, Cost-Effective Fertilizer ApplicationMany fertilizer spreaders are available and easy to use. Two types of spreaders for granular materials are available: "drop" and "rotary" spreaders. Drop spreaders distribute the fertilizer directly below the hopper in a well-defined pattern. Rotary spreaders throw the fertilizer material out beyond the spreader in several directions and are satisfactory for most lawns. Although rotary spreaders give a less precise distribution, they are much faster and are less apt to leave a striped pattern on the lawn where areas were missed. With either type of spreader it is a good practice to fertilize one-half the desired application rate in one direction, then the second-half perpendicular to that direction.

Fertilizer spreaders will apply different materials at different rates. Ideally, you should calibrate your spreader for your pace and the fertilizer used. To calibrate a spreader with a given fertilizer, adjust the spreader setting to a selected level, and measure the ground area covered with fertilizer in the process. It may be convenient to do this on a sheet of plastic or hard surface area that can be swept. To calibrate according to the pounds of nitrogen/100 ft², make the following calculation:

lbs. of fertilizer/100ft² x N in fertilizer (fractional bases) = lbs. N/ft²

then:
lbs. of N/ft² x 1000 = lb N/1000 ft²
For example, if the spreader is set at 8, you find 0.6 pounds of fertilizer covers an area of 100 ft², and the fertilizer has a grade of 23-3-3 (the fertilizer is 23% N), then

0.6 lbs. of fertilizer/100ft² x 0.23 lb N = 0.14/100ft² = 0.0014 lb N/ft²

^{0.0014 lb N/ft2 x 1000 = 1.4 lb N/1000 ft2 applied at spreader setting of 8}

This spreader, at a setting of 8, applies this fertilizer at the rate of 1.4 lb N/1000 ft². Next, make the same measurement at several spreader settings. Then develop a chart for that fertilizer with the rate of nitrogen application at various spreader settings. You can choose the setting for any desired application rate. Realize that different fertilizers have different calibration curves. Manufacturers of lawn fertilizers often recommend a setting for specific fertilizer spreaders.