Understanding Fertilizer: What's in the Bag?

Fertilizer additions help produce a thick, weed-free lawn, abundant flowers, and an ample vegetable harvest. Usually, everything we need to know about the fertilizer is on the box or bag. Too often, however, gardeners do not really understand what the package says, or do riot pay attention to the directions. Proper application is important - too little is ineffective, too much can be damaging or wasteful. The ratio of the three major nutrients fertilizer contains is one of its most important characteristics. This ratio indicates the relationship between the nitrogen, phosphorus and potassium (potash) in the fertilizer.

Numbers on the bag indicate the percent of each nutrient by weight. These amounts must be guaranteed by the manufacturer, and are subject to periodic inspection. These three percentages are indicated somewhere on the package whether a fertilizer is organic, inorganic, slow release or a combination of these types. If one or more of these three nutrients is not included, the label contains a zero in that position. The three major nutrients on a fertilizer label are always listed in the same order; 1 - nitrogen, 2 - phosphorus, 3 - potassium. Ratios may be expressed in their simplest form, such as 1-1-1-, 1-2-1-, 3-1-2- or many others.

Converting these simple ratios into more familiar fertilizers, they would become 10-10-10 or 12-12-12, common garden fertilizers; 5-10-5, commonly used on flowering garden plants; and 12-4-8 or 24-8-16 which are analyses found in some lawn fertilizers. There are two approaches to fertilization, the first is to improve the soil that has had little or no care. Usually a soil test is needed to determine how much fertilizer is needed. The second is to maintain a good fertility level once the basic soil has been improved.

Remember that soil improvement requires more than fertilizer addition. Such factors as working soil at proper moisture levels, good drainage and addition or organic matter or lime may also be part of soil improvement. In some cases, especially lawns, maintenance fertilization is common and frequent. The amount to add is based largely on the nitrogen content of the fertilizer. As a general rule, not more than one pound of actual nitrogen should be added in a single application.

To determine how many pounds of a certain fertilizer is needed to add this much nitrogen, remember that the numbers on the bag indicate the percent of each nutrient by weight. Therefore, 100 pounds of a 12-4-8 or a 12-12-12 will each contain 12 pounds of nitrogen. Using this simple figure, we then know that if the fertilizer comes in a 25-pound bag, it will contain three pounds of actual nitrogen. To apply a rate of one pound actual nitrogen per 1,000 square feet of lawn, we know that we will need one-third of the 25-pound bag, or about 8% pounds.

The fertilizer that contains twice as much nitrogen (such as the 24-8-16) will require only half this amount to apply the same one pound of nitrogen. Not all ratios and formulations are this simple, however. When slowly available sources of nutrients are contained, we may be able to apply more, since they are released over a much longer period of time. These are excellent fertilizers and can reduce the frequency of application, but pay close attention to manufacturer directions before using them.

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