Fertilizers are one of the most important weapons in the arsenal of any successful gardener. By properly choosing and applying the right fertilizer, you can supplement nutrients that your outside soil would otherwise lack, thereby increasing your chances of successful gardening and expanding on the number of plants you can grow at home. Understanding the ways that fertilizers are rated -- using NPK ratings, for example -- is essential to choosing the right fertilizer for your gardening needs.
Nutritional Content of Fertilizers
Fertilizers are designed to provide any of a number of nutrients to plants, but, primarily, they are used to deliver the three nutrients that are most essential to plant growth: nitrogen (N), phosphorous (P) and potassium (K). It is the different concentrations of each of these three essential nutrients that distinguish different types of fertilizer from one another.
An NPK rating on a fertilizer is a series of three numbers printed on the front of the bag itself. The three numbers describe the concentration, by weight, of nitrogen content, phosphorous content and potassium content in that fertilizer. All fertilizers must list an NPK rating somewhere on the packaging, and it is usually found on the front of the bag near the brand name.
If a bag of fertilizer has an NPK rating of 10-15-20, for instance, that would mean that that particular fertilizer contains 10 percent nitrogen, 15 percent phosphorous and 20 percent potassium, by weight, respectively. The remaining 45 percent of bag weight is usually made up of other nutrients that can assist in plant growth as well as filler material that aids in the delivery of the nutrients to plants.
NPK Ratings in Perspective
Each plant responds well to a specific type of fertilizer. When you read gardening guides or consult local university extension services, it is quite common to see something to the effect of "roses respond well to a 10-5-10 fertilizer," or the like. It is essential to understand what these NPK ratings mean when choosing a fertilizer for your plants. Sometimes these same sources will use a sort of shorthand to describe types of fertilizers; a "balanced fertilizer" for instance, is one that contains equal concentrations of all three nutrients; a nitrogen-rich fertilizer is one that contains a higher amount of nitrogen than the other two components, and so on.