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Understanding Fertilizer Components

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With the end of summer quickly approaching, now is the time to start thinking about the final fertilization of your lawn this autumn. For many folks, a walk down the fertilizer aisle of the local garden center can be one of the most confusing parts of lawn and garden care. But it doesn’t have to be Greek to you. Understanding the basic components of fertilizer will help you find the right products to help your lawn grass reach its full potential. 

The NPK Ratio

First it’s important to understand that all fertilizers, regardless of type and specific use, have something called a NPK ratio. The NPK ratio will be prominently labeled on the package and indicates the percentage of primary nutrients the fertilizer contains. For plants, the primary nutrients are nitrogen (N), phosphorus (P) and potassium (K). For example, a fertilizer with a NPK ratio of 12-8-6 will contain 12% nitrogen, 8% phosphorus and 6% potassium. You may also see fertilizers referred to as “complete” or “incomplete”. A complete fertilizer will contain all three primary nutrients, while an incomplete fertilizer lacks one or two primary nutrients. Both complete and incomplete fertilizers have their uses, so it shouldn’t be assumed that a complete fertilizer is better than an incomplete fertilizer. 


fertilizer pouring from hand2Lawn grass requires nitrogen more than any other nutrient. Nitrogen can be found in all living cells and is responsible for a number of factors including overall growth, color, photosynthesis and seed production. Nitrogen is available in many forms, including quick-release synthetic types and slow-release organic types. Natural sources of nitrogen can be found in grass clippings which can be left on your lawn as a natural fertilizer. White clover overseeded into lawn is another excellent source of natural nitrogen. Clover is known as a nitrogen “fixer”, and can transform atmospheric nitrogen into usable forms in the soil. While quick-release synthetic nitrogen can produce fast results, if not applied correctly it has the potential to pollute waterways or even damage your lawn. On the other hand, slow-release organic forms of nitrogen do not have to be applied as often, tend not to leach into waterways, and pose little threat of damaging your grass. 


Phosphorus is a little more controversial when it comes to lawn fertilizer. While it’s still considered a primary nutrient and is required for photosynthesis and root development, research has shown that most soils already contain enough phosphorus, especially when grass clippings are mulched and left on the lawn. In fact, many major fertilizer producers are now leaving phosphorus out of their products to help prevent waterway pollution. However, it’s important to realize that not all sources of phosphorus are the same. Organic fertilizers contain phosphorus derived from compost, plant and animal sources and won’t leach the way synthetic sources of phosphorus will. By using organic fertilizers you can still receive the benefits of phosphorus, which is especially important for new lawns, without the pollution potential of synthetic sources. 

slow release fertilizer2 by Brian Edgar


Potassium is important for building protein, photosynthesis, traffic tolerance and disease prevention. Out of all the primary nutrients, potassium is the least likely to leach or move throughout the soil. Like the other primary nutrients, potassium is available in synthetic and organic forms. 

The Importance of Soil Tests

Before adding any type of fertilizer, be sure to conduct a soil test to find out exactly what your soil is lacking. Don’t let the “test” part scare you off though. It’s actually a very simple, inexpensive process that any homeowner can participate in with help from their local Cooperative Extension service.

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