Photo by Fernando Montalvo for Gardenipedia.com.

Since many of the future posts here in Gardenipedia will refer to fertilizer selection and use, I thought I would take some time to explain one of the most basic aspects when looking at different fertilizers.  Whether the fertilizer will be used for lawns, shrubs, annuals, trees, etc., they all have three little numbers of importance labeled somewhere in the bag or box that every gardener should know about.  The picture above shows these numbers depicted on a bag of fertilizer as they usually are, three numbers separated by dashes (some fertilizer packaging features the numbers in different formats).  These numbers, part of the fertilizer’s guaranteed analysis of included plant/soil nutrients, are known as the NPK ratio.  The NPK ratio shows the amounts of nitrogen, phosphorus, and potassium (also referred to as Potash) that the fertilizer contains.  The numbers are basically what percentage of the fertilizer mix each of the nutrients comprises.  In other words, in a bag of 6-8-6 fertilizer, 6% of the mix in the bag would be the nitrogen, 8% would be the phosphorous, and 6% potash.  But, why don´t these numbers add up to 100%?  That is because the remaining 80% of the bag (in this example) is comprised of other plant nutrients, as well as components designed to deliver the fertilizer adequately.

Knowing the NPK ratio is important as you select fertilizer because different quantities and ratios of each of the three major plant nutrients will yield different results in your garden.  In some cases, the use of the wrong NPK numbers will result in plant injury or death.  Also, overuse of some of the elements leads to groundwater pollution, a growing problem (it is also an unnecessary waste of money).

Knowing which fertilizer to use is something we will talk about in future posts as I discuss each individual situation.  As I´ll mention now and on further posts, each garden’s soil is a unique environment and in order for you to correctly fertilize a garden or lawn, you need to perform a soil test.  However, I do know that this is something the majority of gardeners will never do, even if its offered free or at a low cost, so I’ll be giving some pointers in the future on how to “wing it”.

Going back to N, P, and K, they each have different uses by plants and while the interactions between these elements and other microelements are a little bit more complicated than the following explanation, it can generally be used as a guideline that the elements are used as follows:

1)      N:          Nitrogen is important for the plants “greenery”.  In other words, when you want to encourage a greener grass or more foliage, you generally want to be looking at using nitrogen.

2)      P:           Phosphorous helps plants have stronger root systems and produce more blooms.  Heavy flowering plants such as annuals benefit from fertilizers with phosphates.

3)      K:           Potassium is helps with overall plant health and works in similar ways than salts do in humans.

(NOTE: Not all fertilizers contain N, P, or K and may completely lack the N-P-K designation when all three of these elements are absent.  An example of this would be a micronutrient only fertilizer.)

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