It is important to understand NPK because chemical fertilizers should not be used indiscriminately
Not all types of plants have the same nutrient requirements, and you can sometimes do more harm than good when applying chemicals haphazardly. For example, applying a fertilizer high in nitrogen (indicated by the fact that the first number on the package is high) will cause certain plants to put all their energy into producing foliage, at the expense of flowers.
If you do not have a good grasp on how well your soil is meeting the nutritional needs of a plant (but still feel the need to feed it at a particular time), your best bet is probably to:
· Use compost instead of a chemical fertilizer
· And/or have your soil tested
· And/or use a slow-release fertilizer (which is less likely to harm plants to any great degree)
The NPK Breakdown: What Those Plant Nutrients Actually Do
To advance this discussion from the academic to the practical, let's take a brief look at the roles that the constituents of NPK play in plant growth:
Nitrogen promotes leaf development. As Kelly Burke writes, "Nitrogen is a major part of chlorophyll and the green color of plants." As mentioned above, there can be such a thing as "too much nitrogen"; at the opposite end of the spectrum, gardeners sometimes encounter the problem of nitrogen depletion.
Meanwhile, phosphorus plays a key role in the growth of roots, blooming and fruiting, which is why it is an essential nutrient for your plants in spring. Potassium also plays a part in root growth, as well as in stem development.
Take a look at the picture that I have provided on this page. It shows a portion of the back of a fertilizer bag. Specifically, it is a Scotts Turf Builder product intended to help your grass in summer. The NPK value is listed as 28-0-8. That means it contains 28% nitrogen, no phosphorus, and 8% potassium (potash). Why would the company leave phosphorus out in this case? This productused to contain phosphorus. But Scotts explains that they have removed it for environmental reasons. Phosphorus was retained in their "Starter" fertilizer (designed for spring use), however, because "phosphorus is essential to the initial root development of grass plants...."
More info: http://sonefworld.com