Walker: A primer on 3 things plants crave: N, P and K

U of I Master GardenerAugust 1, 2013 

Two weeks ago, I wrote about soil tests. Several readers asked where to get soil tested. There are two choices for Treasure Valley residents: The University of Idaho and Western Laboratories in Parma.

To have the university test your soil, go to the Ada County Extension Office (5880 Glenwood St, Boise) and pick up a soil test kit. You’ll receive instructions on how to select the soil for testing, how to package the soil for shipping to the lab and the cost of the test.

To have Western Laboratories test your soil, go to their website (www.westernlaboratories.com), and you’ll find all the information you need to have your soil tested by them.

It’s been a long time since I’ve written about the meaning of NPK on a soil test or package of fertilizer. So let’s do a short refresher course.


Plants use a lot of nitrogen, taking it from the soil into the roots and up the plant into the leaves. Nitrogen is part of chlorophyll and is a big part of the photosynthesis process. When plants are starved for nitrogen, they can’t efficiently create the food they need to survive. Fertilizers made for use on lawns usually contain only nitrogen.


This element is also used in the photosynthesis process, and plants use quite a bit of it. But more importantly, phosphorus promotes root growth, flower production and helps plants endure the stress of living in hot weather. If you look at the NPK ratio on products such as Miracle-Gro Quick Start fertilizer and Miracle-Gro Bloom Booster, you’ll see that the phosphorus content is much higher than the other two elements.

In the vegetable garden, producing more blooms means more food.


Photosynthesis also requires a lot of potassium. Potassium helps plants fight off diseases and produce quality fruit.

There are 10 other mineral nutrients and three nonmineral nutrients that plants need to stay healthy and produce flowers, fruits and seeds. The North Carolina Department of Agriculture has an informative, well-organized website that lists all of these nutrients and their purposes (www.ncagr.gov/cyber/kidswrld/plant/nutrient.htm).

If you have particular questions about gardening you’d like to see addressed in this column, send them to highprairielandscapedesign@yahoo.com.

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