NPK what?

     If you’ve even gardened for a little while chances are you’ve purchased fertilizer. One of the top 5 questions I’m asked is “what should I feed my plants”? Well, in order to answer that you need to understand some of the basic physiology of plants.

     First, there are two types of nutrients plants need, macro and micro. The primary, or macro, nutrients necessary are nitrogen, phosphorus and potassium, commonly referred to as “N-P-K”.  Others that are necessary in small, or micro, quantities are minerals such as calcium, magnesium, sulfur, boron and copper. In total there are 13 mineral nutrients, 3 macro and 10 micro necessary for plants to survive. Newer research has shown that there could be 11 micros, however most of the “professional” books I have say 10.

     So, NPK means “Nitrogen, Phosphorus and Potassium” right? On the bottle of fertilizer manufactures usually don’t say “NPK” but rather they represent the total percentage of each mineral in that substance in a format like 10-4-3. That means the fertilizer you’re looking at is 10% Nitrogen, 4% Phosphorus and 3% Potassium. Ok, great, what does this mean and how can I use it?

     An easy way to figure out how much of each you need is to go back to the plant physiology. Nitrogen is used by the plant primarily for foliar (leaf) production. Phosphorus is used for root and flower production. Potassium is used for fruit production and disease resistance. My simple shorthand:

   N = Leaves
   P = Roots & Flowers
   K = Fruit & Disease

     A good example would be Tomatoes. During the initial growth cycle you would want a fertilizer higher in N and P to help establish roots and promote foliage development. During the latter stage, as in the garden, you would want lowered N and higher P & K. High levels of N for things like peppers and tomatoes produces beautiful foliage on the plants but very little to actually eat.

Some common NPK’s are:

   Bone meal – 1-13-0
   Blood meal – 13-0-0
   Mushroom compost – 2-1-1
   Fish emulsion – 2-4-1

     I have found that, much like everything else gardening, simple is better. In the ground most of the micronutrients are supplied by Mother Nature, what you really need to augment are the NPK minerals. Over the past couple years I’ve settled into Daniels 10-4-3 for everything as it includes micro’s. These are critical to me since we grow primarily in pots and not in the ground.

Some good reading on the subject:

http://www.ncagr.gov/cyber/kidswrld/plant/nutrient.htm
http://en.wikipedia.org/wiki/NPK_rating
http://www.planetnatural.com/site/xdpy/kb/n-p-k-organic-fertilizers.html

And if you want to get really geeky on the subject:

http://www.ballpublishing.com/ballpub/_RedBook.aspx