How Do I Use an N-P-K ratio?

If you're into gardening on any scale, at some point you run up against having to amend your soil. Feeding plants is easy; you sprinkle a little here, a little there, using boxed rose food or tomato food or azalea food. When you realize that your whole garden needs a lift, though, a couple of little boxes won't cut it.

What to do? You could get a truckload of manure or compost, and that will certainly do something. But will it solve your problem?

The first step in problem-solving is to define the problem. Get a soil test kit, with phosphorus (P), potassium (K), and pH (acidity) included. They'll throw in a nitrogen test -- ignore it (no chemical test for N is accurate). Run your soil through that, check the symptoms your plants are showing, and ask a few experts at your local extension office or university. These things should give you an idea as to what you need to add.

Plants need a specific set of nutrients in specific ratios. Fortunately, the vast majority are supplied by normal soil, in more than sufficient amounts. That leaves a handful that may run low; among those are the Big Three, nitrogen, phosphorus, and potassium. Other minerals, such as magnesium, are pretty easily spot-treated when they crop up, so I won't go into them here.

The Big Three, however, are so key that it's likely you will run into a deficiency of at least one of them several times during a gardening lifetime. They're listed on every packaged fertilizer as three numbers, in the order NPK (K is potassium), and those three numbers are what I'll be getting into in a moment.

First, some info on the elements themselves. Nitrogen is the most volatile element of the three, which is why chemical tests for it are useless; the levels fluctuate so much, so rapidly, it's impossible to pin them down. That also makes it the most likely to be deficient, as it evaporates and leaches out with watering. It's essential for leaf growth and chlorophyll production, however, and the plants will turn yellow and lethargic without it. The best test for nitrogen levels is to plant some sort of grass -- veggie gardeners often sow sweet corn, others use lawn grass -- and see how green it is. Grasses are sensitive, and will show a host of deficiencies rather quickly to the experienced eye... but it's pretty easy for anyone to tell when the corn or lawn turns a sickly yellow color.

Phosphorus is the middle brother; if you think of it as being stuck in the middle, unable to move anywhere, it's pretty accurate. Phosphorus doesn't move around much at all, as it doesn't dissolve easily in water, and so it will stay where you put it. That's both a blessing and a curse. So long as you remember to put phosphorus in the root zone of a needy plant, where it can easily get to it, the phosphorus will only run out when the plant has used most of it. What it helps with is root growth and fruit development.

Potassium is the all-around helper -- it deals with cell-division, which makes it necessary for healthy growth. It also deals with food transport in the plant, allowing it to make the most of the sun, water, and other nutrients. Last, it increases overall health and helps with disease resistance. Moderately water-soluble, it's not going to run off like nitrogen, but you can count on water distributing it somewhat.

Now, on to the useful part.