Some gardeners are pretty proud of themselves, and rightfully so, because they don't just grab "a spray" when a plant seems unhappy. They identify the problem and chose the right solution. Good for them.

Yet these same gardeners will just grab any fertilizer when a plant is hungry. But just like pesticide sprays, there are different fertilizers for different plants and purposes. And you should have a selection just as you have different pesticides.

That doesn't mean you must have "Tomato Fertilizer" for tomatoes and "Rose Food" for roses. These are pure marketing hype. They take a decent fertilizer, put it in a box with a specific plant on the label, and triple the price.

One exception to this is azalea and rhododendron fertilizer. It is formulated specifically for acid loving plants. You can get away with regular fertilizer, but the specialty product is better.

That means you need to know a little bit about fertilizers. And the first thing is what those numbers mean.

Fertilizers come prominently labeled with three numbers, like 10-10-10 or 15-30-15. The first is the percentage of nitrogen in the box, the second phosphorus, the third potash, the three main nutrients a plant needs. There are others, but I don't want to scare you. If you went to elementary school before 1970, you can use these percentages to figure out which fertilizer is the best buy.

Nitrogen promotes green growth, usually a good thing. But if your tomatoes and peppers and carrots have vigorous green growth but poor fruit or roots, they could be getting too much nitrogen. Onions like nitrogen the first half of their life, phosphorus the second.

Phosphorus promotes fruit and root growth. A high phosphorus liquid fertilizer like the aforementioned 15-30-15 makes a big difference when you water in transplants.

Potassium doesn't have a specific assigned task like the others, but it promotes general health. It's pretty rare to have a potassium shortage.

A balanced fertilizer is one that has the three elements in ... well, not necessarily equal but close amounts. Unlike, say, lawn fertilizer which is very high in nitrogen, something like 26-6-6. It is intended to make your grass grow fast so you can mow more often. I don't use lawn fertilizer.

Well, yes I do. A well known brand name in soluble fertilizers had a formula of 15-30-15 for half a century. A couple of years ago they changed it to 24-8-16. No fanfare. They learned their lesson from New Coke.

I had realized many years ago that the original formula had too little nitrogen for many purposes, just by observing my plants. So I started mixing lawn fertilizer with this Something-Gro, one tablespoon of 26-6-6 with two tablespoons of 15-30-15 in a three gallon watering can, giving me (check my math) 19-11-13. Balanced. Not too much nitrogen.

Store brands still have that original 15-30-15 formulation, and that's what I buy. I use my mix for vegetables and roses and the straight stuff for flowering annuals.

These are all liquids, but the workhorse is granular garden fertilizer, which has lower numbers like 5-10-5 or 10-10-10. You should be throwing this around already for newly emerging plants and spring bulbs. You can get into arguments with gardeners about which formula is best, but frankly it doesn't make a whole lot of difference for the home gardener. Just get a heavy bag of what the store has.

Or you can use compost. Do you know what blend of nutrients your compost has?

- Duane Campbell, a nationally known agricultural expert, can be reached at R6, Box 6092, Towanda, PA 18848, or by e-mail at dcamp@chilitech.net for questions or comments.

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