It’s a Numbers Game

Fertilizer is confusing, and no wonder. Shelves are stacked with more types of fertilizer than supplements in a health food store. All are in brightly coloured packaging adorned with pictures of gorgeous flowering plants and unblemished vegetables. 

There are fertilizers for tomatoes, ones for roses, another for perennials and so on. All you need to do is match the plant to the fertilizer, right? It couldn’t be easier, except if you grow roses, tomatoes, and dozens of other types of plants, you’ll soon have a full shopping cart. The truth is, you could get by with only one type of commercial fertilizer, or even none if it’s for the garden. By using compost and mulch there, you’d still be fertilizing, but as nature does it.

You’ll note packages of fertilizer always have three numbers. These represent the three main nutrients plants require — nitrogen, phosphorous, and potassium, noted as N-P-K. If the K is throwing you off, that’s because it comes from the scientific term, kalium. These numbers indicate the percentages of each element in the package or bottle. 10-15-10 means it contains 10% nitrogen, 15% phosphorus, and 10% potassium.

You might be wondering why there are so many different proportions listed, and why manufactures can’t agree on those proportions. Next time you’re shopping for fertilizer, ignore the glossy images that were shot in a studio and compare those numbers.

You’ll immediately see they differ from brand to brand even though they’re specified for the same type of plant. I’ve seen brands of fertilizer formulated for tomatoes with the numbers 6-12-12, 4-6-8, 8-24-8, and 18-18-21. No wonder the consumer is confused.

Now you might be wondering how they came up with the numbers in the first place. When plants were first analyzed, it was found they contained different proportions of these three nutrients. It was then assumed that each type of plant required fertilizer in the same proportion, except plants don’t use nutrients the same way at the same time as they’re growing. They take up what they need from the soil when they need it.

It’s much like going to the grocery store when you’re out of milk, butter, or eggs. You buy what you need rather than stuffing the refrigerator. A balanced fertilizer, that is one with equal percentages, say 5-5-5 or 10-10-10, is fine in most situations, but by juggling the numbers manufacturers were able to make their products appear unique. And that’s when marketing with numbers began. But do you really need all that fertilizer? If you’re growing in pots and planters, indoors or out, yes you will need it, as most soil-less mediums have little or no nutrients, unlike real soil.

In the garden it’s a different matter. Remember that middle number, the one representing the phosphorus percentage? The soil in this area began with a limestone base, and as it degraded over time we were left with plenty of phosphorous in the soil. Garden soil doesn’t need any extra. In fact, so much has been added waterways are being polluted by it. 

As for the last number representing potassium, it’s only likely to be deficient in light, sandy soils, not in the typical clay soils we have in our gardens. This leaves nitrogen, the first of the trio. Nitrogen does not stick around in soil, which is why we’re forever fertilizing lawns. By adding compost and mulch, nitrogen and other nutrients are returned to the soil as the organic material is broken down.

Bottom line: If your plants are growing poorly, fertilizer is rarely the solution, and too much can be deadly.Your pots and planters will need it, so use the fertilizer of your choice, ideally, one with a ratio corresponding to 3-1-2 (24-8-16 or 12-4-8, for instance). Also look for additional micro-nutrients that are usually found in the fine print of contents.

Finally, do heed the directions. Like soap powder in the washing machine, more is not better.