Buying fertilizer may seem like a numbers game

The excitement of spring planting has faded, the risk of frost has passed and it’s time to focus on caring for those plants, which means food and water. Ensuring plants have enough water is understandable; food in the form of fertilizer is not.

It 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 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 percentages of three main nutrients plants require — nitrogen, phosphorus, and potassium, noted as N-P-K. For example, if a package or bottle has the numbers 10-1510 that means it contains 10 per cent nitrogen, 15 per cent phosphorus and 10 per cent potassium.

You might be wondering why there are so many different combinations listed, and why manufacturers 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 the numbers instead. You’ll 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 those numbers. Plants contain different proportions of these nutrients and it was 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 while they’re growing. They take up what they need from the soil when they need it. It’s much like going to the store when you’re out of milk and buying what you need rather than stuffing the fridge.

A balanced fertilizer with equal percentages like 5-5-5 or 10-10-10 is fine in most situations, but by juggling the numbers manufacturers are able to make their products appear unique.

But do you really need all that fertilizer? If you’re growing in pots and planters indoors or out, you will need it, as most soil-less mediums have little or no nutrients.

In the garden it’s a different matter. Remember that middle number, 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 phosphorus in the soil. Garden soil doesn’t need any extra phosphorus.

As for potassium, it’s only likely to be deficient in light, sandy soils and 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 or mulch, nitrogen and other nutrients return to the soil as the organic matter breaks 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, one that lists additional micronutrients. These are important and usually found in the fine print.

Finally, do heed the directions. Like soap powder in the washing machine, more is not better. ••• To chat with local gardeners, share tips, pics and discuss fertilizers, see Grand Gardeners on Facebook.

David Hobson gardens in Waterloo and is happy to answer garden questions, preferably by email: garden@gto.net. Reach him by mail c/o Etcetera, The Record, 160 King St. E. Kitchener, Ont. N2G 4E5