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5 Things You May Not Know About Fertilizer

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By BETH BOTTS Chicago Tribune

The Tampa Tribune

Published: August 14, 2008

New gardeners often are daunted by fertilizing - or they shrug, buy some blue chemical and apply it lavishly without really knowing why.

What do you need to know about fertilizer? We assembled a cast of experts - Jon Rosenthal, vice president of business development at Florikan, a Florida fertilizer-maker; Tom Tiddens, supervisor of plant health care at the Chicago Botanic Garden; and Chris Paisley, service programs manager for Mariani Landscapes in Illinois - to point you in the right direction. Here are five things you might not know about fertilizer.

1. It's not plant food. Plants make their own food by photosynthesis, recombining carbon, oxygen and hydrogen atoms from water and air with energy from sunlight. But they also need small amounts of other elements, such as nitrogen and potassium, just as we need certain vitamins and minerals in addition to protein and carbohydrates. If the soil doesn't have enough, fertilizer helps, but the first step should be to improve the soil.

2. Not all plants need it. Plants evolved to collect nutrients through their roots from the soil. Some nutrients are minerals in the soil. Others are produced when plant matter is digested by soil microorganisms. If your soil is healthy, with lots of plant matter in it and thriving life underground, it should provide enough nutrients without added fertilizer.

Because many of our plants aren't in the soil they evolved for, some can use fertilizer to bridge the gaps. Plants in pots will need fertilizer because they quickly exhaust the available nutrients. Lawns and annuals, such as petunias and impatiens, do best with some added nutrients too.

3. More is not better. Plants don't speed up when they get more fertilizer the way an engine responds when you give it more gas. A plant can use only as many nutrients as it needs; too much can harm it or force it to grow in ways we don't like.

4. Slower is better. Synthetic water-soluble fertilizers, such as most lawn foods, deliver elements to plants in a rush. That's not how plants are designed to work and it makes it easy to give them a damaging overdose. Instead, use slow-release fertilizers (also called controlled-release), which offer nutrients to plants at a slow, steady, safe, digestible rate.

5. There's a price to pay. Synthetic water-soluble fertilizers are cheap, but much of their fast-release nitrogen and phosphorus washes away (and pollutes waterways). Slow-release fertilizers cost more up front but don't have to be applied as often.

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