Five things you should know about fertilizer before adding it to your garden. Plus, tips on how to read the numbers on fertilizer packages.

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New gardeners often are daunted by fertilizing — or they shrug, buy some blue chemical and apply it lavishly without really knowing why. But there’s no need to let your garden’s need for nutrients trip you up. 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 the soil is healthy, with lots of plant matter in it and thriving life underground, it should provide enough nutrients without added fertilizer. But sometimes fertilizing is needed. Since 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 annual flowers, 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 fertilizer actually can harm it or force it to grow in ways we don’t like. Most lawns, for instance, do fine with less nitrogen and phosphorus than mass-marketed lawn fertilizers contain. Too much at one time can dehydrate or “burn” the grass leaves and habitual overfertilizing causes thatch.

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. Organic fertilizers, created from living material such as bones, manures, alfalfa meal and fish, work most like nature, with soil microorganisms releasing nutrients to plants’ roots. 5. There’s a price to pay. Synthetic water-soluble fertilizers are cheap to buy, 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. And then there’s the ecological cost: synthetic fertilizers, whether fast- or slow-release, require large amounts of natural gas and energy to capture their nitrogen, and the minerals in fertilizer may be strip-mined. Organic fertilizers often are made of agricultural byproducts, but even they take some energy to produce and transport. The cheapest, most sustainable source of nutrients is compost you make yourself from garden and kitchen waste. For some free lawn fertilizer, leave the clippings on the lawn when you mow.

Sources: Tom Tiddens, supervisor of plant health care at the Chicago Botanic Garden in Glencoe; Jon Rosenthal, vice president of business development at Florikan, a Florida fertilizer-maker; and Chris Paisley, service programs manager for Mariani Landscapes in Lake Bluff, Ill.