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.
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.
How to do it
1. Compost.The foundation for healthy plants is rich, lively soil with lots of decaying plant matter and microorganisms to release its nutrients. Compost gets you there. See www.urban ext.uiuc.edu/compost for advice on making and using compost. If you don't have room for composting, give your soil regular infusions of mushroom compost or other bagged composted organic matter.
2. Get a soil test.At a cost of $25 to $40, it will tell you what nutrients your soil already has, so you don't waste money or cause problems adding nutrients your plants don't need. See www.urbanext .uiuc.edu/soiltest for a list of soil testing labs; many garden centers also handle soil tests. (Soil tests don't include nitrogen, since it has a short life in soil.)
3. Have a plan.Know why you are fertilizing -- to provide a lawn with nitrogen, to give annuals enough juice to keep blooming all season, to replenish nutrients washed out of containers by constant watering. Choose the right fertilizer for the task.
4. Follow package directions.Err on the side of less fertilizer rather than more.
Do you speak NPK?
27-3-6? 10-10-10? 8-14-8? What do those numbers on fertilizer packages mean? They refer to the percentages of the three primary nutrients -- nitrogen, phosphorus and potassium, or N, P and K. Nitrogen supports the growth of green parts -- stems and leaves. Since lawns are nearly all leaf, they need ample nitrogen. But too much nitrogen can make plants stressed and weak or push them to make leaves at the expense of flowers.
Phosphorus helps develop roots and seedlings. Most Chicago-area soils have ample phosphorus, though, and excess phosphorus from fertilizer adds to water pollution problems.
Potassium affects the way plants use water and the chemicals in their systems. It is important for overall health. Single-digit percentages indicate the fertilizer probably is slow-release. In lawn fertilizer, look for a higher nitrogen number, such as 6-1-2, but a low middle number to avoid adding to phosphorus pollution. For most other plants, the fallback is a balanced slow-release fertilizer such as 8-8-8 or 6-6-6.
Many other elements are important to plants. Look for "micronutrients" on the package, or check the "guaranteed analysis" section on the label for a wide array of elements.