Fertilizer Facts

By Jeff Gillman

June 17, 2008

Q Do I need a fertilizer?

A If you're a composter and add an inch or so of completed compost to your garden every year or two, you probably don't need fertilizer. Fertilizer might give your plants a bit of a boost, but when compost is used regularly it typically provides all the nutrients most plants need.

Q Do I need to fertilize my trees if I've already fertilized my lawn?

A Probably not. The grass won't be able to take up all of the fertilizer that you apply. Some will make it through the ground to the roots of the trees in or near your lawn.

Q Should I add a high-phosphorus "starter" fertilizer when I plant?

A No. There are several fertilizers that contain high levels of phosphorus for promoting root growth at planting. And while it's true that plants need some phosphorus, most of the soils in and around Minnesota contain plenty of phosphorus. So the extra shot of phosphorus that you're adding with this special fertilizer is not only useless, it also may be dangerous for new plantings. Fertilizer is a salt and it may burn the plants' roots.

When you're transplanting, watering -- not fertilizing -- is what's important. Make sure the soil around the plant stays moist, but not wet. For most plants, a thorough watering at planting, then once or twice a week for two to three months (depending on rain) should be good.

Q Should I use a liquid or a slow-release fertilizer?

A That depends. Liquid fertilizers are readily available to the plant, but tend to move through the soil quickly. Slow-release fertilizers aren't as quickly available, but feed the plant for a longer period. In most cases, slow-release fertilizers are better because they need to be applied less frequently. However, if you want plants to grow fast, then liquid fertilizers are fine.

Q What nutrients should I look for in a fertilizer?

A On the front of every container of fertilizer you should find three numbers, such as 10-0-5. These numbers indicate the percentage of nitrogen, phosphorus and potassium in the fertilizer.

The numbers themselves are less important than the ratio of the numbers. In most cases, a ratio of about 5-1-2 is right for most situations. (Lawn fertilizers usually include no phosphorus because it can lead to pollution in lakes and streams.)

Generally, nitrogen will make plants grow faster. But phosphorus and potassium also are important, especially if your plants are growing in containers. Other nutrients can be important as well, so it's wise to buy fertilizers that include micronutrients (nutrients that plants need in smaller quantities than nitrogen, phosphorus and potassium). Two micronutrients to look for, especially for containers, are calcium and magnesium.

Q What about organic fertilizers?

A There are plenty of great organic fertilizers as well as a few bad ones. One reason people use organic fertilizers is to avoid wasting natural resources, such as coal and natural gas, which are used to make synthetic fertilizers. However, some organic fertilizers use natural resources, too. Guano (a special aged manure), greensand and rock phosphate -- all of which are mined -- are ingredients in some organic fertilizers.

The best organic fertilizers are from renewable sources such as corn gluten meal (which also is used for turf and will help control weeds), seaweed extracts and fish emulsions. These fertilizers also contain more micronutrients than many synthetic fertilizers. And they tend to release their nutrients more slowly than most synthetic fertilizers. They can cost more than synthetics, though.

Jeff Gillman is an associate professor of horticulture at the University of Minnesota. He's also the author of two books, "The Truth About Garden Remedies" and "The Truth About Organic Gardening" (Timber Press, $12.95).