CHICAGO - Jeff Gillman believes in the bedrock idea of organic gardening: that maintaining healthy soil, full of organic matter and beneficial microorganisms that release nutrients to plants, is the way to make plants thrive.

But he's bothered by what organic gardening has become. Over the last 40 years or so, the concept has evolved to demonize the use of all synthetic chemicals. Meanwhile, Internet message boards crackle with recipes for supposedly "natural" home-brewed pesticides and plastic spray bottles labeled "organic" shoulder their way onto the insecticide shelf at the garden center.

Gillman, a professor of horticulture at the University of Minnesota, caused a small stir with a previous book, "The Truth About Garden Remedies" (Timber Press, 212 pages, $19.95), for which he tested and debunked much cherished garden folklore. An entomologist by training, he teaches, among other things, classes on pesticide use. In his own garden, he says, he avoids using any pesticides, but on the rare occasions when he has to spot-treat a problem he chooses a chemical based on how safe it is, not how "natural" it is.

"Assuming that something is safe because it is organic is completely wrong," he says. The right chemical - if you feel you must use one - is the precise one that will deal with the problem as safely and with as little environmental risk as possible, he believes.

Kathy Purdy, who runs the blog "Cold Climate Gardening" out of upstate New York, says she likes both Gillman's books because "he's trying to look carefully at the actual evidence rather than taking a position and trying to defend it. He's trying very hard to get to the truth and not the hype. And he's in a position to be informed."

In his new book, "The Truth About Organic Gardening: Benefits, Drawback and the Bottom Line" (Timber Press, 208 pages, $12.95), Gillman takes a science-based look at a wide range of organic and non-organic gardening substances, ingredients and techniques.

His bottom line: "Organic is usually better." But not always. To be the best gardener, tend your soil, plant a diverse garden, tolerate some imperfection and make informed case-by-case choices to deal with problems. His book can help.

Here are some misconceptions Jeff Gillman addresses in his new book:

* The idea: "Organic gardening" means "safe" and "enviromentally friendly."

The truth: Unlike organic farming, there's no clear definition of "organic gardening." Some think it means using no added fertilizers or pesticides at all; others use a wide variety of chemicals labeled "organic," some of which have hazards and cause problems.

The advice: Focus on the core message of organic gardening - constantly replenishing the soil with lots of organic matter so it provides plenty of nutrients, keeps plants healthy and reduces the need for any chemicals.

* Fertilizers labeled "organic" give plants better nutrients.

The truth: It makes no difference to a plant whether it gets a molecule of nitrogen from alfalfa or from a factory. Nitrogen is nitrogen and, ultimately, it all comes from the air. But organic fertilizers often contain a wider range of elemental nutrients and make them available slowly, so the plant can actually use them.

The advice: Use organic fertilizers where you need to add nutrients, in containers. But don't add fertilizer at all unless you need it. If you regularly enrich your soil with organic matter such as compost, it may provide plants enough nutrients without fertilizing.

* Synthetic fertilizers trash the enviroment. Organic fertilizers are benign.

The truth: Synthetic fertilizer manufacturers take nitrogen from the air through a chemical process that consumes natural gas and uses electricity generated by burning coal or by nuclear power. If you use too much, surplus water-soluble fertilizers can leach away to pollute waterways or build up to kill off soil organisms.

But some organic fertilizer ingredients, such as rock phosphate and potassium, can leach away or build up if over-applied.

The advice: Use slow-release fertilizers, and don't add potassium or phosphorus unless a soil test shows your garden needs them.