Not all soils are alike: Know what fertilizer you need, when and how much

You won't need a dirt doctor to tell you when the soil in your yard is ailing; the vital signs will be obvious.

Turf grass becomes thin and crowded with weeds. Garden vegetables die back. Flowers fade before blooming. Fruit is slow to ripen.

Chances are your planting beds are starved for nutrients - some fertilizer. But what kind and how much? That's when the horticultural equivalent of a physical exam comes into play: a soil test.

"Whether you choose to buy manure or commercial fertilizer is up to you, but a soil test will determine which nutrients are needed," said Steve Heckendorn, manager of the soil-testing lab at Virginia Tech. "You're not fertilizing the plant. You're fertilizing the soil that feeds the plant. In effect, you're putting food into the plant's refrigerator."

There are hundreds of different soil types in North America, each with its own characteristics. A soil test rates them for many things, ranging from texture and water retention ability to their yield potential for specific crops. Nutrient deficiencies are identified along with recommendations for changes.

Used to be, shopping for fertilizer blends was a challenge in chemical code breaking, but the standardized three-digit grade labels now required on bags show at a glance what you need to know.

The first number represents the amount of nitrogen available in the mixture, the second the available phosphate and the last is potash. If the bag label reads "10-10-10," that means a mixture of 10 per cent nitrogen, 10 per cent phosphate and 10 per cent potash.

"Nitrogen is associated with colour, phosphorous with flowering and potassium with growth," said Dennis Lukaszewski, urban gardens director for the University of Wisconsin-Extension. "There are other micro-nutrient blends out there for feeding specific plants, but those are the big three."

Here are some fertilizing do's and don'ts:

-Manure has been the fertilizer of choice since the first gardeners turned their first clumps of earth. But be cautious if you have access to the nitrogen-rich farmyard variety. "Make sure it's fully composted," said Mike Goatley, the extension turfgrass specialist at Virginia Tech. "Fresh manures can burn up plants. Odours can be a problem. It may be something you want to avoid if it's at a stage where it's unattractive to the touch or smell."

-Beware synthetic fertilizers blended with herbicides or insecticides. "The fertilizers we use for lawns are not intended for the vegetable garden," Goatley said. "They may contain ingredients that don't belong in the food chain."

-If some fertilizer is good, more is not necessarily better. "You can overload the system," Goatley said. "You'll get too much green and too much growth but too little flowering. All that nitrogen will produce is vegetation when what you really want is fruit."

-Slow-release fertilizers can be effective over long periods of time but they're not a quick fix. You may want to apply a faster-acting product if your ground is nutrient-deficient.

-Know when to apply. "Fertilizing a lawn in the fall will benefit you in the spring," Goatley said. Also, "Fertilize gardens when something is actively growing. That's when you get the most benefit."

-Organic products generally are a safer way to fertilize. "You won't run the risk of a plant taking off" with leggy growth, Wisconsin's Lukaszewski said. "You'll get a good nutrient balance."