Imagine people walking around their yards on a spring day throwing dollar bills into the wind. Then imagine those dollars disappearing into thin air.

What bad joke is this? No joke at all when we apply fertilizers in early spring, especially when it’s done without benefit of a soil test and a soil thermometer.

To see why this is the case, think of it from the plants’ point of view.

“The plants are actually in charge of the system,” says Todd Harrington, president of Harrington’s Organics in Bloomfield, a longtime provider of soil tests and soil enrichment products. He describes the little-known process by which plants, as they wake to the greater sunlight and higher temperatures of spring, send messages to soil microbes through root emissions called “exudates.”

The exudates encourage population growth among the soil microbes — which, in turn, process nutrients such as nitrogen until it’s in a form that plants can use.

“It takes several levels of conversion by bacteria before a plant can use the nitrogen in the soil,” says Harrington. “If the microbes are too few or just waking up in the spring, the nitrogen just stays in the soil.”

If fertilizer is spread while soil is cold, plants and microbes are relatively inactive and not calling for food. Grass, for example, can’t use the nitrogen in fertilizer until soil is more than 55 degrees for at least three days. If it rains, unconsumed fertilizer may be washed away. If exposed to sun, part of the nitrogen may be dissipated into the atmosphere. This is one way we inadvertently throw away money on fertilizer.

Blossoming forsythia are a great indicator of soil temperature—which may be happening by the time you read this article. But if you want to be sure, buy a soil thermometer, which costs less than $20.

“Most soils around the world actually have all the nitrogen, phosphorous and potassium they need,” says Harrington. “What’s most often missing in a lawn or garden environment is the microbial soil life that makes it possible for plants to use the nutrients.”

Organic matter is key to feeding soil microbes; it should be 4 to 7 percent in most gardens and lawns.

Compost and compost tea are rich sources of food for soil life. The majority of compost applications are best done in the fall, but a ¼” layer in spring can be helpful. (There’s evidence that too much compost on a spring lawn can provide a favorable environment for egg-laying by female beetles, which results in more grubs!) Compost tea can be safely used in spring once temperatures are right.

“There’s actually a predator-prey relationship between soil organisms that releases a predictable amount of plant-available nitrogen,” says Harrington. That’s why his lab provides biological soil tests based on the soil food web, in addition to basic chemistry tests. “If you concentrate on increasing the most beneficial soil organisms, after a period of time the system becomes self-sustaining,” he concludes.

So avoid wasting dollars and get a soil thermometer and a soil test. Measure the areas where you want to improve soil and spread lime and fertilizer at the rate recommended, usually stated in pounds per 1,000 square feet. Wait until the soil is warm enough, at least 55 degrees for three days. Don’t forget that shady areas stay cool much longer.

At the same time, make a plan to feed the living soil organisms and let them feed the plants. Within a few seasons, microbes are likely to take over the jobs once performed by bags of fertilizers. The system that results is much closer to what nature intended.

In Connecticut, we have several soil testing resources:

UConn’s Soil Nutrient Analysis Lab offers a basic chemistry test that includes pH, macro and micronutrients, and lead. Request the percent of soil organic matter for a small additional fee. If desired, request instructions for organics. Download instructions from http://www.soiltest.uconn.edu/sampling.php or call 860-486-4274.

The Connecticut Agricultural Experiment Station offers a free basic test. Be sure to request the amount of organic matter. Download instructions from http://ct.gov/caes. Search on “soil test.” Call (203) 974-8521.

Harrington’s Organics provides soil biology and basic chemistry tests with organic recommendations. See http://harringtonsorganic.com/ or call 800-675-8733.

Kathy Connolly is a garden writer, landscape designer and speaker from Old Saybrook. Visit her web site at www.SpeakingofLandscapes.com.