Digging Up Dirt — And Having It Analyzed

By Betsy Lehndorff
Scripps Howard News Service

When it comes to dirt, looks are deceiving. A rich composted soil can conceal huge quantities of mineral salts that kill everything within days, while a sample of crunchy, dry earth can contain all the elements of soil perfect for gardening.

"Soil testing is a good starting point for people who may just be getting into gardening," says Jim Self, head of Colorado State University's soil-testing laboratory in Fort Collins. "Mainly, the testing allows them an idea of how much fertilizer and how much compost to put in."

Too much of either can inhibit or kill plant growth, especially over time, he says.

To show how soil testing works, Self processes a sample of dirt scooped from the front yard of a 100-year-old home. The dirt doesn't look promising; it consists of light brown pellets the size of flaxseeds.

Self grinds it in a mixer, reducing it to a fine powder. He measures 10 grams of it into a glass flask to test for nitrates, phosphorous, potassium and micro-nutrients. Twenty grams goes into a plastic cup to test pH level, or acidity, as well the presence of mineral salts. Next, liquids are added to create a muddy solution, and everything is stirred by machine.

An electronic probe reveals insignificant levels of mineral salts. A pH scanner determines that the soil is slightly acidic at a reading of 6.8.

Next, a lab technician places dry dirt in the palm of her hand over a sink. After adding a small amount of water from the tap, she tries to work the mud ball into a crude ribbon. But it breaks up. She rubs another tiny bit of the mud between her fingers under the stream of water, feeling its gritty texture. She also looks for hints of limestone.

"It's sandy loam," Self says, identifying it as a highly desirable Colorado soil. Clay, which is more common, often clumps when wet and can be tough on plants.

Next, dry soil is weighed into a heat-proof crucible and roasted for hours in an 800-degree furnace. It's reweighed after cooling, and the figures indicate that it contains 4.9 percent organic material. That means it's root-friendly and absorbent.

Another lab assistant measures and mixes bits of the dirt with special chemicals, then filters the liquid into test tubes and adds more chemicals, timing everything. Results show the presence of essential minerals such as phosphorus and potassium, as well as traces of zinc, iron, manganese and copper. He also comes up with nitrogen levels. All for $20.

"This is a really good soil," says Robert Cox, Colorado State University Cooperative Extension agent for Jefferson County.

Contrary to popular belief, soils that have been cultivated for a long time, such as the sample from a 100-year-old home, can be richer than those laid down in the back yards of new subdivisions, he says.

"Most all the time, soils are low in organic matter," he says.

To remedy that, Cox suggests that gardeners mix in compost or other organic material, such as old leaves or rotted hay. "But you don't want to overdo it. Five percent organic material is ideal," he says.

Although CSU supplies some general recommendations with the soil-test results, it helps to take the findings to a master gardener, a garden center or a nursery, he says. These experts can direct the gardener to the right amendments and fertilizers.

(Contact Betsy Lehndorff of the Rocky Mountain News at www.rockymountainnews.com.)