The first delicate, white fingers of frost appeared on impressions in the garden soil this morning, the fine pattern of crystals a harbinger of colder weather to come over the next few months. The exposed soil looks vulnerable to the elements compared to those plots covered with a winter crop of oats. Winter weather is relentless and difficult; even the soil seems to know that.

Autumn is an excellent time of year to have soil tested. A soil-testing kit is available through your local University of Maine Cooperative Extension office. The kit provides instructions for taking a soil sample and will tell you where to send it for analysis. Testing soil is easy and worthwhile and has potential to dramatically improve your gardening practices and yields.

A determination of the soil’s pH is one of the most valuable pieces of information resulting from a soil test. The acidity or alkalinity of soil is measured by the pH, which stands for “potential of hydrogen.” The pH scale runs from 0 to 14. A pH value of 7 is neutral. Most garden plants grow well in the slightly acidic 6.0-6.5 range. Some landscape plants and a handful of vegetables prefer a slightly alkaline pH, a shade higher than 7.0. Some plants, for example members of the Ericaceae family (including rhododendron, azalea, mountain laurel, cranberry and blueberry), prefer a rather acidic pH of 4.5 to 5.5.

Generally speaking, geographic regions with a high amount of rainfall each year have acidic soils. Eastern and coastal northwestern United States, for example, have naturally acidic soil: Leaching rains “wash” liming agents downward through the soil profile over time. On the other hand, arid areas, such as the southeastern United States and parts of the Midwest, have naturally alkaline soils, due mostly to the absence of regular, leaching rains.

Manipulating or adjusting the pH of a soil is critical for improving a garden. The pH of soil affects the nutrient availability for plants. When the pH is kept within the optimal range for any plant or plant group, nutrients – food for the plant – are available for absorption by the plant. When the pH is outside the optimal range, nutrients immediately within the absorption zone are unavailable.

The results of a soil test will make recommendations possible for adjusting the pH. The most common liming materials used by gardeners include limestone or dolomitic limestone. These materials require several months before a measurable adjustment in the pH is observed. Wood ash also effectively adjusts pH and yields more immediate results.

In addition to altering the pH, limestone or dolomitic limestone boast added benefits, including the nutrient magnesium if dolomitic lime is used.

Lime improves calcium, phosphorus and nitrogen availability and may accelerate the biological and chemical breakdown of organic matter in the soil.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, 512 North Ridge Road, Montville 04941, or e-mail them to dianagc@midcoast.com. Selected questions will be answered in future columns. Include name, address and telephone number. For a soil testing kit, contact the local Extension office or visit the Web site: http://www.umext. maine.edu/.