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Soil thermometers take the guesswork out of gardening

A soil temperature gauge turned up in my Christmas stocking a couple of years ago - the result, my daughters told me, of an intensive search for something that was both ``different and useful.'' They couldn't have chosen better. That eight-inch steel stem with a gauge on top has dramatically changed my approach to gardening. I no longer sow seeds according to the calendar. Instead, I let the thermometer tell me when it's appropriate to plant. Equally important, it has taken the guesswork out of mulching. In short, it has become an important soil-management tool.

Research has shown clearly that plant growth is more responsive to soil temperatures than to air temperatures. Warm roots, like warm feet on people, make for comfortable, flourishing plants. The microscopic soil life that processes organic and mineral matter into nutrients a plant can absorb don't begin to function until the soil reaches about 45 degrees F., and they prefer temperatures that are considerably warmer than that. So even cold, hardy plants such as cabbage, peas, petunias, etc. won't grow when the soil is in the low 40s.

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Heat-loving plants - tomatoes, melons, beans, etc. - have still higher minimum temperature needs.

On the other hand, soil that is too hot will also stop a plant in its tracks. Corn, for instance, grows vigorously right up to 85 degrees F., yet stops growing as soon as that temperature is exceeded.

A general rule of thumb for soil temperatures is: Plants that are frost-tolerant prefer soil temperatures ranging from the low 60s to the mid-70s; and the root-temperature needs of heat-loving plants range from the low 70s to the mid-80s.

To get the most out of a garden, it is important to raise soil temperatures into the ideal range as soon as possible in the spring, and then to keep the soil from getting too hot when the torrid summer months come along. One way to accomplish this is to mulch the garden, using both artificial (plastic) and natural (hay, straw, leaves, etc.) mulches. While the former raises soil temperatures, the latter keep the soil cool.

Jeff Ball, an intensive backyard vegetable grower and author of ``Sixty Minute Vegetable Garden'' (Rodale Press), has found that black plastic readily raises soil temperatures between six and eight degrees above the surrounding soil; clear plastic, on the other hand, can raise temperatures between 20 and 40 degrees, depending on how far north or south you live. Raymond Poincelot, author of ``No-Dig, No-Weed Gardening'' (Rodale Press), detailing his use of plastic mulches, concurs.

In contrast, an organic mulch four to six inches deep can keep the garden soil as much as 18 degrees cooler than unmulched soil at the height of summer.

Here's my approach to guesswork-free backyard vegetable gardening:

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In the early spring I fertilize the soil, preferably with an inch of finished compost, and immediately cover this with black plastic, punching holes in the plastic to allow water to drain through into the soil. This could just as easily be done in the late fall.

The garden beds, often including the surrounding paths, are then covered with clear plastic to speed up the soil-warming process still further. The sun shines through the clear plastic just as it does in a greenhouse, and heats the black plastic beneath. In turn, the black plastic warms the soil by conduction. The addition of the clear plastic dramatically speeds up soil warming in my region.

When the temperature probe indicates that the soil has adequately warmed up, the clear plastic is removed. Planting is done by cutting holes or slits in the black plastic (for transplants or seeds, respectively).

Additional fertilizer is placed at the bottom of each transplant hole. At this stage, the plastic prevents unwanted weeds from sprouting and retains moisture while continuing to absorb the sun's heat.

Should soil temperatures rise to the upper limit of plant preferences, I would cover the black plastic with several inches of organic mulch.

In my region, however, soil temperatures seldom get too hot if the plants are close enough so that their leaves shade the plastic once they are mature.

On the other hand, I mulch the paths between beds heavily with leaves and/or straw so that unwanted heat doesn't creep in from the sides.

In the fall, any organic mulches are removed from between the plants so that soil heat can radiate up from the plastic when cooler nights prevail.

For some frost-tender plants, this can occasionally make the difference between survival or succumbing when light frosts occur.

Finally, all organic mulches are gathered up and thrown onto the compost heap to ultimately feed another generation of plants.

Soil thermometers are not readily available in garden centers, but mail-order garden supply companies and many seed companies stock them. Here's a few that do: Burpee Seed Co., Warminster, PA 18974; Gardener's Supply, Intervale Rd., Burlington, VT 05401; and Stokes Seeds, Box 548, Buffalo, NY 14240.

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