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Keeping tabs on soil temperature

At last, I have a soil thermometer -- one of the more imaginative gifts awaiting me under the Christmas tree a few weeks ago. It will be put to much more frequent use than the inevitable necktie I usually receive from the same relative each year.

It's not unlike a meat thermometer which you stick into a roast to tell when it's done. Unlike the meat thermometer, however, the readings are on the lower end of the temperature scale. It will prove most useful, particularly in the spring, when it will tell me when the soil is warm enough to promote good seed germination and vigorous growth.

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Now a vast majority of gardeners raise backyard crops successfully year after year without testing soil temperatures. A soil thermometer is not a vital gardening item. Rather, it's a helpful one. Moreover, 12 years of observation at Lincoln, Neb., has shown gardeners how to approximate soil temperatures by comparing them with average daily air temperatures.

The researchers found that the top six inches of soil always remained a few degrees warmer than the average air temperatures for the period. At 12 inches or more, the soil stayed above air temperatures during fall and winter but at or below average air temperatures during spring and summer.

These were the Lincoln findings in degrees F.:

Season Air 1 inch 3 inches 12 inches Winter 25.9 28.8 29.5 32.2 Spring 49.9 54.8 53.6 48.5 Summer 73.8 83.0 79.1 73.8 Fall 53.9 56.4 57.1 59.3

In other words, by taking daily maximum and minimum temperatures each day in the weeks leading up to the planting season, and then finding the average, you will arrive at figures a little below the temperature of the soil. If you don't have a maximum-minimum thermometer, take a note of the temperatures on each day's radio or TV weather report in your area.

It is important to note soil temperatures before planting because germination is rapid and vigorous only when the soil is warm enough. In contrast, seeds sown too soon spend much of their energy just surviving the cold. There's simply not enough vigor left over to grow quickly once germination takes place. How often we see that happen in our own gardens when the second planting of peas catches up with those sown two weeks earlier.

In his book ''How to Grow More Vegetables'' (Ten Speed Press), John Jeavons lists the soil temperature needs (in degrees F.) for the more common vegetable seeds:

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Crop Min. Optimum Max. Asparagus 50 75 95 Bean 60 80 95 Bean, lima 60 85 85 Beet 40 85 95 Cabbage 40 85 100 Carrot 40 80 95 Cauliflower 40 80 100 Celery$40 70 85 Chard 40 85 95 Corn 50 95 105 Cucumber 60 95 105 Eggplant 60 85 95 Lettuce 35 75 85 Muskmelon 60 90 100 Okra 60 95 105 Onion 35 75 95 Parsley 40 75 90 Parsnip 35 65 85 Pea 40 75 85 Pepper 60 85 95 Pumpkin 60 95 100 Radish 40 85 95 Spinach 35 70 85 Squash 60 95 100 Tomato 50 85 95 Turnip 40 85 105 Watermelon 60 95 105

Some other thoughts to remember: Sandy soils warm up faster in the spring than clay soils, and darker-colored soils than those of a lighter color.

Damp, sandy soil also heats up faster than dry sand, because water in the soil conducts heat more rapidly than air. In contrast, clay soil holds so much water that its specific heat (the amount of heat it needs to raise the temperature one degree) is much higher than other soils. On the other hand, once warmed, clay holds heat well, so that it is a great soil for fall gardens.

Remember, also, if your garden slopes gently toward the sun (to the south, in other words), it will warm up much faster than a north-sloping garden.