The Temperature Of The Soil

The soil must be warm in order to produce crops. Most farm soils of the United States are not likely to become too warm for ordinary crops; there is far greater likelihood that they may be too cold. This is especially true in the Northern States, where the season is short, and it is very often desirable to make the soil warmer, particularly in early spring. The seeds of most cultivated plants will decay before they have had time to germinate if the temperature of the soil is below 45° ; the colder the soil, the slower the seeds germinate. Only after the soil has reached a temperature of 65° to 70° do most crops grow well in it. The soil temperature that is considered most favourable for the germination of barley has been determined by experiment to be 61° to 70° F.; of clover, 77° to 100° F.; of pumpkins, 100° F.; of tomatoes, 100° F. The growth of a crop after germination is influenced fully as much by the temperature of the soil as is the sprouting of the seeds. The farmer knows that certain crops, as onions, barley, turnips, parsnips, peas and potatoes, are "cool plants"; they can be sown early when the ground is cold, and thrive in the coolness of spring.

Others, as corn, tomatoes, melons and squashes, are "hot plants"; seeds of these do not sprout well if sown very early, and the plants do not begin to grow satisfactorily until there have been summer days to warm the soil thoroughly and deeply. The Temperature of Different Soils. — The temperature of a soil depends upon many factors, most of which are beyond the control of the farmer, but some of them he can regulate by comparatively simple means. The temperature of every soil varies widely with the season, and from day to night. The surface soil becomes warm on a hot day and cools several degrees at night, but this fluctuation rarely extends below two and one-half feet. At a depth of thirty feet the soil temperature changes little if any throughout the year, even in the Northern States. Much also depends upon the materials of which the soil is composed. The coarser it is, the warmer it gets, and the better it holds the heat; hence gravelly and sandy loams are among the earliest and warmest of soils. In Europe, gardeners sometimes put loose gravel around grape vines to keep them warm during the night. But a soil in which the particles are very small, as in clay, warms much faster than sand because the particles lie so close together that the heat passes more readily from grain to grain than in sand where the grains lie loosely.

For the same reason a clay soil loses more heat by radiation than a sandy soil. Moreover, a clay soil holds more water than a sandy soil and so loses more heat because of the larger amount of evaporation. Hence, fine-grained soils, though they absorb more heat than coarse-grained soils, are colder. Sandy soils are "warm," clay soils are "cold." Draining a Soil Warms it. — The warmth of a soil comes chiefly from the sun and incidentally from the fermentation and decay of the vegetable matter and other refuse that it contains. The temperature of a soil is modified most by the amount of water it contains. Wet soils are cold. The wetter a soil is the colder it is, at least during the summer, when warmth is needed most. It is the coolness as much as the excess of moisture and lack of air that makes corn with "wet feet" grow poorly. The chief reason for this is that it takes a large amount of heat to evaporate the excess water from a soil, and also much heat to warm the wet soil that remains, water being a poor conductor of heat, the evaporation of one pound of water from a cubic foot of clay soil makes it 10 degrees cooler. There may be a difference of 7° to 10° in the temperature of a well-drained loam and a poorly drained soil of the same character.

There is one exception to the statement that the wetter a soil is the cooler it is. In early spring we frequently have warm rains that raise the temperature of the surface soil several degrees. It is after these rains that "things just jump." Fortunately the means of controlling this factor is largely in the hands of the farmer. The excess water may be removed, and the soil warmed by draining it. The draining of land by deep plowing, ditching, tiling and other methods is considered in Chapter IX. Influence of Exposure on Warmth of Soil. — The "lay of the land" with reference to the compass, and the steepness of the slope, have an important influence on the warmth of the soil. The soil on a northern slope — which receives about one-third less sunshine than a southern slope, depending upon its steepness — may average 7° to 10° cooler in summer than the soil on a southern slope. The soil of a gentle southern or western slope may be 3° to 5° warmer than the same kind of soil is on a level. In the northern part of the United States the sun is always more or less in the south, so that its rays never strike level soil squarely. It is farthest in the south when the need of greater soil warmth is most likely to be felt. In early spring a slope of 12 to 15 feet in a hundred will catch the largest number of the sun's rays, being most nearly at right angles to them. Many of the rays glance off from the level land because they strike it obliquely. The practical conclusion is that a moderate slope to the south or southwest is the best site for a crop when earliness is desired; which is what husbandmen, especially fruit growers and gardeners, have known and practised for centuries. Dark-coloured Soils Absorb More Heat. — The colour of a soil is often some index to its agricultural value and has an important influence on its temperature. A dark-coloured soil is usually warmer and earlier than a light-coloured soil. All dark substances absorb more of the sun's rays than light substances. That is why we wear light-coloured clothes in summer, and partly why snow melts faster on the dark-coloured, plowed ground than on the meadow. In Switzerland farmers sometimes hasten the disappearance of the snow by strewing it with black, powdered slate. Gardeners sometimes sprinkle a light-coloured soil with peat, charcoal and bog mould; these are called "sun traps." Melons are ripened in Saxony with the aid of a layer of coal dust. But although colour has an important influence on the power of a soil to absorb heat, it has not ability to retain heat. Schubler states that, other things being equal, a dark-coloured soil is about 8° warmer near the surface than a light-coloured soil. This difference in the temperature of soils, due to colour, may have a marked influence upon the growth of a crop, especially on its germination. When earliness is a prime consideration, as it is with most market-garden crops, the colour of a soil may become very important. Dark, sandy loams, rich in humus, are preferred by market gardeners. Light-coloured soils may be made dark by filling them with humus. Two or three green-manuring crops plowed under will darken a light-coloured soil quite noticeably. I have a neighbour who, in three years, has transformed a poor, yellow soil into a black, retentive and productive loam by plowing under four inches of composted manure every fall. Another neighbour, under similar circumstances, has accomplished nearly as good results by plowing under muck drawn from a near-by swamp. The chief reason for adding humus to a soil is to improve its texture, but another benefit, and one that is often quite important, is to improve its colour. The buff yellow and yellowish-brown colours of soils are usually due to the presence of iron oxides. These soils are most common south of the glaciated part of the United States, particularly in the southern Appalachian states.

The Influence of Tillage on Soil Temperature. — The way in which a soil is handled has much to do with its warmth. Uneven, ridged soil, like that left by fall plowing, loses more heat than smooth, level soil. However, ridging may warm the soil by drying it, and this usually more than counterbalances the loss of heat because of the greater surface exposed. Rolling land in fair and warm weather makes it warmer, but rolling it in cloudy and cold weather, especially if it is wet, makes it colder. Deep plowing makes the soil cooler, because loose soil is a poor conductor of heat. The decay and fermentation of farm manure plowed into a soil may raise its temperature several degrees; it produces as much heat in the soil as it would if burned in the open air. Manured soil is usually about 2° warmer in spring than unmanured soil. Thorough tillage, especially in the preparation of a seed bed, has a marked influence on soil temperature; it prevents the evaporation of soil moisture and hence keeps in the soil the large amount of heat that it takes to evaporate water. Good tillage saves heat, then, as well as water, especially in early spring. This means that the soil for early crops should be plowed early and tilled often.