Soil Temperature

The soil temperature is a factor of vital concern.

It has been determined that biological and chemical activities are an energy expression and that these changes will not continue with the right intensity unless certain temperatures are maintained. This makes the temperature of the soil of major concern. The most favorable limit of temperature are 80 to 90 degrees Fahrenheit as nitrification does not begin until the soil reaches around 40 degrees Fahrenheit. To have seeds germinate and plants to grow the temperature varies for example as bluegrass needs a low temperature and corn needs a high temperature. The soil gets its energy for normal activity from the sun.

The amount of energy entering the soil is contingent largely upon the color, the slope, and the vegetative cover of the soil under consideration. Dark soils will absorb more energy from the sun than light colored soils and red or yellow soils will show a more rapid temperature rise tan those that are white. Studies have shown that the nearer the angle of incidence of the sun's rays approaches the perpendicular, the greater will be the absorption. This means that a southeasterly inclination is warmest in the early season, a southerly slope during midseason, and a southwesterly slope in the fall. This is not true in all locations universally, but it does indicate that slopes vary in their desirability depending on the crop that you choose to grow.

Most gardeners prefer south or southeasterly slopes.

Another important factor in the temperature of the soil is determined if the soil is bare or covered with vegetation as this affects the amount of insolation that is received. A field of grass doesn't have much effect on temperature fluctuations but a forest does. Bare soils will warm more quickly and cool off more rapidly that those covered with vegetation or even with mulches. During the winter, frost penetration is considerably greater in noninsulated land.

Mineral soils will not only vary in respect to the energy necessary to raise their temperature but also that this variation is largely in proportion to the amounts of water present. Moisture is one of the major factors in respect to the heat capacity of a soil, and hence, has much to do with its rate both of warming up and cooling off.

The vaporization of soil water is caused by an increased molecular activity and requires the expenditure of a certain amount of energy which results in a cooling effect especially at the surface where most of the evaporation occurs. The low temperature of a wet soil is due partially to evaporation and partially to high specific heat.

Drained and undrained soils in different areas will show maximum differences. The influence upon the surface layers is, therefore, marked.

The significance of conduction in respect to field temperatures provides a means of temperature adjustment. In temperature regions, for example, we expect surface soils in general to be warmer in summer and cooler in winter than the subsoil, especially the lower horizons of the latter.

We should remember that the maximum temperature of a dry surface soil may definitely exceed that of the air. In Winter even surface soils do not fall greatly before freezing. The surface soil, usually doesn't reach its maximum temperature until late in the afternoon. The lower subsoil shows little daily or weekly fluctuations as its temperature change is usually monthly or seasonal.

In effect we need to realize that the influence of moisture upon soil temperatures is great. Loss of energy to the atmosphere or the movement of heat to and fro within the soil, the percentage of water present is always important.

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