If a kernel of corn be placed in the ground in early spring before the soil has become warm, the seed will not germinate. Abundance of moisture and oxygen may be present, but the third requisite for germination, proper temperature, is lacking. The soil is very slow to become warm in the spring, and this is due to the large amount of water which must be evaporated. During the winter and spring, the rain and melting snow have saturated the soil. The under-drainage is deficient so there is no way for the escape of the surplus water except by evaporation, and evaporation is a cooling process. A well-drained soil is thus warmer than a poorly-drained one.
The atmosphere is much quicker to respond to changes in temperature than is the soil. In the spring, the air becomes warm while the soil continues cold, and the rains which fall during this time are warmed by passing through the warm air. Then in sinking through the soil the rain-water parts with some of its heat which makes the soil warmer. During mid-summer the soil becomes very warm, and it is not affected by cool nights, as is the atmosphere. Consequently as a summer rain may be several degrees cooler than the soil, the water in passing through the soil takes up some of the heat; thus the soil conditions are made more favorable for plant growth. Therefore, soil temperature is regulated somewhat by the rainfall.
Experiment No. 10.—The color of a soil also affects its temperature, a dark soil being warmer than a light colored soil. By having thermometers as a part of the school room equipment, interesting experiments may be conducted in determining the effect of color and moisture upon the temperature of soils.
Air in the Soil.
Although that part of the plant which we can see is entirely surrounded by air, it is also necessary that the soil be in such a condition that it can be penetrated by the air. Indeed, growth cannot begin in a soil from which the air is excluded.
Fig. 40. The clover roots penetrate the soil deeply.
Experiment No. 11.—To prove this, put clay soil in a pot and plant seeds; then wet the surface of the soil and puddle or pack the clay while wet and watch for the seeds to germinate and grow. At the same time put seeds in another pot filled with loose, mellow, moist soil.
Frequently, after the farmer has sown his grain, there comes a heavy, beating rain, and the surface of the soil becomes so packed that the air is excluded and the seeds cannot germinate. If plants are grown in pots and the water is supplied at the top, the soil may become so hard and compact as to exclude the air and the plants will make a sickly growth. The surface soil must be kept loose so that the air can penetrate it.
