This has the effect of warming the soil, and water in it, causing both to expand and one of them (water) to evaporate. As water is driven out of the soil in this way air enters, and thus makes the soil warmer than it was before. As the temperature of the air varies greatly between midday and midnight, sometimes as much as 60° F., one can readily imagine a kind of opening and closing - or expanding and contracting - movement going on continually on the crust of the earth, much in the same way that the tides rise and fall, although not so conspicuous. This variation of temperature has its effect in splitting up the soil into smaller particles. Of course the temperature varies according to altitude, season, and climate, but it seems to be universal that night temperatures are always lower than day temperatures.

The temperature of the soil itself, as distinct from that of the air, varies according to the nature of the soil and the depth at which it is cultivated.

All heat is derived from the sun, and the gardener seeks, in temperate climes at least, to secure as much as possible for his crops. Thus he likes to have his land with a gentle slope between the south-east and the south-west, because a larger surface is thus exposed to the direct rays of the sun. Even on level ground, if he is wise, he will always arrange his rows of fruit trees and bushes, Potatoes, Lettuces, Tomatoes, Peas, Beans, etc, running as near north and south as possible, so that the sun shall shine in between the rows at midday to warm the soil about the roots. If the rows run east and west, one will shade the other, with the result that the soil will have a lower temperature, the effect of which is less feeding activity of the roots.

The annexed diagram, from The Standard Cyclopaedia of Modem Agriculture, shows the variation of temperature in clayey, sandy, and chalky soils. It will be noticed in each case that the temperature of the soil at 4 ft. deep is always higher than that of the soil at 1 ft., and higher than the air, during the first three months of the year (January, February, March), and (in the case of clay and sand) during the last four months of the year (September, October, November, December). During the other months, April, May, June, July, August, the soil at 4 ft. deep is generally several degrees cooler than the air. The diagram shows the variations for the three soils.