This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Soils lose moisture in four ways: (1) by natural evaporation from the surface; (2) by bad and shallow cultivation; (3) by transpiration from the leaves of the crops grown; and (4) by the leaves of weeds allowed to grow.
The loss by natural evaporation will depend upon the temperature of the atmosphere and the dryness or otherwise of the season. The higher the temperature and the drier the atmosphere the greater the evaporation from the surface. This is so well known to gardeners who grow produce under glass that special care is taken to counteract the heat and dryness by saturating both soil and atmosphere with moisture with the hose or water pot. The only market gardeners who do the same thing in the open air in a systematic manner are the intensive cultivators or maraichers in France and Holland. British fruit-growers and market gardeners, owing to the large areas they crop, find it physically impossible to apply sufficient moisture to their crops; hence they suffer great losses in dry seasons.
The diagram (fig. 89) will give an idea as to how water either rests on the surface of the soil if hard and caked, or how it sinks down to a depth of 1, 2, 3, or more feet if the soil has been broken up to such a depth. It is obvious from the diagram, in which the soil has become hard and baked, or is of a clayey nature and uncultivated, that most of the rain that falls remains on the surface, and will be soon evaporated. In the diagram, where a similar soil has been cultivated and turned up more or less deeply, more water will sink into the soil, and it may be taken that the powers of absorption will be as stated on p. 120, according to the nature of the soil. The deeper, therefore, a soil is cultivated, the more moisture it will hold for the benefit of the crops.
Fig. 89. - Diagram showing Soil dug 1 ft. (A), 2 ft. (B), and 3 ft. (C) deep at the shaded portions. The unshaded portions show the hard, impervious, and unbroken subsoil.