The tremendously large number of soil grains found in even a small amount of soil makes it possible for the soil to hold very large quantities of capillary water.  To illustrate:  In one cubic inch of sand soil the total surface exposed by the soil grains varies from 42 square inches to 27 square feet; in one cubic inch of silt soil, from 27 square feet to 72 square feet, and in one cubic inch of an ordinary soil the total surface exposed by the soil grains is about 25 square feet.  This means that the total surface of the soil grains contained in a column of soil 1 square foot at the top and 10 feet deep is approximately 10 acres.  When even a thin film of water is spread over such a large area, it is clear that the total amount of water involved must be large It is to be noticed, therefore, that the fineness of the soil particles previously discussed has a direct bearing upon the amount of water that soils may retain for the use of plant growth.  As the fineness of the soil grains increases, the total surface increases’ and the water-holding capacity also increases.

Naturally, the thickness of a water film held around the soil grains is very minute.  King has calculated that a film 275 millionths of an inch thick, clinging around the soil particles, is equivalent to 14.24 per cent of water in a heavy clay; 7.2 per cent in a loam; 5.21 per cent in a sandy loam, and 1.41 per cent in a sandy soil.

It is important to know the largest amount of water that soils can hold in a capillary condition, for upon it depend, in a measure, the possibilities of crop production under dry-farming conditions.  King states that the largest amount of capillary water that can be held in sandy loams varies from 17.65 per cent to 10.67 per cent; in clay loams from 22.67 per cent to 18.16 per cent, and in humus soils (which are practically unknown in dry-farm sections) from 44.72 per cent to 21.29 per cent.  These results were not obtained under dry-farm conditions and must be confirmed by investigations of arid soils.

The water that falls upon dry-farms is very seldom sufficient in quantity to reach the standing water-table, and it is necessary, therefore, to determine the largest percentage of water that a soil can hold under the influence of gravity down to a depth of 8 or 10 feet—­the depth to which the roots penetrate and in which root action is distinctly felt.  This is somewhat difficult to determine because the many conflicting factors acting upon the soil-water are seldom in equilibrium.  Moreover, a considerable time must usually elapse before the rain-water is thoroughly distributed throughout the soil.  For instance, in sandy soils, the downward descent of water is very rapid; in clay soils, where the preponderance of fine particles makes minute soil pores, there is considerable hindrance to the descent of water, and it may take weeks or months for equilibrium to be established.  It is believed that in a dry-farm district, where the major part of the precipitation