Which soils have the greatest capacity for film water?
Experiment.—Place in a tumbler or bottle one-half pound of pebbles about the size of a pea or bean; pour a few drops of water on them and shake them; continue adding water and shaking them till every pebble is covered with a film of water; let any surplus water drain off. Then weigh again; the difference in the two weights will be approximately the weight of the film water that the pebbles can carry. Repeat this with sand and compare the two amounts of water. A striking illustration can be made by taking two slender bottles and placing in them amounts of colored water equal to the amounts of film water held by the pebbles and sand respectively. In the accompanying illustration ([Fig. 29]), A represents the amount of water that was found necessary to cover the pebbles in tumbler B with a film of moisture. C is the amount that was necessary to cover with a film the particles of sand in D. The finer soil has the greater area for film moisture. It has been estimated that the particles of a cubic foot of clay loam have a possible aggregate film surface of three-fourths of an acre.
CHAPTER VI[ToC]
Loss of Soil Water
LOSS OF SOIL WATER AND MEANS OF CHECKING THE LOSS
We noticed in previous paragraphs that soil might at times have too much water in it for proper ventilation and so check the growth of the roots of the plant. Now is it possible that soil water may be lost or wasted and if so can we check the loss?
In the experiment to find out how well the soils would take in the rainfall ([page 40]) we noticed that the clay soil took in the water very slowly and that on a field of clay soil part of the rain water would be likely to run off over the surface and be lost. Free water may be lost then, by surface wash.