Now I think we can answer the question which was asked when we were studying the habit of growth of roots but was left unanswered at the time (see page 14).  The question was this:  Of what value is it to the farmer to know that roots enter the soil to a depth of three to six feet?  We know that roots will not grow without air.  We also know that if the soil is full of free water there is no air in it, and, therefore, roots of most plants will not grow in it.  It is, therefore, of interest to the farmer to see that free water does not come within at least three or four feet of the surface of the soil so that the roots of his crops may have plenty of well ventilated soil in which to develop.  If there is a tendency for free water to fill the soil a large part of the time, the farmer can get rid of it by draining the land.  We get here a lesson for the grower of house plants also.  It is that we must be careful that the soil in the pots or boxes in which our plants are growing is always supplied with film water and not wet and soggy with free water.  Water should not be left standing long in the saucer under the pot of a growing plant.  It is best to water the pot from the top and let the surplus water drain into the saucer and then empty it out.

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

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.