Fig. 35. The glass of water at the right has received lime and the clay has been flocculated; the other was not treated.
Clay soils are also frequently treated with lime to cause them to remain in good condition and be more easily tilled. Lime causes the fine particles to flocculate, or to become granular, i. e., several particles unite to form a larger particle, and these combinations are more stable and do not so readily puddle, or run together. A mud-puddle in clay soil will remain murky until the water has evaporated entirely. Let a little water-slaked lime be mixed with the muddy water, and the particles of clay will be flocculated and will settle to the bottom; thus the water will become clear.
Experiment No. 5.—Into two glasses of water put some fine clay soil; thoroughly stir the mixture ([Fig. 35]). Into one glass thus prepared put a spoonful of water-slaked lime; stir thoroughly, then allow both glasses to remain quiet that the soil may settle. Notice in which glass the water first becomes clear, and note the appearance of the sediment in each.
The Moisture in the Soil.
In Leaflet VI has been given the history of a thunder shower. We are not told much about the history of the water after it reaches the earth. If we go out immediately after a heavy shower, we find little streams running alongside the road. These little streams unite to make larger ones, until finally the creeks and rivers are swollen, and, if the rain was heavy enough, the streams may overflow their banks. In all these streams, from the smallest to the largest, the water is muddy. Where did this mud come from? It was washed largely from the cultivated fields, and the finest and best soil is certain to be the first to start on its voyage to the valleys or to the sea. If the farmer had only learned better the lesson from nature and kept his fields covered with plants, a large part of the loss might have been prevented. A rain gauge should be kept in every school yard, so that every shower can be measured. It can then be easily determined by the pupils how many tons of rain fall upon the school grounds, or how much falls upon an acre of land. It will be a matter of surprise that the amount is so great.
Fig. 36. a. Soil too dry. b. Soil in good condition. c. Soil too wet.
Not all the water which falls during a summer shower is carried off by surface drainage, since a considerable part sinks into the soil. As it passes down, each soil grain takes up a portion and surrounds itself with a little film of water, much as does a marble when dipped into water. If the rain continues long enough, the soil will become saturated and the water which cannot be retained, will, under the influence of gravity, sink down to the lower layers of soil until it finally reaches the level of the free water. From this free water, at varying depths in the soil, wells and springs are supplied. If the soil were to remain long saturated, seeds would not germinate, and most cultivated plants would not grow because all the air passages of the soil would be filled with water ([Fig. 36]). The water which sinks down deep into the soil and helps to supply our wells is called free water. That part which is held as a film by the soil particles (as on a marble) is called capillary water. After the rain is over and the sun shines, a part of the moisture which is held by the particles near the surface is lost by evaporation. The moisture which is below tends to rise to restore the equilibrium; thus there is created a current toward the surface, and finally into the air; the moisture which thus escapes aids in forming the next thunder storm.
Experiment No. 6.—Humus enables the soil to take up and hold large quantities of water. To illustrate this, two samples of soil should be obtained, one a humus, or alluvial, soil, rich in organic matter, and the other a sandy soil. Put the two samples where they will become thoroughly air dry. Procure, say five pounds each of the dry soils, and put each into a glass tube over one end of which there is tied a piece of muslin, or fine wire gauze. From a graduated glass pour water slowly upon each sample until the water begins to drain from the bottom of the tube. In this way it can be shown which soil has the greater power of holding moisture. Both samples should then be set away to dry. By weighing the samples each day, it can be determined which soil has the greater power of retaining moisture. This experiment may be conducted not only with sand and humus, but with clay, loam, gravel, and all other kinds of soil.
Experiment No. 7.—A finely pulverized soil will hold more film-moisture than a cloddy soil. To illustrate the importance of texture as related to moisture, soil should be secured which is cloddy, or lumpy. One tube should be filled, as heretofore described (Exp. No. 6), with the lumpy soil, and the other tube with the fine soil which results from pulverizing the lumps, equal weights of soil being used in each case. From a graduated glass pour water upon each sample until the drainage begins from the bottom. Notice which soil possesses greater power of absorbing moisture. Put the samples away to dry, and by careful weighing, each day, it can be determined which soil dries out more readily.