Experiment.—We now have knowledge and means for making simple tests of soils. Repeat the last three experiments with the garden soil. We will find, perhaps, that it is dark in color and some of it burns away when placed in the fire, therefore it contains organic matter or decaying vegetable matter or humus, as it is called. This sample has perhaps fine particles and coarse particles; part of it will settle quickly in water while part settles very slowly, and it is sticky. Therefore we conclude that there are both clay and sand in it. If we shake a sample of it in a bottle of water and let it settle for several days, we can tell roughly from the layers of soil in the bottom of the bottle the relative amounts of sand and clay in the soil. Also if we weigh a sample before and after burning we can tell roughly the amount of organic matter in the soil. Test a number of soils and determine roughly the proportions of sand, clay and organic matter in them.

Experiment.—Take the pans of soil used in our first soil experiment and separate the soils in the pans into two parts by a trench across the centre on the pan. Now wet the soil in one side of the pan and stir it with a stick or a spoon, carefully smooth the surface of the soil in the other side of the pan and pour or sprinkle some water on it, but do not stir it. Set the pans aside till the soils are dry. This drying may take several days and in the meantime we will study the classification of soils.

FIG. 16.
To show that roots need air. Bottle A was supplied with fresh water, and bottle B with water that had been boiled to drive the air out and then cooled.[ToList]

FIG. 17.
Bottle A contains fresh water, bottle B contains boiled water. Notice the air bubbles in bottle A.[ToList]