=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. [Illustration: 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.] [Illustration: FIG. 17. Bottle _A_ contains fresh water, bottle _B_ contains boiled water. Notice the air bubbles in bottle _A._] [Illustration: FIG. 18. Tumblers _A_ and _C_ contained moist sand, _B_ and _D_ contained puddled clay. Cuttings in _B_ and _D_ died, because there was not sufficient ventilation in the clay for root-development.] CLASSIFICATION OF SOILS Soil materials and soils are classified as follows: _Stones._--Coarse, irregular or rounded rock fragments or pieces of rock. _Gravel._--Coarse fragments and pebbles ranging in size from several inches in diameter down to 1/25 inch. _Sand._--Soil particles ranging from 1/25 of an inch down to 1/500 of an inch in diameter. Sand is divided into several grades or sizes. Coarse sand 1/25 to 1/50 of an inch. Medium sand 1/50 to 1/100 of an inch. Fine sand 1/100 to 1/250 of an inch. Very fine sand 1/250 to 1/500 of an inch. These grades of sand correspond very nearly with the grains of granulated and soft sugar and fine table salt. _Silt._--Fine soil particles ranging from 1/500 to 1/5000 of an inch in diameter. It feels very fine and smooth when rubbed between the fingers, especially when moist. A good illustration of silt is the silicon used for cleaning knives, a small amount of which can be obtained at most any grocery store. By rubbing some of this between the fingers, both dry and wet, one can get a fair idea of how a silty soil should feel. Silt when wet is sticky like clay. _Clay._--The finest of rock particles, 1/5000 to 1/250000 of an inch in diameter, too small to imagine. Clay when wet is very soft, slippery and very sticky. Yellow ochre and whiting from the paint shop are good illustrations of clay. _Humus_, or decaying vegetable and animal matter. This is dark brown or almost black in color--decaying leaves and woods soil are examples. Soils composed of the above materials: