Humus contains plant food and is also an excellent absorbent of the poisonous excretions. It is added as barn-yard manure, leaves, or as a green crop ploughed in. The chemicals commonly used comprise nitrate of soda, bone meal, sulphate of potash, chloride of potash, lime, ashes, cotton-seed meal, dried blood, super-phosphate, rock phosphate, and basic clay. EXPERIMENTS: 1. Sow wheat on the same plot year after year and note the result when no fertilizer is used. Sow wheat on another plot, but use good manure. 2. Try the various commercial fertilizers on the school plots, leaving some without treatment. 3. Examine the roots of clover, peas, or beans, and look for nodules. These show the presence of bacteria, which convert the atmospheric nitrogen into a form in which the plants can use it. Scientific farmers have learned the value of inoculating their soil with these germs. A crop of peas or clover may produce the same result. 4. Observe Nature's method of supplying soil with humus. SOIL-FORMING AGENTS There was once a time when the surface of the earth was bare rock. Much of this rock still exists and in many places lies on the surface, but it is usually hidden by a layer of soil. Soil is said to be "rock ground to meal by Nature's millstones". The process is very slow, but it is constantly going on. The pupils should be directed to find evidences of this "grinding". 1. RUNNING WATER.--Brooks, creeks, rain, and the tiny streamlets on the hills all tell us how soil is carried from place to place. Get some muddy water from the river after a heavy rain. Let it settle in a tall jar and observe the fine layer formed. Wash some pebbles clean, place them in a glass jar with some clear water, and roll or shake the jar about for a few minutes. Note that the water becomes turbid with fine material worn from the stones. A process similar to this is constantly going on in rivers, lakes, and seas. Account for the presence of gravel beds now situated far away from any water. 2. ICE GLACIERS.--How do these act on rocks? Show evidences in Ontario as far as these can be illustrated from the surroundings, such as polished rocks, boulders, beds of clay, sand, or gravel, small lakes, grooved stones, etc. 3. FROST AND HEAT.--See "Expansion of Solids", pages 189, 190. Look for splintered or cracked stones. Why do farmers plough in the fall? 4. WIND.--In sections near the lakes the action of the wind in moving the sand may be seen and appreciated. There are other places where this work is going on on a smaller scale.