Using a balance, compare weights of equal-sized boxes of different soils, dried and powdered fine. Note the comparative lightness of humus. Weigh a box of earth taken fresh from the field, from this compute (1) the weight of a cubic foot of such soil, (2) the weight of the soil to the depth of a foot in a ten-acre field. Repeat the experiment, making it an exercise in percentage. Fill two glass tubes (lamp chimneys will do), one with finely powdered clay, the other with sand. Set the tubes in a pan containing water. Note the rise of the water due to capillarity. Through which soil does it rise faster? Farther? Try with other soils. Try with fine soil and also with the same soil in a lumpy condition. From this give a reason (1) for tilling soil, (2) for rolling after seeding. SUBSOILS Procure samples of soil from different depths, four inches, eight inches, twelve inches, sixteen inches, etc. Note how the soil changes in colour and texture. In which do plants succeed best? In most fields the richest part of the soil is contained in the upper nine inches; the portion below this is called subsoil. This extends to the underlying rock and is usually distinguished from the upper portion by its lighter colour, poorer texture, and smaller supply of available plant food. The difference is due largely to the absence of humus. The character of the subsoil has an important bearing on the condition of the upper soil. A layer of sand or gravel a few feet below the surface provides natural drainage, but if it be too deep, it may allow the water to run away rapidly, carrying the plant food down below the roots of the plants. A hard clay subsoil will render the top too wet in rainy weather and too dry in droughts, because of the small amount of water absorbed. Such a soil is benefited by under-draining. A deep and absorptive subsoil returns water to the surface, by capillary action, as it is needed. The subsoil finally contains a large amount of plant food, which becomes gradually changed into a form in which plants can make use of it. Pupils should find out the character of the subsoil in their various fields at home and its effect on the fertility of the field. FERTILIZERS Along with water, the roots take up from the soil various substances that are essential to their healthy growth. Potash, phosphoric acid, nitrogen, calcium, sulphur, magnesium, and iron are needed by plants, but the first three are particularly important. If land is to yield good crops year after year, it must be fertilized, that is, there must be added chemicals containing the above-mentioned plant foods. Land becomes poor from two causes: the plant food in the soil becomes exhausted, and poisonous excretions from the roots of one year's crops act injuriously on those of the next season. Rotating crops will improve both conditions for a while, but eventually the soil will require treatment.