The Romance of Modern Chemistry

CHEMISTRY AND AGRICULTURE the origin of which we have discussed, the elements which are essential to the building up of the plant are derived from the soil itself. Compounds containing potassium, magnesium, calcium, iron, sulphur, and phosphorus are found in the rocks of the earth’s crust, and it is through the breaking down of these rocks that the various in- gredients of soils have been produced, except, indeed, the humus, which has quite a different origin. The humus is that part of the soil which represents the decayed vegetation of an earlier age; it is organic in origin, and contains the ruins and remains of the nitro- genous compounds which were built up in that vegetation. Hence arises the fertility of virgin soil from which no crops have ever been taken; it is rich in nitrogenous humus, and is practically a storehouse of food for the first crop which the new settler grows upon it. When the crops which, grow on a given piece of ground are removed year after year, the soil must obviously become impoverished in the chemical materials on which the crops have fed. There need be no anxiety about the supply of carbon; the source of this element is the atmosphere, and fresh quantities of carbon dioxide are always being produced. Nor is there likely to be any shortage of hydrogen and oxygen ; they come from water, and we are not often seriously troubled, in this country at least, with a deficiency of that commodity. It is really in regard to nitrogen, phosphorus, lime, and potassium that the soil becomes most rapidly impoverished, and if the crops are to be kept up in quality and quantity we must replenish the store of these elements; that is, manuring becomes essential. The necessity for this was, of course, recognised long before the agricultural chemist came on the scene, but since his appearance the materials 223