Long before agricultural chemistry was thought of there were practically only two kinds of manure in use - farmyard or stable manure and lime. These constituted the stock of the farmer and market gardener, but other odds and ends were added to soil in the way of waste materials. With the advance of botanical and chemical science, however, plant-growers have been made aware of the different constituents of plants, and numerous experiments proved that from twelve to thirteen different ingredients (see p. 108) were always found in plants, and had to be supplied. Of these the most important are the nitrates, phosphates, potash, and lime. Hence manures are now classified in accordance with the amount of food they supply as nitrogenous, phosphatic, potassic, and calcareous. Natural manures supply all these foods in small quantities in proportion to their bulk, but they must not be despised on this account. The advantages of complete, bulky manures are discussed under the heading of "Farmyard Manure" below, and these advantages exist to a certain extent in all organic material placed in the soil for manurial purposes. Artificial manures, on the other hand, supply large quantities in proportion to their bulk of one or more fertilizers, and therefore have to be used with caution. And they possess not only this disadvantage, but others. They supply no humus to the soil, and consequently are incapable of generating bacteria. Their application is often of what may be termed a purging nature, because they liberate too freely large quantities of valuable foods that cannot be absorbed by the roots of plants, and are therefore lost either in the drainage or as gas that escapes into the air. Thus it may happen that a soil, instead of being enriched by applications of chemical manures, may be quickly impoverished and rendered sterile. In practice this is actually the case when chemical manures are applied injudiciously or indiscriminately.

From a practical standpoint it may be more convenient to consider the various manures under the following headings: -

1. Complete Manures

Those supplying not only nitrogen, potash, phosphates, and lime, but also the other essential foods like sulphur, iron, magnesia, soda, chlorine, etc.

2. Nitrogenous Manures

Those chiefly supplying nitrogen.

3. Phosphatic Manures

Those chiefly supplying phosphoric acid.

4. Potash Manures

Those chiefly supplying potash.

5. Calcareous Manures

Those supplying lime or chalk.

6. Miscellaneous Manures

Miscellaneous Manures, such as sulphate of iron, salt, etc.