Different Classes of Manures.
In the first place, we can divide manures into two great classes,—(1) those supplying to the soil necessary plant-food constituents, and thus contributing directly to fertility; and (2) those influencing soil-fertility in an indirect manner. The first class we may call direct manures, and the second indirect. Those two classes admit further of being subdivided into other smaller classes. Among the direct manures we have a number of subdivisions in use. They may be divided into general manures and special manures, according as they contain all the elements necessary for plant-growth, or only some of them; or they may be divided according to their source into natural and artificial, mineral and vegetable. Similarly we have a number of subdivisions among the second class, depending on the special nature of the action they exert. Some manures act in both capacities—both directly and indirectly—and in order that their value be fully appreciated must be studied under both heads. The most striking example of such a manure is farmyard manure. There are other manures which may in certain circumstances act in two different ways. Such a substance is lime. There are soils which are actually lacking in a sufficiency of lime for the needs of crops. On such soils an application of lime would act both as a direct and also as an indirect manure. There may also be cases of an exceptional nature, in which magnesia salts or even iron salts may act as direct manures. Many manures commonly regarded as purely direct manures would exert an indirect influence were the quantities in which they were applied sufficiently large. This is the case, indeed, with many artificial manures, such as guano, bones, nitrate of soda, and basic slag. It has been claimed for nitrate of soda that it not merely promotes fertility by supplying nitrogen in its most available form to the soil, but that the soda it contains exerts a valuable indirect influence in consolidating the soil and increasing its absorptive powers. When we reflect, however, on the small quantity of this manure which is applied per acre, its mechanical influence must be insignificant. The same applies to basic slag, which contains a considerable quantity of free lime in its composition. As this manure, however, is sometimes applied in considerable quantities, it is reasonable to suppose that its indirect value may not be altogether insignificant. Indeed we have proof of this in the fact that its most favourable action has been found to be on soils rich in organic matter.[62] The action of bones and guano, and indeed of all other manures containing a large percentage of decomposable organic matter, is likewise of a double nature, inasmuch as their decomposition or putrefaction in the soil gives rise to the formation of carbonic and organic acids, which are capable of exerting a chemical action on the soil ingredients. There is one point in connection with the action of these manures which is worthy of notice, and it is that, however slight their indirect value may be, their action as a direct manure is very much accelerated by the way in which their organic matter putrefies. In short, they may be described as providing, to a certain extent, the solvents which render them available for the requirements of the plant. It may be here convenient to classify the manures which we intend subsequently to deal with.
I. Manures, action of which is both direct and indirect—e.g., green manures, farmyard manure, composts, and sewage.
II. Manures which may be regarded as having only a direct action—e.g., guano of all kinds, bones in all forms, nitrate of soda, sulphate of ammonia, dried blood, superphosphates, mineral phosphates of all kinds, horns and hoofs, shoddy, wool-waste, fish-guano, muriate of potash, sulphate of potash, and kainit.
III. Manures which may be regarded as having only an indirect value—e.g., lime, mild and caustic, marl, gypsum, salt, &c.
We shall now proceed to discuss the nature and action of these different manures, starting with those exercising both a direct and indirect influence. Before doing so it may be well to consider the occurrence and natural sources of the three important soil constituents, nitrogen, phosphoric acid, and potash, with a view of seeing to what extent these are being removed from our soils by the various natural processes constantly going on, as well as by the crops, and how far their natural sources are capable of making good this loss—in short, to clearly understand the economic reasons for the application of artificial manures.
FOOTNOTES:
[62] See Chapter on Basic Slag.