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