FOOTNOTES:

[61] Composition of the earth's solid crust in 100 parts by weight:—

Oxygen44.0 to 48.7
Silicon22.8 to 36.2
Aluminum 9.9 to 6.1
Iron 9.9 to 2.4
Calcium 6.6 to 0.9
Magnesium 2.7 to 0.1
Sodium 2.4 to 2.5
Potassium 1.7 to 3.1

(Roscoe's 'Lessons in Elementary Chemistry,' p. 8.)

CHAPTER II.

FUNCTIONS PERFORMED BY MANURES.

Having now considered the general conditions on which fertility of soil depends, we are in a position to deal with the nature and function of manures.

Manures may be classified in several different ways, and a considerable amount of confusion is sometimes caused by the variety of classification adopted by different writers on this subject.

Etymological meaning of the word Manure.

Let us, in the first place, clearly understand what we mean by a manure. The word manure comes from the French word manœuvrer, which simply means "to work with the hand," hence "to till," and this etymological meaning of the word illustrates the old belief in the function of manures. We have already seen in the historical introduction that, according to Tull, the true and only function of manures was to aid in the pulverisation of the soil by fermentation. In advancing his system of thorough tillage, he claimed that since tillage effected the pulverisation of the soil, where it was practised, manures could be dispensed with.

Definition of Manures.

We no longer, of course, attach this old meaning to the word. The word manure is now applied to any substance which by its application contributes to the fertility of a soil. As has been shown in the previous chapter, the substances necessary for plant-growth which are apt to be lacking in a soil, are only generally three in number—viz., nitrogen, phosphoric acid, and potash. A manure, therefore, is understood to be any substance containing these ingredients, either singly or together, and its commercial value is determined by the amount it contains of these substances. But while this is so, it must not be forgotten that if we define a manure to be a substance which contributes in any way to the fertility of the soil, substances other than these above mentioned may be fairly regarded as manures. The fertility of a soil, we have seen, depends not merely on the presence of certain constituents, but also on their chemical condition—i.e., whether they are easily soluble or not. It further depends, as we have also seen, on the possession by the soil of certain mechanical and biological properties. Thus there are substances which act upon the soil's inert fertilising matter, and by their action convert it into a more speedily available form. There are other substances which by their application exert a considerable effect on the texture of the soil, and thereby influence its physical and biological properties. All such substances, according to the above definition of a manure, must be included under the term. It will thus be seen that since fertility in a soil can be promoted in a variety of ways, and the functions performed by manures are of different kinds, we can divide them into different classes, according to their respective action.

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.