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. CHAPTER III. THE POSITION OF NITROGEN IN AGRICULTURE. Of manurial ingredients, nitrogen is by far the most important, and on the presence and character of the nitrogen it contains, the fertility of a soil may be said to be most largely dependent. Most soils, as a rule, are better supplied with available ash ingredients than with available nitrogen compounds. The expensive nature of most artificial nitrogenous manures also gives to nitrogen the first position from an economic point of view. A thorough study, therefore, of the different forms in which it exists in nature, of the numerous and complicated changes it undergoes in the soil, by which it is prepared for the plant's needs, of the relation of its different forms to plant-life, and of the natural sources of its loss and gain, is of the highest importance if we are to hope to understand the difficult question of soil-fertility. _The Rothamsted Experiments and the Nitrogen question._