[27] Further reference is made to this subject in Chapter III., p. 136. [28] See p. 6. [29] See Phil. Trans., Part II., 1861, pp. 444-446. Lawes & Gilbert. Schloesing has found in the air in the neighbourhood of Paris 1 lb. of ammonia in 26,000,000 cubic yards; while Muentz found only about half that amount in a similar quantity of air on the top of the Pic du Midi. [30] See Chapter III., pp. 119, 120; Appendix, p. 155. [31] Some recent experiments by Dyer and Smetham would seem to show that comparatively small quantities of ammonia in the air prove actually hurtful to plant-life. Thus they found that one volume of ammonia in 1000 volumes of air was fatal to hardy plants; while one volume in 3000 volumes killed tender ones. [32] According to the experiments of Hellriegel and Wollny. The quantity, it may be added, varies with the leaf-surface and the length of the period of growth of the plant. It is greatest with clovers and grasses, and least in the potatoes and roots. PART II. PRINCIPLES OF MANURING CHAPTER I. FERTILITY OF THE SOIL. It is necessary to clearly understand to what the fertility of a soil is due ere we can hope to master the theory of manuring. _What constitutes Fertility in a Soil._ The question, What constitutes fertility in a soil? is by no means an easy one to answer. If we say, The presence of a plentiful supply of the constituents which form the plant's food, our answer will be incomplete. Similarly, if we reply, A certain physical condition of the soil--here, again, it will be found equally unsatisfactory; for fertility of a soil depends both on its physical condition and on its chemical composition, and indeed even on other circumstances. It may be well, then, before proceeding to treat of the nature and action of the different manures, to offer a brief statement of the conditions of fertility so far, at any rate, as we at present know them. For it may be well to warn the reader that, despite the great amount of work carried out on this subject by experimenters, we still have much to learn before we shall be in a position fully and clearly to understand the subject of soil-fertility in all its bearings. Apart altogether from the influence exerted by climate, latitude, altitude, and exposure, the fertility of a soil may be said to depend on the following properties. These we may divide into three groups or classes:-- 1. Physical or mechanical. 2. Chemical. 3. Biological.