Lastly, we give in the Appendix[78] the amount of nitrates found in wheat and barley soils, differently manured, at Rothamsted. From a perusal of these tables, it will be seen that the amount (under various conditions of manuring) of nitrates in the first 27 inches varies from 21.2 lb. per acre to 52.2 lb. for the wheat-soils, and 20.1 to 44.1 lb. per acre for the barley-soils. THE SOURCES OF SOIL-NITROGEN. We shall now consider the sources of soil-nitrogen, the conditions which determine its increase, and the amount of that increase, as well as the sources of loss, and the conditions which determine this loss. _That dissolved in Rain._ The natural sources of the soil-nitrogen are several. We have first of all the atmospheric nitrogen. Of this let us first consider that present as combined nitrogen. This, as we have already seen, consists chiefly of nitrates, nitrites, and ammonia, and reaches the soil dissolved in rain or in other meteoric forms of water, such as snow, hail, fog, hoar-frost, &c. _That absorbed by the Soil from the Air._ It is also absorbed by the soil from the air, especially when the soil is in a damp condition, as has been proved by Schloesing's experiments, already referred to. The total amount which falls dissolved in the rain, per acre per annum, varies very considerably in different parts of the world, but in any case only amounts yearly to a few pounds per acre.[79] That absorbed by the soil from the air may be probably very much more considerable. Schloesing in his experiments found that this latter might amount to 38 lb. per acre per annum. These results, however, were obtained under circumstances most favourable for absorption--viz., with a damp soil and in the vicinity of Paris, where the air is presumably richer in combined nitrogen than it is in the country. The nitrogen absorbed, it may be mentioned, was almost entirely in the form of ammonia. It is to be noted that the nitrogen the soil obtains in this way from the combined nitrogen of the air is not all pure gain. With regard to the nitrates and nitrites, no doubt most of these are formed by electrical discharge, although a small portion of them may be formed by the oxidation of ammonia by means of ozone and peroxide of hydrogen. With regard to the ammonia and the combined nitrogen present in the organic particles in the air, a not inconsiderable proportion is probably derived from the soil. Schloesing considers the chief source of the ammonia present in the air to be the tropical ocean; but we must remember that the source of much of the nitrogen in the tropical ocean is, after all, the soil. Leaving aside for a moment the question of the availability of the free nitrogen of the air, let us consider the other sources of soil-nitrogen. _Accumulation of Soil-nitrogen under Natural Conditions._