On the whole, however, as we have said, nitrogen is chiefly found in the surface-soil. The amount found in the subsoil at Rothamsted seems to vary very slightly at different depths, the percentage amounting to from .06 to .03.[71] Unlike the nitrogen of the surface-soil, that in the subsoil seems to be of very ancient origin, being probably derived from the remains of animal and vegetable life in the mud deposited at the bottom of the ocean. It is more abundant in the case of a clay subsoil than in a sandy subsoil.
Nitrogen of Surface-Soil.
Nitrogen has a tendency to collect on the top layers of the surface-soil, the first 9 inches or foot containing by far the largest proportion of it. In the table given in the Appendix,[72] the rate at which it decreases in amount the further down we go is clearly shown. Determinations of the respective amounts of nitrogen in every 3 inches of the soil, taken to a depth of one foot of the experimental wheat-field at Rothamsted, showed that the percentage between the first 3 inches and the second 3 inches varied very slightly. A more marked difference, however, was shown to exist between the nitrogen in the second and third 3 inches; while the fourth 3 inches were distinctly poorer—differing very little in their percentage of nitrogen from the subsoil. This was the case in unmanured soil. In the case of heavily manured soil, the increase in the soil's percentage, due to manure, was shown to be felt to the depth of a foot, but not much below it.[73]
A careful perusal of the tables in the Appendix will show that the quantity of nitrogen in the case of both arable and pasture soils steadily decreases for the first 3 feet, but that below this depth little decrease is seen, the percentage evidently becoming fairly constant.
The amount of Nitrogen in the Soil.
Very considerable difference exists in the amount of nitrogen present in different soils. The majority of analyses refer only to the amount found in the surface-soil—generally in the first 9 or 12 inches. As the soil, further, is not a body exactly homogeneous in its character, very considerable difficulty exists in obtaining reliable results. A great deal depends, therefore, on the method of sampling and the basis of calculation adopted; and it may be that this may occasionally explain, to some extent at least, the great discrepancies in the estimation of the quantities of nitrogen present in different soils as found by different investigators.
Peat-soils richest in Nitrogen.
Of all soils, peat-soils are richest in nitrogen. Professor S. W. Johnson found the nitrogen in fifty separate samples of peat to range from .4 per cent to 2.9 per cent, the average being 1.5 per cent. On the other hand, marls and sandy soils are poorest, the analyses of a number of these soils showing only from .004 to .083 per cent for the former, and .025 to .074 for the latter. As a general rule most arable soils contain over one-tenth per cent of nitrogen, or, say, over 3500 lb. per acre. A good pasture-soil, taken to a depth of 9 inches at Rothamsted, was found to contain about a quarter per cent. In ten samples of soil, taken to a depth of 9 inches, from different parts of Great Britain and Ireland, Munro found from .128 to .695 per cent of nitrogen, the average being .3278 per cent. The Rothamsted soils, it may be pointed out, are probably poor in nitrogen compared with most soils. A. Müller's investigations showed that in some of the soils he has analysed, the nitrogen amounted to little short of one per cent, while for the others the average was over half a per cent; even the poorer soils he examined contained about one quarter per cent on an average. Anderson's analyses of Scottish wheat-soils showed a variation of from .074 to .22 in the surface-soil, while he found in their subsoil from .15 to .92 per cent. Boussingault's results are also very much higher. The amount of nitrogen in a number of loams coming from widely different localities he examined contained from 6000 to 30,000 lb. per acre—the soil taken to a depth of 17 inches.[74]
Nature of the Nitrogen in the Soil.
When we compare the amount of nitrogen removed by different crops (which, even in the case of those most exhaustive of nitrogen, does not often amount to more than 150 lb. per acre), with the amount contained in the soil, the former amount seems very insignificant when compared to the latter. Such being the case, it would seem at first sight that the addition of nitrogen in the form of manures is quite superfluous. We must remember, however, that while the total amount of nitrogen is relatively large when compared to that removed by crops, only a very small proportion is in a condition available to the plant. This leads us to consider the different forms in which nitrogen is present in the soil, and their respective quantities.