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. Mueller'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._