_Amount of combined Nitrogen falling in the Rain._
The importance of the combined nitrogen in the air as a source of soil-nitrogen is best gauged by the amount falling annually on the soil dissolved in rain. This has been found to vary considerably. In the rain falling in the vicinity of large towns the amount is greater than in rain falling in the country. Thus at Rothamsted, in England, the average amount for several years was only 3.37 lb. nitrogen per annum per acre, of which 2.53 lb. were as ammonia,.84 being as nitric acid. At Lincoln, in New Zealand, 1.74 lb. fell annually per acre--as ammonia,.74, as nitric acid, 1.00; while at Barbadoes the amount was 3.77 lb., of which .93 was as ammonia, and 2.84 as nitric acid.[68] That the combined nitrogen derived from the air by the soil may be considerably in excess of this is highly probable. Soils, especially when damp, may absorb much larger quantities from the air of the combined nitrogen it contains. We must remember that the air in contact with the soil-surface is constantly being changed, and that there is thus a constant renewal of the air passed over the ground. The result is that the amount of air from which combined nitrogen may be removed is very great.[69]
It has been remarked as a fact worthy of notice that nitrogen is essentially a superficial element. By this is meant that it is only found, as a rule, on the earth's immediate surface. This statement can only be admitted to be true within certain limits. The chief source of nitrogen, in addition to the atmosphere, is, of course, vegetable and animal tissue.[70] As vegetable and animal tissue are only found to any extent on the earth's surface, nitrogen is therefore chiefly found there. The natural deposits of nitrogen salts, such as the nitrate-fields of Chili and the saltpetre soils of India, &c., also only occur superficially. Notwithstanding these facts, however, the amount of nitrogen which exists at probably considerable depths from the surface must be very great. There are few sedimentary rocks which do not contain it. At Rothamsted a sample of calcareous clay, taken from a depth of 500 feet, contained .04 per cent--that is, as much as is found, on an average, in the Rothamsted clay subsoils.