| Beech forest, | about | 44 | lbs. | of | nitrogen; |
| Silver fir forest, | " | 35 | " | " | " |
| Spruce forest, | " | 33 | " | " | " |
| Scotch pine forest, | " | 30 | " | " | " |
Thus, only the ammonium salts and the organic compounds of nitrogen formed in the process of decay are available for the roots as sources of nitrogen. The amount of nitrogen supplied to the soil through atmospheric precipitation, either in the form of nitrates or ammonia, is not sufficient to supply the needs of trees for nitrogen.
There remains still another source, and this is the organic compounds of nitrogen formed in the process of decay of litter. In fact, Ebermayer has recorded strongly developed roots of spruces and firs on the Bavarian Alps that grew in pure humus one meter thick, from which he concludes that the dark forest humus furnishes all the nitrogen and other mineral nourishment required by trees.
If, therefore, the source of nitrogen in forest soil is nitrogenous compounds resulting from the decay of the litter, one would expect in a forest which is managed on a business basis (that is, in which trees are removed when ripe), a gradual decrease of the contents of nitrogen in the soil, as occurs on a larger scale in agriculture. In agriculture, where the annual harvesting of crops deprives the soil of almost all the nitrogen which is assimilated by the plants, and returns to the soil only a small part of it by the decay of the roots of the plants, and where the easily soluble nitrates are washed out by rains and carried away from the fields, or deposited in layers inaccessible to the roots, the exhaustion of nitrogen in the soil sets in soon, and the artificial introduction of nitrogen becomes a necessity.
One of the most common ways of replenishing the nitrogen taken up by crops is manuring and the growing of leguminous plants which have the capacity of absorbing atmospheric nitrogen. These plants are plowed under during the period of blooming, and when they decompose they give their nitrogen to the soil. In the forest, it is true, a considerable part of the nitrogen is returned to the soil in the form of shed leaves, and only part of it, which is contained in the trunk of the tree, is removed. The washing out of nitrates from forest soil does not occur, because no nitrates are formed in it, and those which are brought in by atmospheric precipitation are decomposed under the influence of a special microorganism known as Bacillus dentrificans, which is formed in soils with acid reaction.
But forest soil, though it loses less nitrogen than does arable land, nevertheless loses it; and more remarkable yet, forest soils not only do not become poorer in nitrogen, but, on the contrary,
