438 POPULAR SCIENCE MONTHLY. 



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 com- 

 pounds 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 nitrog- 

 enous 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 agri- 

 culture. In agriculture, where the annual harvesting of crops de- 

 prives 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 de- 

 composed 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, 



