NITROGEN IN FOREST SOIL. 437 



other species only by gradual adaptation. Leguminosae which lack 

 proper bacteria are backward in development. 



If, for instance, black locusts are grown in soil which contains no 

 bacteria or only bacteria taken from the tubercles of leguminosse some- 

 what less closely allied to them, they thrive only moderately; whereas 

 under the influence of the locust bacteria they develop luxuriantly. 

 The organism living in the root tubercles of the alder is also, accord- 

 ing to Hiltner, instrumental in the absorption of nitrogen. It dis- 

 plays its activity particularly whenever the available nitrogen of the 

 soil begins to be exhausted on account of its vigorous consumption 

 by the tree. The fungi which cover the suction rootlets of oaks, 

 beeches, birches, and other cupuliferse, as well as conifers and many 

 other woody plants, probably also assist the trees in the taking up of 

 nitrogen. They do not, however, absorb the atmospheric nitrogen, 

 but only decompose, among other organic substances, the nitrogenous 

 compounds of humus, and thus convert them into chemical combina- 

 tions more accessible to the tree. 



Thus, excepting the leguminosse and a few species of other families, 

 plants depend for their supply of nitrogen upon the nitrogenous com- 

 pounds in the soil. And it is with these plants that the question as to 

 the source of nitrogen in the soil becomes important, inasmuch as there 

 are no minerals containing nitrogen so widely distributed as to satisfy 

 the demand of trees for this element. Atmospheric precipitations and 

 the product of the decay of animal and vegetable refuse are usually 

 supposed to be the sources of nitrogenous compounds in the soil. The 

 chemical compounds in which nitrogen occurs in the soil are chiefly 

 nitrates, ammonia, and organic nitrogenous compounds resulting from 

 the decomposition of organic substances. The best sources of nitrogen 

 for green vegetation are generally supposed to be the nitrates; but 

 for forest trees, at least, these seem to be of small account, because, 

 as Ebermayer says, in forest and marshy soils nitrates are present, 

 if at all, only in small traces. Also, in the interior of stems nitrogen 

 is found in the form of nitric acid only when they are grown on a 

 soil containing nitrates; for instance, on cultivated land. Among 

 the compounds of nitrogen which are supplied to the soil through 

 atmospheric precipitation, some nitric acid is found, but two to five 

 times as much ammonia is present. The whole amount of nitrogen 

 which is held in combination and supplied to the soil in this manner 

 through snow and rain amounts, according to Eamann, to nine pounds 

 per year per acre, and even less. This is insufficient to supply 

 the demands of trees for nitrogen, as they store up a much greater 

 quantity. Ebermayer states the average amount of nitrogen taken 

 up by various kinds of trees per year and acre as follows: 



