THE ACCUMULATION OF NITROGEN IN FOREST SOILS. 187 
per cent. of organic matter. This organic matter or humus 
contains about 1°5 percent. of nitrogen, which means in an 
acre of soil of 6 inches depth, about 240 lbs. of nitrogen. 
This represents the accumulation due to fifty-six years’ growth, 
exclusive of what has been lost, of what has been stored up in 
the trees, and of what is present in the surface layer of 
decaying vegetable matter. It must further be noted that 
the soil in question would, for a long time, have little or no 
absorptive or retentive power for soluble nitrogenous compounds 
such as ammonia or nitrates. 
The power of soils to absorb and retain nitrogen is very 
varied, and depends on a great variety of circumstances, such as 
the fertility of the soil, the composition of the principal stock, 
of the vegetable soil-covering, mode of working, climate, 
abundance of micro-organisms, etc. In the case of an old 
forest on fertile soil, the rate of increase of nitrogen is much 
greater, and the example quoted above may fairly be taken 
to represent the minimum rate of increase, 
M. Henry next considers in turn the various means by which 
the soil can gain and lose nitrogen, and shows that the main 
source of increase is undoubtedly the nitrogen of the atmosphere ; 
how is this “fixed” and ultimately converted into a form 
suitable for plant-food? That leguminous, and, to a lesser 
extent, certain other plants, usually abundant in the undergrowth 
of a forest, can fix the nitrogen of the air by means of the 
bacteria on their roots is now well known. The growth of 
these plants, then, gradually increases the amount of nitrogenous 
matter in the soil. 
In addition to this, however, the announcement made by 
M. Henry in 1897, that certain micro-organisms, working on the 
dead leaves of various trees (oak, beech, hornbeam, aspen, 
Austrian pine, etc.), especially in the presence of moisture, 
have also the power of fixing the free nitrogen of the air, is now 
confirmed, not only by the later results of M. Henry himself, 
but also by the independent investigations of H. Siichting in 
Germany and D. L. Montemartini in Italy. 
It is thus seen that the leaves at their fall, in addition to 
returning to the soil all the mineral matter not fixed in the body 
of the tree, form a substratum each year for the life of the 
micro-organisms which play such an important part in the 
fertilisation of the soil. 
