CVITI HENRIK HESSELMAN. 
stances we shall find azotobacter in practically all soils with alkaline or 
possibly neutral reaction, but not in the sour soil, inasmuch as, like other 
bacteria, they spread with great ease, but can only live and attain any -. 
further development under certain circumstances. 
It seems too as if this way of looking at things could properly be adapted 
to the behavior of the saltpetre-forming bacteria. It is only under certain 
circumstances that they can develop. If these circumstances exist, one can, 
as a rule, find traces of the saltpetre-forming bacteria; but where the circum- 
stances are unfavourable, there one seeks for them in vain. 
In my previous treatise (HESSELMAN, 1917), I have shown that saltpetre- 
forming types of humus are formed on a ground which is fairly rich in salts 
(electrolytes), or under such circumstances that the humus-covering becomes 
exposed to the influence of electrolytic salts. The formation of humus that 
takes place accompanied by a great removal of electrolytes, or on ground 
poor in electrolytes, leads to types of humus where the nitrogen is not nitri- 
fied. As examples of this may be mentioned on one side the typical mould 
where worms or insects see to it that the humus substances are mingled with 
mineral particles, so that the humification is influenced by electrolytes. On 
the other side, we may call to mind the tough peat-like covering of raw 
humus that is formed in the sparse spruce forests of Norrland on a ground 
with a deep layer of bleached sand, that is to say on ground the surface 
of which is poor in electrolytes. In the mould nitrification is constantly 
going on: in order to set nitrification going in a covering of raw humus 
the ground must be burnt or the humus must be mixed with mineral soil. 
These essential differences between the formative conditions of mould and 
raw humus may, it seems to me, form the starting-point for a discussion 
concerning the changes, favourable to the saltpetre-forming bacteria, that the 
humus-covering often undergoes in consequence of the measures that we take 
for the regeneration of our forests. 
This explanation is, of course, most obvious when we are concerned with 
the preparation of the ground by machines or by burning. In the former 
case, the humus-covering is mingled with mineral soil, which always contains 
a number of soluble electrolytes; in the latter case, soluble salts are formed 
through the fire, which, when they are of a basic nature, are able to neutral- 
ize any humus acids that occur. In both cases the electrolytes affect the 
structure of the humus-covering. Through the coagulation of the colloids (ef. 
HESSELMAN, 1917, page 305) the structure becomes more mould-like. 
On cut areas in the mossy coniferous forests of central Sweden, where the 
mere increase of the access of light can bring about an active nitrification in 
the ground, the humus-covering undergoes changes of structure similar to those 
which occur when the ground is prepared or burnt. The humus-covering as- 
sumes a more and more mould-like structure. As worms or insects scarcely 
occur in such numbers that the very rapid appearance of a change in struct- 
ure can be explained in a purely biological way, it seems to me that we 
ought to seek the cause in the increased introduction of electrolytes brought 
about by physical and chemical causes. 
This introduction of electrolytes can be thought to come from two causes, 
namely, (1) from the humus-covering itself, and (2) from the underlying 
mineral soil. The types of humus which soon begin to nitrify on the clear- 
