REGENERATION MEASURES AND THE FORMATJON OF SALTPETRE CTIX 
ings or in gaps are marked by a more rapid turning to mould. As the humus 
always contains a number of chemical compounds which in oxidation or 
mouldering give rise to salts, the amount of electrolytes or salt contained in 
the humus covering must be increased by the increased mouldering; and this 
can then in itself be sufficient for a transformation of the structure of the 
humus-covering. One can further think of the possibility of the introduction 
of electrolytes from the ground. The evaporation from the surface of the 
ground is increased by clear chequerboard cutting; this must result amongst 
other things in the water from the underlying mineral soil being drawn up 
towards the surface, and this water always takes with it a number of electro- 
lytes. In the wood, on the other hand, the evaporation from the surface of 
the ground is reduced, so that the movement of water it mainly directed 
downwards. A further reason for thinking that such a movement of electro- 
lytes can play an important part is that the types of humus which become 
nitrificent through increased access of light alone seem only to occur on 
slightly podsolized ground, that is to say, where the layer of bleached sand is 
not very deep, while on grounds with a strongly marked bleached sand stronger 
measures than mere cutting are necessary to produce nitrification. 
With regard to nitrification in a tough peat-like covering of raw humus 
also, it seems to me we can adopt the same method of looking at things con- 
cerning the great importance of the introduction of electrolytes. In the clear 
cutting of the sparse spruce forests or mixed coniferous forests of Norrland 
the raw humus covering begins to moulder, but the mouldering proceeds 
slowly, and therefore the introduction of electrolytes cannot be so strong as 
in the more rapid mouldering that is produced by clear cutting in the mixed 
coniferous forests of central Sweden. Moreover, the bleached sand in these 
forests is much deeper and much more strongly formed; and consequently 
the introduction of soluble salts from the mineral soil ought to be weaker than 
on the usually weakly podsolized ground in central Sweden. Possibly, too, in this 
raw humus covering we have to deal with other nitrogenous compounds than 
in the better coniferous forest lands of central Sweden. There the humus- 
covering is formed of mosses and berry refuse: in the sparse spruce forests 
of Norrland berry shrubs have greater importance. But even if this does play 
some part, yet the introduction of electrolytes would seem to be of more im- 
portance; and this belief is supported by the fact that a mixture of raw 
humus and mineral soil very soon produces an active nitrification. 
The account which I have now given concerning the causes of the changes 
in the humus-covering where nitrification appears, has in part the character 
of a working hypothesis. It may serve as a foundation for further investiga- 
tions, and constitutes an attempt to offer a similar explanation for several 
phenomena which are different rather in externals than in their inner course. 
The question how the salts work may be disputed: one may imagine that the 
humus acids are neutralized or lose their power, or that the salts serve as 
foods for the nitrifying agents and play some part in the neutralization of the 
newly formed nitric acid. In this latter respect it is possible that the iron 
compounds that occur in the podsolized ground may be of importance. ASHBY 
(1907), in fact, has shown that recently precipitated ferric-oxo-hydrate, like iron 
rust, can to some extent take the place of carbonate of lime for the neutral- 
ization of the newly formed nitric acid. But no complete neutralization of 
