CX HENRIK HESSELMAN. 
the humus acids need take place: thus strongly saltpetre-forming humus types 
on clearings in Jönåker communal forests show a strong and reaction to 
litmus paper. 
Another thing which perhaps may play a part in nitrification is the stronger 
current of air, but this would not seem to be of any decisive importance. 
Experiments that have been carried out have clearly shown that the supply 
of free oxygen is of less importance than one might be inclined to believe 
on the strength of the course of nitrification in the culture liquids commonly 
used, where one must either employ a very thin layer of liquid or else use artificial 
means to produce an active current of air (SCHLÖSING; LÖHNIS 1910, page 615). 
Nor do the notions which are usually entertained concerning the humus- 
covering as being a strong obstacle to the aöration of the ground seem to 
be justified. ALBERT of Eberswalde (1912, page 667 foll.) has with suit- 
able instruments taken samples of air both from mould and from ground 
with a covering ot raw humus. The content of oxygen was in both kinds 
of soil the same and very high, nearly 203, on the average, as against 21 & 
in the air of the atmosphere. 
There is also another factor to take into consideration. According to the 
investigations of KocH, which I have previously summarized (HESSELMAN, 1917, 
page 392) there are certain substances thrown off from the needles of conifer- 
ous trees which hinder the activity of the bacteria of nitrification. When the 
forest is felled, there are no fresh supplies of such substances; they gradually 
disappear, and with them disappears also this hindrance to the bacteria. 
Of late years what is called the partial sterilization of soil has begun to 
be studied with great eagerness. Soil is heated up to a not too high temper- 
ature, for instance 65”—98?” C, or is treated with volatile antiseptic substances, 
such as chloroform, sulphide of carbon, toluol, etc., which, after they have worked 
on the soil for a shorter or longer time, can again be removed. Soil treated 
in this way often shows that it contains greater quantities of available plant- 
foods than before the treatment. Above all this seems to hold good of nitrogen- 
ous compounds: the plants in the partially sterilized soil become stronger 
and acquire a darker green colour than in the unsterilized soil, bearing wit- 
ness to an increased supply of nitrogen (HILTNER, 1908). This interesting 
phenomenon has been very thoroughly studied by different investigators. The 
greatest attention seems to have been aroused by a theory put forward by 
two Englishmen, RUSSEL and HUTCHINSON (1909 and 1913), at the well- 
known experimental agricultural station at Rothamsted in England. These try 
to explain the phenomena by suggesting that, besides the bacteria, certain 
lower animals, the protozoa, exercise an important influence on the fertility of 
the soil. These protozoa live on the bacteria of the soil, and eat them up so 
that they reduce their multiplication in the ground. Through this partial 
sterilization the protozoa are killed, after which the bacteria, who have greater 
power of resistence, can develop unhindered, and increase the rapidity of de- 
composition among the organic compounds in the ground. By this means 
one might be able to explain its increased fertility. This theory, however, 
has not gained any general adhesion; many different explanations have been 
put forward; and at the present moment much work is being done in this 
department (see, for instance, KOPELOFF, LINT and COLEMAN, 19106). 
Burning or denshiring also implies a kind of partial sterilization: the ground 
