CIV HENRIK HESSELMAN. 
the formation of saltpetre begins in such a humus-covering; but as early as 
the autumn of the second year the nitrification is extremely active. Germ- 
inating plants, ÅLuzula pilosa and Epilobium angustifolium, then give a very 
strongly marked nitrate reaction, and very considerable quantities of saltpetre 
are. formed in soil samples! on storing; (see, too, table 14,n0:07) mmm 
time during which nitrification continues is assuredly very various, depending 
upon the intensity with which the fire has passed over the ground. I will 
not here adduce any observations illustrating this matter. 
A clearing (Krokmyrshygget) in the parish of Indalsliden, Medelpad, den- 
shired in the spring of 1907, was at the time of my examination in Sep- 
tember, 1915, Overgrown with a very fine forest of young pines, sown in 
patches. A moss carpet of Polytrichum juniperinum covered great parts of the 
ground, and a sparse colony-like burn flora was still found amongst the young 
pines (see detailed description on page 1050). Amongst the species comprised 
in the ground-covering may be noticed raspberry and pilobium angustfolwum. 
Samples of soil for investigation were taken from two spots, one a slightly 
burnt patch with a fairly good covering of humus, distinguished among other 
things by the occurrence of raspberry, the other a more severely burnt part with 
very scattered and meagre Kpilobium angustifolium and a very dense carpet of 
Polytrichum guniperinwn. "The samples differed from one another only slightly 
with regard to their content of humus and nitrogen, but very substantially 
with regard to capacity for nitrification. While the first-named formed 100 
mg. saltpetre nitrogen in the course of two months, the latter formed during 
the same period and under the same conditions only o.4 mg., both calcu- 
lated per kg. soil. The first-named sample of soil was taken from a small 
hollow in the ground, which of course both through its position and greater 
moistness had been better protected against the fire, whereas the latter was 
on a little elevation, which, owing to its greater dryness, was naturally burnt 
more severely. Where the ground had not burnt very badly, the formation 
of saltpetre is thus extremely active, even eight years after the burning. But 
it may continue even longer. Fig. 20 shows a clearing denshired for culture 
and recently cultivated. The ground was burnt off in the spring of 1902 and 
in the autumn of 1914 several samples of soil were taken there for bacterio- 
logical investigation. Though a colony-like burn flora still survived amongst 
the young pines, the ordinary forest-land covering was beginning to immi- 
grate (see, too, page 1049). The samples of soil were taken either under a 
carpet of Agrostis vulgaris or else at another spot under a swelling covering 
of Polytrichum commune. Both samples show a very considerable power of 
nitrification and denitrification (table 10, nos. 7 and 8), and, especially 
as regards the first sample, a very considerable power to produce 
ammonia. (Table 4, nos. 1 and 2.) The condition of the samples on stor- 
age have not been closely examined; but to judge by the bacteria flora the 
capacity of nitrification should be considerable. As much as twelve years 
after the burning, therefore, the ground can retain its capacity to 
transmute the organically bound nitrogen into saltpetre. This, how- 
ever, is by no means a maximum. In the neighbourhood of Selsjön there is 
a twenty-five-year-old burn with very scattered remains from the time of the burn 
flora. Samples of earth from this area still nitrify a solution of sulphate of ammonia 
(see, too, page 1047), and there also occur denitrifying agents (seetable ro; no.5): 
