CXIV HENRIK HESSELMAN. 
The nitrogen problem of forest trees has been treated by several people, 
including VATER (1909 b), who has shown that in manuring experiments in 
nurseries better results have been obtained as a rule with ammonia than with 
nitrates, although it has also been found that the pine can also acquire nitric 
acid with great ease, provided that it is obtained in a very diluted form (a 
solution of 0o.o2 2, MÖLLER, 1904). But all manuring experiments suffer from 
one definite error: it is not known whether the ammonia in the ground is 
first converted into nitric acid before it is absorbed by the roots ofthe pine- 
plants. So much seems to be certam, however, that saltpetre in a somewhat 
more concentrated form, for instance in a o.r 2, solution, is injurious to the 
pine, while it is assimilable in a very weak solution. MÖLLER (1914) and, 
quite recently, MÖLLER and ALBERT (1916) have shown, however, that raw 
humus, when it is broken into small pieces and mixed with sand or is de- 
posited in a layer under a crust of sand, forms an excellent source of nitro- 
gen not only for pine and spruce but also for many other kinds of trees, 
including the oak. In this respect raw humus excels mineral manuring substan- 
ces containing nitrogen, such as nitrate of calium and nitrate of calcium, sulphate 
of ammonia, etc. The investigators mentioned have not examined in what form 
the nitrogen in these raw humus experiments reaches the tree-plants; but my 
own experiences tell me that such treatment of raw humus must pro- 
duce a nitrification of the raw humus. nitrogen. I shall now attempt 
to show, by means of some experiments and observations in nature, the im- 
portance that this possesses, for instance, for the pine. In 1910 an experi- 
ment was made by ANDERS HOLMGREN at the Bispgården School of Forestry 
to throw light on the difference between the humus which is formed under 
the trees and that which arises on the more open bare areas in a very much 
sparse pine-heath. The samples of soils were put in wooden boxes, which 
were buried in sand up to the edge of the boxes, after which pine seed 
was sown in the different boxes. On a visit to the Bispgården School of 
Forestry in September, 1915, I was struck by the beautiful green colour which 
distinguished the pine-plants in the humus which was taken from under the 
trees, while the pine plants in the humus collected between the trees had a 
more yellowish-green colour. Another thing, too, aroused my attention: in 
the boxes with the first-named kind of humus there appeared as weeds Leon- 
todon autumnalis, Spergula arvensis, Poa pratensis, Agrostis vulgaris, Betula ver- 
rucosa, and Rumex acetosella; while in the other box there were only found 
some small germinating plants of bilberry, heather and Betula odorata. «Leon- 
todon showed a marked saltpetre reaction on examination. The vegetation in 
the boxes thus led one to assume a great difference between the two differ- 
ent kinds of humus. In figs. 29 and 30 pictures are given showing the 
appearance of the pine-plants in the autumn of 1916, that is to say at the 
age of seven, as the experiments were begun in 1910. The plants which 
had been grown in the humus taken under the trees are taller 
and have longer, broader, and darker needles than thefplamnmts 
grown in humus taken from between the trees. 
The appearance of the plants gives reason to suspect that the 
cause of their different development lies in a difference in the 
supply of nitrogen. The strong, broad, dark-green needles indicate 
an abundant supply of nitrogen, while the shorter, narrower and 
