REVIEWS 71 



Cultivation brings to the surface the mineral soil which contains salts 

 in sufficient quantity to promote nitrification. 



Burning supplies salts which occur in the ashes. Another possible 

 explanation for the beneficial effect of burning lies in the theory of 

 partial sterilization, which in recent years has acquired many adherents. 

 Experiments have shown that soils heated to moderate temperature, 

 ranging from 65 to 98 degrees Centigrade, afterwards show a notice- 

 ably higher content of food elements, especially nitrates, than before 

 treatment. Plants grown in such soils are more vigorous and of richer 

 color than those growing in untreated soil. This phenomenon has been 

 explained by the English investigators, Russell and Hutchinson, on the 

 following theory : The soil contains, in addition to bacteria, large num- 

 bers of protozoa which feed on the bacteria. Moderate heat kills the 

 protozoa, but does not injure the bacteria, which thus are given an 

 opportunity to increase in number. If this theory is correct, we might 

 expect the same result after a ground fire in the forest, providing it is 

 not so severe as to destroy the bacteria and the organic matter. 



Good reproduction is everywhere associated with an adequate supply 

 of available nitrates. A high nitrate content, however, is not necessary 

 or even desirable. Old trees are relatively independent of soil nitrifica- 

 tion, since in some of the most productive stands nitrifying agencies are 

 inactive. Young growth under such stands, however, shows distinct 

 signs of nitrogen starvation. Numerous instances are cited where pine 

 and spruce saplings apparently suffering from lack of light under an 

 old stand began to grow vigorously after a light fire which burned the 

 litter and ground cover, thus starting nitrification, without damaging 

 the trees. Typical two-storied pine forests are often produced in this 

 manner. 



Although fire is beneficial under proper conditions, it may also prove 

 harmful when not rightly used. Due to a lack of understanding of 

 the fundamental principles governing the use of fire, there has been 

 considerable dissension on this subject among Swedish foresters. 

 Harmful eft'ects may result from too hot a fire. Which consumes a large 

 part of the organic matter and thus reduces the main source of nitrogen. 

 On the other hand, if nitrification is already active, fire may stimulate 

 it to such an extent that the humus is quickly converted into nitrates 

 which are dissipated in a few years and then followed by a decline in 

 growth. Again, an abundance of available nitrates often produces an 

 overluxuriant herbaceous growth, resulting in the suppression of young 

 seedlings. The rule laid down by Hesselman is, therefore, to use fire 

 only when distinctly necessary. Obviously the question asked by the 



