242 



If humus is mixed with dense soils, they are loosened and made warmer 

 and more easily worked. In sandy soils the humus acts as a hinder and in- 

 creases the water capacity, whereby the fluctuations in temperature become 

 less marked. These favorable peculiarities, which arise from the mixing 

 with mineral elements in the soil, disappear as soon as the humus 4s de- 

 posited on the soil in impervious layers, i. e., is not broken up by abundant 

 decomposition and the micro-organisms. In compact humus layers, the con- 

 lent in free acids is almost always greater. The forest soils, which are most 

 rapidly decomposed and worked up, are the best. In warm climates the 

 work progresses very quickly of itself. 



With a favorable humus decomposition, we find that in forest soils the 

 porous forest debris, which forms the layers of litter, is not so thick and 

 merges gradually into a friable, strongly decomposed, structureless humus 

 layer. If in any region the factors contributing to decomposition are ab- 

 sent, these layers of litter are retained, settle only gradually and become a 

 firm, fibrous humus mass, which is deposited on the subsoil and remains 

 more or less sharply separated from it. Such cases may be observed in poor 

 sandy soils, especially those containing meadow ore. 



This process, in which therefore the organic substance acquires no 

 earthy composition, will occur everywhere where conditions unfavorable to 

 decomposition exist, — as, for example, when the air is excluded by water, or 

 conversely, with too great drought in the hot seasons or in places exposed to 

 constant strong winds. 



Our forest tracts, where heather (Calluna vulgaris), cranberries and 

 huckleberries (Vaccinium) the pteris and aspidium brakes and the cushion- 

 forming mosses grow, are most inclined to the formation of such fibrous and 

 but slightly earthy humus layers, the undecomposed elements of which are 

 deposited in dense masses on the soil and in this way form the so-called 

 "raw-humus." The upper layer of such raw-humus deposits still shows the 

 interwoven structure of the plant debris, the lower layer, in which the plant 

 parts are but slightly distinguishable from one another, has a fibrous dark 

 humus substance interwoven with roots. In moist beech, pine and spruce 

 tracts, such raw humus may become peat-like. 



Ramann (loc. cit. p. 162) states as his opinion of the change in the soil 

 beneath a covering of raw humus, — that, besides the exclusion of air, the 

 humic acids especially form the injurious factors. These act on the un- 

 weathered silicate, decomposing it energetically, bringing into solution al- 

 kalis and alkaline earths and, since at the same time the amount of acid 

 solutions absorbed in the soil is slight, leaches the soil, i.e., the soluble sub- 

 stances are carried down to greater depths. If raw humus lies on sandy 

 soils, the grains of the uppermost layer appear to be strongly bleached and 

 milk-white, the intermixed silicate rock is greatly weathered and usually 

 transformed into white kaolin. The humus admixture still richly present on 

 the upper surface decreases more and more from the top downwards so that 



