68 SOIL CONDITIONS AND PLANT GROWTH 



6. Although the group is essentially transitional it has a certain 

 degree of permanency and only slowly disappears from the soil. 



The group of substances possessing these properties is generally 

 called " humus," and so long as the word is used in a collective sense 

 as a convenient label it may be retained. But the practice has been 

 responsible for a good deal of loose thinking, because it obscures the 

 fact that the group is an indefinite and complex mixture, and gives 

 instead the impression that it is a single definite substance. 



From these half-dozen general properties we may infer that humus 

 is a brown, slowly oxidisable colloid, but unfortunately we cannot get 

 much further. Careful examination of a number of soils in their 

 vegetation relationships has shown that there must be several dis- 

 tinct types of humus, but the laboratory methods are not yet as 

 sensitive as the growing plant and fail to indicate some of the differ- 

 ences. We have to look to field observations for the facts on which to 

 base a scheme of classification, and, unfortunately, these are not yet 

 very numerous. 



An admirable series of studies has been made by P. E. Miiller 

 (205) of the types of humus occurring in the Danish forests. In beech 

 forests he found two types, which he called mull and torf, our nearest 

 equivalents being mould and peat. On mull the characteristic plants 

 were Asperula odorata with its associated Mercuriah 's perennis, Milium 

 effusum^ Melica uniflora, Stellaria nemorum, and others, moss being 

 absent. The mull itself was only a few inches thick, and was under- 

 lain by I to 5 feet of loose soil, lighter in colour than mull, but almost 

 equally rich in organic matter ; still lower came a compact but porous 

 layer of soil. The surface of the soil was covered by a layer of leaves, 

 twigs, etc. Earthworms were numerous throughout ; their potent 

 influence in the soil had recently been shown by Darwin (75). De- 

 tailed chemical examination was not made : it was shown, however, 

 that mull was free from acid and contained about 5 to I o per cent, 

 of organic matter completely disintegrated and most intimately min- 

 gled with the mineral matter. 



Torf differed completely. The characteristic plant was Trientalis 

 etiropcea with the associated Aira flexuosa and moss, but surface vegeta- 

 tion was not very common. The loose layer of leaves was absent, and 

 the torf itself was so tough and compact that rain water could not 

 readily penetrate. Below it was a layer of loose, greyish sand (blei- 



the soil. So great has been the subsequent shrinkage that over 10 feet of the pillar is now 

 out of the ground, and the process has not yet reached its limit, for a perceptible shrinkage 

 took place during the dry season of 1911. 



