70 SOIL CONDITIONS AND PLANT GROWTH 



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 Mercurialis 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 10 per cent. 

 of organic matter completely disintegrated and most intimately min- 

 gled with the mineral matter. 



Torf differed completely. The characteristic plant was Trientalis 

 europcea 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- 

 sand), and lower still a layer of reddish soil (roterde), or else a pan 

 (ortsteiri). Practically no earthworms were found in the torf, but there 

 were numerous moulds and fungi, Cladosporium humifaciens Rostrup 

 and Sorocybe Resince Fr. being perhaps the commonest. 



Torf was acid, contained about 30 per cent, of organic matter not 

 completely disintegrated, nor well mixed with the mineral matter. 

 It was not very favourable to the growth of young trees, and the 

 forest tended to become an open heath as the old trees died. 



The distribution of mull and torf did not seem to be determined 

 by the nature of the soil, or by the amounts of soluble alkali salts or 

 calcium carbonate present, but rather by the nature of the living 

 organisms in the soil. Animals, especially earthworms, gave rise to 

 mull, fungi produced torf. If the conditions were favourable to earth- 

 worms mull was therefore found, if not, torf was produced. The 

 nature of the vegetation was also a factor: oak only rarely formed 

 torf but commonly gave rise to mull, at least two varieties of which 



