SOIL ORGANISMS 245 



humus by the individual communities is primarily due to climatic 

 conditions. 



In the accumulation of humus the less easily decomposed plant 

 products, such as Hgnin and cutin, are chiefly concerned. We do not 

 know the annual production of these materials by any plant com- 

 munity; hence we cannot bring it into relation with the process of 

 humus decomposition. A general idea of the effect of chmate is 

 given by the percentage of humus in soils of similar grassland com- 

 munities which grow under climatically different conditions. 



Plant Waste: Litter. — The raw material out of which humus is 

 made, the plant Utter (Swedish Forna), is chemically different for 

 different plant communities and dominant species. This litter con- 

 sists of dead remains of plants which are still recognizable as such 

 without the aid of the microscope. They have not gone through a 

 transforming process in the soil but consist of dead leaves, twigs, 

 fruits, straw, mixed with animal remains. 



While the pH value of the top layer upon raw soils with very little 

 or no humus depends upon the petrographic nature of the substrata, 

 that of soils covered with vegetation is determined by the nature of the 

 plant waste (in the broadest sense as Forna). Fresh Picea excelsa 

 Utter showed pH 3.8; Larix decidua litter pH 3.9, that is, a strongly 

 acid reaction. The leaf litter of Quercus ilex has a pH of 6.5 ; while that 

 of Ulmus scahra indicated pH 7.3, the latter a distinctly alkaline reac- 

 tion, a condition which favors nitrification. 



This win explain the fact that even small accumulations of raw 

 humus from Larix and Picea Utter usually shift the pH of the upper soil 

 layer to the acid side,^ while alkaline Utter may change the acid reac- 

 tion toward the alkaline. Thus a 1 to 2 cm.-thick layer of undecom- 

 posed Juniperus nana needles in the Val Scarl, at 2,500 m., gave a reac- 

 tion' of pH 6.0, the granitic soil immediately beneath, on which the 

 juniper bush grew, showed pH 5.3. These changes caused by plant 

 litter directly affected the moss and herb layers. The floristic com- 

 position of the lower layers varies greatly in different woods, and even 

 within a definite forest association differences occur due to the invasion 

 of certain species where the litter is more abundant, e.g., Vaccinium 

 vitis idaea under Pinus montana and P. silvestris. 



Formation and Destruction of Humus. — The accumulation of 

 humus through plant waste depends upon the annual production of 



1 This explains also the fact, determined by Nemec and Kvapil (1925, 1926) and 

 Frank (1927) that clearings in the spruce forest always show lower acidity than 

 the forest soil, and that the soil of young spruce woods with its heavy layer of 

 needle litter is more acid than that of the older woods. 



