248 



PLANT SOCIOLOGY 



Humus and Total Nitrogen. — The total nitrogen of the soil runs 

 approximately parallel to the humus content : the H : N ratio in a tem- 

 perate cHmate is about 10:1 according to Waksman (1924). In a cool 

 climate it is somewhat greater or about 12:1 ; in a warmer climate some- 

 what smaller — 9:1 (Jenny, 1929) (see Fig. 80). The dependence of the 

 nitrogen content, and therefore of the humus content of the soil, on 



m 20 do % 60 60 70 80 90 WO%HumuS 



Fig. 124. — Decomposition curve (also the humus distribution curve) for the Curvule- 

 tum. (After Braun-Blanquet and Jenny.) 



temperature and relative humidity (N.S. quotient, see p. 143) is 

 brought out in the accompanying table by Jenny (1930). 



Table 28. — Mean Annual Tempeuature and the Humidity Factor Related 

 TO THE Nitrogen Content of Soils 



Forms of Humus. — In the collective term "humus," which com- 

 prises all organic bodies which are in a state of decomposition, there are 

 really included numerous substances that are still unknown chem- 



