INSOLUBILITY OF HUMUS. 131 



the brown soluble matter into the insoluble coal of 

 humus, but serves for the formation of carbonic 

 acid. This change takes place very slowly, and in 

 some instances the oxygen is completely excluded 

 by it ; and whenever this happens, the soil loses its 

 fertility. Thus, in the vicinity of Salzhausen (a 

 village in Hesse Darmstadt, famed for its mineral 

 springs,) upon a meadow called Griinschwalheimer, 

 unfruitful spots are seen here and there covered with 

 a yellow grass. If a hole be bored from twenty to 

 twenty-five feet deep in one of these spots, carbonic 

 acid is emitted from it with such violence that the 

 noise made by the escape of the gas may be dis- 

 tinctly heard at the distance of several feet. Here 

 the carbonic acid rising to the surface displaces 

 completely all the air, and consequently all the oxy- 

 gen, from the soil ; and without oxygen neither seeds 

 nor roots can be developed; a plant will not vege- 

 tate in pure nitrogen or carbonic acid gas.* 



Humus supplies young plants with nourishment 

 by the roots, until their leaves are matured sufficient- 

 ly to act as exterior organs of nutrition ; its quan- 

 tity heightens the fertility of a soil by yielding more 

 nourishment in this first period of growth, and con- 

 sequently by increasing the number of organs of 

 atmospheric nutrition. Those plants which receive 

 their first food from the substance of their seeds, 

 such as bulbous plants, could completely dispense 

 with humus ; its presence is useful only in so far as 

 it increases and accelerates their development, but it 

 is not necessary, — indeed, an excess of it at the 

 commencement of their growth is in a certain mea- 

 sure injurious. 



The amount of food which young plants can take 

 from the atmosphere in the form of carbonic acid 

 and ammonia is limited; they cannot assimilate more 

 than the air contains. Now, if the quantity of their 

 stems, leaves, and branches has been increased by 



* See note p. 79. 



