INSOLUBILITY OF HUMUS. 



acid is emitted from it with such violence that the noise made by 

 the escape of the gas may be distinctly heard, at the distance of 

 several feet. Here the carbonic acid rising to the surface dis- 

 places completely all the air, and consequently all the oxygen, 

 from the soil ; and without oxygen neither seeds nor roots can be 

 developed ; a plant will not vegetate in pure nitrogen or carbonic 

 acid gas. 



Humus supplies young plants with nourishment in the form 

 of carbonic acid by the roots, until their leaves are matured 

 sufficiently to act as exterior organs of nutrition ; its quantity 

 heightens the fertility of a soil by yielding more nourishment in 

 this first period of growth, and consequently by increasing the 

 number of organs of atmospheric nutrition. Humus acts in 

 this respect as a source of carbon to plants ; but vegetable mould 

 contains other substances which are equally necessary to plants. 

 Vegetable mould contains invariably carbonate of ammonia, 

 besides the salts and alkalies left behind by the putrefaction of 

 former plants.* Those plants which obtain their first food from 

 the substance of their seeds, such as bulbous plants, could com- 

 pletely 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 measure injurious. 



The amount of food capable of being extracted by young 



* Some vegetable mould taken from the interior of a hollow oak, yielded 

 i" o"3"o~ °f residue after incineration ; of this residue 100 parts contained 24 

 parts of soluble salts with alkaline bases, 10*5 parts of earthy phosphates, 

 10 parts of earthy carbonates, and 32 parts of silica. The aqueous extract 

 gave 66 per cent, of soluble salts. (Saussure.) One thousand parts of 

 the extract obtained by hot water from vegetable mould formed by the de- 

 cay of the Rhododendron Fcrrugineum gave 140 parts of ashes, which 

 contained, according to Saussure : 



Carbonate of potash - - - 14 

 Chloride of potassium - - 23 

 Sulphate of potash - - - 16 

 Earthy phosphates ... 17*25 

 Earthy carbonates - - - 21 '50 



Silica 3*25 



Metallic oxides and loss - - .V00 





