INSOLUBILITY OF HUMUS. 93 



CHAPTER IX. 



The Art of Culture. 



The conditions necessary for the life of all vegetables have been 

 considered in the preceding part of the work. Carbonic acid, 

 ammonia, and water, yield elements for all the organs of plants. 

 Certain inorganic substances — salts and metallic oxides — serve 

 peculiar functions in their organism, and many of them must be 

 viewed as essential constituents of particular parts. 



The atmosphere and the soil offer the same kind of nourish- 

 ment to the leaves and roots. The former contains a compara- 

 tively inexhaustible supply of carbonic acid and ammonia ; the 

 latter, by means of its humus, generates constantly fresh carbonic 

 acid, whilst, during the winter, rain and snow introduce into the 

 soil a quantity of ammonia, sufficient for the development of the 

 leaves and blossoms. 



The complete, or it may be said, the absolute insolubility in 

 cold water of vegetable matter in progress of decay (humus), 

 appears on closer consideration to be a most wise arrangement 

 of nature. For if humus possessed even a smaller degree of 

 solubility than that ascribed to the substance called humic acid, 

 it must be dissolved by rain-water. Thus, the yearly irrigation 

 of meadows would remove a great part of it from the ground, 

 and a heavy and continued rain would impoverish a soil. But 

 humus is soluble only when combined with oxygen ; it can be 

 taken up by water, therefore, only as carbonic acid. 



When moisture is absent, humus may be preserved for cen- 

 turies : but when moistened with water, it converts the surround- 

 ing oxygen into carbonic acid. As soon as the action of the air 

 ceases, that is, as soon as it is deprived of oxygen, the humus 

 suffers no further change. Its decay proceeds only when plants 

 grow in a soil containing it ; for they absorb by their roots the 



