USE OF HUMUS. 127 



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 nourishment to the leaves and roots. The former 

 contains a comparatively 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 con- 

 sideration 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, which lasts 

 for several weeks, would remove a great part of it 

 from the ground, and a heavy and continued rain 

 would impoverish a soil. But it is soluble only when 

 combined with oxygen; it can be taken up by water, 

 therefore, only as carbonic acid. 



When kept in a dry place, humus may be preserved 

 for centuries ; but when moistened with water, it 

 converts the surrounding 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 the soil containing it; for they ab- 

 sorb by their roots the carbonic acid as it is formed. 

 The soil receives again from living plants the car- 

 bonaceous matter it thus loses, so that the proportion 

 of humus in it does not decrease. 



The stalactitic caverns in Franconia, and those in 

 the vicinity of Baireuth, and Streitberg, lie beneath 



