HUMUS A SOURCE AND RESERVOIR OF CARBONIC ACID. 83 



only perceptible smell be that of burnt straw, it indicates 

 humus formed from decayed, vegetable substances. 



284. Humus is always favorable to vegetation, except 

 when it has been produced by the decay of plants under 

 water, or has been very long lying under water. This is 

 often the case with peat, bog earth or marsh mud. These 

 are almost entirely humus ; but when they have been 

 long beneath the surface of water, they are considered 

 cold, and possess acid properties, which render them 

 unfavorable to the nourishment of plants, until corrected 

 by long exposure to the influences of the atmosphere, and 

 to the alternation of the sun s light and of frost. 



285. Humus not only acts as a reservoir of carbonic 

 acid, holding it ready to be given to the roots of plants, 

 but, as it consists mostly of carbon and water, and has an 

 attraction for oxygen, it is constantly receiving oxygen 

 from the air. By the progressive decay thus produced, the 

 vegetable and animal remains are constantly turning into 

 carbonic acid and ammonia, and the ammonia into ammo- 

 niacal salts, thus rendering the soil rich in these precious 

 elements of vegetable food. In soil abundantly supplied 

 with humus or other rich manure, the air is sometimes 

 found to contain four hundred times as much carbonic 

 acid as an equal quantity of the air in the atmosphere. 



286. The carbonic acid formed in vegetable soil by 

 the oxygen, not only serves directly as food for plants, 

 but it decomposes the silicates and thus sets the potash 

 and other salts free to be dissolved by water and taken 

 up by the roots. Another portion of the oxygen absorbed, 

 combines with the hydrogen of the humus, and produces 

 water. This is a very valuable property, especially in 

 dry seasons, and is one reason why soils abundantly 

 supplied with humus suffer so little from drought. 



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