42 SOILS AND HUMUS. 



undergo two processes after death ; one is fermentation and the other 

 is putrefaction, decay, or eremacausis. Decay is a slow process of 

 combustion, in which the combustible parts of plants unite with the oxy- 

 gen of the air. The decay of woody fibre in contact with oxygen 

 converts the gas into an equal volume of carbonic acid. During this 

 process water is necessary ; alkalies promote, but acids retard it. The 

 woody fibre in a state of decay is called humus, and the remaining 

 coal-like substance is called mould, the product of complete decay. 

 Decay is the great process of nature by which oxygen, assimilated by 

 plants, is returned to the air. The oxygen of all organic matter is 

 given off combined with carbonic acid. Acids yield more carbonic 

 acid than neutral compounds ; while fatty acids, and resins do not 

 putrify. 



Soils are considered a magazine of matter which is variously pre- 

 pared by vegetables for their nutrition. The rich virgin soils of 

 many parts of our country contain a very large proportion of vegeta- 

 ble matter, and hence is called vegetable mould. To this is attributa- 

 ble the great fertility of these soils. 



Humus, which is the active and important principle in manures, is 

 considered the product of vegetable decomposition. It is believed to 

 be the principal nutriment of plants and to be extracted by them from 

 the soil, and to contain some of their constituents during life. It is a 

 brown substance, soluble in alkalies, but slightly so in water ; it is 

 produced by the action of acids and alkalies during vegetable decom- 

 position. It is artificially obtained by treating peat, woody fibre, soot 

 or coal with alkalies by decomposing sugar, starch &c. by acids, or 

 exposing alkaline solutions of tannic and 'gallic acids to the action of 

 the air. Its modifications are humic acid, humin, coal of humus, &c. 

 Geine is stated by some to be the basis of all the nourishing parts of 

 vegetable manures. It is the decomposed organic matter of the soil 

 and the product of putrefaction. Humus does not nourish plants in 

 its unaltered state, but as a source of carbonic acid, it is absorbed by 

 the roots, principally when young and destitute of leaves and unable to 

 extract food from the atmosphere. It is a constant source of carbonic 

 acid for the supply of vegetables. 



The carbon of plants is derived from the atmosphere and in part 

 from the humus of manures. Plants possess the power of decompos- 

 ing carbonic acid gas of the air, and of appropriating the carbon to 

 their use. The oxygen of the acid is therefore returned to the air, by 

 which its deficiency, from various causes, is in part supplied. This office 

 is performed by the leaves and green parts of the plant, and this they 

 will do independently of and when separated from the stem. Plants, 

 however, yield to the soil more carbon than they extract. 



The life of plants is evidently connected very closely with that of 

 animals. Vegetation may exist without animal life, but the existence 



