ORGANIC PORTIONS OF THE SOIL. 215 



Earths, Mkalies, 



Silica 66. per cent. 



Alumina 16. " 



Magnesia 1. " 



Lime 2. " 



Oxides, 

 Of Iron 

 Of Manganese 



per cent. 



Potash 2. per cent. 



Soda .5 " 



Ammonia .5 " 



Salts and Urets, 1.5 per cent 

 Organic matter, 6.5 " 



II. Organic constituents of the soil. A proper mixture of 

 organic matters with the mineral ingredients, is essential to 

 the fertility of the soil, and hence vegetable or animal sub- 

 stances are always found in soils capable of cultivation. These 

 organic matters are derived from the roots and other parts of 

 plants, or from the application of manures. The substance 

 which is formed by the decay of these organic products, and 

 which has been supposed to give fertility to the soil, is called 

 by several names, as humus, geine, vegetable mould, etc. and 

 is intended to include all the decaying organic matter of the 

 soil. It is a brownish or black substance, and when it be- 

 comes intimately mixed with the mineral ingredients, it im- 

 parts a black color to the soil, a greater power of absorbing 

 water and gaseous substances, renders it more permeable to 

 air and to the roots of plants, improves its texture, and in- 

 creases its power of absorbing and retaining heat. 



This remarkable substance, a history* of which has al- 

 ready been given, p. 138, is composed, as we have seen, 

 of the following substances : Jiumin, extract of humus, 

 humic acid, crenic and apocrenic acids, which are generally 

 combined with the bases, lime, magnesia, soda or potash, 

 ammonia, alumina, oxide of manganese, and per-oxide of iron. 

 When therefore we examine the organic constituents, by anal- 

 ysis, we find the following substances. 



1. Humic acid, which is identical in composition with hu- 



* Dr. Dana, in his Muck Manual, has given a history of this sub- 

 stance, and advocates the use of the terra geine. 



