DESCRIPTION OF THE ORGANIC SUBSTANCES OF 

 THE SOIL. 



The names of these in the order in which they occur in rela- 

 tive abundance in an average fertile soil, are 



Silica (sand), Alumina, Oxide of Iron, Lime, Magnesia, Pot- 

 ash, Soda, Sulphur, Phosphorus, Chlorine, Fluorine. 



Silica is a compound of 53.34 per cent, of oxygen, and 46.66 

 per cent, of a metal called silicon. This metal never occurs in 

 nature, but was discovered by Sir Humphrey Davy, in 1813, 

 and is obtained as a brown powder or as scaly crystals, like 

 graphite. Silica has acid properties, that is it combines with 

 alkalies, and exists in three forms, crystalline, amorphous, and 

 jelliform ; when crystalized it forms hexagonal (six-sided) trans- 

 parent, colorless prisms, which are called rock crystal ; when 

 amorphous, it is .white, gritty and tasteless, as in flint and sand. 

 Neither of these forms when pure are acted on by any of the 

 acids except the hydrofluoric, nor even by the strongest fires. 



The jelliform variety is called soluble silica, and is a combina- 

 tion of silica and water, which is slightly soluble in water and 

 freely so in acids, even carbonic. This is the immediate source 

 of the silica in plants, and it is formed from the akaline silicates 

 of the soil ; thus, if we have a powder composed of silica, lime, 

 potash, etc., such as powdered feldspar, and expose it to the ac- 

 tion of water containing carbonic acid, as rain water, the latter 

 will unite with the lime, potash, etc., and leave the silica in the 

 soluble form, when it is readily appropriated by the plants. 



From the want of enough of this in the soil, those plants 

 that need it largely in their straw, as wheat, oats, rye, etc., are 

 unable to stand up, and fall down and rot. 



As the amount of soluble silica in soils is small, all the straw 

 of the crops, and all the weeds that grow in swampy places, or 

 running water, should be returned to the soil best through the 

 compost pile. 



Alumina. This substance, which is a compound of the metal 

 aluminum and oxygen, exists in nature in two forms, crystal- 

 line and .amorphous ; when crystallized, it forms the precious 

 stones, as the ruby and the sapphire ; when amorphous, it is 

 very like silica, white, gritty, hard. In the soil, however, it 

 usually occurs as clay, which is a compound of silica and 

 alumina. This substance does not form plant food, for it is 

 seldom absorbed by the roots of plants, but its office is chiefly 

 to absorb and retain moisture and all the soluble salts of ferti- 

 lizing substances. When clay is present in large quantity the 

 land must be drained, or it will be wet, cold, and heavy ; if de- 



