42 PRACTICAL FARM CHEMISTRY. 



The compound sulphate of copper (blue vitriol, 

 bluestone, copper vitriol) is valuable as a spore 

 killer and preventive of fungous diseases of plants 

 and now largely used for the prevention of grape 

 mildew and rot, potato rot, tomato blight and rot, 

 melon blight, and many other fungous enemies to 

 our crops. So also is carbonate of copper. 



Another, and most important use made of sul- 

 phuric acid, is in the treatment of bones and phos- 

 phatic rocks by which they are transformed into so- 

 called superphosphates, and rendered soluble and 

 consequently immediately available for plant food. 



The compounds of silica constitute a large pro- 

 portion of the earth's surface. The tiny, glossy 

 pieces of "sand" or quartz which abound everywhere, 

 are almost pure silicious acid (silica); 

 and the rocks, sandstones, the soil — all 

 contain it in abundance. So we have the silicates 

 of alumina, potash, soda, etc. The element silicon, 

 although not exactly indispensable to plant growth, 

 enters quite largely into plant structure, especially 

 near the outside, and is the substance which gives 

 the glossy, hard surface to the stems of cereals and 

 grasses, thus adding stiffness to the stalks and 

 making them self-supporting. It is also true that a 

 vast store of unavailable potash is held in some of 

 these silica compounds. This potash is gradually 

 rendered soluble and fitted for the use of plants by 

 the slow process of disintegration and decompo- 

 sition, through the action of carbonic acid, etc . , as 

 the years roll by. I mention this merely to show 

 that we can expect at least some assistance from 

 every soil in furnishing us this needed substance of 

 plant food. 



