82 THE CHEMISTRY OF THE FARM. 



verted into basic phosphate of iron, a substance attacked 

 with difficulty by plants. 



Superphosphates are naturally more speedy in their 

 effect than manures consisting of undissolved phosphate. 

 A small quantity of phosphoric acid applied as superphos- 

 phate will have as great an effect as a considerable quantity 

 applied as bones or gi-ound phosphate. 



Superphosphate is chiefly employed for turnips, for 

 which it is invaluable ; it is also of considerable use for 

 com crops, especially barley. Its use tends to early 

 maturity in the crop. 



Gypsum. — This manure is one of limited value. It is 

 composed of calcium and sulphuric acid, and is most suit- 

 able for crops, such as clover and turnips, which require a 

 considerable amount of sulphur. As superphosphate 

 always contains much gypsum, special applications of 

 gypsum will be unnecessary where superphosphate is 

 employed. 



Lime, Chalk, and Marl are frequently manures of the 

 greatest importance. On soils naturally destitute of lime, 

 as is the case with many clays and sandstones, these 

 manures will supply an indispensable element of plant 

 food. Some marls will also supply a notable quantity of 

 phosphoric acid. In most cases, however, the beneficial 

 influence of these manures is due to the chemical actions 

 which lime performs in the soil ; the chief of these have 

 been already glanced at under the head of "Soil," (see 

 page 15). 



Burnt lime is much more powerful in its action on vege- 

 table matter than chalk or marl ; it should be used with 



