GYPSUM. 41 



as a considerable quantity applied as bones or ground 

 phosphate. 



Superphosphate is chiefly employed for turnips, for 

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

 corn crops, especially barley. Its use tends to early 

 maturity in the crop. 



Gypsum. Tnis manure consists of sulphate of cal- 

 cium it is of very limited value. Gypsum 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. Finely-powdered gypsum is sometimes 

 employed in stables to hinder the volatilisation of ammo- 

 nium carbonate. Crude potassium salts act in the same 

 manner, owing to the magnesium salts which they con- 

 tain. 



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 quan- 

 tity of phosphoric acid. In most cases, however, the 

 beneficial influence of these manures is due to the che- 

 mical actions which lime performs in the soil ; the chief 

 of these have been already glanced at (see p. 19). 



Burnt lime is much more powerful in its action on 

 vegetable matter than chalk or marl; it should be used 

 with discrimination, lest the humus of the soil be unduly 

 diminished. Heavy clays, or soils rich in humus, are 



