268 



MATERIALS OF INDIRECT VALUE 



[chap. 



gypsum is similar to that of potash salts, though to a 

 less degree. Some crops, like Swede turnips and 

 cabbage, may take up more sulphur compounds than 

 the soil can normally supply and so be benefited by the 

 sulphur in the gypsum, but in the main its value is as a 

 liberator of potash in the soil. 



Table LXXXIV.— Composition of Clover Ash. 



Potash 



Soda ..... 



Magnesia ..... 

 Lime ..... 



Oxides of Iron and Manganese . 

 Chlorine . . . . . 

 Phosphoric Acid 

 Sulphuric Acid .... 

 Silica . . , . . 



Without 

 Gypsum. 



23-6 

 I -2 

 7-6 



28-5 



1-2 



4-1 

 9-7 



3-9 



20'0 



With 

 Gypsum. 



35-4 

 0-9 



6-7 

 39-4 

 i-o 

 3-8 

 9-0 



3-4 

 iO'4 



Salt. — The use of salt, alone or as an adjunct to 

 other fertilisers, is a common farming practice ; for 

 example, in growing mangolds it is customary to give 

 them 2 or 3 cwt. per acre of salt as a top dressing with 

 or without nitrate of soda. On the fen land of Lincoln- 

 shire potatoes are generally grown with farmyard 

 manure, superphosphate, and a liberal dressing of salt, 

 and in barley growing, salt alone is sometimes used 

 where roots have been folded off, with an idea of 

 stiffening the straw. Though, in the first case, the value 

 of salt is often ascribed to the fact that the mangold 

 has been derived from a maritime plant, it is really 

 due to the dependence of mangolds upon an abundant 

 supply of potash (see p. i68), because the soluble 

 sodium chloride will bring into solution the reserves of 

 insoluble potash in the soil and the manure. Potatoes 

 again are much in need of potash, and the straw 



