OTHER MANURES. 169 



IY. MISCELLANEOUS MANURES. Under this heading 

 may be classed substances which contain none of the three 

 chief manurial ingredients, potash, phosphoric acid, and nitro- 

 gen, but convey to the soil some other constituent of plant 

 food, or effect by their action upon the soil the liberation of 

 some necessary plant food from the insoluble and unavailable 

 condition. Among the most important are the following: 



Common salt. 



Gypsum. 



Lime and chalk. 



Ferrous sulphate. 



Gas lime. 



Common Salt is usually regarded as possessing no real 

 manurial value, but as owing what merit it possesses to its 

 action in promoting the decomposition of the potash, lime, and 

 magnesia compounds already present in the soil. It has an 

 undoubtedly good effect on certain crops, particularly mangolds 

 and cabbages. It probably also has an effect, in common with 

 all soluble salts, upon the physical properties of the soil and 

 upon its power of retaining water. Waste salt from fish-curing, 

 bacon-curing, &c., has a value because of the nitrogen and 

 potash which it contains, these substances being derived partly 

 from the organic matter and partly from the saltpetre which is 

 also used in ''curing." Salt is sometimes used in mixed 

 manures ; solutions of common salt undoubtedly have a greater 

 solvent action than water upon phosphates and silicates. 



Gypsum, or Land Plaster (American), CaS0 4 .2H 2 O, has 

 been found to give good results when applied to fields of clover 

 or turnips. It may act as a source of sulphur, but in all pro- 

 bability its action is indirect, and its good effects are due to the 

 liberation of potash from the double silicates in the soil. Its 

 action in promoting nitrification has already been mentioned.* 

 Where superphosphates (of which calcium sulphate is a large 

 constituent) are employed, its application is not required. 



Lime, Chalk, Marl, or Limestone. These substances 



* v. p. 69. 



