158 SALT GYPSUM 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 pro- 

 perties of the soil and upon its power of retaining water. Waste salt 

 from fish-curing, bacon-curing, etc., has a value because of the 

 nitrogen and potash which it contains, the 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 0, 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 probability 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. 1 Where superphosphates (of which 

 calcium sulphate is a large constituent) are employed, its application 

 is not required. 



Lime, Chalk, Marl, or Limestone. These substances consist- 

 ing mainly of oxide, hydrate, or carbonate of lime, but always con- 

 taining small and varying quantities of magnesia, phosphoric acid and 

 iron oxide -are employed as manures. The important function per- 

 formed by calcium carbonate in the process of nitrification has 

 already been discussed. 2 



The chief effect of the application of lime or its carbonate to a soil 

 is to accelerate nitrification and thus to enable the crop to draw upon 

 the nitrogenous stores already present in the soil. Another action of 

 value is its neutralising effect upon the organic acids in peaty soils, 

 the presence of which is unfavourable to any but coarse, undesirable 

 plants. It also acts by replacing the potash in the silicates. 

 Although caustic lime (i.e., CaO or CaH 2 O.,) speedily becomes con- 

 verted into the carbonate when applied to the soil, it always has a 

 more energetic action than chalk or limestone. This is due to its 

 solubility producing a more uniform distribution throughout the soil 

 before precipitation as calcium carbonate occurs, than it is possible to 

 obtain by the mere mechanical admixture of the soil with even finely 

 powdered chalk or limestone. 



Lime made from magnesian limestone, and therefore containing 

 magnesia, is not so suitable for agricultural purposes as a purer 

 product. This is usually stated to be due to the fact that so long as 

 the bases are in the caustic state, i.e., as hydroxide, they have an in- 

 jurious effect upon vegetation, and magnesia is said to combine with 

 carbon dioxide much less readily than lime does. Consequently, a 



1 Vide p. 64. 2 Vide p. 64. 



