CHAPTEE VII. 

 SPECIAL MANURES. 



IN the previous chapter the chief organic manures have been de- 

 scribed ; it remains to consider the other substances, generally of 

 mineral or artificial origin, which are employed as fertilisers. They 

 may be conveniently divided into four groups : 



I. Nitrogenous manures. 

 II. Phosphatic manures. 



III. Potash manures, 



IV. Miscellaneous. 



Many of the organic manures already described contain variable 

 quantities of all the chief manurial substances, but those about to be 

 dealt with are, as a rule, intended to supply only one important item 

 of plant food. Their employment gives the farmer, therefore, the power 

 of applying exactly what he thinks may be necessary, without the in- 

 troduction of other plant food with which his land may already be 

 abundantly provided. Their general introduction into farming prac- 

 tice has thus rendered easy a far more scientific treatment of the soil 

 than was possible with complex manures only. 



1. NITROGENOUS MANURES. These include the two import- 

 ant substances, sodium nitrate and ammonium sulphate, the recently 

 introduced calcium cyanamide and nitrate of lime, and also the less 

 abundant and more expensive potassium nitrate. 



Sodium Nitrate, " Chili Saltpetre," occurs in the enormous 

 nitrate deposits of Peru, Chili and Bolivia. It is found in rainless 

 districts and comparatively near the surface, covering a huge desert, 

 devoid of both animal and vegetable life. The raw product, known as 

 caliche, is found beneath a covering consisting of two layers, the upper 

 one of sand and gypsum and the lower of baked clay and gravel ; be- 

 neath the caliche is a soft earth known as cova. The thickness of 

 the caliche varies from a few inches to 12 ft. It is extracted by boring 

 through the upper layers and introducing a charge of gunpowder, 

 which, when fired, exposes a considerable quantity of the material. It 

 is then broken up by means of picks and carried to the refinery. 1 

 There it is purified by crystallisation. This is done by dissolving in 

 water by the aid of heat, allowing the solution to settle, and then run- 



See Article by Aikman in Blackwood's Magazine, March, 1892, and Report on 

 the Nitrate Trade of Chili, Jour. Soc. Chem. Ind., 1890, 664. 



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