XIII.] POTASH MANURES 257 



of the guanos contain so little else that they may be 

 practically regarded as phosphatic manures, whose 

 action is very similar to that of the steamed bone flour 

 and bone meal mentioned above. 



The third group of fertilisers are those which supply 

 the element potash, but as this substance is naturally 

 abundant in many soils, especially those containing much 

 clay, the need for potassic fertilisers is not so much recog- 

 nised, and they are less generally employed by farmers 

 than either nitrogenous manures or phosphates. For a 

 long time the only source of potash was the ashes of 

 wood and other vegetable matter like kelp, but these 

 sources have been entirely superseded by the opening 

 up during the last half century of enormous mines of 

 potash salts near Stassfurt, in Germany, whence the 

 world's supply of potash is now almost wholly derived. 

 Various grades of manures are put upon the market, 

 the most common being an impure material containing 

 about 12 per cent, of potash, which is called kainit, 

 though nowadays it has little right to the title, since it 

 consists almost entirely of chlorides of potash, soda, and 

 magnesia. As a rule, kainit is just as valuable, potash 

 for potash, as the more concentrated fertilisers, because 

 the salt and other impurities wash out of the soil 

 without doing any harm. In some cases, however, it is 

 desirable to use pure materials, and sulphate of potash 

 of two grades of purity, and muriate of potash, the most 

 concentrated of all, can then be obtained. There is a 

 general idea that with crops like potatoes, for which 

 potash salts are largely employed, the sulphates give rise 

 to better quality of the product than do the chlorides. 



Of the compound fertilisers the oldest and 

 still the most widely employed are the guanos, of 

 which the Peruvian guanos, obtained from some of the 

 rainless islands off the west coast of South America, are 



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