MINERAL MATTERS SUPPLIED BY IllUlOATlOX. 403 



obtaiu reliable iuformation on the matter, from various countries, 

 more especially from the torrid zone, a careful examination of 

 the facts ascertained would appear to refute ever3'\vhere the old 

 wide-spread error that a very fruitful soil, under favourable 

 climatic conditions, in the tropics for instance, will continue in- 

 exhaustible, even mthout receiving back from the hand of man 

 the mineral matters removed in the crops. Even in the most 

 enchanting lands of the tropical zone, on the most fruitful vol- 

 canic earth, such as is found in the old country of the Incas, 

 the tableland of Quito, Imbabura, Riobamba, Cuenca, &c., a 

 long-continued succession of crops drained the soil wherever it 

 was impracticable to convey to the fields by artificial irrigation 

 the mud carried down by the torrents of the Andes. In those 

 regions water, aided by the wide-spread old volcanic mud 

 streams (Lodozales), plays the part, which guano and farm-yard 

 manure do elsewhere, of restoring to the soil the mineral con- 

 stituents removed by a continued succession of crops. In most 

 of the provinces of Persia, more especially in Aserbeidschan and 

 in a great portion of Armenia and Asia Minor, the irrigation 

 canals everywhere met with serve the piurpose, not so much of 

 moistening the ground, as of conveying to the land in the 

 valleys the mineral detritus washed from the mountains at the 

 time of the melting of the snow. This method of artificial 

 manuring by irrigation is commonly applied also in those 

 countries where there is no lack of rain and dew. It subserves 

 the same purpose as the mud of the Nile in Egypt, viz. to 

 replace the action of farm-yard manure. Where the mineral 

 constituents removed by a long succession of crops are not 

 restored to the ground either by animal manure, or by irrigation, 

 the soil is almost completely drained of its productive powers, 

 as is the case, for instance, in certain parts of the extensive 

 taljle-lands of Tacunga and Ambato (in the South-American 

 State Ecuador), where barley will often barely give a two or 

 threefold return, notwithst-anding the frequent alternations of 

 rain and sunshine. From the most reliable information ob- 

 tained by me, even the most fertile estates in San Salvador and 

 Chiriqui, in Central America, with their most fruitfid, loose, tra- 

 chytic soil, abounding in potash and silica, cannot show a single 

 field on which maize has been grown for thirty years running 



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