48 BOARD OF AGRICULTURE. 



into this line of investigation, it is enough to say that most of 

 our cultivated crops require potassium and phosphorus, or 

 phosphate of lime, in much greater quantities than many 

 other elements, and hence their cultivation involves the 

 natural exhaustion of these important elements and the 

 necessity of supplying them wherever they are, or become, 

 deficient. 



The modern use of concentrated fertilizers has grown up 

 chiefly from this necessity. It was met, to a certain extent, in 

 the use of Peruvian and other guanos, rich in phosphoric 

 acid. The value of these articles was based largely on the 

 amount of soluble phosphates they contained, and this is the 

 case also with the more common fertilizers sold under the 

 name of superphosphates, a technical term applied to phos- 

 phoric acid when it appears in a form soluble in water. A 

 common phosphoric acid, as is well known in chemistry, is a 

 tri-basic acid ; that is, its atoms unite with three atoms or 

 molecules of some substance as a base, such as lime. In 

 bones, the base is always lime. But if two parts or atoms of 

 lime can be taken out and their place substituted by water, 

 we have the phosphoric acid and the lime in a form soluble 

 in water, and this is superphosphate. The phosphoric acid in 

 bones, being united with lime, is insoluble in water, but if 

 the bones are treated with sulphuric acid, or the common oil 

 of vitriol of commerce, the sulphuric acid will take from the 

 bones two-thirds of the lime, and water will take its place, 

 when the mass is brought into a soluble condition. The sul- 

 phuric acid which has effected this change, will unite with the 

 lime which it has thus removed from the bones, and form 

 sulphate of lime or gypsum, commonly known as plaster, 

 and this latter is always a constituent of true superphosphates, 

 made in this way. 



Now guano was rich in soluble phosphoric acid. The 

 rainless islands, on the coast of Peru, had preserved it age 

 after age, till its value as a fertilizer was recognized, when its 

 use became general all over the civilized world. Scientific 

 men had called attention to the richness of the deposits on the 

 Peruvian islands, and in 1840, twenty casks were taken to 

 England for trial, with such surprising results that the impor- 

 tation increased to 2,000 tons in 1841, and to over 200,000 



