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full benefit of the increase in the available amount 

 of this plant-food. This solvent action of carbonic 

 acid shows us the importance of decaying organic 

 matter in the soil ; and, as a consequence, a soil 

 containing 10 per cent, of organic matter will 

 dissolve double as much phosphate of lime as one 

 containing only 5 per cent. ^ This peculiar property 

 of carbonic acid is shared also by other substances, 

 the principal of which are common salt, nitrate of 

 potash, and ammonia salts. The presence of these 

 salts in the soil is therefore conducive to its 

 fertility not only by virtue of being themselves 

 elements of plant-food, but chiefly by possessing 

 digestive powers, as it were, to convert other 

 inorganic food-elements into a fit state for assimi- 

 lation by the plant. 



It is, however, often very desirable to obtain the 

 earthy Phosphates contained in bones in a soluble 

 form at once, and for that purpose crushed bones 

 are subjected to the action of sulphuric acid, which 

 converts the insoluble phosphate of lime into a 

 soluble salt. This preparation is called super- 

 phosphate of lime, and contains generally 25 per 

 cent, of phosphates soluble in water. 



The use of bone-dust or superphosphate of lime 

 is entirely unknown among native agriculturists, 

 while the high prices of English manufacture must 

 have deterred many of the more enlightened land- 

 holders, who were convinced of its advantages, from 



18 



