114 THE CULTURE OF FARM CROPS. 



referred to, but may be usefully recalled in this connection, 

 because nitrate of lime is formed as a result of the combina- 

 tions. This compound rapidly absorbs water, and is never 

 found as a solid in its natural condition, but always in so- 

 lution as a liquid. It is supposed to exist in all fertile soils, 

 and to furnish most valuable plant food ; but being extreme- 

 ly soluble and being rapidly changed to carbonate of lime 

 by a low heat, it escapes detection in the analysis of soils or 

 vegetable substances, while its constituents have entered in- 

 to other combinations. 



PHOSPHATE OF LIME, formed by the combination of 

 lime with phosphoric acid is an exceedingly important ele- 

 ment of vegetable and animal substance. It forms 57 per 

 cent, of the dried bones of an animal and exists to some ex- 

 tent in every part of its body. It is largely contained in 

 the seeds of plants, and in all the grasses. Next to nitro- 

 gen it is the most valuable constituent of manures and fer- 

 tilizers, and its sufficient supply to the soil gives the farmer 

 much care and anxiety in regard to the culture and perfec- 

 tion of his crops. It exists naturally in the rocks as apa- 

 tite, or mineral phosphate of lime, and thus consists of 54 

 per cent, of lime, and 45 $ per cent, of phosphoric acid ; bone 

 phosphate of lime, containing 51 per cent, of lime, and 48 

 per cent, of phosphoric acid. A bi-phosphate of lime is 

 found in animal manures, chiefly in the urine, in which 

 there are 71 i per cent, of phosphoric acid and 28? per 

 cent of lime. The phosphate of lime and bones, furnish 

 the basis for the manufacture of superphosphate of lime 

 which is one of the most valuable fertilizers. 



MAGNESIUM, is a metal having many points of similar- 

 ity to those above mentioned. It is white, easily inflamma- 

 ble, and when burned in the air unites with oxygen form- 

 ing a compound or earthy oxide known as magnesia. It is 

 of no direct interest in relation to vegetable growth. Its 

 compounds enter into vegetable and animal substance, at 

 times to a considerable extent. 



CHLORIDE OF MAGNESIUM, exists in the water of the 

 ocean to a larger extent than chloride of sodium and gives 



