MANURES. 



27 



of the weiglit of water ; with carbon, it composes carbu- 

 retted hydrogen, and with nitrogen, ammonia. The main 

 supply of hydrogen is derived from water, a portion, how- 

 ever, is obtained from the ammoniacal and other gases 

 which pass from decomposing organic substances, and 

 from the absorption of soluble organic matters in the soil. 



Of all the substances that minister directly to the growth 

 of plants, by entering into their composition, nitrogen is 

 perhaps the most necessary to supply. Though it forms 

 four-fifths of the atmosphere, yet in this state plants can- 

 not appropriate it. It unites with hydrogen forming am- 

 monia, and with oxygen forming nitric and nitrous acids, 

 from which compounds plants derive the small proportions 

 they appropriate. Though the proportion of nitrogen 

 existing in plants is small, yet it is not the less essential ; 

 but in fact, is that which is most needful to place within 

 their reach. Ammonia is the substance from which plants 

 derive most of their nitrogen. Ammonia or hartshorn is 

 the gas given out by the common smelling salts. It is 

 that which gives to animal manures their peculiar value. 



This gas enters immediately into the circulation of 

 plants. We all know how luxuriantly plants grow . in 

 putrid animal manure. 



This is owing to the ammonia given off by the manure, 

 and appropriated directly by the plants, supplying them 

 with both its constituents nitrogen and hydrogen. Indeed 

 the value of manure is measured by the amount of am- 

 monia it contains, not because ammonia is more essential 

 than, some of the other constituents of plants, but because 

 more difficult to obtain in sufficient quantity. Manures 

 that are rich in nitrogen, readily pass into a state of fer- 

 mentation, and the abundant ammonia given off will often 

 burn the plants with which it comes in contact ; hence 

 they are called hot manures, as guano, horse and pig 



