MANUAL FOR SUGAR GROWERS. 25 



time the decaying vegetable matter increases the 

 humus and returns to the soil those mineral mat- 

 ters which entered into its own structure, thus 

 increasing the soil's fertility. These soils are gen- 

 erally spoken of as virgin soils, and will often bear 

 luxuriant crops for years without manures ; but 

 sooner or later they become more or less ex- 

 hausted, and the art of the farmer is required, 

 that by proper manuring they may be kept in a 

 condition to produce remunerative crops. 



The constituents of mineral plant-food which are 

 usually first exhausted are phosphoric acid and pot- 

 ash. Hence the importance attached to the pres- 

 ence of these substances in manures. To these in 

 certain cases may be added lime and iron. 



Thus far attention has been directed to the min- 

 eral plant-food, there yet remains a great deal to be 

 said on the subject of manures supplying nitrogen. 



Nitrogen is an essential constituent of the food 

 of both plants and animals, just as carbon and the 

 various mineral constituents are. It has been seen 

 how the mineral matters are obtained from the soil, 

 and how the carbon is obtained from the atmosphere ; 

 now, four-fifths of the atmosphere consists of nitro- 

 gen, which would thus appear to provide an abun- 

 dant supply of this important plant-food ; yet it is 

 found that plants cannot derive their nitrogenous 

 food from the air, but that it all comes from the 

 soil. (As to the nitrogen in the case of leguminous 

 plants, see page 28.) All organic matter contains 

 more or less nitrogen, and thus organic matters, 

 such as animal excreta, pen manures, green dress- 



