26 MANUAL FOR SUGAR GROWERS. 



ings, dried blood, etc., are nitrogenous manures, 

 several of these being humus-supplying manures as 

 well. The need for humus and its action on the 

 soil has been already dwelt upon. 



The various nitrogenous compounds in the or- 

 ganic substances, and in the ammonia salts above 

 mentioned, are not in a fit condition to serve as 

 plant-food without undergoing change ; careful ex- 

 periments have demonstrated that plants usually 

 absorb their nitrogen in the form of a salt of nitric 

 acid, or a nitrate, as it is termed.* Now, when 

 organic matters or ammonia salts are mixed with 

 soil, chemical changes take place, and the greater 

 part of the nitrogen is converted into nitrate by 

 a process very similar to fermentation. The re- 

 searches of recent years prominent among them 

 being those of Warington, Schlosing, and Muntz 

 have proved that this conversion into nitrate is the 

 work of a minute microbe or germ which is present 

 in all fertile soil. This microbe is found in greatest 

 number and in greatest vigour near the surface of 

 the soil, and penetrates to various depths according 

 to the character of the soil. For its active growth, 

 and in order that it may carry on its useful function 

 of converting organic matter into plant-food, sev- 

 eral conditions are necessary : 1. A substance con- 

 taining nitrogen and capable of being acted on by 

 the microbe; such things as organic matter or 



* Nitric acid is a compound of hydrogen, nitrogen, and oxygen, 

 having the formula HNO 3 ; the hydrogen may be replaced by 

 metals, as in the case of potassium nitrate or saltpetre, KNO 3 , oV 

 sodium nitrate, NaNO 3 . 



