42 THE POTATO 



age, deep plowing, and thorough tillage are necessary 

 to bring air into the soil and stimulate bacterial activ- 

 ity. Humus, lime, and tillage are three important fac- 

 tors in maintaining a fertile soil, 1 and the farmer who 

 understands the value of these is the one who will de- 

 rive the most benefit from the use of fertilizers. 



Purchasing and Applying Fertilizers. Fertil- 

 izers may be divided into three classes viz. : 



() Nitrogenous, or those rich in nitrogen. 



(^) Phosphatic, or those rich in phosphorus. 



(c) Potassic, or those rich in potassium. 

 Nitrogen occurs in fertilizers, as : 



(1) Nitrates e.g., nitrate of soda, nitrate of 



potash. 



(2) Ammonium salts e.g., sulphate of am- 



monia. 



(3) Organic nitrogen e.g., dried blood, tankage, 



hoof meal, etc. 



Nitrogen as nitrates is immediately available 

 as plant-food, is soluble in water, and if 

 not taken up quickly by plants is liable 

 to be lost in the soil water; hence small 

 quantities applied at short intervals give the 

 best results. 



Nitrogen as ammonium salts soon becomes 

 available in warm weather, and is not so 

 liable to be washed out of the soil as when in 

 the form of a nitrate. 



Nitrogen as organic matter is more slowly 

 available. 



Del. Bui. 66, "Soil Bacteria and Nitrogen Assimilation.' 



