Tillage, Fertilizing and Rotation 99 



fertilizer standards must be studied, the forms of plant- 

 food required, the sources of supply, as well as present 

 day methods of preparation, commercial sale and appli- 

 cation. 



Kinds of fertilizers. 



Fertilizers may be direct or indirect ; complete or in- 

 complete. A fertilizer that supplies available plant- 

 foods directly to the soil is known as direct. !Ritrate of 

 soda is a good example of this type. 



A fertilizer that benefits growth through its effects on 

 the availability of the plant-food already contained in 

 the soil, through its action on soil acidity or by im- 

 proving the mechanical condition of the soil, is known as 

 indirect. Strictly in this sense, drainage, proper cul- 

 ture or humus might be considered fertilizers. Lime is 

 the most commonly used commercial fertilizer of the 

 indirect type. 



Although plants ordinarily require about fourteen 

 different food elements for their normal growth, only 

 four and usually only three ever need be applied arti- 

 ficially. These three are nitrogen, phosphorus and 

 potassium; the fourth is calcium in the form of lime. 

 A complete fertilizer is one containing nitrogen, phos- 

 phorus and potash while an incomplete one supplies 

 only two of these plant-foods. 



Function and form of plard-foods. 



l^itrogen, phosphorus and potassium are known as 

 the " essential " fertilizing constituents since they are 

 the specific plant-foods most likely to be deficient in 

 soils or more quickly exhausted by the production and 

 removal of crops. Nitrogen functions most largely in 



