THE USE OF FERTILIZERS 213 



tation should be followed and the system of fertilization should 

 be adjusted to the rotation as explained in § 271. 



In order that the kind and quantity of fertilizer shall be a con- 

 trolling factor, the plats should be well drained and well tilled 

 and should not be in need of lime, which may be ascertained 

 by either of the tests described in §§ 145, 146. 



270. Calculation of the results. — Each test plat has, on 

 one side of it, a plat that has not been fertilized. The non- 

 fertilized or check plats will not all give the same yield be- 

 cause the soil differs in various parts of the field. If the 

 variations in yield between check plats are not greater than 

 one bushel to the acre, they may be considered as being equal. 

 If a greater difference exists, the yield from each check plat 

 must be subtracted from the yields of the test plats beside 

 it and the result may then be considered to be the increase 

 due to the fertilizer application. 



If the yield is as good, or nearly as good, on a check plat 

 as it is on the corresponding test plat that lacks one of the 

 fertilizing constituents, it may be concluded that the use of 

 that constituent would not be a profitable investment. On 

 the other hand, the very beneficial substances will be indi- 

 cated by the increased yields wherever they are used. Fi- 

 nally the desirable quantities will be indicated by a compari- 

 son of the rates of increase on the plats receiving the full 

 quantity and those receiving the half quantity of complete 

 fertilizer. The tests should be continued for a period of 

 three to five years in order that they shall be indicative of 

 the fertilizer needs of the soil, and a rotation of crops should 

 be used, with an adjustment of the fertilizer treatments to 

 suit the different crops. 



271. Fertilizing the rotation. — In a rotation of crops 

 fertilizers need not be applied every year. For instance a 

 rotation consisting of hay, two or three years, corn, oats 

 and wheat would probably not receive any fertilizers on 



