322 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



(NHJ 2 S0 4 + Ca(OH) 2 =: 2NH 8 + CaSO 4 + 2H 2 O. 



(2) Chemical changes convert the phosphoric acid into less 

 soluble forms. Acid phosphate should not be mixed with Thomas 

 slag, lime or ashes. 



CaH 4 (PO 4 ) 2 -f 2CaO == Ca 3 PO 4 + 2H 2 O. 

 Lime is, however, sometimes mixed with moist acid phosphate 

 to improve its physical condition, so that the resulting mixture 

 may be applied with a fertilizer drill. 



(3) Certain mixtures will harden or cake and thus become 

 difficult to distribute if kept for some time after mixing. Hence 

 they should be applied soon after mixing. This applies to mix- 

 tures of lime or Thomas slag with potash salts, nitrate of soda, 

 and kainit. 



Conditions which Modify Use of Fertilizers. These are : ( i ) 

 Deficiency of soil; (2) Value of crop; (3) Character of crop; 

 (4) Kind of rotation. 



Deficiency of Soil. A knowledge of the nature of soils with 

 respect to the deficient elements is important, in order that those 

 elements which are present in abundance may not be added to, but 

 that they may be supplemented by such quantities of the deficient 

 elements as to permit maximum profitable crops. This matter 

 of soil deficiencies has been treated elsewhere. 



An opinion as to the deficiency of the soil may be based on : 



(a) The chemical composition of the soil. 

 (&) The behavior of the crop. 



(c) Previous experience in the use of fertilizers. 



(d) Field tests to ascertain needs of the soil. 



The value of the crop is of importance in deciding the profit- 

 able application of fertilizers. Crops may roughly be divided into 

 two classes : the first class have a relatively low commercial value 

 per acre, the second have a high commercial value per acre. 



Wheat, corn, oats, cotton, etc.,, belong to the first class. These 

 crops remove large amounts of plant food in proportion to their 

 value. For example, a ton of wheat removes 38 pounds of nitro- 

 gen, 19 pounds phosphoric acid, and 13 pounds of potash. With 

 nitrogen at 20 cents and phosphoric acid and potash at 6 cents, the 



