CHAPTER XVI. 



PURCHASE AND USE OF FERTILIZERS. 



Commercial fertilizers consist; first, of acid phosphate, cotton- 

 seed meal, potash salts, and other commercial substances contain- 

 ing plant food; and secondly, of mixtures of these substances, 

 made to secure a product of a desired composition. The mixture 

 usually contains all three kinds of plant food, though a number of 

 mixtures are on the market which contain only two, phosphoric 

 acid and potash, or phosphoric acid and nitrogen. 



Mixed Fertilizers. Mixed fertilizers are of two classes dry 

 mixed and wet mixed. In dry mixing, the materials are weighed 

 out, ground when necessary, mixed thoroughly, then passed 

 through a screen so as to make them of uniform size. The nitro- 

 gen is generally in two or more forms, one highly available, the 

 others less so. A filler is added when the sum total of the in- 

 gredients containing plant food do not make up the required 

 weight to give the desired composition. Any substance which 

 contains no plant food, or quantities much lower than the content 

 of standard fertilizer ingredients, should be considered as a filler. 

 The filler is usually sand or dirt, but sometimes objectional fillers 

 are used, such as limestone, lime, or pyrite cinder, which con- 

 tains oxide of iron. Small quantities of lime are sometimes used 

 to dry the fertilizer. 



In wet mixing, the organic nitrogenous material is first mixed 

 with the sulphuric acid, then the phosphate rock is added, and the 

 mixture is dumped out and allowed to harden as in the manu- 

 facture of acid phosphate. Potash salts or nitrate of soda, if 

 either is used, is added while the product is being ground or 

 otherwise prepared. In wet mixing, the nitrogenous material is 

 to a certain extent acted on by the acid. There is no doubt that 

 this treatment increases the availability of low-grade nitrogenous 

 materials, but little experimental work has been done to show the 

 availability of the product. Street 1 found the following changes 

 in the nitrogen of one sample commercially treated in this way : 

 1 Report Connecticut Exp. Sta., 1911, p. 14. 



