DAVIS] STUDIES OF COMMERCIAL FERTILIZERS 123 



at home. A still better practice wovild be for him to supply his 

 own. nitrogen by means of manures and rotation of crops, buying 

 only the necessary phosphoric acid in form of ground phosphoric 

 rock, and potash. 



The arithmetic of fertilizer calculations is within the capacity 

 of the pupils of the seventh and eighth grades. All that is neces- 

 sary is an intelligent presentation of the problems by the teacher. 

 However, teachers seem to have much difficulty in understanding 

 such problems themselves. The two following exercises dealing 

 in considerable detail with the application of commercial fertilizers 

 have been presented to several hundred teachers in county insti- 

 tutes and in my own classes. Problems arising from such appli- 

 cation have the two-fold value (i) of affording a practical use of 

 arithmetic, (2) of giving direct service to a farming community. 

 A school might well undertake to make all necessary fertilizer 

 calculations for its patrons. " •' 



Commercial Fertilizers 



Explanation: . ^ 



The plant derives certain substances from the soil which are 

 necessary for its growth. All but three of these (nitrogen, phos- 

 phorus, and potassiimi) are generally found in the soil in sufficient 

 quantities for the needs of the plant. The "essential ingredients" 

 of a fertilizer are subscances containing these elements; i.e., 

 substances which supply (a) nitrogen as nitrate of soda, dried 

 blood, hoof meal, etc., (b) phosphorus in form of phosphoric 

 acid as bone meal (raw or steamed), mineral phosphates, etc., 

 (c) potassivim in form of potash as wood ashes, kainite, muriate 

 or sulfate of potash, etc. 



A complete fertilizer is one that contains nitrogen, phosphoric 

 acid, and potash in proportion supposed to be suited to the needs 

 of certain crops. Such a fertilizer is made by mixing substances 

 containing the basic ingredients so as to give the desired propro- 

 tion of nitrogen, phosphoric acid, and potash. 



It is often the practice to use substances rich in these ''essential 

 ingredients" and dilute the mass to the desired strength by means 

 of some inert material such as dry earth. Materials used in this 

 way are called fillers. A 2-8-4 fertilizer means one that contains 

 two per cent nitrogen, eight per cent phosphoric acid, and four 



