238 SOILS AND FERTILIZERS 



NATURAL PLANT FOOD, $25 TO $28 PER TON 



COMPOSITION 



PER CENT 



Total phosphoric acid 

 Insoluble phosphoric acid 

 Available phosphoric acid 

 Potash soluble in water . 



22.21 



20.81 



1.40 



0.13 



Actual value per ton, $1.52 



285. Mechanical Condition of Fertilizers. In pur- 

 chasing a fertilizer its mechanical condition should be 

 considered. The finer the fertilizer, as a rule, the bet- 

 ter it is for promoting crop growth. Some combina- 

 tions of plant food produce fertilizers which become so 

 hard and lumpy that it is difficult to crush them before 

 spreading. They should be pulverized so they may be 

 evenly distributed, otherwise the plant food will not be 

 economically used. A fertilizer that passes through a 

 sieve with holes 0.25 mm. in diameter is more valuable 

 and can be used to better advantage than one of the 

 same composition with particles 0.5 mm. in size. 



286. Forms of Nitrogen in Commercial Fertilizers. - 

 Nitrogen is present in commercial fertilizers in three 

 forms : (i) Ammonium salts, (2) nitrates, and (3) 

 organic nitrogen. The organic nitrogen is divided 

 into two classes : (a) available, and (b) unavailable. 

 Pepsin and also potassium permanganate are used as 

 solvents for determining the availability of the organic 



