COMMERCIAL FERTILISERS 377 



farm. There are several advantages in doing 

 this as compared with buying commercial brands. 

 The most important one is that of being able to 

 use but one or two of the plant foods, as is found 

 necessary, and to gauge the proportions of each 

 to suit the needs of the soil and the crop. There 

 is also a saving in the cost of plant food, because the 

 raw materials are mostly concentrated, and there 

 is less expense in handling them. Added to this 

 is the difficulty of always determining with absolute 

 certainty the exact amount of plant food in a brand 



of commercial fertiliser owing to the ambiguous 



i- f & 



wording ot many guarantees. 



On me other hand, it is sometimes difficult to 

 buy these raw materials; they are not so generally 

 distributed over the country, as brands of mixed 

 fertilisers. Again, the mixed fertilisers are apt 

 to be ground more finely than the unmixed. If 

 a man uses but little fertiliser, it is likely that he 

 will find it more expedient to buy a commercial 

 brand; but if he uses a considerable amount it 

 may be cheaper for him to buy plant food in the 

 raw materials, not mixed. Most fertilizer dealers 

 sell the raw materials as well as mixed fertilisers. 



The following raw materials are most com- 

 monly used: 



SOURCES OF NITROGEN 



Nitrogen is the most costly of the three plant 

 foods. For this reason special attention should 

 be given to the means of producing it on the farm, 

 as discussed in the two preceding chapters. The 

 chief commercial sources fall into two classes: 

 the "nitrates," which are salts; and "organic 

 nitrogen," which is the nitrogen in plant and 



