Nitrogenous Fertilizers 55 



Such products as dried blood, dried meat, dried fish and 

 concentrated tankage change rapidly, and are, therefore, 

 good forms, while products like raw leather and horn 

 meal are very slow to change. 



The practical point, and the one of prime importance 

 to the farmer, is, then, to know how to estimate the rela- 

 tive value or usefulness of these different products, what 

 is the rate of availability as compared with nitrate, and 

 thus the relative advantage of purchasing the one or the 

 other, at the ruling market prices. Relative values, 

 however, cannot be assigned as yet, though careful studies 

 of the problem have been made, chiefly by what are known 

 as " vegetation tests " ; that is, tests which show the actual 

 amounts of nitrogen that plants can obtain from nitrog- 

 enous products of different kinds, when they are grown 

 under known and controlled conditions. An enormous 

 amount of work has been devoted to the comparison of 

 the availabilities of the nitrogen of the different substances 

 under different conditions and with different crops, and 

 tables have been prepared showing the relations thus ob- 

 served. The comparative availabilities are established in 

 these tables by taking the yield from nitrate nitrogen at 

 100, and using this as a standard for measuring the yields 

 from other substances. It will be seen, therefore, that 

 the actual availabilities are smaller than the comparative 

 availabilities, since the return from nitrate nitrogen never 

 actually reaches 100 per cent. Furthermore, while practi- 

 cally all of the effect from the application of nitrate or 

 ammonia nitrogen is obtained in the first season, the 

 effect from manure nitrogen or other forms of organic 

 nitrogen is often considerable in the second and even the 

 third season a fact which must not be overlooked in 

 the study of availabilities. The following tables give 



