SOURCES AND COMPOSITION OF FERTILIZERS 



299 



applied. If there are other limiting conditions, the series to 

 which they apply should be rejected. 



Some workers have taken the weight of the crop as a measure 

 of the availability of nitrogen, but since the object of the work is 

 to ascertain what proportion of the nitrogen of the fertilizer can 

 be taken up under the most favorable conditions, it is obvious that 

 the amounts of nitrogen recovered is the only correct measure. 

 The nitrogen taken up by the crops is not necessarily in propor- 

 tion to their weights. For example, Johnson, Britton, and Jenkins 1 

 secured the following results, with oats : 



In this experiment, the quantities of nitrogen taken up are 

 nearly in proportion to the amounts of nitrogen applied, but the 

 weights of the crops are not. 



Conditions which Affect Availability. The conditions which 

 affect nitrification also affect availability, since nitrification is 

 a necessary process for the preparation of active nitrogen. The 

 length of the growing season is an important factor ; a long grow- 

 ing season being relatively more favorable to the slowly nitrified 

 materials. If a crop is grown and harvested and then a second crop 

 grown without any addition of fertilizer nitrogen, the slowly act- 

 ing nitrogen will appear relatively more effective than if one crop 

 only is considered. For example, Voorhees found the availability 

 of fresh solid manure (compared with nitrate of soda equal to 

 100) to be 12 when one crop (oats) alone was considered, but 43 

 when the nitrogen in two crops, oats and millet, was taken. 



On account of the effect of the conditions of the experiment, 



and also on account of the error inherent in the method of work, 



considerable differences in the availability of the nitrogen of 



fertilizers are observed by different workers. The following 



1 Report Connecticut Exp. Sta., 1893. 



