FOOD FOR PLANTS 173 



About 10 years later the subject began to receive 

 attention in this country and it is a satisfaction to 

 find that the results obtained here are fairly in accord 

 with the findings of the European investigators. 



Fairly complete revievi^s of this early work have 

 been given in recent publications (2,3) and no at- 

 tempt will be made here to cover this field. 



The completion in 1917 of 20 years' work in which 

 a comparison is made of the materials mentioned in 

 the title of this paper would seem to justify the pub- 

 lication at this time of a brief summary of the work. 

 A detailed account covering the first 15 years of this 

 work has already been published (3). Much of this 

 need not be repeated, but the results of the last five 

 years are of value as confirming the earlier work. 



Experimental. 



The work was originally outlined under the broad 

 heading ''Investigations Relative to the Use of 

 Nitrogenous Materials," and this included: (a) a 

 determination of the yield of dry matter and nitrogen 

 in crops from soils receiving various treatments 

 under controlled conditions; (b) the percentage of 

 nitrogen in the crop as affected by the treatment; 



(c) the utilization of nitrogen in different materials; 



(d) the relative efficiency of nitrogen in different 

 materials; (e) the residual effects of nitrogenous 

 substances; (f) denitrification and (g) the effect of 

 special treatment on the income and outgo of nitrogen 

 in the soil. 



As the work has progressed, more and more atten- 

 tion has been given to the utilization and relative 



