174 FOOD FOR PLANTS 



efBciency of nitrogen in different materials, and it 

 is this phase of the work which is to receive consid- 

 eration in this paper. 



In order that the work might be under more per- 

 fect control, it was carried out in galvanized iron 

 cylinders, open at both ends and having a diameter 

 of 2311. inches and a depth of 4 feet. These cylinders 

 were set on the ground so that about 2 inches 

 remained above the ground level. Thus the contents 

 of the cylinders are isolated so that the roots of the 

 crops growing in them are prevented from getting 

 mineral plant-food from outside sources. The sub- 

 soil is a gravelly sandy material such as occurs where 

 the cylinders are located, but the top soil is a loam 

 (Penn loam) brought from another source, an 8-inch 

 layer of which was placed in each cylinder on top 

 of the subsoil, each cylinder receiving the same 

 weight of the thoroughly mixed soil. 



When the work was begun all the soils were given 

 a liberal treatment of lime in the form of ground 

 limestone and with the exception of one series which 

 does not enter into this discussion, all have received 

 annual dressings of acid phosphate and potassium 

 chloride at the rate of 640 pounds and 320 pounds 

 per acre, respectively. Thus nitrogen is made the 

 limiting factor insofar as human control can provide. 

 Various combinations of manure and fertilizer were 

 arranged, but it is sufficient to report here only on 

 the four nitrogenous materials mentioned in the title. 



One series received the phosphoric acid and potash, 

 but no nitrogen, in order that it might be used as a 

 check. Thus if a certain amount of nitrogen is recov- 

 ered in the crop from the nitrogen-treated cylinder, 



