184 FOOD FOR PLANTS 



tent, while the carbon content of these soils was 

 slightly increased. 



The recovery of nitrogen in the four different treat- 

 ments for the 20 years is shown by the curves in 

 figure 1. A study of these curves shows that the 

 high points are generally reached in either the first 

 or second year of oats, and in the wheat year, while 

 the low points occur almost invariably in the corn and 

 timothy years. It is not entirely clear whether this 

 is a seasonal variation or a crop characteristic. 



It is certain, however, that the utilization of the 

 residual nitrogen by the corn crop which follows the 

 oats, helps to explain the high recovery for the years 

 when oats are grown. 



Conclusions. 



In a 5-year rotation on Penn loam soil well sup- 

 plied with phosphoric acid, potash and lime, crop 

 yields were better maintained over a period of 20 

 years with Nitrate of soda at the rate of 320 pounds 

 per acre than with an equivalent amount of ammo- 

 nium sulfate or dried blood. For several years the 

 latter gave results about on a par with the Nitrate, 

 but an average of the second 10-year period shows a 

 considerable falling off with these materials as com- 

 pared with the Nitrate. This is no doubt due in part 

 to the fact that the Nitrate, being immediately avail- 

 able, gives the plant an early start which tends to 

 keep it in the lead and to the further fact that in the 

 transformation of the ammonium salt and the organic 

 material into nitrates, there is a considerable loss of 

 nitrogen, possibly as ammonia gas or gaseous nitro- 



