III.] IMPORTANCE OF NITROGENOUS MANURES 79 



nitrogenous manure alone, and in nearly all cases by a 

 mixture of nitrogenous and phosphatic manures. The 

 special value of nitrogen in this connection is well seen 

 in the Rothamsted experiments ; on the wheat field, for 

 example, we may compare the yield of the unmanured 

 plot with that receiving nitrogen alone and minerals 

 alone, and again that which receives nitrogen and 

 phosphoric acid against that which receives nitrogen, 

 phosphoric acid, and potash. 



From Table XIX. it will be seen that plot 5, which 

 is nitrogen starved but which receives an excess of all 

 the other elements of nutrition, only yields 1-9 bushels 



Table XIX.— Average Yield of Wheat. Broadbalk, 

 Rothamsted. 56 years (1852-1907). 



more grain than the unmanured plot ; whereas plot 10, 

 which receives an excess of nitrogen but has had to 

 rely solely upon the original reserves of minerals in 

 the soil, has produced on the average 7-6 bushels of 

 corn more than the unmanured plot. The minerals only 

 increased the yield by 14-7 per cent, but nitrogen by 59 

 per cent, and these differences would have been much 

 more pronounced had they been calculated on the 

 results of the first year or two of the experiments only, 

 instead of over a period so long that the mineral reserves 

 of the soil are also highly exhausted. It is this greater 

 relative deficiency of available nitrogen than of available 



