56 The Rothamsted Wheat Experiments. 



his immediately available supply of mineral constituents, but 

 also pays very dearly for his increase, if he tries to obtain it 

 by means of purely nitrogenous manures when his soil is 

 already unduly exhausted of mineral constituents. 



The above table is taken from a more comprehensive one, 

 which also gives the result on each plot for every one of the 12 

 years. As showing the difference of effect of a given amount of 

 ammonia in one season as compared with another, the results 

 on plot 6, receiving 501b. of ammonia (in 2001b. of ammonia- 

 salts) each year, and always with mixed mineral manure, are 

 instructive. Taking the average of the 12 years, it required 

 4*861b. of ammonia in manure to yield 601b. of increase of 

 grain and its proportion of straw ; whereas, in the remarkably 

 productive season of 1863, it required only 2*421b., but in 

 1853, 7'131b. ; in 1860, 8'851b. ; and in 1852, 12'451b. 



The conclusion on the point under notice is that, great as 

 is the difference of effect of a given quantity of ammonia, 

 according to the amount applied per acre, to the mineral con- 

 dition of the soil, and to the season, still, when only moderate 

 quantities were used, when there was a sufficient supply of 

 mineral constituents, and taking the average of many seasons 

 that is, under the conditions the most comparable with 

 those of the average of common practice, the result was in 

 marked accordance with the early estimate, that almost 

 exactly 51b. of ammonia were required to be expended to 

 obtain an increase of one bushel of wheat grain and its pro- 

 portion of straw. 



THE EOTHAMSTED EXPEEIMENTS ON THE CON- 

 TINUOUS GEOWTH OF WHEAT COMPAEED 

 WITH THOSE AT WOBUEN, HOLKHAM, AND 

 EODMEESHAM. 



It may fairly be objected that these experiments on the 

 continuous growth of wheat furnish results true of one 



