Or Agricultural Chemistry, 



23 



In Table V. we have compared together the produce of the 

 nnmanured plot with that of Plot 10a, which, excepting in 

 1844, when superphosphate of lime and silicate of potass were 

 used (giving, however, less than one bushel of increase)^ was 

 manured in every succeeding season by ammoniacal salts alone. 



It is a remarkable fact that from Plot 3 (the unmanured one), 

 of this previously unusually corn-exhausted soil, we have carried 

 from the land seven successive crops of wheat grain, and of straw, 

 without any manure whatever ; and that under this treatment 

 there are, at present, no signs of diminished fertility ; for the 

 average of the seven seasons collectively is about 17 J bushels of 

 dressed corn, and about 16 cwts. of straw, or more than was ob- 

 tained in the first experimental year. Indeed, there is little 

 doubt that upon a soil of any given quality the produce will only 

 vary with the character of the climate and the variations of the 

 seasons, which must materially affect the amount of ammonia 

 available from natural sources ; and upon this again depends the 

 assimilation of other constituents, which in the case of our experi- 

 ments were proved to have existed in ample relative quantity 

 within the reach of the plant. Thus, the results of Plot 10a, as 

 seen in the 2nd column of the table, are alone sufficient to show 

 that, whatever the deprivation by the previous cropping, the 

 soil still contained, relatively to the ammonia available from natural 

 sources^ an excess of the necessary mineral constituents. We 

 shall presently show that this must be the condition of most if not 

 all cultivated land, where grain and meat constitute — as they do^ 

 as the rule, in Great Britain — almost the exclusive exports from 

 the farm ; the straw of the grain and the excrements of the animals 

 fed upon the farm, finding their way into the home manures, 

 and eventually back again to the fields from whence they came. 



But w^e must not be understood to say that all soils wdll yield 

 continuously \1\ bushels of grain and 16 cwts. of straw per acre, 

 without manure ; on the contrary, we know full well that they will 

 not, and that what are termed light soils, but which, under high 

 cultivation, give good crops of wheat, would give but a small pro- 

 portion of this quantit}'. That the heavier ones do possess a 

 native fertility beyond what might at first sight be supposed, there 

 can be little doubt ; were it not so, we should find it difficult to 

 explain how those who sell off their land almost all its produce 

 without return, are enabled to live and pay their rent. But what 

 we say is, that by the ordinary methods of practical agriculture, 

 by which any soils are made to yield a fair produce of grain and 

 meat only, for sale, their characteristic exhaustion, as grain-pro- 

 ducers, wdll be that of nitrogen ; and that the mineral consti- 

 tuents, will, under this course^ relatively to nitrogen, be 

 in excess. To this point, however, we shall recur further on. 



