172 FAEM-YAED MANUEE, 



year of crops. After the exliaustioii of this excess of 

 mineral matter, the period of diminishing crops would 

 commence for our field, and the interposition of fallow 

 years would, after this, no longer exercise the least influ- 

 ence on the production of larger crops. 



If the excess of phosphoric acid, sihcic acid, and 

 potash, which we have assumed in the case under consi- 

 deration, were not unequally but uniformly distributed, 

 and everywhere perfectly accessible and available to the 

 roots of the plants, our field would be able to yield 

 thirty full average crops in thirty successive years, with- 

 out the intervention of a season of fallow. 



Let us return to our field, which we have assumed to 

 contain 25,000 kilogrammes of the ash-constituents of 

 wheat, equally distributed through the soil, and in a suit- 

 able state for absorption by the roots. Suppose we were 

 to cultivate wheat upon it year after year, but instead of 

 removing the entire crop we were merely to cut off the 

 ears, leaving the straw on the ground and immediately 

 ploughing it in ; the loss sustained by the field would, in 

 this case, be less than before, as all the constituents of the 

 straw and the leaves would be left in the field, the mine- 

 ral constituents of the grain alone having been removed. 



The straw and leaves contain, among their constituent 

 elements, the same mineral substances as the grain, only 

 in different proportions. If the total quantity of phos- 

 phoric acid conveyed away in the straw and corn be 

 designated by the number 3, the loss will be only 2, if 

 the straw is left in the ground. The decrease of produce 

 from the field, in tlie following year, is always in propor- 

 tion to the loss of mineral substances occasioned by the 

 preceding crop. The next produce of grain will be a 

 little larger than it would have been had tlie straw not 

 been left in the ground ; the produce of straw will be 



