170 FARM-YAKD MANURE. 



nutritive substances as in the preceding years of culti- 

 vation sufficient, therefore, for an average crop ; we 

 should, in that case, be able to obtain full average 

 crops for a number of years by always letting a year 

 of fallow intervene after a year of cultivation. Instead 

 of thirty progressively decreasing crops, we should in 

 that case reap thirty full average crops in sixty years, 

 if the excess of mineral matter in the soil were suffi- 

 ciently^ large to replace everywhere the phosphoric 

 acid, silicic acid, and potash taken away in each year 

 of crops. After the exhaustion of this excess of min- 

 eral matter, the period of diminishing crops would com- 

 mence for our field, and the interposition of fallow 

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

 fluence on the production of larger crops. 



If the excess of phosphoric acid, silicic acid, and 

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

 sideration, were not unequally but uniformly distrib- 

 uted, 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, without 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 

 suitable state for absorption by the roots. Suppose we 

 were to cultivate wheat upon it year after year, but in- 

 stead of removing the entire crop we were merely to 

 cut off the -ears, leaving the straw r 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 mineral constituents of the grain alone 

 having been removed. 



The straw and leaves contain, among their constitu- 

 ent elements, the same mineral substances as the grain, 

 only in different proportions. If the total quantity of 

 phosphoric 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 



