RETARDATION OF THE PERIOD OF EXHAUSTION. 171 



produce from the field, in the following year, is always 

 in proportion to the loss of mineral substances occa- 

 sioned by the preceding crop. The next produce of 

 train will be a little larger than it would have been 

 ad the straw not been left in the ground ; the produce 

 of straw will be nearly the same as in the preceding 

 year, because the conditions for the formation of straw 

 have been but slightly altered. 



Thus, then, by taking away from the ground less 

 than formerly, we increase the number of remunerative 

 crops, or the sum total of grain produced in the whole 

 series of corn harvests. Some of the straw-constituents 

 are converted into corn-constituents, and are now re- 

 moved from the field in the latter form. The period 

 of final exhaustion, though sure to come in the end, 

 will, under these circumstances, occur later. The con- 

 ditions for the production of grain go on continually 

 decreasing, because the substances removed in the corn 

 are not replaced. 



It would make no difference in this respect, if the 

 straw were cut and carted about the field, or used as 

 litter for cattle and then ploughed in ; the supply thus 

 bestowed upon the field, having been originally taken 

 from the field, cannot enrich it. 



Considering that the combustible elements of the 

 straw are not supplied by the soil, it is clear that in 

 leaving the straw in the ground we leave nothing more 

 than the ash-constituents of the straw. The field re- 

 mained somewhat more fruitful than before, because a 

 little less had been taken away. 



If the corn or its ash-constituents were ploughed in 

 with the straw, or if, instead of it, a corresponding 

 quantity of some other seed containing the same ash- 

 constituents as wheat, e. g. ground rape-cake, that is, 

 rape-seed freed from the fatty oil, were given in proper 

 proportion to the ground, its composition would remain 

 the same as before : the next year's crop would equal 

 that of the preceding year. If after every harvest the 

 straw is always in this manner returned to the field, 

 the further consequence will be an inequality in the 



