SOIL EXHAUSTION AND ROTATION IN CROPS. 83 



two exceptions, the regular yield, within two or three bushels, 

 of this piece of land. A field, for example, which had been, 

 this last summer, twenty-eight years in continuous cultivation 

 under wlieat, has averaged about 16 bushels ; on one occasion, 

 it went up to 23, and on one occasion it dropped down to five. 

 These variations were due to the season, but otherwise the 

 yield ranged between 12 to 17 bushels, so tliat this pro- 

 ductive power of IG busliels may be considered as the capa- 

 city of that soil in respect to the wheat crop. I do not see 

 any reason why he and his successors should not go on for a 

 hundred years and get the same amount of wheat, within 

 about the same limits. Perhaps it would fall off somewhat. 

 There is a little falling off in the last half of the period just 

 completed. The yield is perhaps a bushel less than during 

 the first half; but that may be accidental, and due to the 

 character of the seasons. Tiicre is no reason in my mind why, 

 for the next twenty-five years, the yield should not be a 

 bushel or two more ; but we have not lived those other twenty- 

 five years, and we cannot tell positively. 



The worst soil we can point out has a certain natural capa- 

 city. Take our rocky hill ranges in this State : if we sliould 

 give a little care to them, we could harvest every twenty-five 

 or thirty years, a certain crop of wood from them ; and if we 

 should begin that culture now, and carry it on for a hundred 

 years, we should get the same crop the last thirty years that 

 we did the first thirty. If we carried it on for a thousand years, 

 the climate and circumstances generally, remaining as they 

 are, we could depend upon getting from them three uniform 

 wood crops every century. So in tlie poorest pasture, we have 

 a certain natural productiveness, which remains the same, so 

 long as the state of the soil is unaltered. The field may be- 

 come a swamp, or its natural water-supply may be dried up 

 by local changes, but independently of accidents like these, it 

 will manifest a certain nearly uniform natural strength from 

 generation to generation. All production of vegetable matter 

 in the soil, of any kind, is the result of change — the result 

 of chemical and physical change. Natural strength depends 

 upon changes in the soil which act in a nearly invariable 



