176 FAEM-YARD MANURE. 



ploughed it in. We have assumed the field to be in tlie 

 best physical condition, which therefore could not be 

 improved by the incorporation of the organic substances of 

 the clover and the potatoes. Even if we were to take the 

 potatoes from the field, to mow down and dry the clover, 

 giving both to cattle in the farm-yard or making any 

 other use of them, and then to bring all back to the field and 

 plough them in, so as to restore to the soil all the mineral 

 constituents contained in both crops, yet by all these 

 operations the field would not produce, in thirty, sixty, or 

 seventy years, a single grain of corn more than with- 

 out this alternation. The conditions required for the 

 production of grain are not improved in the field during 

 the whole of this period, and the causes of decrease in the 

 crops remain the same. 



The ploughing in of the potatoes and the clover could 

 have a beneficial effect upon those fields only which have 

 an inferior physical condition, or in which the mineral 

 constituents are unequally distributed, or are partially 

 inaccessible to the roots of plants. But this effect is 

 hke that of green manuring, or of one or more years of 

 fallow. 



By the incorporation of the clover and the organic con- 

 stituents with the soil, its store of decaying substances 

 and nitrogen increased year by year. All that these 

 plants received from the atmosphere remained in the 

 ground ; but the increase of these otherwise so useful 

 substances cannot make the soil produce a larger amount 

 of grain than before ; since the production of grain depends 

 upon the right proportion of ash-constituents in the soil, 

 and these, so far from being increased, have been gradually 

 reduced by the removal of the corn crops. The aug- 

 mentation of nitrogen and of decaying organic substances in 

 the soil might possibly lead to an increase of produce foi 



