174: FARM-YAKD MANURE. 



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 without 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 like that of green manuring, or of one or 

 more years of fallow. 



By the incorporation of the clover and the organic 

 constituents with the soil, its store of decaying sub- 

 stances 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 use- 

 ful 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-constit- 

 uents in the soil, and these, so far from being increased, 

 have been gradually reduced by the removal of the 

 corn crops. The augmentation of nitrogen and of de- 

 caying organic substances in the soil might possibly 

 lead to an increase of produce for a number of years ; 

 but the period when this field will cease to give remu- 

 nerative crops will in that case come all the sooner. 



If we take three wheat fields, and cultivate wheat 

 upon the one, potatoes and clover upon the other two ; 

 and suppose we remove the corn alone from the wheat 

 field and heap upon it and plough in all the crop of 

 clover and all the potato tubers, then the wheat field 

 will be more fertile than before, for it has been en- 

 riched by all the mineral constituents which the two 

 other fields had furnished to the potatoes and the clo- 

 ver. It has received three times as much phosphoric 

 acid and twenty times as much potash as was contained 

 in the corn crop it produced. 



