A POTATO-CROP AND THE MINERALS IN THE SOIL. 199 



The potato plant draws its principal constituents 

 from the arable surface soil, and from a somewhat 

 deeper layer than the rye plant ; and the crops reaped 

 show the condition of the layers more accurately than 

 could be ascertained by chemical analysis. 



In the fields at Mausegast and Cunnersdorf the nu- 

 tritive substances available for the potato plant were 

 about equally close ; in Kotitz they were one-ninth 

 closer to each other ; at Oberbobritzseh they were twice 

 as far asunder ; while at Oberschona they were one-fifth 

 closer than in Oberbobritzsch. 



The largest potato crop was obtained from the field at 

 Kotitz. Potash (for the tubers) and lime (for the herb- 

 aceous parts) are the predominant constituents of the 

 potato plant : but a certain amount of nitrogen and 

 phosphoric acid is as necessary for the development of 

 the potato as it is for cereals ; and the effective quan- 

 tity of the transmuted potash and lime is essentially 

 determined by the phosphoric acid and nitrogen ab- 

 sorbed at the same time. Where one of the two latter 

 elements which, as we have remarked, are equally con- 

 stituents of cereals, is deficient in the soil, the potato 

 crop will be proportionate to the available quantity of 

 these two substances, and the greatest excess of potash 

 or lime in the soil will have no influence whatever upon 

 the amount of the produce. 



The arable surface soil of the field at Oberbobritzsch 

 is much richer in phosphoric acid and nitrogen than 

 that of the Kotitz field ; yet the potato crop yielded by 

 the former was only half that given by the latter. 



Accordingly, nothing can be more certain than that 

 the field at Oberbobritzsch contained much less potash 

 or lime in an available state, than the Kotitz field ; and 

 by manuring with lime alone, or with wood-ashes (pot- 

 ash and lime), it might readily be ascertained in which 

 of the two substances the ground was deficient. 



But from the inferior potato crop given by the field 

 at Cunnersdorf, we cannot infer that it was poorer in 

 potash or lime than the field at Kotitz ; the latter de- 

 cidedly contained, as the preceding corn crop shows, 



