

ROTATION OF CROPS. 243 



of the carbohydrates. Consistently with this it was found 

 that by far the larger proportion of the potash of the turnip 

 crop was in the roots, where was the great accumulation of 

 sugar. Again, of the total potash of the barley crop, the 

 larger proportion was found in the straw where there was the 

 greatest accumulation of carbohydrate — as cellulose ; and 

 now, in the wheat, with a larger proportion of straw to grain, 

 and a proportionally larger amount of the total carbohydrates 

 accumulated in the straw, we have in it a still larger pro- 

 portion of the total potash of the crop. It is, however, to be 

 borne in mind, as has been pointed out, that the straw of both 

 barley and wheat frequently contains, besides the mineral 

 constituents actually essential for the organic formations and 

 changes, a more or less surplus amount taken up as the result 

 of liberal supply, and retained by the plant. 



Although there is doubtless clear foundation in fact for the Functions 



conclusion that the role of phosphoric acid is more in con- fP ota f h 

 • Tin . r , r . . „ . and phos- 



nection with the formation and activity of the nitrogenous phorw 

 bodies, and that of the potash with those of the non-nitro- aeid ' 

 genous compounds, yet it is obvious that in such a view we 

 have only a partial and imperfect explanation of the function 

 of these mineral constituents. Thus, in the case of the beans 

 there was, consistently enough, much more phosphoric acid 

 in the corn than in the straw — that is, the more where there 

 was the more nitrogen ; but there was also by far the larger 

 proportion of the potash accumulated in the corn, although 

 the greater part of the dry matter of the crop, and with this 

 of its carbohydrates, was in the straw. Indeed, although the 

 leguminous crops are pre - eminently highly nitrogenous, a 

 liberal supply of potash is essential for their luxuriance; 

 whilst they contain a higher proportion of it in their dry 

 substance than do the cereals, with their higher proportion of 

 carbohydrates. 



Eeference to the figures shows that the application of super- 

 phosphate, without potash, enabled the wheat plant, whether 

 grown in rotation or continuously, to take up an increased, 

 but not a much increased, amount of potash, compared with 

 that in the unmanured crops ; and that the direct application 

 of it increased the assimilation of it still further, though the 

 increased amount of it stored up represented only a small 

 proportion of that supplied in the manure. 



"Without manure, the rotation wheat crops contained an Rotation 

 average of about 27 lb. of potash, but the continuously grown and contm - 

 ones scarcely 13 lb., or only about half as much. With super- and the 

 phosphate, without potash, the rotation crops gave an average e ^ ec< °f 

 of nearly 30 lb., and the continuously grown ones little more 

 than 15 lb.; or, again, only about half as much. That is, 



