SPECIAL MANUKES. 323 



turnips, can avail itself for a longer period of the resources of the 

 soil; therefore in most cases the phosphoric acid disseminated 

 through the soil is amply sufficient to meet the requirements of the 

 wheat crop ; whilst turnips, depending on a thinner depth of soil 

 during their shorter period of growth, cannot assimilate sufficient 

 phosphoric acid, to come to perfection. This is, I believe, the 

 main reason why the direct supply of readily available phosphates 

 is so beneficial to root-crops, and not to wheat." 



"This reason," said I, "has never been entirely satisfactory to 

 me. If the roots of the turnip have such a limited range, how are 

 they able to get such a large amount of potash? 



" It is probable that the turnip, containing such a large relative 

 amount of potash and so little phosphoric acid, has roots capable 

 of absorbing potash from a very weak solution, but not so in re- 

 gard to phosphoric acid." 



" There is another way of looking at this matter," said the Doc- 

 tor. " You must recollect that, if turnips and wheat were grow- 

 ing in the same field, both plants get their food from the same so- 

 lution. And instead of supposing that the wheat-plant has the 

 power of taking up more phosphoric acid than the turnip-plant, 

 we may suppose that the turnip has the power of rejecting cr ex- 

 cluding a portion of phosphoric acid. It takes up no more potash, 

 than the wheat-plant, but it takes less phosphoric acid." 



But it is not necessary to speculate on this matter. For the 

 present we may accept the fact, that the proportion of potash, 

 phosphoric acid, and nitrogen in the crop is no indication of the 

 proper proportion in which these ingredients should be applied to 

 the soil for these crops in manure. 



It may well be that we should use special manures for special 

 crops ; but we must ascertain what these manures should be, not 

 from analyses of the crops to be grown, but from experiment and 

 experience. 



So*far as present facts throw light on this subject, we should 

 conclude that those crops which contain the least nitrogen are the 

 most likely to be benefited by its artificial application ; and the 

 crops containing the most phosphoric acid, are the crops to which, 

 in ordinary practical agriculture, it will be unprofitable to apply 

 superphosphate of lime. 



" That," said the Doctor, "may be stating the case a little too 

 strong." 



" Perhaps so," said I, " but you must recollect I am now speak- 

 ing of practical agriculture. If I wanted to raise a good crop of 

 cabbage, I should not think of consulting a chemical analysis 



