290 THE TUKtflP CROP. 



In replacing the mineral ingredients abstracted from 

 the field by the growing crops, there are only two or three 

 out of the nine which they have made use of, about which 

 we need concern ourselves, as the remainder are those 

 of which the mass of the soil is commonly made up. Of 

 these the potash and phosphoric acid are the most im- 

 portant, as all our "Farm Crops" draw largely upon 

 both for their requirements, while they rarely are met 

 with in our soils except in very minute proportions. If 

 we look at the analysis of the turnip (p, 328) we see that 

 the proportion of potash it abstracts from the soil is 

 about three times that of the phosphoric acid ; and yet we 

 are inclined to consider that " superphosphate," rarely 

 containing a particle of potash, is the most judicious 

 application, ' whether for supplying the wants of the 

 growing crop or of replacing in the soil the substances it 

 takes from it. Noiu, it should be distinctly understood 

 that fertility in a soil is not governed so much by the 

 maximum as by the minimum proportions of its consti- 

 tuents. If out of the nine substances the plant requires 

 for its growth, eight of them be largely in excess of its 

 wants, #nd one be in a less proportion than the crop 

 requires, its returns will be determined altogether by the 

 producing power of the minimum, and not in the least 

 affected by that of the other substances in excess. It is 

 true that plants have, like animals, a great power of 

 suiting themselves to the conditions under which they are 

 placed, and of substituting, to a limited extent, one food 

 material for another for instance, soda for potash, mag- 

 nesia for lime; but this variation does not affect the 

 above rule, neither do the plants, under these forced 

 conditions, carry on their processes in such a vigorous 

 and healthy manner. In the use of all artificial manures 

 these points should not be forgotten, and a due propor- 

 tion in all the composing ingredients be secured. 



