AS ILLUSTRATED BY THE POTATO MURRAIN. 



191 



On a perfect comparison of the contents in ashes of different 

 kinds of manure with those in a sound tuber, it appears tliat if 

 the alkalies are put down at unity, the magnesia and phosphoric 

 acid in the dung are too great. We must, however, remember 

 that sound potatoes are not at present m their normal condition 

 in consequence of the established mode of culture. 



The following table, compiled from the details given by Dr. 

 Schleiden, will exhibit the matter clearly and usefully, the alkalies 

 being in every case placed at unity. 



If then what has been said about the malady and its causes be 

 correct, the practice of planting potatoes in newly manured soil, 

 and of employing the kinds of manure just enumerated, the dis- 

 position to disease must increase more and more, and slight cir- 

 cumstances will be sufficient to cause its eruption. If very 

 favourable years allow us to hope for a sound harvest, such ex- 

 pectation will be more and more deceptive every year, and a 

 time will at last come when potatoes, instead of affording the 

 surest crop, will be the most precarious. 



Comfortless as this view is, when we reflect how large an in- 

 fluence a favourable potato harvest has, not only on the welfare 

 of most people in Europe, but even on tiie possibility of their 

 existence, yet, on the other side, the way is thus laid open which 

 must be taken to avert with certainty the tiireatening evil. 



This consists in a general alteration of our mode of culture 

 and in the whole system of rotation. Potatoes must henceforth, 

 if we hope to avoid the apprehended consequences, be universally 

 excluded from the commencement of any system of rotation. The 

 particular place in the course I will not attempt to decide, since 

 so many purely practical circumstances must be regarded which 

 cannot be measured by theoretic principles, and distinguished 



