Disease in Plants. 193 



to increase his produce in the direction wliicli lie considers most 

 advantageous, 



Tlie ascertained facts, I think, cstablisli this point : that, in 

 arable land containing- the ordinary amount of nitrogen, am- 

 monia maybe excluded as manui-e for potatoes without prejudice 

 to the crop ; and that, moreover, in soils rich in potash the addi- 

 tion of phosphates, and in soils poor in potash, but containing a 

 sufficient quantity of phosphoric acid, the addition of wood-ashes 

 is absolutely essential to an increased yield of tubers. 



Theory, indeed, would presuppose these conditions in any 

 given case ; and these experiments were quite unnecessary to 

 establish the principle that, lor obtaining a full crop of potatoes, 

 every element of food of the plant ought to be present in the soil 

 in proper proportion and in sufficient quantity. Their value 

 consists in this, — that by them a definite notion has been obtained 

 of the extent of the influence which the defect or excess of one 

 nutritive substance can exert upon special functional develop- 

 ments ; and that without the application of ammonia (that chief 

 element in animal manure) the produce of a field in potato- 

 tubers can be made, under the most favourable relations, to 

 exceed in great degree the largest yield which has hitherto been 

 obtained on the best of soils. Theory alone could never have 

 settled this point. 



It appears, therefore, unquestionable from these experiments 

 that a farmer may, in the culture of potatoes, dispense altogether 

 with animal manure, and with great advantage employ in its 

 stead a j)roperly-selected combination of phosphates, wood-ashes, 

 and sulphate of lime. 



Although this is significant enough in itself, it, nevertheless, 

 is not the most important result of these experiments. One still 

 more important yet remains. 



Of the potatoes grown in the two sorts of soil (Boxes I,, II.), 

 where the elements essential to conditions of growth were not 

 present in sufficient quantities or in due proportion, every tuber 

 was attacked by the potato-disease. The eyes first turned black ; 

 then, after a few Avecks, decomposition set in, which extended 

 from these inwards. This decay only appeared, as has been 

 already observed, in those tubers which were grown in the 

 natural turf, or in that which was dressed with ammonia. 



Every one of the tubers, on the other hand, which were grown 

 in the soil dressed with the fixed ash-constituents has remained 

 so far perfectly sound ; not a trace appeared in any one ol them 

 of what is usually described as the potato-fungus. 



It follows incontestably, from these experiments, that the coil' 

 ditions ichich favour the normal development of the plants are 

 precisely those ichich ward off the disease ; and hence, as whatever 

 external harmful influences there might have been acted equally 



VOL. II. — S. S. O 



