190 Disease in Plants. 



From these figures it would seem that there is a simple law 

 with reference to the quantity of water and of dry vegetable 

 matter in the leaves and in the tubers of the potato-plant, for 

 the confirmation of which further experiments are required. 



An inverse relation between the two may be gathered from 

 the above experiments. Whereas in Boxes I., III., the tubers 

 were watery, the tops of those plants were rich in solid matter ; 

 on the other hand, in Box II., the tops contained much water, 

 whilst the tubers Avere rich in solid vegetable matter. 



It has been mentioned that our turf unmanured formed a good 

 barley-soil (at least, for one crop). The development of the 

 potato-plants, and the yield of tubers, prove that it may be 

 called fruitful for this crop likewise, as the produce amounted to 

 two-thirds of that which would be given by a soil of the best 

 quality under ordinary cultivation. From these facts we gather 

 that the food-materials for the barley and the potato-plants were 

 contained in this turf in such quantity and so distributed that 

 they were sufficient to mature a full crop of barley and a 

 moderate growth of potatoes. The food, however, absorbed by 

 the two kinds of plants was not regularly but irregularly diffused 

 throughout the soil ; and hence is immediately explained the 

 influence which the addition of ammonia, phosphoric acid, and 

 sulphuric acid in Box II. exercised upon the increase in the 

 produce of tubers and tops. 



To estimate this influence justly it is well to imagine the case 

 of a piece of ordinary arable land, where the plant-food is always 

 unequally distributed. In such a soil, at one place molecules of 

 phosphoric acid, potash, lime, magnesia, silica, &c., Avill occur 

 in such juxtaposition and proportion that the fibres of the roots 

 of a plant which requires them for its growth can, on arriving at 

 this spot, absorb of them all in appropriate proportion ; whilst 

 at another spot in the same ground all these elements do not occur, 

 or not in immediate proximity ; but here may be phosphate of 

 lime without potash, magnesia, or silica ; there again alkalies, 

 alkaline earths, and silica, but no phosphoric acid. 



It is clear that on such a soil an increase in the crop Avould be 

 obtained by the addition of manures of totally opposite characters . 

 If, for example, it were manured by wood-ashes, many spots 

 would receive an excess of potash, which there would be in- 

 operative ; but at another place this addition would supply 

 the existing lack, and render effective the phosphoric acid and 

 other elements, which without the potash would be useless. 

 A consequence of this would be an increase of crop. Exactly 

 the same would follow from manuring with phosphates. Wher- 

 ever phosphoric acid already existed in sufficient quantity 

 in the soil, that which was added would, of course, be unem- 



