Disease in Plants. 191 



ployed ; but where all other necessary elements of food except 

 phosphoric acid were present, there the supply of phosphoric acid 

 would make the others effective. Thus, by using- phosphates as 

 manure an increased crop would be obtained. 



If, by the use of phosphoric acid in a perfectly homogeneous 

 soil (which, however, does not exist in nature), a larger crop 

 were produced, it would be impossible to gain a similar result 

 by alkalies or alkaline earths : for the beneficial effect of the 

 phosphoric acid would depend on an existing excess of other 

 elements of food everywhere in the soil, which, before inactive, 

 would be rendered available by the added acid. To increase in 

 such a field the inactive element would clearly not increase the 

 yield. 



Our turf-soil contained altogether in each box 277 grammes 

 of potash. A full crop of barley would withdraw from a surface 

 of 1-8 square metres (20 feet) the superficies of our boxes 9 

 grammes of potash, or about l-30th of that in our soil. 



This amounts to nearly two-thirds of the quantity required to 

 produce a full potato-crop both in tops and tubers. There was 

 just double as much phosphoric acid present in the turf as 

 potash, but unequally distributed ; as, by increasing the quantity 

 of phosphoric acid, the yield of tubers (as appears by the Tables) 

 was increased 21 per cent, that of tops 92 per cent., over the 

 produce of the unmanured turf. 



Again, our soil contained ten times as much lime and about 

 the same quantity of magnesia as of potash. Potato-tops are rich 

 in lime and magnesia, and poor in potash ; for in 100 parts by 

 weight of ash they contain 60 parts of alkaline earth and 4 parts 

 of potash. On the other hand, the tubers abound in potash 

 and are poor in alkaline earths, containing in their ash about 

 86 per cent, of alkalies and soluble alkaline salts, but only 14 per 

 cent, of alkaline earths. 



The weights of tops and tubers grown in Boxes I., II. — the 

 one containing, as will be remembered, turf alone, the second 

 turf, Avith the addition of ammonia and phosphoric acid — were 

 in the following ratios : — 



Tops. Tubers. 



Bos 1 7-2 .. .. 10 



Box II 11-0 .. .. 10 



As from II., 542 grammes more of tubers, and 1698 grammes 

 more of tops were obtained than from I. (pure turf), the propor- 

 tionate increase was : — 



Tubers. Tops. 



Box II 10 .. .. 31 



The use of phosphoric acid and salts of ammonia as manure 

 had unquestionably brought a certain quantity of lime, magnesia, 



