VI.] RETENTION OF POTASH BY SOIL 165 



removal will in any case depend upon the relative 

 masses of the potassium salt and the zeolite, so that 

 it is practically complete when a few hundred pounds 

 of fertiliser are applied to the great weight of soil 

 which represents an acre. Voelcker's analyses of the 

 drainage waters collected below the various plots at 

 Rothamsted showed that the amount of potash in the 

 water flowing from the drain below the unmanured 

 plot was 1-7 parts per million, and was only increased 

 to 2-9, 3-3, 4-4, and 5-4 parts respectively on other plots 

 which are annually manured with 300 lb. per acre of 

 potassium sulphate. Dyer's examination, also, of the 

 soils from the same Broadbalk wheat field show that, 

 of the potash applied as manure during fifty years and 

 not removed in the crops grown during the same 

 period, about one-half was still to be found in the top 

 9 inches of soil, much of it in such a combination 

 as to be soluble in i per cent, solution of citric acid, 

 while further quantities of the applied potash, also 

 soluble in the weak citric acid, were to be found 

 in the second and third 9-inch layers of soil. Thus, 

 from the point of view of practice, no loss of potash 

 need be apprehended through its application in the 

 winter before the crop is occupying the land, except 

 on the lightest sands where clay and humus are 

 lacking. 



To understand the use of potassic fertilisers in the 

 ordinary routine of farming, it is necessary to enquire 

 into the function of potassium in the nutrition of the 

 plant, for the water culture experiments hitherto 

 quoted only demonstrate that it is one of the indis- 

 pensable elements. 



Further enquiry goes to show that in some way 

 potash is an essential part of the mechanism of the 

 process of assimilation ; when it is deficient the manu- 



