I64 THE POTASSIC FERTILISERS [chap. 



to be washed away, kainit and the crude salts are as 

 effective, potash for potash, as the concentrated salts in^ 

 which the unit of potash is more expensive. But foj 

 potatoes, malting barley, and similar crops where qualit 

 is of moment, especially when the manure is put 

 near the time of seeding, sulphate of potash is advisable, 

 especially upon heavy soils. 



Muriate of potash has often been shown to yield a 

 greater weight of potatoes than sulphate, though the 

 tubers are more watery, and this result has been 

 associated with the chlorine, which is supposed to 

 assist in the migration of starch about the plant, but 

 the facts are by no means certain as yet. 



All the compounds of potash found in these fertilisers 

 are freely soluble in water, so that some danger of loss 

 by washing out might be apprehended when the 

 manures are applied in the winter. As early, however, 

 as 1850, Way found that ordinary soils possessed con- 

 siderable powers of reacting with potash salts to produce 

 insoluble potassium compounds, the chlorine or sulphuric 

 acid of the salt remaining in solution combined with 

 calcium and other bases derived from the soil. The 

 absorptive power was found to be greatest with soils 

 rich in clay and humus, and the retention of the 

 potash is chiefly effected by interaction with the zeolitic 

 double silicates of the clay, potassium being exchanged 

 for calcium, magnesium, or sodium in the zeolite. To 

 a certain extent a similar exchange of calcium for 

 potassium takes place in the humus, a comparatively 

 insoluble potassium humate being precipitated and 

 calcium sulphate or chloride going into solution. 



When experiments are made in the laboratory, by 

 treating a soil with a weak solution of potassium 

 sulphate or chloride, the removal of the potassium 

 from solution is never complete ; the extent of the 



