POTASH MANUEES 279 



defloculation of clay, and its effects on seeds, if brought 

 into contact with them, would be very injurious. The 

 effects of plant ashes are, of course, much milder because 

 they are not so concentrated, and also because the pres- 

 ence of the lime would tend to correct the defloculating 

 effects of the alkali. Noxious weeds collected in the 

 process of cleaning the land should, of course, always be 

 burned, as that is the only way to make sure of destroying 

 them. The quantity is usually so small that the loss of 

 nitrogen is inconsiderable, and so also are the effects- 

 beneficial or otherwise of the ashes which are spread on 

 the land. 



POTASH SALTS. 



Occurrence. The extensive saline deposits which were 

 discovered in 1839, near Stassfurth, in Germany, were at 

 first, worked only for common salt. The occurrence of 

 layers, consisting of potassium chloride and other com- 

 pounds, was not suspected until some twenty years later. 

 It was known that potassium, magnesium and calcium 

 compounds were present in the salt in considerable quan- 

 tities, but as these substances are commonly found in 

 deposits of rock salt they were regarded merely as ordin- 

 ary impurity. The importance of the discovery was at once 

 apparent. The potash salts have a much higher value than 

 common salt. The mines are now worked chiefly for the 

 former, and it is the latter that is regarded as impurity. 

 All other sources of potassium compounds, both for 

 agricultural and other purposes, have since become rela- 

 tively insignificant. 



The deposits extend over an area of many square 

 miles, and are estimated to be, in places, upwards of 

 5,000 feet thick. The supply may be therefore regarded 

 as practically inexhaustible, and since the geological 



