Manuring and Rotations 81 



Wood ashes have for generations been known to be high 

 in potash. They are often applied directly to land, but 

 are sometimes leached to obtain the potash in a more 

 concentrated form. In some countries where sunshine 

 is abundant, sea water is evaporated and potassium ob- 

 tained by fractional crystallization. During the last 

 few years much potash has been obtained from kelp, which 

 is harvested in the sea with special boats. This is a 

 promising source of potash. 



The mineral alunite is also being used to a considerable 

 extent as a source of potash. Rather extensive beds occur 

 in Utah and other parts of the West. Other minerals, 

 such as orthoclase feldspar, have a rather high potash- 

 content, but cheap methods of making it available have 

 not yet been developed. 



INDIRECT FERTILIZERS 



Many soils, particularly in humid regions, have an acid 

 reaction which is not conducive to the best growth of 

 most crops. It is necessary to neutralize this acidity be- 

 fore sugar-beets will thrive. This is best done by the use 

 of some form of lime. Burned lime has been used ex- 

 tensively, but it is gradually giving way to finely ground 

 limestone which is much easier to handle and much 

 cheaper. The effectiveness of limestone depends to a 

 great extent on the fineness of grinding. 



Many substances are added to the soil because of their 

 stimulating action. Among the most common of these 

 are common salt, gypsum, iron sulfate, soot, and man- 

 ganese salts. It may be advisable to use some of these 







