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 
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