80 The Sugar-Beet in America 
steamed or burned to remove fat and nitrogenous ma- 
terials which are used for other purposes. The fine 
grinding of bone makes its phosphorus more readily avail- 
able. Tankage relatively high in bone is used largely for 
its phosphorus; if high in flesh scraps it is valuable for 
its nitrogen. Bone is sometimes treated with sulfuric 
acid to render its phosphorus more available. 
Mineral phosphorus is found in several kinds of rock, 
which usually have the phosphoric acid in combination 
with lime, iron, and aluminum. The presence of the 
last two elements reduces the availability of the phos- 
phorus. Rock phosphates are used in various ways. 
Formerly practically all of the rock was treated with 
sulfuric acid, to form super-phosphate, or acid phosphate 
as it is often called; but of late years the use of finely 
ground raw rock-phosphate has increased, especially in 
soils rich in organic matter. The acid phosphate is doubt- 
less more immediately available than the raw rock, but 
it is also much more expensive. 
In the manufacture of steel from pig-iron, much phos- 
phorus is removed with the basic slag, called Thomas 
slag. It is often ground and used as a fertilizer. 
Potassium. 
Most of the potash fertilizers used in the world have in 
the past come from the Stassfurt deposits in Germany. 
Here many minerals rich in potash are found. Some of 
these are ground and put directly on the land; others are 
leached with water to concentrate them before being 
used. Kainit and silvinit are among the most common 
of these minerals. 
