SULFATE OF POTASH 119 



names of superphosphate or acid phosphate. This form of phos- 

 phate is chiefly soluble and is quickly available to plants. It may 

 contain from fourteen to twenty per cent of phosphoric acid. 



Superphosphates (acid phosphates) are sometimes, though 

 rarely, made from bone, and bone tankage, by treating them with 

 sulfuric acid. The change which takes place when rock phosphate 

 is treated with sulfuric acid is here shown: 



Ca 3 (PO 4 ) 2 + 3 H ? SO 4 = 3 CaSO 4 + 2 H 3 (PO 4 ) 

 calcium phosphate + sulfuric acid = calcium -j- phosphoric 



sulfate acid 



If all of the calcium is not removed from the original rock by 

 the sulfuric acid the resulting phosphate may be represented by 

 the formula 



CaH 4 (PO 4 ) 2 or Ca 2 H 2 (PO 4 ) 2 



Basic slag, sometimes called Thomas slag, is a by-product of the 

 manufacture of steel from ores containing much phosphorus. 

 The amount of phosphoric acid present varies widely, seventeen 

 or eighteen per cent being commonly found. Its availability for 

 plant growth is usually slow. In the older methods of manufac- 

 ture this product contained much lime, but in the more recent 

 methods very little lime is left in the slag. 



Potash is not so generally needed as phosphoric acid and nitro- 

 gen, but a little is often combined with the other ingredients in 

 commercial fertilizers. It is particularly valuable on sandy soils, 

 on soils containing an abundance of humus and other soils that 

 have been used many years. We have already seen that it may 

 be derived to a limited extent from unleached hard wood ashes. 

 The chief commercial sources, however, are certain mineral salts 

 from mines in Germany. The natural material exists in the mines 

 in the form of kainit, sylvinit and other less common forms. From 

 these salts two chief products are made by a process of separation 

 and purification: (1) Sulfate of potash containing about 48 per 

 cent of actual potash, K 2 O. (2) Muriate of potash containing 

 about 50 per cent of actual potash. Both of these forms are 

 soluble in water and are easily absorbed by plants. 



Muriate of potash is slightly cheaper than the sulfate and is 

 recommended for use when potash is required for such staple 

 crops as cotton, potatoes, tobacco and others. It is usually con- 

 sidered the better form to use if the soil is light and sandy. 



Sulfate of potash is perhaps more commonly used than it 

 otherwise would be, because of its physical condition, which 



