SOIL IMPROVEMENT 83 



before it is used on the soil. This makes the phosphoric 

 acid more quickly available for the use of growing plants. 

 In this form it is called superphosphate, or acid phosphate^ 

 which is a soluble phosphate; this may contain from 

 fourteen to twenty per cent of phosphoric acid. Super- 

 phosphates may also be made from bone and bone tankage. 



Exercise. — To Study Phosphorus. — Roll the head of a 

 match between the moistened fingers. The white fumes are 

 caused by the phosphorus in the match uniting with the 

 oxygen in the air. They are called phosphoric acid. This 

 is a form of the plant-food we have been studying. 



Potash is the third essential element in commercial fertil- 

 izers, and is not so generally required. It is often needed 

 on soils that have been farmed for a long time and also on 

 sandy soils and peaty soils. It is derived from wood ashes 

 to a limited extent. The largest source is in the form of 

 mineral salts from the Stassfurt Mines of Germany. The 

 chief names given to these salts are car-nall-ite, kain-it, 

 and syl-vin-it. These may be ground fine and used on the 

 soil; but they are more commonly refined or concentrated 

 before being shipped to this country. The two concentrated 

 forms of potash salts are the (1) muriate of potash, contain- 

 ing fifty per cent of actual potash, and (2) the sulphate of 

 potash, containing forty-eight per cent of actual potash. 

 In all of the forms potash is soluble in water, and is easily 

 absorbed by plants. 



Value of Fertilizers. — In buying commercial fertilizers 

 the purchaser should remember that the agricultural value 

 is based not upon the brand but more upon kind and form 

 of the elements and upon their proportion in the mixture. 

 Many farmers are inclined to buy fertilizers that have a 

 low price per ton. These are invariably more expensive 

 sources of plant-food elements than those more highly con- 

 centrated, because of the necessity of diluting the mixtures 

 with ''make-weight" or low-grade materials. High-grade 



