t-" 



essential to plant growth than are the other substances, but be- 

 cause of the deficiency of them in many soils and their corres- 

 ponding commercial importance. 



8 . Phosphoric acid {P^O^ is one of the prime essen- 

 tials. — Kver}^ farmer, whether he has used phos- 

 phates or not, has seen phosphoric acid. When 

 a match is ignited the little curl of white smoke 

 which first appears is pure phosphoric acid, P.O.. 

 The kind of match for this purpose is the old 

 fashioned one known as a sulfur match, easily reco- 

 gnized by the blue, flickering flame and the odor 

 of burning sulfur. The kind known as crack or 

 parlor matches do not show phosphoric acid. 



In the blue-black or red tip of this match there 

 is a small amount of the substance called phos- 

 phorus. For convenience and shortness this name 

 is represented by the letter P. When this phos- 

 phorus is warmed up by friction it burns, and in 

 burning it unites with oxygen from the air. Oxy- 

 gen is represented by O. The figures 2 and 5 in 

 this symbol PjO^, mean that 2 parts of P are united 

 with 5 parts of O. The result is the white sub- 

 stance of the smoke, or pure phosphoric acid. such 

 as is sold in phosphates. 



9. Farmers bny phosphoric acid in combinations 

 with lime. — Phosphoric acid does not exist in the 

 soil in the free state, that is, not as one sees it on 

 igniting a match. It readily unites with lime to 

 form phosphates of lime. The chemical name 

 of lime is calcium oxide (CaO) and the phosphates 

 of lime are called calcium phosphates. Water is H,0. 



lyime and phosphoric acid unite in three different proportions 



CaO) 



. .CaO - pa 



Showing what passes 

 off when a bhie- 

 headea match is first 

 ignited. 



I. One part phosphoric acid and 3 of lime. 



II. One part phosphoric acid and 2 of lime. . 



III. One part phosphoric acid and i of lime, . 



CaO \ 



H,0) 



.CaO \ P.O. 

 CaO) 



HO) 

 •HpfP.O, 



CaO \ 



