24 SCIENCE AND SOIL 



hydrogen atom is replaced by potassium, and the compound 

 formed is acid potassium sulfate (HKSO 4 or KHSO 4 ). The charac- 

 ter of this compound is one half acid and one half salt. If applied 

 to the soil, it would tend to make the soil sour, or acid, which is also 

 true of the common acid phosphate of the fertilizer trade, which is 

 further discussed under phosphorus. 



Common acids are so few in number that it is well worth while to 

 memorize the formulas. The following are important in the study 

 of soil fertility : 



HC1 . . . Hydrochloric acid (no oxygen). 



HNO 8 . . Nitric acid. 



HNO 2 . . Nitrous acid (less oxygen). 



H 2 SO 4 . . Sulfuric acid. 



H 3 PO 4 . . Phosphoric acid. 



H 2 CO 3 . . Carbonic acid. 



H 2 SiO 3 . . Silicic acid. 



H 2 S . . . Hydrosulfuric acid (no oxygen). 



The salts made from these acids, and their derivatives, by reac- 

 tion with the bases represented by the six metals (iron, aluminum, 

 calcium, magnesium, potassium, and sodium) constitute, in large 

 part, the solid crust of the earth and the salts of the sea. One non- 

 metallic oxid (SiO,j) and two oxids of metallic elements (Fe 2 O 3 and 

 ALjO 3 ) are also found native in very considerable amounts. 



Some acids are strong and some are weak. The weakest acid is 

 carbonic (H 2 CO 3 ), but silicic (H 2 SiOg) and hydrosulfuric (H 2 S) 

 are also weak acids; while hydrochloric (HC1), nitric (HNOg), 

 sulfuric (H 2 SO 4 ), and phosphoric (HgPOJ are all very strong acids. 

 A strong acid may take a base away from a weak acid, thus : 



CaCO 8 + H 2 SO 4 = CaSO 4 + H 2 CO 8 . 



Here we have the strong sulfuric acid reacting with calcium 

 carbonate (limestone) to form a neutral salt, calcium sulfate, and 

 the weak carbonic acid. The carbonic acid is so weak that it may 

 break in two, forming water and carbon dioxid: 



H 2 CO 3 = H 2 O + CO 2 . 



