MASS ACTION 175 



Other things being equal it has been found that there is a 

 difference in the ability of one base to replace another. 

 Potassium will replace magnesium and be replaced by 

 ammonium in turn. Sodium will replace calcium and be 

 replaced by magnesium. In other words, the order of 

 replacing power beginning with the strongest is: Ammo- 

 nium, potassium, magnesium, sodium, calcium. Each element 

 will replace any of those following it and be replaced by any 

 of those preceding it. In the case of acid radicles, those 

 which form insoluble compounds the most readily are the 

 ones absorbed the quickest. Phosphates of calcium (tri) 

 and of iron and aluminium are insoluble. Calcium phosphate 

 forms soluble acid phosphates, but the others do not dissolve 

 easily. Carbonates of calcium and magnesium are insoluble, 

 but form soluble bicarbonates very readily. Sulphates, 

 except of barium, are rather soluble. Chlorides and nitrates, 

 particularly the latter, are all readily soluble and hence do 

 not form compounds which can be retained chemically. 



But a part at least of the above discussion seems to be at 

 some variance with what has been said in Section 135 about 

 the solubilitv of soil minerals. The fact is that under some 

 conditions elements are rendered soluble and under other 

 conditions the same elements are rendered insoluble, even 

 when the reacting substances are exactly the same, and this 

 difference of reaction depends on the active mass of the 

 reacting substance. This is called the law of mass action. 



139. Mass Action. — To illustrate this important chemical 

 law, the equations for the solubility of phosphorus and for 

 the fixation of phosphorus may be written together thus: 



Ca 3 (P04) 2 + 4CO-2+ 4H 2 CaH4(P0 4 )2+ 2CaH-2(C0 3 )2. 



— v* 



This is called a reversible reaction, for it will go in either 

 direction depending on the active mass of the reacting sub- 

 stance. Tricalcium phosphate continues to go into solu- 

 tion as long as the supply of carbon dioxide is continuous, 

 and the monocalcium phosphate or calcium bicarbonate is 

 removed from the solution. In the soil, decomposition of 

 organic matter supplies carbon dioxide. Growing root hairs 



