IV. THE REACTIONS OCCURRING IN SOILS 81 



This result means that equilibrium is attained when the ratio of 

 potassium sulphate to potassium carbonate reaches a certain fixed 

 value. 



The same is true (with different values for the ratio) with all other 

 reactions of similar type where there are two soluble and two little 

 soluble or insoluble substances concerned. It applies, for example, to 

 a change which is often made use of in agricultural practice and 

 affords an explanation of what may appear puzzling and contradictory 

 to the student. Calcium sulphate is used as a means of lessen- 

 ing the loss of ammonium compounds from manure heaps ; this it is 

 said to do by undergoing double decomposition with the volatile 

 ammonium carbonate, yielding non-volatile ammonium sulphate and 

 calcium carbonate. 



On the other hand, when ammonium sulphate is, applied as a 

 manure to a soil, calcium sulphate is found in the drainage water, and 

 ammonium carbonate is apparently held back in the soil until nitrifi- 

 cation occurs. 



As represented in the ordinary textbook manner the reactions 

 are 

 in the case of the manure heap 



CaSO 4 + (NH 4 ) 2 CO 3 = CaC0 3 + (NH 4 ) 2 S0 4 

 and in the case of ammonium sulphate applied to soil 

 CaCO 3 + (NH 4 ) 2 S0 4 = CaSO 4 + (NH 4 ) 2 CO 3 . 



Now, in the light of what has been said, it is obvious that neither 

 of the equations really represents what happens in either case. 



In the first place the carbonates involved in the reactions are 

 probably not normal carbonates but acid carbonates, since excess of 

 carbon dioxide is present both in the manure heap and in the soil. 



The reaction might be represented 



CaSO 4 + 2NH 4 HC0 3 ^ Ca(HC0 3 ) 2 + (NH 4 ) 2 SO 4 



equilibrium being attained when the ratio of the ammonium sulphate 

 to the ammonium carbonate attains a certain value, if the solutions are 

 so concentrated that separation of CaSO 4 and Ca(HC0 3 ) 2 in the solid 

 state can occur (as perhaps may happen in a manure heap) ; but the 

 conditions of equilibrium will, in dilute solution, be also affected by 

 the quantities of calcium sulphate and calcium bicarbonate present in 

 a dissolved state, for it must be remembered that their influence only 

 becomes constant when they are present in such quantities that they 

 separate partially in the solid state. 



It is only fair to say, however, that the tendency for the soil to 

 absorb and retain ammonium carbonate will also play an important 

 part in the case of the action in the soil, that compound being with- 

 drawn from the sphere of action as fast as it is formed, thus diminish- 

 ing the speed of the reaction represented by the equation read from 

 left to right. 



