THE CONSTITUTION OF THE SOIL 



55 



washes out in the drainage water. Potassium sulphate reacts in the 

 same way, the potassium being precipitated and an equivalent amount 

 of calcium going into solution. Potassium phosphate undergoes a 

 more complete precipitation, since calcium phosphate is insoluble. The 

 precipitated potassium compound dissolves somewhat in water, but it 

 has no definite solution pressure, instead the amount of potassium 

 dissolving increases with the amount present. It can also be decom- 

 posed by sodium salts ; hence addition of sodium sulphate to the soil 

 increases the amount of soluble potassium ions and to this extent acts 

 like a dressing of potassic manure. Magnesium salts have a similar 

 effect, and, like sodium salts, lead to an increase in the amount of 

 potassium available for the crop. Some of Lawes and Gilbert's results 

 (i 66) are given in Table XXV. : 



TABLE XXV. EFFECT OF SODIUM AND MAGNESIUM SULPHATES IN INCREASING THE 

 SUPPLY OF POTASH TO THE PLANT. LAWES AND GILBERT (166). 



In the twenty years the sodium sulphate has enabled the plant to 

 take up an additional 263 Ib. of K 2 O, whilst the magnesium sulphate 

 has furnished it with an extra 320 Ib. over and above what the crop on 

 Plot 1 1 can get. 



The interaction between ammonium sulphate and the clay has been 

 found by Hall and Gimingham (121) to follow the ordinary law of mass 

 within a certain restricted range of concentration, but, according to the 

 old experiments of Weinhold (300) and the recent ones of Cameron 

 and Patten (67), not over a wider range. 



