534 VARIOUS FERTILITY FACTORS 



phosphate. Another possible reaction may result in the liberation 

 of potassium or magnesium from polysilicates, as roughly indicated 

 by the following equation: 



AlFeMgNaK x+2 (SiO 3 ) y (H 2 O) Z + CaSO 4 



= AlFeMgNaK x Ca (SiO 3 ) y (H 2 O) 



With large supplies of potassium present in polysilicates, heavy 

 applications of calcium sulfate would doubtless liberate some 

 potassium sulfate, although under the opposite conditions mass 

 action would force the reverse reaction; that is, heavy appli- 

 cations of potassium sulfate to a soil containing much calcium in 

 polysilicates would liberate some calcium sulfate. 



The potassium sulfate liberated from the insoluble silicate, as 

 indicated above, may serve directly as plant food, or it may react 

 to increase the availability of phosphorus, thus : 



Ca 3 (PO 4 ) 2 + 2 K 2 SO 4 = CaK 4 (PO 4 ) 2 + 2 CaSO 4 . 



These equations are given to show some of the possible reactions 

 that may occur when a soluble salt is added to the soil. A dozen 

 different reactions may be taking place at the same time within 

 the same cubic inch of soil, and it is easily possible that, while 

 one reaction is taking place in one part of the cubic inch, the 

 reaction is running in reverse order in another part, depending 

 upon the mass, composition, and concentration of the insoluble 

 and soluble salts. 



If a solution of ammonium sulfate or potassium chlorid is per- 

 colated through a stratum of soil, an examination will usually 

 show that, while ammonium or potassium passed into the soil, 

 calcium and magnesium have passed out in the percolate. If 

 tricalcium phosphate be shaken with pure water, practically no 

 phosphorus will be found in the filtrate; but if a solution of some 

 neutral salt, such as sodium chlorid or potassium nitrate, be sub- 

 stituted for the pure water, very appreciable amounts of phosphorus 

 are dissolved. 



Land-plaster has been much used in some parts of the North 

 Central and Eastern states, and for a time it usually gives quite 

 profitable results; but finally, the element of real value that has 

 been liberated by the action of the land-plaster becomes so depleted 



