196 MOVEMENTS OF SALTS IN THE SOIL 



salt is distinctly diminished. The relative diffusibility of 

 the two salts is also sometimes affected by the strength 

 of the solution employed. In some cases as in mixtures 

 of potassium nitrate with potassium chloride, or of sodium 

 nitrate with sodium chloride the more dilute is the solution, 

 the more nearly do the rates of diffusion of the two salts 

 approach each other. In other cases as in mixtures of 

 potassium nitrate and carbonate, or of the chlorides of sodium 

 and magnesium the amount of dilution does not affect the 

 relative diffusibility of the salts. The subject is thus one of 

 some complexity. 



Some of the facts just mentioned are of considerable 

 agricultural importance. The salts having the greatest value 

 as plant food the nitrates, and the salts of potassium and 

 ammonium are also those which will diffuse most rapidly in 

 the soil. In selecting manures it is well to remember that 

 the chlorides of potassium, sodium, and ammonium will dis- 

 tribute themselves much faster when applied to a moist soil 

 than the corresponding sulphates. The differences in the 

 diffusibility of the bases are also of practical importance. 

 Nitrate of sodium will clearly diffuse in the soil much more 

 rapidly than nitrate of calcium. This fact indicates an 

 additional difference between the behaviour of nitrate of 

 sodium and ammonium salts when applied as manure to a 

 soil, the nitrification of the latter in the soil resulting, as is 

 well known, in the production of nitrate of calcium. 



The different diffusibility of different salts will of course 

 occasion their separation in the soil, the most diffusible salt 

 always travelling fastest. Graham diffused a mixture con- 

 taining equal weights of chloride and sulphate of sodium ; 

 in the most distant part of the solution the proportions found 



