194 MOVEMENTS OF SALTS IN THE SOIL 



For our present purpose we need only notice those substances 

 which may occur in solution in the soil *. Colloid substances 

 have the lowest diffusibility. The diffusibility of a salt is deter- 

 mined partly by its acid, and partly by its basic constituent. 



The most diffusible salt of any metal is the chloride ; the 

 nitrate is but little less diffusible ; the sulphate is considerably 

 less diffusible than either chloride or nitrate ; the carbonate 

 is rather less diffusible than the sulphate. The diffusibility 

 of phosphates does not seem to have been determined; the 

 subject is however of little agricultural importance, as phos- 

 phoric acid occurs only in very minute quantity in soil 

 solutions, and when applied in a soluble form as manure it 

 is quickly precipitated upon the surface of the soil particles. 



Of the ordinary bases, the most diffusible is potassium ; 

 then follow, in a decreasing order, ammonium, sodium, calcium, 

 and magnesium. As potassium and ammonium are somewhat 

 readily removed from solution by a fertile soil, their dis- 

 tribution by diffusion cannot generally be long continued. 



Some idea of the comparative diffusibility of the acids and 

 bases just named will be gathered from the results of some 

 of Marignac's experiments (Ann. Chim. Phys. [5], 2, 579), 

 given in Table XXXI. In each of these experiments 

 equal weights of two salts were dissolved together in water 

 in the cases selected 2-5 grams of each salt in 100 of water 

 and the solution placed in an open, wide-mouthed bottle, 

 which it nearly filled, pure water being added till the upper 

 edge was reached. The bottle containing the mixed salt 

 solution stood in the middle of a much larger vessel, which 

 was subsequently carefully filled with water. At the end of 



1 A good general account of the phenomena of diffusion will be found in 

 Mr. Pattison Muir's translation of W. Ostwald's Solutions. 



