150 SOIL CONDITIONS AND PLANT GROWTH 



quantities of electrolytes. The former state is called a "sol," 

 the latter a- "gel". 



The effect of these properties has not yet been fully studied. 

 It will, therefore, be most convenient to deal with a few direc- 

 tions in which work has been done, and this plan will have 

 the double advantage of showing how the new ideas have 

 developed and how far reaching is likely to be their effect in 

 soil chemistry. 



Absorption by Soil. 1 



The facts of absorption by soil have long been known : 

 they were established, indeed, by the end of the sixties. 

 Soluble salts, such as ammonium or potassium sulphate, 

 which might be expected to wash out of the soil with rain, 

 do not, as a matter of fact, do so, but are kept back in such 

 form that the plant can get them. In becoming absorbed, 

 however, these substances displace something else, and con- 

 versely they can themselves be displaced by another salt. 

 Hence there arise a series of interchanges which profoundly 

 affect the nutrition of the plant. 



The first quantitative investigation was made by Thompson 

 (282) who showed that ammonium sulphate is decomposed 

 when dissolved and shaken with soil, ammonia being fixed 

 and calcium going into solution. The problem was taken up 

 by Way (298) who found that the quantities of ammonia ab- 

 sorbed and of calcium displaced were equivalent. Further 

 experiments by A. Voelcker (2890 and b} and others, showed 

 that the same action takes place in the soil itself when am- 

 monium sulphate is added in the ordinary way as manure, an 

 insoluble nitrogen compound being formed which remains in 

 the soil, while the calcium sulphate washes out in the drainage 

 water. Potassium sulphate reacts in the same way, the potas- 

 sium being precipitated and an equivalent amount of calcium 

 going into solution. Potassium phosphate undergoes a more 



1 The literature of this subject has been summarised by J. A. Prescott in 

 Journ. Ag. Sci., 1916, 8, 111-130. 



