M— AGRICULTURE 245 



showed that the ammonium of ammonium sulphate, or the potassium 

 of potassium sulphate, was retained by the soil while an equivalent amount 

 of calcium went into solution and could be washed away as sulphate. He 

 also showed that this power resided in the finest mineral part of the soil, 

 the clay, and he regarded the action as an ordinary case of double de- 

 composition between clay and the soluble, neutral salt in solution. Though 

 there was much discussion about these phenomena, which were regarded 

 as of the greatest practical importance because they showed that valuable 

 manurial bases when applied in a soluble form could be absorbed and 

 retained in the soil, and though soil investigators of last century were 

 divided into two camps, one regarding this fixation of bases as a chemical 

 precipitation by double decomposition and the other looking upon it as a 

 physical process of absorption, little further advance was made till the 

 present century. By that time considerable advance had been made in 

 our knowledge of colloid chemistry and we also knew that there were two 

 types of colloid complexes found in soils, one mineral and the other organic. 

 The mineral colloid material, sometimes known as the alumino-silicic 

 complex, is found in the clay fraction of the soil, while the organic colloid, 

 known as the humus complex, is found in the decomposed vegetable 

 matter or humus matter of the soil. The soil and these colloid consti- 

 tuents of the soil were studied by the methods of colloid chemistry late 

 in last century and early in this one by the Dutchman, van Bemmelen, 

 and later by his fellow countryman, Dr. D. J. Hissink, by the late Professor 

 G. Wiegner of Zurich, whose recent death at the height of his powers we 

 all deplore, and by the famous Russian worker, the late K. K. Gedroiz, 

 whose very valuable work in this subject became generally known only 

 after the great war. 



We now know that this process of base exchange is a colloid phenomenon, 

 and follows the laws of colloid chemistry. It is not confined, as Way 

 supposed, to the fine mineral matter of the soil but is a property of the 

 organic colloids also. The old controversy as to whether this is a chemical 

 or a physical phenomenon is thus cleared up and both sides are shown to 

 be right or both wrong, according to your taste, for both sides knew nothing 

 of that border-line field of colloid phenomena where Physics and Chemistry 

 blend, and, in the best modern manner, tend to become indistinguishable. 

 We are now on firmer ground than we were a few years ago as to the 

 nature and properties of soil colloids, both mineral and organic, and the 

 new knowledge has shed fresh light on certain matters of great practical as 

 well as of great scientific importance, which were wrapped in gloom at the 

 beginning of this century. We can now not only estimate with a high 

 degree of accuracy the degree of intensity of soil acidity, or alkalinity, 

 as well as the amount of such acidity or alkalinity, but we have also a sound 

 theoretical picture of the nature of that acidity or alkalinity. 



It was a common statement in our text- books till quite recent years that 

 a supply of calcium carbonate was necessary in a healthy soil. It was 

 useless to point out that there are fertile soils in which no recognisable 

 amount of calcium carbonate can be found. It had to be there. We now 

 know that the part which was ascribed to calcium carbonate is played by 

 the exchangeable bases of the soil, and that in our fertile soils the principal 



