246 SECTIONAL ADDRESSES 



exchangeable base in combination with the soil colloids is lime. Other 

 bases, magnesia, potash and soda, are also present in smaller amount. 

 When a base is required to combine with any acid or to exchange with and 

 fix other bases these are the ones primarily drawn upon, and the exchange 

 with other bases or the combination with acids which takes place is primarily 

 the settling of an equilibrium between these bases and the acids present, 

 the electro-negative clay and humus colloids themselves acting as acids. 



I would like to suggest that some of our methods of soil analysis require 

 revision in the light of this new knowledge of exchangeable bases and the 

 constitution of the clay colloids of soil. Our old methods, or certain of 

 them, were based on the view that calcium carbonate was essential to the 

 soil and that it or the ' lime requirement,' which meant calcium carbonate 

 requirement, were among the more important things to determine in a soil 

 analysis. It seems more important nowadays to set up standard methods 

 of determining exchangeable bases and the requirement of the soil for these. 



Our knowledge of the chemistry of humus, in spite of the great amount 

 of work which has been done upon it in recent years by workers in many 

 countries, is still in a state of doubt and darkness, but in the last few years 

 we have learned a great deal of the chemical structure of clay. The 

 application of X-ray methods of analysis has shown that much clay 

 material exhibits a definite lattice structure, and that there are several 

 different minerals, showing at least two different types of lattice structure, 

 to be found in clays. Some light has also been thrown by this work on 

 the nature of the base exchange capacity of clay and on the great differences 

 in base exchange capacity which are found in different types of clay 

 substances. 



The X-ray method has supplied us with a very valuable new method of 

 attacking the problem of the structure of clay, and taken along with other 

 methods is clearing up many of the gaps in our knowledge of clay. There 

 is a great deal of work still to be done on this subject but it seems we are 

 now well on the road to success. I may point out that a valuable summary 

 of recent research on the structure of clay has been given by our Recorder, 

 Dr. E. M. Crowther, in The Annual Reports on the Progress of Applied 

 Chemistry for 1935. 



One cannot give so hopeful an account of the progress of our knowledge 

 of humus. We have not yet found any clear method of unravelling the 

 structure of humus and of showing what is the nature of the colloid mole- 

 cules which build up the main part of this very important soil constituent. 



Both the clay colloids and the humus colloids are acid substances 

 which, when uncombined with bases, render the soil acid, and require to be 

 combined with bases before they can be neutralised and produce a neutral 

 soil, while when fully saturated with strong bases they are alkaline in 

 reaction and can produce a soil of alkaline reaction. To the chemists of a 

 generation ago it would no doubt have appeared rather shocking to apply 

 to indefinite substances of large, undetermined and variable molecular 

 structure the name of acids, but the evidence cannot be otherwise explained, 

 and the recent X-ray work is supplying, in the case of clay at any rate, 

 further evidence of a complex molecular structure which justifies the views 

 which have gradually gained acceptance as to the constitution of these 



