27 



as to how they act or what they really indicate, it is impossible 

 to arrive at any satisfactory conclusion. 



The method hitherto used in this country was devised in 1913 

 by Drs. Hutchinson and MacLennan in these laboratories. It 

 has served a useful purpose, but it suffers from the drawback 

 that it is considerably affected by three soil factors, none of which 

 it accurately measures : the hydrogen ion concentration : the 

 " buffer action M : and the neutral salt action in the soil. These 

 are separated in the modern electrometric method used by Mr. 

 E. M. Crowther in the Physics Department. The older method, 

 however, has the merit of convenience, and it has now been 

 improved by the introduction of certain empirical corrections. 



Measurements at Rothamsted and at Woburn have shown 

 that the effects of soil acidity induced by long-continued and 

 excessive use of sulphate of ammonia are manifested as far down 

 as 3 or 4 feet in the soil, and are not confined to the surface 

 9 inches. 



Soil Chemistry, Physics and Microbiology. 



In the Chemical Department work has been done 

 under Mr. Page on the organic matter of the soil, which 

 plays so important a part in soil fertility. Mr. du Toit has 

 adduced evidence that humus is formed from lignin and 

 not from the carbohydrate materials, cellulose, etc., to which 

 its origin was formerly assigned. It is true that these substances 

 can be made in the laboratory to yield black products looking 

 like humus, but chemical examination shows that only the lignin 

 product closely resembles the substance actually present in the 

 soil. The problem is difficult and necessitates much further study, 

 but the information is needed in order to discover what are the 

 useful organic constituents of the soil. It is expected that this 

 work will find application to green manuring. 



Another important line of enquiry is in connection with the 

 bases in the soil. It is shown that many of the important soil 

 properties depend on the presence of a complex calcium com- 

 bination : indeed, of all elements in the soil, calcium is probably 

 the chief in agricultural significance. This calcium can be 

 replaced by hydrogen under conditions of high rainfall : the 

 soil then readily becomes acid. Alternatively it can be 

 replaced by sodium in dry regions where irrigation water con- 

 taining sodium salts is used (as not infrequently happens). The 

 sodium combination differs chemically and physically from the 

 normal calcium combination, and it is infertile when treated by 

 the normal agricultural methods : it might conceivably be fertile 

 if treated by methods specially suited to its properties. But its 

 gravest defect is that it is easily hydrolysed, forming sodium car- 

 bonate, a very serious plant poison. Or, again, the calcium may 

 be wholly or partially replaced by magnesium or potassium. 

 Each of these products behaves unlike the calcium product when 

 subjected to ordinary treatments and therefore is regarded as 

 infertile. This new knowledge will undoubtedly prove useful in 

 devising means of dealing with difficult soils : the Weald and 

 Lower Lias clays deserve study from this point of view. 



