CONCLUDING REMARKS 153 



The second field of research which should afford valuable 

 results in the control of deficiency diseases is the investigation of 

 the relationship of soil and climatic factors to the incidence of 

 these diseases. It is, for example, well known that the man- 

 ganese available in the soil for absorption by plants may be only 

 a fraction of the total manganese present, and that one of the 

 factors determining the degree of availability of this element is 

 the acidity (pR) of the soil, the availability in general de- 

 creasing with decreasing acidity (increasing pH). Hence liming 

 the soil, by bringing about an increase in the pR value of the soil, 

 may seriously affect the availability of manganese. This appears 

 to be particularly so when the soil contains much humus, 

 though the relations between pH, humus content and con- 

 centration of available manganese are at present far from being 

 completely understood. Liming may also adversely affect the 

 absorption of boron by plants, perhaps again because of its 

 effect in lowering the availability of this element, perhaps partly 

 because of the relations between calcium and boron. Again, it is 

 well established that the severity of deficiency diseases may vary 

 in different years, a variation which must be attributed to the 

 differences in climate experienced in different seasons; the 

 symptoms of boron deficiency, for example, are generally much 

 more severe in periods of drought than in wetter seasons. It is 

 to be presumed that the different water contents of the soil in 

 the different seasons must affect the availability of the trace 

 elements concerned. 



Nor must the micro-organisms of the soil be forgotten. These 

 may themselves play a part in determining the availability of 

 various elements by bringing about oxidations and reductions of 

 compounds of the different trace elements and so affecting their 

 solubility, or they may play a more direct part in the deficiency 

 diseases themselves as suggested by the work of Gerretsen and 

 Ark referred to in an earlier chapter. There is here a wide field 

 of investigation which at present has scarcely been entered. 



In the physiological field the outstanding fact is, of course, 

 that the trace elements as their designation implies are needed 

 in very minute amount. This does not make the solution of the 

 problems of their functions any easier. Because of the small 

 quantities of them involved in the organism it has been usual to 



