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SCIENCE PROGRESS 



rests on less trustworthy evidence, and has indeed been modified 

 in some of its details by Hiltner himself. 



The effect of heat on the productiveness of the soil was first 

 noticed by the early bacteriologists. It had at first been 

 assumed that heat simply sterilised the soil and produced no 

 other change, until Frank in 1888 showed that it increased the 

 soluble mineral and organic matter and also the productiveness. 

 Later work by Pfeffer and Franke and by Kriiger and Schneide- 

 wind showed that plants actually take more food from a heated 

 than from an unheated soil. Heat undoubtedly causes decom- 

 position of some of the soil constituents quite apart from its 

 effect on the soil flora. 



Experiments by Dr. F. V. Darbishire and the writer at Wye 

 showed that the rate of oxidation was considerably reduced 

 after the soil had been heated to 130 C, but was more than 

 doubled after it had been heated to ioo° C. ; it was also in- 

 creased by treatment with small quantities of volatile antiseptics. 

 It follows from what has already been stated that the bacterial 

 activity is increased and consequently the amount of decom- 

 position. The increased quantity of plant food thus formed is 

 shown by the amounts taken up by the plant : the following 

 is a typical series of results : 



Fig. 1 shows photographs of wheat plants grown in heated soil 

 and in soil treated with toluene. 



The process has been more fully traced out in experiments 

 made at Rothamsted in conjunction with Dr. H. B. Hutchinson. 

 The most striking change that sets in after partial sterilisation 

 is an accumulation of ammonia as shown in fig. 2. Nitrification 

 ceases, but the ammonia is much in excess of the sum of the 



