PRESIDENTIAL ADDRESS. 47 



however, cause an increased decomposition of the soil 

 nutrients, not only of the nitrogenous organic matter but 

 also of the minerals. 



The Action of Dry Heat. 



The effect of sterilisation by dry heat appears to have 

 been first seriously considered by Pickering 1 in 1910. Like 

 earlier observers he found that heating the soil at 100° 

 rendered them more fertile, but he also found that a higher 

 temperature caused them to become less fertile. The com- 

 paratively low temperature of 100° occasioned an increase 

 of the soluble fertilising matter, while a higher temperature 

 resulted in the additional formation of substances of an 

 actively toxic nature and which tended to arrest the growth 

 of plants. These disappear in time when the soil is kept 

 moist and aerated, the alteration being apparently due to 

 oxidation. The effect of the toxin towards the classes of 

 plants was variable. For example, tomatoes, spinach, 

 clover and other plants were injuriously affected, and the 

 higher the temperature the greater was the quantity of 

 toxin produced and the greater was the toxic effect. 

 Grasses, however, behaved quite the other way, the- higher 

 the temperature to which the soil had been subjected the 

 more luxuriant was the growth. 



The action of dry heat varies with the temperature 

 applied; a low heat 2 produces ammonia and amines, while 

 a higher (135° - 150°) causes an increase in all the water- 

 soluble substances and the acidity. Several definite 

 chemical substances of a toxic nature, such as guanine, 

 arginine, dihydroxystearic acid, etc., are formed. Jensen 3 

 found that heat bad no effect upon the mineral portions 

 of the soil. 



1 Journ. Agri. Sci., 3, 277. 



3 Schreiner and Lathrop, through Chem. Soc. Repts., 1913, 218. 



3 Jensen, Journ. Roy. Soc. New South Wales, 45 (1911), 169. 



