132 EFFECT OF HEAT ON SOIL BACTERIA 



10. Although strongly contested by many experimenters, there 

 are some workers who have produced strong evidence that the heat- 

 ing of a soil destroys toxins which have been formed in it due either 

 to bacterial activity or to plant growth. Fletcher grew plants in 

 a nutrient solution until he considered it to be made toxic by plant 

 excreta. He then steamed it at a pressure of 150 pounds for two 

 hours. Flakes of an organic substance were thrown down, which 

 did not dissolve on removal of pressure. This substance he con- 

 sidered to be the toxic excreta. 



The principal changes produced in and by sterilizing soil are thus 

 summarized by Johnson : 

 I. Destruction of Life: 



(a) Normal soil flora and fauna, desirable and undesir- 

 able forms of bacteria, fungi, protozoa, and higher 

 animals. 

 (6) Plant parasites, especially pathogenic bacteria, fungi, 



nematodes, and injurious soil-infection insects, 

 (c) Propagative organs of higher plants, especially weed 



seeds. 



II. Immediate Chemical Action (formation of toxic and bene- 

 ficial compounds) : 



(a) Decomposition of organic material resulting in the 

 formation of ammonia, carbon dioxid, and various 

 new and complex organic compounds. 

 (6) Decomposition of inorganic material, reduction of 

 nitrates and nitrites to ammonia and increased solu- 

 bility of potassium, phosphorus, and other salts. 



III. Biochemical Action : 



(a) Increased ammonification, particularly, and modified 

 nitrification, denitrification, and nitrogen-fixation. 



IV. Physical Action : 



(a) Absorption capacity of soil modified for water, gases 



and salts. 

 (6) Increased concentration of soil solution. 



(c) Modified capillarity, colloidal state, and mechanical 



condition. 



(d) Modified color and odor. 



V. Action on Organisms Growing in Sterilized Soils: 

 (a) Lower organisms. 



1. Increased development due to reduced competi- 



tion, increased food supply, destruction of 

 "bacterio-toxins," "stimulation," byproducts 

 added or formed, or other causes. 



2. Retardation in growth in rare cases due to inju- 



rious conditions produced. 



