120 Journal of A f/riculture, Victoria. [11 Feb., 1918 



Such treatment is sufficiently drastic to completely alter the bacterial 

 flora and the relations between the groups of bacteria ; some will be 

 destroyed, while others will be unaffected. Those gfroups which 

 ammonify organic nitrogen are little affected, while the nitrifying bac- 

 teria, according to the recent work of Gainey, are only temporarily dis- 

 abled. 



In demonsti'ating the presence of bacterial toxins in soils, I have 

 made use of aqueous extracts of soils which, after filtering through 

 porous porcelain have been seeded with known quantities of bacteria. 

 Generally, Bac. j^'f^dw^'^stts has been employed as a test organism. It 

 is more sensitive than mixtures of soil bacteria, and is easily grown, 

 detected and counted. Tests have shown that extracts which destroy 

 Bac. prudigiosus retard the growth of mixed soil-bacteria. We are 

 justified in considering that an extract which destroys Bac. ■prodigiosus 

 is also capable of destroying some of the soil-bacteria. 



The bacterial toxins are not always easily demonstrated, as they are 

 frequently overshadowed by the sodl-nutrients, but investigation has 

 pointed out some of the conditions under which they may. not be expected 

 to show a direct action in aoil-extra<ct&. For example, they are destroyed 

 by exposing the soil to the sun, by heating the soil, by storing the soil 

 in the air-dry condition; they decay rapidly in aqueovis solution, and are 

 destroyed upon boiling. They are soluble in water and are washed out 

 of the soil by rain. Direct evidence of their presence should not, there- 

 fore, be expected in arid soils, in soils during a drought, or in soils 

 after rain. Much of the so-called fertilizing effect of the sun may be 

 due to the destruction of the soil-toxins. Indirect evidence of their 

 presence is easily obtained by boiling the soil-extract, seeding it with 

 bacteria, and comparing the growth with that obtained in the unboiled 

 extract. A greatly increased growth of bacteria is usuallv obtained in 

 the boiled extract. A direct diminution is only obtained under certain 

 conditions. These have not been fully investigated, but enough has 

 been done to show that one of these depends upon the ratio of the soil 

 to the water used for extraction. Equal parts of soil and water — that is, 

 100 grams of soil and 100 c.c. of water — generally give the maximum 

 toxic effect. 



The toxic effect is not evident after rain, but becomes pronounced 

 after a few days of dry weather. Similarly, a soil which has been ex- 

 tracted with water, and found to be toxic, will, upon further extraction, 

 give a nutritive extract. If the same soil, after extraction, be incubated 

 at 22 degrees for some time, and then extracted with water, the extract 

 will be found to be toxic. Thus toxins are developed upon incubating a 

 nutritive soil. 



While the extracts, of soils show an enhanced nutritive effect after 

 boiling, those of the subsoil become more toxic. It appears, therefore, 

 that tliere are at least two kinds of toxins in soils — one, predominating 

 in the soil, is thermolabile ; the other, predominating in the subsoil, is 

 thermostable. 



The action of the volatile antiseptics upon soils is to so alter them 

 that, while before treatment they yielded extracts directly bacteriotoxic, 

 after Treatment the extracts became nutritive. Thus the partial steriliza- 

 tion of soils, whether by heat or by volatile antiseptics, causes them to 

 give extracts, in which there can develop a greater number of bacteria. 



