734 CONTRIBUTIONS TO OUR KNOWLEDGE OP SOIL-FERTILITY, vii. -xi., 



In this and the preceding experiment, the bracketed numbers 

 are probably excessive. It is frequently found, in a series of tests, 

 that one is out of what appears to be the normal sequence. For 

 this reason, the last experiment was made in duplicate, so that a 

 discrepancy might be allowed for. 



It is seen that the toxicity of the soil, manifest in Experiments 

 ix., and x., has disappeared by the twelfth day of storage, but 

 although now nutritive, there is some confirmation of the toxicity 

 being most evident when about equal parts of soil and water are 

 taken. 



The return of the toxins to the soil, after dry weather, shows 

 that, though they may be disguised by the soil-nutrients, they 

 should be reckoned with, when considering the seasonal variation 

 of bacteria in soils. The seasonal variation has been noted by 

 many writers. Conn found that the numbers were high in Feb- 

 ruary, and, rising in the summer, fell again in the autumn. Hiltner 

 and Stormer showed that the bacteria did not tend to increase as 

 the temperature rose, the August counts being no higher than those 

 of February, and, in some cases, they were less. It is true that one 

 cannot trace a direct relationship between the rainfall and the 

 bacterial numbers of some investigations,* but the question may 

 have to be determined in tropical or subtropical countries, where 

 the rain falls during regular monthly or quarterly periods. The 

 removal of toxin, by drainage-waters, is another question that 

 deserves consideration. 



From the irregularity of the results obtained by diluting the soil- 

 extracts, it would appear that this method offers no means of deter- 

 mining the toxicity of soils. As the toxins are thermolabile, the 

 action of heat might prove more successful; and, accordingly, ex- 

 periments were made in this direction. Two years previously* it 

 had been found that, in a certain soil-extract, 1,000 bacteria be- 

 came reduced to 73 ; but when the extract had been raised to boil- 



* As, for example, Engberding, Centrl. Bakt. 2te., 23, 569. 

 t These Proceedings, 1911, 815. 



