728 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL FERTILITY, vii.-xi., 



ments upon the comparative rate of evaporation of water, in soils, 

 bore this out. The trouble is in getting the treated soil thoroughly 

 moistened, for unless this is done, the evaporation, as compared 

 with that of an untreated soil, is slower or faster, according to 

 whether the soil is wetted from below or from above. A thoroughly 

 wetted and chloroformed soil lost slightly more during the first 

 three days, but afterwards kept pace with the untreated soil. 



viii. The Toxins of Soils. 



In former papers of this series, I have shown that there are, in 

 soils, substances which act as toxins towards bacteria. Their effect 

 is not so clearly shown in soils themselves as in extracts obtained 

 from them. The toxic extract, obtained by digesting soils with 

 water and filtering through porcelain, showed its activity by either 

 reducing the numbers of a sensitive bacterium, such as Bac. prodi- 

 giosus, or by retarding the speed of multiplication of the ordinary 

 soil-bacteria. The sensitive Bac. prodigiosus was used, because it 

 is typical of a class of soil-bacteria ; it can be readily counted when 

 grown upon plates, it can be evenly distributed in water, and it 

 grows well in fluid and solid media. 



The growth of bacteria, in soil-extracts, depends upon at least 

 two factors, the nutriment in the extract, and the toxins. The 

 former acts as an accelerant, increasing the number ; while the lat- 

 ter behaves as a depressant, either destroying or hindering growth. 

 In all extracts, these two play their respective parts, so that the 

 final result will depend upon the relative preponderance of the 

 one or the other. So far as is known, the nutrients are stable, and 

 their effect is, therefore, constant. The toxins, on the other hand, 

 are unstable, and according to the temperature, decay with greater 

 or less rapidly. They are destroyed by heat and by sunlight. 



As further information regarding the behaviour of the toxins 

 should be interesting, and possibly useful, it was decided to deter- 

 mine the effect of extracting the soil with water under varying con- 

 ditions; and, if possible, of devising a method for measuring the 

 toxicity of soils. 



