190 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL-FERTILITY, xvi. 



The point to be noted is the change in the reaction of the ex- 

 tracts. It changed from being comparatively strongly acid on 

 the third day, to strongly alkaline on the fifty-third. When the 

 extract of the latter was treated with lactic acid to neutralise 

 the apparent alkalinity, the number obtained was 007 as against 

 100 for the non-neutralised extract; that is, for every seven 

 bacteria in the neutralised, there were 14,000 in the alkaline 

 extract. The acid had clearly been added in excess, tlie —OS' 

 being far from a true indication. 



This confirms the result obtained in another place, that the 

 reaction of a soil-extract, as judged bj^ methyl-orange, is not the 

 true reaction. Mobile amoebae were seen in the soil on the fifty- 

 third day. 



Conclusion. — It has been shown that certain soil-bacteria, 

 moulds, and araoebse, all reasonably supposed to be capable of 

 furnishing substances of a toxic nature, were grown in various 

 media and under varying conditions; and, in all cases, the signs 

 of toxicity which became manifest could be attributed to an 

 alteration in the reaction of the media. 



The test-organism, Bac. prodigiosus, grows best in a neutral 

 medium, and an indicator is required which will indicate strict 

 neutrality. The methyl-orange numbers are too high, and the 

 phenolphthalein too low. Small divergences from the neutral 

 point strongly affect the growth. 



The humus of leaf-mould contains two types of humic acid; 

 one absorbs alkali from alkaline carbonates, and the other from 

 alkaline carbonates and hydrates. These were present to the 

 extent of one part of the former to three of the latter. Heating 

 the humus increases the amount of acid, and the increase is 

 largely soluble in water. 



The effect of reaction is quite of a different order from the 

 evidence of toxic action obtained in former researches. 



I am indebted to Mr. W. W. L'Estrange for kind assistance 

 during the later stages of the work. 



