160 CONTRIBUTIONS TO OU K KNOWLEDGE OF SOIL-FERTILITY, xvi., 



into a nutritive condition, not only in the raw extract, but also 

 in the 80% dilution. The numbers obtained in the extracts of 

 the control and of the infected leaf-moulds are of the same order, 

 and indicate that the toxic property is inherent in the sterilised 

 medium. The suspicion is strengthened, that the toxins are 

 really acids developed during the sterilisation of the organic 

 matter. It is evident that the treatment of the leaf-mould with 

 lime-water or with ammonia was not suflacient to neutralise the 

 excessive acidity of the humus, for the numbers show a position 

 intermediate between the acid and the fully neutralised extracts. 



In a new set of tests, the sifted mould was treated with water 

 to eliminate the sand with which it was mixed. It was then 

 dried, and again sifted through a finer sieve to remove the bulk 

 of the fine, light fibres. It contained moisture 10-2%, and ash 

 43-8%, leaving 46% for the organic matter. Portions containing 

 10 grams of organic matter were put into 4-ounce bottles, and 

 sterilised at 130° for two hours. During the sterilisation, an 

 odour of burnt sugar was noticeable. 



A portion of the sterilised leaf-mould was tested against a 

 portion of the unsterilised for acidity. Each was shaken 300 

 times with 500 c.c of water, and filtered. This was repeated a 

 third time. The acidity of the filtrates was tested with N/lOO 

 soda, using phenolphthalein as an indicator. The figures repre- 

 sent the number of c.c. required to neutralise the 500 c.c. of the 

 extract. 



The curves of these numbers were plotted, and it was seen that 

 the curve of the heated mould would meet that of the unheated 

 mould at the fifth 500 c.c, and that, at the fourth, there would 



