162 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL-PERTILITY, xvi., 



maining portions of the tests were discarded until further work 

 had been done. 



The addition of the bicarbonate of soda to the portions of leaf- 

 mould did not neutralise the soluble acidity, for when the raw 

 extracts were tested, it was found that the T.P.2 test had an 

 acidit}^ equal to + 62° in the extract. The control had +0-63'', 

 and lihizobium +0-35". It is, perhaps, to be expected that, in 

 a substance like rotted leaf-mould, there will be a mixture of 

 humic acids, some soluble in water, some partially soluble, and 

 some insoluble in water. The humates are so complex that it 

 was a mistake to consider that even all the water-soluble acid 

 would have been obtained by water-extraction, unless the time- 

 factor had been taken into account, and days instead of hours 

 been occupied in the extraction. 



The necessity for having a neutral vegetable-mould was 

 emphasised, especially in the last two tests, and some experiments 

 were made with the object of getting a better idea of the true 

 acidity. 



The Acidity of heated Leaf-mould. 



A number of portions of the leaf-mould, each weighing four 

 grams, were bottled, and some were heated for two hours at 130°. 

 A sterile and an untreated portion were repeatedly shaken up 

 with a standard alkaline solution at intervals during the time 

 of contact, and filtered. The filtrates were tested for residual 

 alkalinity or, when neutral liquids were used, for acidity. 'J'he 

 numbers represent cubic centimetres of normal acid in, or derived 

 from, 100 grams of the dry, organic matter of leaf-mould. 



Calcium bicarbonate. — The acidity was first tested by the 

 method of Hutchinson and MacLennan,* which consists in 

 having the portions in contact with a solution of bicarbonate of 

 calcium for a certain time, and subsequently determining the 

 amount of lime that had been removed from solution 



Heated 93-3 



Control 91-7 



* Journ. Agric. Sci., vii., 75. 



