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tile they are. We will speak of absorption and decompos- 

 able silicates more at length in the Part devoted to Agricul- 

 tural Chemistry. By absorbent co-eficient is meant the num- 

 ber of cubic centimetres of Nitrogen absorbed in the form of 

 Ammonia from a solution of NH 4 C1 by 100 grammes of soil. 

 II 5- (5) Speaking generally, the greater the proportion of 

 a soil which is dissolved by dilute acids the more fertile it is. 

 The amount of soil substances soluble in water usually varies 

 from '2 to -5%. But solubility in pure water is not a guide 

 to the solubility of plant food actually undergoing in the 

 soil. Some chemists, e.g. Hermann von Liebig, assumed that 

 dilute acetic acid dissolved all those substances available 

 to plants ; but the acid secretions from rootlets are of a 

 complex nature and no absolute guide as to the dilution to be 

 used is possible. Professor Stutzer of Bonn was the first to 

 use i per cent, solution of Citric acid for ascertaining the 

 amount of available phosphoric acid in manures, and Dr. 

 Dyer of London has carried out this method in dealing 

 with soils, and arrived at very important practical results, 

 in determining the proportions of available phosphoric acid 

 and potash in soils. But the method gives no clue to the 

 amount of available nitrogen in soils ; and after all the ques- 

 tion of fertility is mainly concerned with the amount of avail- 

 able Nitrogen present in the soil. Besides, acid secretions from 

 all rootlets are not all equivalent to a one per cent, solution 

 of Citric acid. Some secretions are more acid than others, 

 and some plants therefore are better able to utilize the latent 

 fertility of soils than others. The average acidity of root- 

 secretions in terms of Citric acid, shown by hundreds of 

 plants examined by Dr. Dyer, is not i but about O'86 per cent. 

 Coming to individual plants he found the variation was very 

 great. Strawberry showed about 2% and a geum, (another 

 plant of the order Rosaceae) as much as 5*53%; while the 

 examination of Solanaceae and Liliacese gave very low results 

 about 0-36%. Cruciferae and Leguminosse averaged about 



