170 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



such as dyes and pigments, may be mechanically retained by the soil without under- 

 going any perceptible change, just as dyes are absorbed by charcoal or coagulated 

 egg albumin. According to various researches, including quite recent ones, there 

 can be no doubt that the alkalies and alkaline earths are fixed by the soil in the 

 form of silicates. When all the hydrosilicates present in the soil are destroyed by 

 boiling with hydrochloric acid, no further fixation of alkalies, &c. is possible, nor 

 can an alkali drive out an alkaline earth, whereas caustic alkalies and alkaline 

 carbonates are still absorbed, since they form insoluble compounds with the silica 

 or siliceous acid present in the soil. 



Although it is usually by the formation of silicates that the above bases are 

 fixed, nevertheless other absorptive processes take place in a very different manner. 

 Thus many dyes, tannic acid, and other bodies which do not form silicates, and do 

 not enter into combination with them, are retained by the soil (cf. Sect. 12); 

 hence it does not necessarily follow that the whole of the alkalies or alkaline earths 

 absorbed are held in the form of silicates. It is indeed possible that a formation of 

 insoluble phosphates may take place, and that alumina or ferric oxide when present 

 may fix a certain amount of ammonia. Moreover, the humus acids may aid in the 

 process of absorption by forming insoluble compounds with different saline and 

 other substances. 



For purposes of demonstration, an inverted cylindrical bell-jar, 250 mm. high, 

 having a basal opening through which a tube passes, may be employed. This is 

 filled with garden soil resting upon a layer of sand and cotton wool, and if a little 

 alizarin solution, reddened by the presence of a trace of ammonia, is added, it filters 

 through as an almost colourless fluid, whereas, through a similar depth of sand 

 and cotton wool, the solution soon passes unaltered owing to the feeble absorptive 

 power of the latter. 



The nutritive solution in the soil. The free water of the soil contains on the 

 average only trifling quantities of organic and inorganic substances, as is shown by 

 analyzing spring- or drain-water, which contains as a general rule from o-oi to 0-03 

 per cent, of solid substances. Exceptionally, as for example in saline regions, the 

 water of the soil reaches a high degree of concentration. Direct researches have 

 shown that only traces of the absorbed substances are dissolved by water, and 

 Peters found that to extract one part of potassium 28,000 to 36,600 parts of water 

 are necessary, while Bretschneider found that it required 51,612 parts of water to 

 dissolve one part of phosphoric acid 1 . It is of considerable importance in the 

 economy of the soil that traces only of substances which cannot be retained should 

 be present at a given time, for in this way the loss of saltpetre is largely avoided, 

 although in fertile soils it is continually being formed from ammonia and insoluble 

 nitrogenous compounds (Sect. 63). 



The nature of the soil and of the substances absorbed has naturally a marked 

 influence upon the concentration and composition of the water it contains, and by 

 means of saline solutions, or by the action of carbonic acid, the solution of certain 



' Peters, Versuchsst., iS6o. Bd. IT, p. 135 ; Bretschneider, Jahresb. d. Agriculturchemie, 1865, 

 p. 22. For further details on this and following points, see Sachsse, 1. c., p. 182 ; A. Mayer, 1. c. 



