212 POWER OF THE SOIL TO ABSORB SALTS [chap. 



tained and the acid draining through. Way found 

 that liquid manure from a dung-heap, which contains 

 both organic and ammoniacal compounds of nitrogen, 

 potash, and a little phosphoric acid, when filtered through 

 a short column of soil, parted with almost the whole of 

 its organic matter and much of its salts to the soil; 

 compounds of calcium were, however, more abundant in 

 the filtered liquid than before. Way's observations 

 were extended by Voelcker, who compared the absorb- 

 ing powers of different types of soils, and so obtained an 

 idea of the method by which the absorption of each 

 substance was effected ; and later researches have only 

 served to confirm the results then obtained. It was 

 found that all the organic compounds of nitrogen, 

 ammonia — either free or in combination — phosphoric 

 acid, and potash were wholly removed from solution 

 by ordinary soil, though some soils were more effective 

 than others ; whereas nitrates, sulphates, chlorides, and, 

 among bases, sodium and calcium, were only slightly, 

 if at all, retained. These results are confirmed by the 

 analysis of the water which flows from land drains 

 under normal conditions; this will generally be found 

 to contain nitrates (sometimes in fair quantity), sulphates 

 and chlorides of calcium and sodium, and considerable 

 amounts of calcium bicarbonate, but rarely shows more 

 than a trace of ammonia, phosphoric acid, or potash. 

 The absorptive action of the soil is partly a chemical 

 process, due to interactions with the humus, the zeolitic 

 double silicates, and the calcium carbonate of the soil ; 

 and partly physical, dependent upon the extent of 

 surface offered by the soil particles (for the surface of a 

 solid possesses the power of concentrating molecules of 

 any dissolved substance in the layer of solution with 

 which it is immediately in contact). The mechanism of 

 this physical " adsorption " is but imperfectly understood 



