248 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 almost 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 con- 

 siderable amounts of calcium bicarbonate, but rarely 

 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 zeolitic 

 double silicates and the calcium carbonate of the soil ; 

 and partly physical, dependent upon the extent and 

 nature of the surface offered by the soil particles (for 

 the surface of a solid possesses the power of con- 

 centrating molecules of any dissolved substance in 

 the layer of solution with which it is immediately in 

 contact). Adsorption, as this process is called, is a 



