166 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



should remain turgescent in a soil from which no water can be squeezed 

 by even marked pressure. 



An important property of soil is its power of withdrawing many 

 inorganic and organic bodies from their solutions, so that liquid manure, 

 when filtered through a sufficiently thick layer of soil, passes away as 

 an almost colourless fluid, retaining only traces of certain inorganic 

 and organic substances which were previously present in abundance. 

 Numerous researches have been made with regard to the absorptive 

 powers of different kinds of soils, more especially for inorganic salts, and 

 it has been found that compounds of potassium, ammonium, sodium, 

 calcium, magnesium, phosphoric acid, and commonly silicic acid also, are 

 retained by an ordinary soil, while sulphuric, nitric, and hydrochloric acids 

 are absorbed either to a very slight extent, or not at all. The above men- 

 tioned alkalies and alkaline earths are retained by the soil when presented 

 to it in the form of oxides, and also when they reach the soil as more or less 

 soluble salts. In the last case, a double decomposition takes place in the 

 soil, so that when a sulphate, nitrate, or chloride of an alkali is added, 

 in general a corresponding salt of calcium, or else of another alkaline base, 

 passes away in solution, while alkaline phosphates may be directly retained 

 by the soil without any such change. The order of absorptive power for 

 the oxides and salts of alkalies and alkaline earth is with most soils as 

 follows : potassium, ammonium, magnesium, sodium, calcium. From this 

 it is easy to see that when a solution of common salt is added to an ordinary 

 soil, it will be for the most part a calcium salt that passes away in solution, 

 and hence usually only a trifling absorption of calcium salts is possible. 

 Nevertheless the result obtained depends largely upon the quality of the 

 soil, and also upon the nature and quantity of the salt added. Thus under 

 certain circumstances, magnesium may be replaced and driven out of the 

 soil by calcium. 



In addition to the above, many other substances, such 'as tannin, 

 dyes, &c., may be absorbed, while the energetic absorption of metallic 

 salts, of which humus is capable, enables it very largely to render these 

 innocuous. Gases also may undergo perceptible condensation in humus soil, 

 but it is obvious that in all cases absorption must cease when the satura- 

 tion point is reached. In ordinary farmed land, however, the maximal 

 accumulation of food substances hardly ever occurs, and the power of 

 absorption depends almost entirely on the amount of humus which soils 

 contain, for pure sandy and gravelly soils have only feeble absorptive 

 powers. 



From a physiological standpoint, it is of subordinate interest whether 

 absorption is due to chemical or physical fixation, but on the other hand 

 it is of the utmost importance that absorption is never permanent and 

 absolute, so that the repeated addition of water continually removes traces of 



