IMPORTANCE AND PROPERTIES OF THE SOIL 167 



the absorbed substances. Water containing carbonic acid is a more active 

 solvent than pure water, while dilute hydrochloric or nitric acid are still 

 more energetic in their action and may remove from a soil the whole of the 

 substances it has absorbed. Similarly certain saline solutions, by inducing 

 double decompositions, render particular constituents of the soil soluble. The 

 water in the ground is always a very dilute saline solution, as is evidenced 

 by the composition of drainage water, and its percentage of dissolved salts 

 is increased by the aid of carbonic acid gas derived from respiration and 

 from organic decompositions in the soil. In this solution, not merely 

 are substances present which have been absorbed, but also others derived 

 from the mineral soil particles. As the latter oxidize or decompose, aided 

 by the action of carbonic acid, or saline solutions, various substances are 

 rendered available for the plant's use, either passing directly into solution, 

 or being first absorbed and retained by the soil. The same takes place with 

 those ash constituents set free by the decomposition of organic remains. 

 The slow but unceasing decomposition of humus is of great importance, 

 for with the aid of micro-organisms traces of nitrates are continually 

 produced, so that a supply of this important nitrogenous compound is 

 always available, and at the same time the loss of the valuable nitrates 

 by drainage from the soil is reduced to a minimum. 



The water from a fertile soil always contains the fubstances which 

 the plant requires in the form of a dilute solution, and when a small 

 quantity of any one of these is absorbed, a corresponding amount 

 passes from the absorbed into the freely soluble condition. It is not 

 merely the water and portions of soil, with which the roots are in direct 

 contact, that are of service to the plant, for the disturbance of equilibrium 

 caused by the absorption of any substance induces diffusion towards the 

 absorbing organ, while the currents of water flowing to the roots convey 

 fresh supplies of saline substances. Hence, widely separated areas may 

 be utilized by the plant, provided the character of the soil and the per- 

 centage of water present interpose no obstacles. Terrestrial plants are 

 therefore supplied with soluble nutriment in much the same manner 

 as are aquatic plants by the very dilute solutions in which they grow. 

 As the result of the changes which inorganic nutritive materials may 

 undergo in connexion with absorption, the latter may always be presented 

 to the plant in the same form, whatever salts are added to the soil. 



Absorbed substances are usually distributed only very slowly by 

 currents of water in the earth, so that some time elapses before the localized 

 differences due to absorption are equalized in a normal garden-soil. Hence 

 it is important that the branching root-system shall grow towards the 

 unexhausted areas of soil, and come into close contact with fresh particles 

 of earth. When the root absorbs nutriment, the differences of equilibrium 

 thereby induced will be rapidly readjusted across short distances, with the 



