76 TEXTBOOK OF PLANT PHYSIOLOGY 



general, the higher the valence of the ion, the smaller the concen- 

 tration required for antagonistic effect. 



Choosing different concentrations of the various ions a combina- 

 tion may be obtained that will suit best the development of an 

 organism. Such an optimal combination has been termed a 

 balanced solution. For animals, as well as for marine algae, a 

 balanced mineral medium corresponds closely to sea water, which 

 approaches in its composition of salts to that of the blood and 

 lymph. This caused Loeb to say that we all carry in our bodies 

 a part of the ocean, where once animal life was produced. A 

 balanced medium for plants is represented by the solutions that are 

 used for water cultures. 



27. Mineral Nutrition of Plants. — In soil the mineral nutrition 

 is much more complicated than in an artificial culture, for the plant 

 encounters here a great variety of compounds of different elements, 

 continually reacting with each other. Only a small part of 

 the mineral salts found in the soil are present in water-soluble 

 form and directly available to the plant. The largest quantities 

 of the salts are either adsorbed by the colloidal particles, or else 

 remain in a solid state as water-insoluble minerals or organic 

 substances. 



It has been known for some time that the water solution ob- 

 tained from even the most fertile soils contains such minute quan- 

 tities of nutrient salts that it cannot produce a normal develop- 

 ment of plants. This can be proven easily by growing plants in a 

 solution leached from a layer of soil. From this it is clear that 

 plants must be able to feed upon adsorbed and insoluble mineral 

 substances. 



Adsorption is the mechanism whereby colloids and, in general, 

 all finely divided substances accumulate on their surfaces sub- 

 stances present in a solution. The forces in operation are those of 

 molecular attraction. Usually the phenomena of adsorption are 

 regarded as surface reactions, caused by changes in surface ten- 

 sion. The union between the adsorbing and the adsorbed sub- 

 stances should be clearly distinguished from pure chemical reac- 

 tions. Lately it has been proven that the phenomenon is more 

 complicated than has been represented heretofore. The general 

 idea may be accepted, however, that the adsorbed substance 

 adheres to the surface of the adsorbing substance as a result of a 

 decrease in the surface tension on the boundary between the solid 



