PLANT NUTRITION 261 



If a soil sample is treated with ainnionium acetate solution, a reac- 

 tion will take place in which NH4 i ions displace the previously ad- 

 sorbed cations. The quantity of ammonium ion remaining in the soil 

 after adequate washing may be determined and expressed as mil- 

 liequivalents per 100 g. of soil. (A milliequivalent of an ion will com- 

 bine with or replace 1 mg. of hydrogen.) The values so obtained are 

 taken as a measure of the exchange capacity of a soil. Exchange capac- 

 ity is another term for cation-adsorbing ability. 



The exchange capacity of a soil varies with its composition. For 

 example, the exchange capacity of clay is about 10 to 100 milliequiva- 

 lents per 100 g. The importance of humus in the base-exchange com- 

 plex is indicated by its high exchange capacity, 400 milliequivalents 

 per 100 g. 



Since the exchange of cations on soil colloids is called base exchange, 

 the exchange of anions has been called acid exchange. However, acid 

 exchange is not so well understood as base exchange. It is certain that 

 most anions are not held by soil colloids as tightly as cations, for they 

 are readily lost from the soil by moderate leaching. Nevertheless it is 

 believed that anion as well as cation exchange does take place between 

 the soil solution and soil colloids. 



Soil solution. The components of soil, both organic and inorganic, 

 appear to be difficultly soluble in water. However, soil compounds 

 are soluble to a limited extent in the moisture which surrounds soil 

 particles. The quantity of such dissolved material is always small at 

 any one time, and the concentration of the solution varies greatly with 

 the quantity of water present. 



If soluble materials present in the soil solution are removed by 

 plants or other agencies, the supply of plant nutrients is replenished 

 by the dissolving of more material from the soil. Ions present in the 

 soil solution are in equilibrium with ions adsorbed on the soil col- 

 loids. The maintenance of this ionic equilibrium by adsorption and 

 release of nutrient ions is one of the important functions of soil col- 

 loids. 



The soil solution of fertile soils contains a sufficient (juantity of 

 nutrients to supply the plants growing upon them. Excessive depletion 

 because of overcropping or leaching leads to lower crop yields, and 

 nutrients must be restored by the addition of manine or artificial ferti- 

 lizer or by allowing the soil to lie fallow. 



Absorption of plant nutrients. Our knowledge of the way in which 

 nutrients are absorbed from the soil solution by plants is still frag- 

 mentary. Normal osmosis will account for the passage of water into 

 root cells, for the soil solution is more dilute than the root cell solu- 



