300 NATURE AND PROPERTIES OF SOILS 



dioxide, organic or mineral acids and their salts or enzymes 

 — act on tlie colloidal and non-colloidal soil constituents ; and 

 (5) synergistic relationships in the soil solution or the cell- 

 walL 



The concentration and composition of the cell-sap deter- 

 mines not only the osmotic relationship but has much to do 

 with diffusion tendencies. The ability of the plant to obtain 

 water and nutrients is thus directly affected by such condi- 

 tions. The character of the cell-wall has of course an im- 

 portant influence on such phenomena. If the cell-wall is 

 easily penetrated, it may greatly facilitate the absorbing ca- 

 pacity of the plant. If it is slowly penetrated or exerts spe- 

 cial differential influences, it might have a great deal to do 

 with the differences observed between certain plants. The 

 character of the cell-wall has already been shown to be in- 

 volved in the development of certain residues in the soil. 



The rate at which materials are utilized within the plant 

 is also a factor. If ions or molecules are used rapidly and 

 thus removed from solution, the diffusion of similar ions and 

 molecules is hastened. Such activity would also influence 

 osmotic relationships to a marked extent. This has already 

 been discussed under differential diffusion. 



It is readily conceivable that exudates, insofar as they are 

 capable of directly affecting the solubility of nutrients, might 

 produce marked differences between plants as far as their 

 absorbing activities are concerned. A crop producing active 

 exudates of any kind should be able, other conditions being 

 equal, to grow to better advantage, especially on a soil in 

 which the necessary nutrients are somewhat unavailable. 



The absorption of electrolytes by plants seems to be influ- 

 enced by the presence of other nutrient ions. True ^ has 

 shown that X.* ions when accompanied by Ca*+ ions are readily 

 absorbed by the seedlings of certain plants. When the same 



^ True, B. H., The Function of Calcium in the Nutrition of Seedlings; 

 Jour. Amer. Soc, Agron., Vol. 13, No. 3, pp, 91-107, 1921. 



