EDAPHIC OR SOIL FACTORS: CHEMICAL 



179 



e.6 



6.8 



Cu Nutritive Salts and Plant Communities 



Nutritive substances are taken up by the roots of plants in the form 

 of ions. Very probably exchange reactions on root colloids are of 

 prime importance in this process. H ions may act in a regulatory 

 capacity, but they are of no food value for plants. Under the influence 

 of hydrolysis in combination with the car- ph^ 

 bon dioxide, CO2, and other (especially or- 

 ganic) acids, the slightly soluble mineral sf 

 substances in the soil are brought into solu- 

 tion. In the absorption of nutritive sub- 

 stances an antagonistic effect of the ions is ^.2- 

 often evident, in that certain ions minimize 

 or offset the effect of other ions. 



In the absorption of mineral nutrients the 

 plant shows a pronounced selective power. 

 In fact, the permeability of plant cells for 

 certain ions appears to be a property char- 

 acteristic of the species. The investigations 72- 

 of Pantanelli (1915), Lundegardh, and others ^^^ 

 show that sometimes the anions and some- 10 203040 so 60103090100% 

 times the cations are absorbed in greater Fig- 100.— The regulation 



. . . T-i 1 1- J. 1 1 of the pH of water by an 



quantity. For example, sugar beets absorb oedogonium grown in a mix- 



the cations of most chlorides more strongly ture of pure water and water 

 / -ii /-I i^i 1- i 1 \ saturated with CO 2. The 



(with CaClo, seven times more strongly) abcissae show the percentage 



than the anions (Stiles, 1923). Conversely, of co^-saturated water in the 



. , ^ . , J 1 • mixture and the curves the 



the Gramineae appear to prefer the anions ^suiting pH values (a) with 

 rather than the cations (Stoklasa, 1924). and (6)^ without the Aiga. 

 The capacity of selective absorption explains 



the earlier known fact that the plant itself is able to change the acidity 

 or alkalinity of its nutritive solution from pH 6.6 to 11.0 (Bode, 

 1925-1926). Ulehla (1923) gives very striking graphs of the pH 

 regulation of the water by an Oedogonium (Fig. 100). 



In culture experiments with the root fungus Rhizoctonia silvestris 

 upon nutritive media of various degrees of acidity the following pH 

 changes resulted under the influence of the fungus, according to Mehn 

 (1924, p. 45) : 



Before experiment pH 3.4 4.1 4.4 4.9 5.7 6.0 7.0 



After experiment pH 3.2 3.5 4.1 4.2 4.4 4.5 6.0 



The possibilities of ionic exchange and the maintenance of the 

 ionic equilibrium within the plant cell have been discussed in detail by 



