PLANT ROOT-SOIL INTERACTIONS 



679 



Figure 6. Diagram of a two- 

 phase experiment by the dou- 

 ble-column technique, with 

 columns arranged in series 

 (B). The upper column 

 contains FcoOg-coated sand 

 grains; the lower column (so- 

 lution phase) contains sand 

 grains only. A shows the ar- 

 rangement for pouring solu- 

 tion in the upper column to 

 prevent flooding of delicate 

 seedlings. 



B 



sand; this is designated as Treatment I in Table III. In this case any 

 uptake of iron by the plants had to come from contaminating iron in 

 the circulating solution or from the white sand grains themselves. 

 Therefore the upper and lower columns had to behave alike. 



In the experiments employing Fe-sand, the excess of iron uptake 

 over that in the check must come from the iron-coated sand grains in 

 the upper column. If solubility of the Fe203 is crucial, the plants in the 

 lower white-sand column should also benefit from it. To rule out the 

 remote possibility that roots in the upper column might deplete it of 

 all the dissolved iron and thus deprive the lower-column partner of its 

 share, in one treatment ( Treatment V ) the upper column had Fe-sand 

 but no plants; the lower column had white sands plus plants. 



Roots are known to contain iron-chelating organic acids, and these 

 have been linked to iron nutrition. Hutner et al. (1950) state: "It is 

 difficult to understand, however, how plants could exist on a neutral or 

 alkaline soil low in organic matter unless the roothairs excreted a 

 metal-solubilizing substance such as citric and malic acid." Presumably 

 the excreted organic acid would diffuse to the Fe-grains, chelate the 

 iron, and diffuse back to the root as Fe-chelate. Since the percolating 

 solution would carry a portion of the diffusing Fe-chelate to the lower 

 column, its plants should exhibit partial growth at least. 



