196 Auxins in Agriculture 



sponsible for adsorption. A correlation between auxin adsorption and 

 exchange capacity has been found for a diverse group of soils by Ogle 

 (1953). However, exchange capacity is measured as the attraction of a 

 colloid for cations, and auxins will probably be adsorbed as anions. 

 It would seem more likely that adsorption is proportional to the free 

 colloidal surface in a soil, which will be reflected in turn by the ex- 

 change capacity. Thus soils with large free colloidal surfaces will be 

 highly eff:ective in adsorbing auxins and will also have high exchange 

 capacities. It seems probable that auxin adsorption by soils is a 

 consequence of van der Waals' forces. These are forces which attract 

 molecules to each other, and they are a function of electric charges 

 on the molecules. Auxins are probably adsorbed as intact anions. 



Because of the capacity of colloidal materials to adsorb auxin, it 

 has been found possible to protect plants from auxins in the soil by 

 the application of colloidal materials such as activated carbon (Arle 

 et al, 1948). This method has been extended by Weaver (1948) who 

 found that activated carbon could be used to protect sensitive plants 

 against a large number of auxins by simple localized applications in 

 the soil. It appears that adsorbed auxins are still active as growth 



LU 20 



O 

 O 



9 



CvT 



Sand 



\Muck 



_l Q 



2 4 



SOIL DEPTH 



10 Inches 



Fig. 81. The distribution of 2,4-D at various depths of the profiles of three 

 different soils following the application of two inches of water. The 2,4-D is 

 quantitatively estimated by the inhibition of growth of crabgrass seedlings (Ogle, 

 1953). 



