. Auxins and plant growth. Auxin; Plant growth promoting substances. 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:ect
. Auxins and plant growth. Auxin; Plant growth promoting substances. 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 C/) {/) 100 U ^ o> .n o ^. o 80 «^. o c o lo 60 -C c 40 LU 20 O O 9 CvT Sand. \Muck _l Q 0 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).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Leopol
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