198 Auxins in Agriculture 



with the use of different soils, adsorptive differences might mask true 

 leaching effects. 



Leaching characteristics can be visualized rather graphically from 

 results of experiments on the complete leaching of auxins from soils. 

 For example in figure 82 it can be seen that the amount of water 

 required to leach 2,4-D completely from three different soils varied 

 immensely with the soil type. Four inches of water completely removed 

 the toxic effects of the auxin from the surface of a sand soil; eight 

 inches of water were required to do the same for a silt loam, and six- 

 teen inches for a muck. Here again the distinction between adsorp- 

 tion and leaching is not clear, but differences in ability of the auxin 

 to move are very clear. The leaching characteristics of various auxins 

 and other growth regulators are quite different from one another and 

 it may be tentatively presumed that such differences are owing to the 

 adsorptive differences and the solubility differences between the auxins. 

 Thus naphthyl pthalamic acid is much less easily leached than 2,4-D 

 (Ogle, 1953) and consequently lends itself more readily to some pre- 

 emergence types of application. This is discussed in more detail in 

 chapter XVI. 



80 DAYS 



TIME OF PERFUSION 



Fig. 83. The time course of destruction of three auxins in soil. Solutions of 

 100 mg./l. of the auxins were continuously perfused through the soil and assayed 

 in the leachate by the cress root test (Audus, 1951). 



