Application and Persistence 195 



malities occur on many more leaves than were present in the bud at 

 the time of treatment. It is estimated that ten leaves which were 

 differentiated after the 2,4-D spray were so affected, and estimating the 

 time of differentiation of each leaf at 2.5 days, one can calculate the 

 persistence of the auxin in cotton plants to have been up to 25 days. 

 When 2,4-D was applied in lanolin it was still extractable from the 

 foliage after 3 months, but of course the lanolin is a source of the 

 auxin continuing in time. 



The persistence of auxin effects in plants varies a great deal with 

 the auxin. For example, naphthaleneacetic acid is effective in prevent- 

 ing pre-harvest drop of apples for up to fourteen days, whereas 2,4-D 

 and 2,4,5-TP are effective for 30 to 50 days. In the citrus species these 

 latter auxins provide effective control of abscission for as long as seven 

 months! It is not clear whether the auxins themselves persist in the 

 plants for this length of time or whether the effects which they 

 generate carry on after the auxins are metabolized away. 



Persistence in Soils 



The persistence of auxin in soils is a subject of very widespread 

 interest, particularly with respect to soil or pre-emergence herbicidal 

 applications. Such persistence appears to be dependent upon three 

 major factors: auxin adsorption, leaching, and destruction by micro- 

 organisms. 



Adsorption. Since the time when auxins were first used as pre- 

 emergence herbicides applied to the soil, it has been recognized that 

 some of the auxin so applied may become adsorbed in the soil. The 

 adsorbing capacity can be observed as a reduced mobility of the auxin 

 in the soil solution. Sample data demonstrating this phenomenon can 

 be seen in figure 81 in which the distribution of 2,4-D in the soil 

 profile is shown after the application of two inches of water. The first 

 curve demonstrates the high mobility, and hence low adsorption, of 

 2,4-D in a fine sand soil. The second taken from a silt loam soil shows 

 a concentration of the auxin near the surface due to greater adsorp- 

 tion. And the last curve shows an even greater restriction of the auxin 

 to the surface in an organic muck soil. 



Adsorption or degree of immobility of auxin varies with soil type 

 and varies too with different auxins. Some observations have led to 

 the suggestion that adsorption is a function of the organic matter con- 

 tent of the soil (Hernandez and Warren, 1950). However, other 

 workers have observed less adsorption in high organic soils than in 

 some mineral soils (Arakeri and Dunham, 1950). It has been suggested 

 by Weaver (1947) that exchange capacity of the soil colloids was re- 



