290 Auxins in Agriculture 



Soil herbicides applied before the crop plant emerges depend to 

 a considerable extent upon positional selectivity. The herbicide is 

 applied to the surface of the soil where the bulk of the weed seeds 

 germinate, while the crop seed is at a lower depth. For this reason 

 herbicides which are readily leached are somewhat less dependable 

 than less mobile materials. If the chemical remains strictly on the 

 surface of the soil, large amounts may be lost by volatilization in the 

 heat of the sun, so some small movement into the soil is desirable. 



Soil herbicides may also be applied after the crop plant has 

 emerged. Most generally this is done by utilizing some device to keep 

 the herbicide from excessive contact with the crop plant. In some 

 cases the herbicide is sprayed onto the soil with nozzles held below 

 the foliage level of the crop plants, or pelleted formulations are some- 

 times used to avoid retention of the herbicide on the foliage. The 

 sodium 2,4-dichlorophenoxyethyl sulfate is advantageous for this pur- 

 pose, as there is no contact injury sustained by the foliage. 



FACTORS IN SUCCESS 



The use of auxin herbicides to remove weeds selectively often 

 represents something of a compromise between reduced competition 

 by weeds, and injurious effects of the sprays on the crop itself. This 

 has been pointed out particularly clearly by Woodford (1950). 



Various types of injuries to crop plants have been observed subse- 

 quent to removal of weeds by auxin herbicides. An interesting set of 

 observations by Andersen and Hermansen (1950) indicates that the 



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DATE OF SPRAY 



Fig. 119. Changes in the type and frequency of abnormalities in wheat with 

 various dates of spraying of 2,4-dichlorophenoxyacetic acid, 0.75 lb. /acre (Ander- 

 sen and Hermansen, 1950). 



