soil, rainfall and irrigation supply water to the soil; and 

 water is essential for the occurrence of many of the processes 

 which promote the dissipation of herbicides from soils. Air 

 movements influence vaporization and, therefore, loss of her- 

 bicides from soils." 



Dr. Sheets, et. al. arbitrarily classified herbicides into four 

 groups based on the time usually required for inactivation in soils: 



(a) to 3 weeks (c) 3 to 12 months 



(b) 3 to 12 weeks (d) More than 12 months 



These are arbitrary classifications, since many factors influence 

 the decomposition of herbicides and therefore a herbicide may fit more 

 than one group . 



Herbicides that are usually inactivated within 3 weeks, which are 

 of interest to fruit growers, are amitrole and dalapon. Both of these 

 materials are labeled for use in apple orchards. 



Simazine and diuron, which are in frequent use in our orchards, 

 generally are inactivated between 3 and 12 months. Under some environ- 

 mental conditions, diuron and simazine have persisted more than a year, 

 however. 



"When organic herbicides applied at recommended rates for selec- 

 tive weed control in crops have persisted for 1 year, the con- 

 centration in the soil has been very low at the end of the 

 year." 



*************** 



