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MISCELLANEOUS PUBLICATION NO. 10 6'5, U.S. DEPARTMENT OF AGRICULTURE 



cals such as common salt, iron sulfate, copper sul- 

 fate, and sodium arsenite were evaluated and 

 found to have promise for selective weed control 

 under selected conditions. 



Following this initial work, interest in use of 

 selective herbicides to control weeds did not thrive 

 in the United States until the forties. During the 

 period 1934-42, scientists at the Boyce Thompson 

 Institute reported the results of their research on 

 the responses of plants to synthetic growth regu- 

 lators. In 1942, they were the first to report (138) 

 that 2,4-dichlorophenoxyacetic acid had properties 

 of a growth regulator when applied to plants in 

 very dilute concentrations. Two years later, a 

 report from research in the U.S. Department of 

 Agriculture (76) indicated that an application of 

 2,4-D killed dandelion, plantain, and other weeds 

 in a bluegrass lawn. 



From this point, research on a wide array of 

 organic chemicals for use as possible herbicides 

 boomed. Commercial use of 2,4-D and related 

 chemicals expanded rapidly during the sixties. 



Persistence in the Environment. — In the sum- 

 mer of 1959 a tolerance of zero was established 

 for the herbicide amino triazole. Research had 

 shown that this compound might be capable of pro- 

 ducing cancer in test rats when it was fed at high 

 levels. Under the Delaney amendment to the Fed- 

 eral Food, Drug, and Cosmetic Act a tolerance of 

 zero was mandatory. 



The registration and recommendations for the 

 use of amino triazole for weed control in cranber- 

 ries specified that the chemical be applied only 

 after the berries had been harvested. Some grow- 

 ers did not follow these recommendations. 



On November 9, 1959, the seizure of lots of cran- 

 berries containing illegal residues of amino tria- 

 zole was announced. Large blocks of cranberries 

 were seized, and the market for cranberries was 

 demoralized within 24 hours. 



Cranberry growers vigorously denounced the 

 manner in which the seizure action was taken. Re- 

 gardless of the pros and cons of the situation, the 

 public was alerted to the problem of the use of 

 pesticides in the production of foods. 



U.S. Department of Agriculture and the State 

 Agricultural experiment stations are continually 

 carrying on studies on how different herbicides 

 might contaminate soil, water, and crops. This re- 

 search begins well before the first experimental 



label is granted and usually continues as long as 

 the herbicide is used. 



Scientists are concerned with several problems 

 involved in the persistence of herbicides in differ- 

 ent kinds of soil : Nature and capacity of the ad- 

 sorption complex, photodecomposition, chemical 

 reactivity, microbial degradation, vapor transfer, 

 and leachability. 



Adsorption of a herbicide by a soil (30) is de- 

 termined by the specific surface of the soil, organic 

 matter content, the nature of the clay mineral, 

 moisture status, temperature, nature and degree of 

 base saturation, and structure and polarity of the 

 herbicide molecule. Specific surface of soils may 

 vary from 300 square centimeters per gram for a 

 sandy soil to over 30,000 square centimeters per 

 gram for clay. Adsorption of organic molecules 

 such as herbicides is also determined by the nature 

 of the adsorbing surface. For example, montmo- 

 rillonite clay, kaolin clay, and colloidal organic 

 matter differ in their adsorbing specificity. There 

 is wide diversity in the extent to which the adsorp- 

 tion mechanism of a soil affects persistence and de- 

 activation of the many different herbicides. 



Amino triazole forms relatively stable com- 

 plexes with metal ions in soils (109). Formation 

 of the complex detoxifies the herbicide. 



Hydrolysis and oxidation of herbicides in differ- 

 ent soils under varying conditions have received a 

 modest level of research attention (50). The wide 

 diversity of conditions involved in this research 

 has not opened up a path to sweeping generaliza- 

 tions (U,78). 



Probably one of the most important factors re- 

 sponsible for the detoxification of herbicides in 

 soils is the soil micro-organisms. Their importance 

 in the detoxification process is readily apparent 

 when one compares the persistence of a herbicide in 

 sterile and nonsterile soils (128) . With few excep- 

 tions, herbicides in the latter are detoxified far 

 more rapidly than in the former. Soil bacteria, 

 fungi, and actinomycetes have been implicated as 

 the most important agents in the degradation proc- 

 ess. Soil micro-organisms either possess constituent 

 enzymes or can be induced to form enzymes to 

 catalyze the breakdown of many diverse organic 

 herbicides. Many of these micro-organisms have 

 been isolated and identified, and in a few cases, the 

 enzyme responsible for carrying out the initial de- 

 toxification reactions have been isolated and char- 



