"WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



83 



acterized. It is largely because of the activity of 

 these soil micro-organisms and other dissipation 

 mechanisms that most herbicide residues in the 

 soil are not showing a progressive buildup. 



Phytotoxic residues of herbicides may persist 

 longer in soils when the herbicides are applied in 

 combination than when each is used individually. 



The degree to which a herbicide is soluble in 

 water or is adsorbed on soil particles determines 

 leachability of the herbicide by rain or irrigation 

 water {127) . Obviously, the amount of water mov- 

 ing through the surface soil is an important deter- 

 minant in the amount of herbicide leached away. 



Klingman (70) has summarized research infor- 

 mation on persistence of herbicides in soils. For 

 some uses, 2,4-D is not completely satisfactory be- 

 cause it lasts only 1 to 4 weeks. On the other hand, 

 simazine applied as a soil sterilant may persist for 

 2 to 4 years or more. The evidence does indicate 

 that the adverse effects of persistence of herbi- 

 cides are those upon succeeding crops. 



Cooperative research by the U.S. Department of 

 Agriculture and the Arizona, California, and Mis- 

 sissippi Agricultural Experiment Stations was 

 carried out over several years on the presistence 

 of herbicides in cotton fields. Use of repeated an- 

 nual application of dichlobenil, trifluralin, di- 

 phenamid, DCPA, monuron, diuron, and linuron 

 on weeds in cotton caused no buildup of the chemi- 

 cals to levels harmful to cotton. By contrast, treat- 

 ment of the cotton fields with diuron, trifluralin, 

 linuron, prometryne, or dimethyl 2,3,5 ,6-tetra- 

 chloroterephthalate (DCPA) resulted in residue 

 accumulations that were toxic to succeeding crops 

 of barley, milo, oats, soybeans, and sugar beets. 

 This research confirmed the need for existing re- 

 strictions on the use of several herbicides. 



Potentially the most serious problems associated 

 with the use of herbicides are their (1) downward 

 movement in the soil to depths of 4 to 15 feet 

 where degradation mechanisms are relatively inef- 

 fective, and (2) movement from large watersheds 

 into drainage systems and our major water 

 resources. 



Inactivation and dissipation rates of herbicides 

 in soils vary widely. Cereals planted in the fall 

 after crops have received summer applications of 

 1 to 4 pounds per acre of such herbicides as atra- 

 zine, simazine, and diphenamid have been injured 

 by phytotoxic residues. Injury to soybeans, sugar 



beets, oats and forage grasses and legumes has 

 been encountered in some instances 10 to 12 months 

 after application of atrazine to corn. Tobacco, cot- 

 ton, peanuts, and soybeans have been injured by 

 fenac residues 1 to 2 years after application at 

 rates used for selective weed control in corn. 



Certain of the benzoic, phenylacetic, and pico- 

 linic acid herbicides persist in soils for several 

 years. These same chemicals, especially 2,3,6-TBA 

 and fenac may often move downward in soils 

 through volatilization and percolating water. In 

 the arid areas of the Great Plains, phytotoxic resi- 

 dues of the benzoic and phenylacetic acids have 

 been detected at depths of 4 to 12 feet in certain 

 soils. Single applications of these herbicides are 

 used to control deep-rooted perennial weeds such 

 as field bindweed (Convolvulus arvensis L.) at 

 rates of 4 to 40 pounds per acre. However, when 

 these herbicides move 4 to 12 feet downward in the 

 soil, they are not readily degraded and thus pose a 

 potential problem, if these uses should become 

 extensive. 



Although low levels of phytotoxic residues of 

 the substituted urea and s-triazine herbicides and 

 some of the phenylacetic and benzoic acids have 

 persisted from one season to the next, data from 

 many sources indicate that accumulation of exces- 

 sive residues has not occurred and is extremely un- 

 likely. Inherent phytotoxicity of herbicides pro- 

 vides a natural indicator for residues and serves 

 as a defense against their accumulation when used 

 for selective weed control in crops. 



There is little evidence of adverse effects of her- 

 bicidal residues on fruit trees and shrubs. Research 

 in Xew Jersey underway over 5 year? has revealed 

 no undesirable residual effects from annual appli- 

 cations of a number of different herbicides on 

 apple, peach, blueberry, and cranberry plant; 



Movement in the Environment. — The winds 

 carry mi^ts and dusts. And the wind does not dis- 

 criminate because of chemical specificity. Mists or 

 aerosols or dusts containing pesticides are carried 

 at the same rate as those that are uncontaminated. 



Water moving downstream is an effective ve- 

 hicle. The amount and physical character of sus- 

 pended material transported depends on velocity 

 of flow. Gravel and even sand settle out as tlow 

 tards. Colloidal material of clay and organic mat- 

 ter may even stay suspended in still water I 

 loidal materials can readily be I of 



