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Herbicide Detection in Soils via Bioassays

Bioassays Have Strengths and Weaknesses

Dr. Michael Pfeiffer

Detection of herbicides in soils via a bioassay isa relatively inexpensive technique for screening soils fora wide range of herbicides. The presence or absence ofherbicides is based on whether indicator plants grown inthe soil show symptoms of herbicide injury. Bioassaysare designed to detect bio-available amounts ofherbicides in soil; that amount which is readily availablefor plant uptake. The bio-available amount and totalamount of herbicides in a specific soil often varyconsiderably. Total amounts of herbicides are generallydetermined by quantitative analysis usually involvingsolvent extraction of the herbicide followed byinstrumental analysis. Quantitative analysis mayindicate phytotoxic levels of herbicide are present in soilwhen in reality equilibria processes in the soil, soilsolution and plant severely limit the bio-availability ofthe herbicide.

Time needed to complete analyses via bioassaysis dependent on time of year, assays performed and thesoil type. Generally bioassays can be completed in 30-45 days. The minimum sample volume needed foranalysis is 750 cc (approx. 0.75 quart) of finish soil; soilminus rock and gravel. As indicated previously, thebioassay techniques are designed ONLY to assess thepresence or absence of herbicides. They are notdesigned to assess the presence or absence of any othertype of organic or inorganic contaminants or biohazards.

Although a bioassay can be a valuable tool fordetecting herbicides, there are limitations associated

with this technique. With the bioassay technique, theexact herbicide present in the soil often can not bedetermined. Herbicides in the same chemical classoften produce the same symptoms on test plants. Forexample, Simazine, Atrazine and Prometon which areall triazine type herbicides produce symptoms on testplants which are indistinguishable. Normally herbicidesdetected via bioassays can only be placed in generalchemical classes based on the symptoms exhibited bythe test plants. Bioassays are not well suited for analysisof coarse textured soils: soils with a high percentage ofsand. Herbicides can leach readily in these coarsetextured soils and thus are not likely to be detectedwhich results in a "false" negative assessment of thepresence of herbicides in that sample. Soil sampling caninfluence the reliability of certifying a particulargeographic location free of herbicides. In soil,herbicides are distributed three dimensionally. Thespacial location of herbicides in the soil profile can beaffected by numerous factors. The chemical character-istics of the individual herbicide, formulation, amountsapplied, date of application, rate of transformation, soiltexture and structure and amounts of rainfall orirrigation are some of the parameters which affectspacial distribution of herbicides in soil. It is oftendifficult to know where to take samples from and howmany samples to take to be assured of actually obtaininga sample of soil with herbicide in it assuming herbicidesare present. Assessing whether herbicides are present orabsent in specific geographic locations often is only asgood as the sampling technique employed.

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