STUDIES OF THE RESPONSE OF PLANTS TO ROOT-APPLIED HERBICIDES

Summary. Applications of several herbicides -were made to roots and to the bases of shoots of peas, cucumber, mustard and barley grown in soil, sand or water culture.
Localized applications (variation horizontal) of atratone and MCPA to roots of peas and barley in soil produced effects similar to those observed in water euluire, described in Pan I. Airatone killed the plants whether available to the whole or to only a portion of the root system whereas MCPA affected only the roots with which it was in direct contact, and growth continued when a portion of the root system was in herbicide-free environment.
In water culture, MCPA was more effective when applied to the lower (younger) roots with the upper (older) roots kept dry than when twice the concentration was applied evenly to the whole root system in water. When all the roots were kept wet the effect of application to the upper roots was greater than the effect of application to the lower roots. The response of plants to atratone was not appreciably altered whether applications were made to the upper or lower parts of the root system in water culture. Variations in water level had little effect.
Even when the herbicide solution was confined to the stem or hypocotyl, atratone and DNOC were little, if any, less effective than when applied to roots. MCPA, both as ester and sodium salt, was significantly less effective.
Partial replacement of solution in the root zone by sand and air did not reduce the activity of atratone at a given concentration. Similar replacement in the zone of the stem or hypocotyl greatly reduced the effectiveness of all herbicides. When sand of low water content was used, atratone and MCPA-sodium became quite ineffective via the stem, but DNOC and MCPA-ethyl ester remained active.
Études sur les réactions de certaines plantes á des herbicides appliqués aux racines II. Observations nouvelles sur l'effet de l'application localisée.     

THE EFFECTS OF SUB-LETHAL DOSES OF MCPA ON THE MORPHOLOGY AND YIELD OF VEGETABLE CROPS

Summary. The effects of 2,4-D, 2,4,5-T, mecoprop and 2,3,6-TBA were compared with those of MCPA on the morphology and yield of lettuce, cabbage, cauliflower, Brussels sprout, carrot, parsnip, French and broad bean, red beet, spinach, turnip and onion. Although the relativie phytotoxicity of MCPA, 2,4-D, 2,4,5-T and mecoprop varied according to the crop, the morphological abnormalities produced by the different compounds were essentially similar, except that in lettuce and cabbage sperilic foliar symptoms appeared to be associated with 2,4-D. 2,3,6-TBA caused less epinasty and usually fewer root and stem abnormalities, but in some crops, notably French and broad bean, red beet and spinach, it produced severe injury symptoms which were different from those produced by the other compounds.
Effets de doses sub-léthales de MCPA. sur la morphologie et le rendement des cultures légumirres VI. Comparaison avec le 2,4-D, le 2,4,5-T, le micoprop et le 2,3,6-TBA     

THE EFFECTS OF MCPA AND 2,3,6-TBA ON GLASSHOUSE CUCUMBERS

Summary. The effect of sub-lethal doses of MCPA-sodium and 2,3,6-TBA-sodium on young cucumber plants was studied in three replicated experiments. The materials were applied either to the leaves of plants in pots or as a soil drench. Epinastic and formative effects were observed. Formative effects included leaf inrolling and blistering, stem and petiole proliferation, and root modification. The symptom pattern resulting from application of each material was described.
Les effets du MCPA et du 2,3,6-TBA sur Us concombres cultivés en serre     

SITE OF UPTAKE OF SOIL-APPLIED HERBICIDES*

Summary. We conducted studies to determine the effects on corn (Zea mays L, var. Indiana 654) and pea (Pisum sativum L. var. Alaska) of localizing various herbicides in the soil, using a double plastic pot technique which ensured separate exposure of the root and shoot zones of the plants to treated soil. Effects on corn and pea were similar in relation to site of uptake. 2,4-D-amine, naptalam, simazine, diuron and dalapon-sodium entered primarily through the roots. Some shoot entry and also severe inhibition of roots occurred in soil treated with 2,4-D and naptalam; these were noticed only to a slight extent with the other three herbicides. EPTC, chlorpropham and trifluraiin were most effective when applied to the shoot zone. Little effect on foliage growth was evident when the root zone alone was treated. However, roots in treated soil were severely inhibited by these three herbicides. Dinoseb displayed a contact type of action, injuring both shoots and roots. Treatment of both zones had an additive effect. Entry of chlorthal-methyl which was tested on a susceptible species, sorghum (Sorghum vulgare Pers.) was mainly through the shoot, with only a slight effect on top growth when roots alone were treated. Roots in treated soil were slightly inhibited. Localisation de l'absorption des herbicides appliqués sur le sol     

HERBICIDAL EFFECTS ON CERTAIN SOIL MICROBIAL ACTIVITIES IN SOME BROWN SOILS OF SASKATCHEWAN

Summary. Amitrole, 2,3,6-TBA, 2,4-DB and di-allate were applied at normal field rates to a loam soil in a preliminary study, The toxic effects, as measured by nitrification studies, persisted for 8 weeks after application. After a further 8 weeks this depressive effect was decreased.
Residual effects were assayed by measuring the nitrification rate of soils following the application of the same four herbicides at two rates to four different soils. The residual effects of amitrole, di-allate, 2,3,6-TBA and 2,4-DB were overcome 34, 28, 32 and 44 weeks respectively after treatment. Total microbial counts were also corelated with the nitrification rate of the soils. The trend of disappearance of the toxic effect of these chemicals was similar in all the soils in these laboratory investigations.
Under field conditions the residual effect of amitrole, 2,3,6-TBA and 2,4-DB had disappeared by the end of 52 weeks, as far as could be determined by studies of nitrate production, ammonification and total counts of microorganisms. A second application of the chemicals 56 weeks after the initial treatment affected nitrification in the laboratory for only half as long, indicating adaptation of' the microorganisms to the herbicides
Effels des herbicides sur certaines activités des microorganismes du sol     

CONTROL OF SONCHUS ARVENSIS (L.) WITH CHEMICALS

Summary. Pot experiments concerning the control of Sonchus arvensis with chemicals showed that the time of application of MCPA or 2,4-D has a dominant influence on the results obtained. Foliage applications at late rosette to early bud stage gave considerably better effects than did earlier or later applications as assessed by the. weight of roots, this assessment being considered the most reliable for final effects. Increasing the rates from 1 to 3 kg/ha was of less significance than the time of application. A good wetting ability of the spray solution is also essential for an effective control of this weed, which has very waxy leaves. The addition of a wetting agent to aqueous solutions of various salts of 2,4-D had a great and significant influence on the effects. This was not the case with 2,4-D acid in Carbowax. When wetter was added, there was no difference in the effects of the sodium, ammonium and triethanolamine salts. The latter were also slightly more effective than MCPA. 2,4-D-ester was much more effective than MCPA by foliage application. When the chemicals were applied to the roots through sub-irrigation pipes, however, MCPA was at least as effective as 2,4-D-ester. The maximum effect by this method of application was obtained at a somewhat later stage of development than by foliage application. By foliage application of MCPA and dinoseb or DNOC at various rates used alone or in mixture, an antagonism between the growth regulator and the contact herbicides was demonstrated as regards the reduction in root weight of S. arvensis . The scorching effects of dinitro compounds were, on the contrary, increased when applied in combination with MCPA. Amitrole at rates of 5, 7.5 and 10 kg/ha applied at late rosette stage gave a complete kill of S. arvensis.
La destruction du Sonchus arvensis L. à l'aide de produits chimiques     

The influence of some biologically active compounds on microbial activity and on the availability of plant nutrients in soils. I. simazine,ioxynil and 2,3,6-tba

The effect of the herbicides simazine (2)-chloro-4,6-bis (ethylamino)-s-triazine, ioxynil (4-hydroxy-3,5-diiodobenzonitrile) and 2,3,6-TBA(2, 3,6-trichlorobenzoic acid) on microbial activity and available plant nutrient content of an acid and an alkaline soil during an incubation period of six weeks under aerobic conditions has been studied in comparison with untreated soils. The herbicides were incorporated with the soils at normal application rates. At intervals determinations were made of carbon dioxide liberated, the content of ammonium and nitrate nitrogen and of available mineral plant nutrient elements. These quantities were not significantly influenced by the presence of 2,3,6-TBA :simazine stimulated microbial activity and ioxynil did so after an initial period of suppression. Both simazine and ioxynil retarded nitrification in the alkaline soil but increased it in the acid soil, attributed in part to stimulation of heterotrophic organisms. Biological oxidation of manganese was retarded by ioxynil but stimulated by simazine. Changes in the availability of some other plant nutrient elements were also found.     

Glasshouse trials with controlled drop application of some foliage-applied herbicides

MCPA, mecoprop, dichlorprop, dicamba, 2,3,6-TBA, bentazone, ioxynil/bromoxynil (a mixture), barban, difenzoquat and chlorfenprop-methyl were applied by spinning disc in controlled drop sizes from 150-350μm and at very low volume rates (5–45 1/ha), to the foliage of some dicotyledonous weed species or wild oats (Avena fatua L.). The same herbicides were also applied by means of conventional hydraulic nozzles at volume rates of about 200 1/ha. Most of these herbicides performed as well at very low volume rates as with the conventional application, the major exceptions being bentazone, ioxynil/bromoxynil (both of which have distinct contact effects) and dichlorprop. Comparisons between oil and water as the diluent at a very low volume rates suggest that there may be situations when oil is preferable and others when water is.     

STUDIES ON THE EFFECTS OF HERBICIDES ON SOIL NITRIFICATION*

Summary. A perfusion technique was used to examine effects of selected herbicides (concentration range 50–1000 ppm) on the rate of oxidation of ammonia to nitrate in garden loam soil. Effects on the nitrification rate of soil, previously enriched with nitrifying organisms, were used to estimate direct action on the processes of ammonia oxidation. The kinetics of nitrification in fresh soil were used to estimate effects on bacterial growth and to detect possible adaptations to herbicide toxicity. The herbicides used could be ranged in the following order of increasing effectiveness as inhibitors of ammonia oxidation: dichlobenil < paraquat < picloram < 2,3,6-TBA = chlorthiamid < bromoxynil < chlorflurazole < ioxynil < propanil. Endothal only stimulated ammonia oxidation, even up to 1000 ppm. Endothal, paraquat and 2,3,6-TBA had no apparent effects on bacterial proliferation; the inhibitory effects of the remaining herbicides were in the order: picloram < dichlobenil = bromoxynil < ioxynil < chlorthiamid = chlorflurazole = propanil. With the exception of paraquat and endothal all herbicides seemed to become less toxic during perfusion of fresh soil, i.e. over the enrichment period of 26–28 days. In the case of propanil and chlorthiamid this could have been due to the destruction of the herbicide by soil micro-organisms. With picloram and 2,3,6-TBA it is probable that there was an adaptive loss of sensitivity of the nitrifying organisms to the toxic action of the herbicides. The evidence available suggests that Nitrosomonas is more sensitive than Nitrobacter to herbicide action. Recherches sur les effets des herbicides sur la nitrification Résumé. Une technique de perfusion a été utilisée pour examiner les effets d'une série d'herbicides (á des concentrations de 50 á 1000 ppm) sur le taux d'oxydation de l'ammoniac en nitrate dans un sol de jardin limoneux. Les effets sur le taux de nitrification du sol, préaiablement enrichi avec des organismea nitrifiants, ont été utilisés pour évaluer l'action directe sur les processus d'oxydation de l'ammoniac. Les données cinétiques de la nitrification dans un sol frais ont été utilisées pour évaluer les effets sur la croissance des bactéries et pour déceler les adaptations possible h la toxicité de l'herbicide. Les herbicides utilisés purent être classés comme suit dans i'ordre croissant d'activité inhibitrice de I'oxydation de I'ammoniac: dichlobénil<paraquat<piclorame<2,3,6- TBA = chlorthiamide<bromoxynil<chlorflurazole<ioxynii<propanil. Scul l'endothal provoqua une stimulation de I'oxydation de I'ammoniac jusquà 1000 ppm. L'endothal, le paraquat et le 2,3,6-TBA ne présentèrent pas d'activité apparente sur la proiifération bactérienne; les effets inhibiteurs des autres herbicides furent, dans I'ordrc: piclorame < dichlobenil = bromoxynil<ioxynil<chlorthiamide = chlorflurazole = propanil. A l'exception du paraquat et de l'endothal, tous les herbicides parurent devenir moins toxiques pendant la perfusion sur un sol frais, c'est-à-dire au deli delà période d'enrichissemenl de 26 à 28 jours. Dans le cas du dichlobénil et du chlorthiamide, ceci pourrait avoir été provoqué par la destruction de l'herbicide par lea micro-organismes. Avec le piclorame et le 2,3,6-TBA, il est probable qu'il y eut une adaptation consistanten une perte dela sensibilité des organismes nitrifiants vis-à-vis de i'action toxique des herbicides. Les résultats obtenus suggèrent que les Nitrosomonas sont plus sensibles que les JVitrobacter kà l'action des herbicides. Untersuchungen zum Einfluss von Herbiziden auf die Nitrifikation des Bodens Zusammenfassung. Mit Hilfe einer Perfusionsmethode wurde der Einfluss von bestimmten Herbiziden (Konzentration zwischen 50 und IOO ppm) auf die Oxydationsrate von Ammonium zu Nitrat in Garteniehmboden untersucht. Der Einfluss auf die Nitrifikationsrate des Bodens, der zuvor mit nitrifizierenden Organismen angereichert worden war, wurde als Mass fur die direkte Wirkung der Herbizide auf die Prozesse der Ammoniumoxydation verwendet. Die Kinetik der Nltrifikation in frischem Boden wurde als Mass für die Beeinflussung des Bakterienwaehstums und zur Feststellung einer moglicherweise auftretenden Adaptation an die Herbizidtoxizität verwendet. Für untersuehte Herbizide ergab sich, geordnet nach zunehnnender Hemmung der Ammoniumoxydation, folgende Reihenfolge: Diehlobenil<Paraquat<Pieloram<2,3,6-TBA = Chlorthiamid<Bromoxynil<Chlorflurazole<Ioxynil<Propanil. Endothal stimulierte die Ammoniumoxydation selbst in Konzentration bis zu 1000 ppm. Endothal, Paraquat und 2,3,6-TBA hatten keinen augenscheinlich Efifekt auf die bakterielle Proliferation; fur die Hemmwirkung der ubrigen Herbizide ergab sich folgende Reihenfolge: Picloram < Dichlobenil = Bromoxynil < Ioxynil < Chlorthiamid = Chlorflurazole = Propanil. Mit Ausnahme von Paraquat und Endothal wurden alle Herbizide wahrend der Perfusion dureh frisehen Boden, d.h. wahrend einer Enrichmentperiode von 26–28 Tagen, of Tensichtlich weniger toxiseh. Im Falle von Propanil und Chlorthiamid kann dies auf den Abbau der Herbizide durch die Bodenmikroorganismen zurüekzuführen sein. Bei Piclorani und 2,3,6-TBA handelte es sich wahrscheinlich um einen adaptiven Verlust der Empfindlichkeit der nitrifizierenden Organismen gegenuber der toxisehen Wirkung der Herbizide. Die Ergebnisse lassen den Sehluss zu, dass Nitrosomonas gegenüber Herbizide empfindlicher ist als Nitrobakter.     

Activité des peroxydase, catalase et glycolate oxydase après traitement avec divers herbicides

Activity of peroxidase, catalase and glycollate oxidase after treatment with various herbicides Various herbicides were applied to 17-day-old cucumber seedlings growing in nutrient solution. After 1, 2 and 3 days, peroxidase, catalase and glycollate oxidase activities were measured in the leaves of seedlings treated with lethal doses of atrazine, chlortoluron, MCPA, alachlor and dichlobenil. The same determinations were carried out after 5 h on seedlings previously sprayed with paraquat and DNOC. Atrazine and chlortoluron inhibited peroxidase activity and slightly stimulated catalase activity. MCPA, alachlor and dichlobenil stimulated both enzymes and after 3 days the peroxidase/catalase relations were respectively 0.7, 7 and 14. DNOC and paraquat inhibited catalase. Glycollate oxidase activity increased in seedlings treated with atrazine, chlortoluron, MCPA and DNOC and decreased in those treated with dichlobenil and paraquat.     

HERBICIDES AS FERTILIZER ADDITIVES

Summary. Granulated compound fertilizers were tested as solid carriers for a variety of herbicides known to be active when applied as liquids.
Pasture and cereal experiments with 2,4-D and MCPA as fertilizer additives indicated considerable loss of herbicidal activity in comparison with spray formulations.
Residual herbicides such as TCA, propham and diuron retained their activity when added to fertilizer for application to root crops. The most effective of the herbicides used as fertilizer additives were TCA and propham, both of which gave good control of Avena fatua in sugar beet, and a mixture of chlorpropham and diuron gave promising results as a general weed control treatment in carrots.
It is concluded that the addition of these or similar herbicides to fertilizer granules may have advantages over conventional spray formulations for those crops which require broadcast pre-sowing applications of herbicides and fertilizer. For such crops the use of a granular fertilizer as the carrier instead of inert material could also have advantages over conventional granular herbicides, particularly by reducing transport and application costs.     

VERTICAL DISTRIBUTION OF HERBICIDES IN SOIL AND THEIR AVAILABILITY TO PLANTS: SHOOT COMPARED WITH ROOT UPTAKE

Summary. Turnip, lettuce and ryegrass seedlings showed toxicity symptoms following shoot exposure to atrazine, linuron and aziprotryne at soil concentrations less than would be obtained from normal field applications. Responses following shoot exposure to simazine and lenacil were much less. Root exposure to all five herbicides caused seedling death at concentrations lower than those required for 'shoot-zone' toxicity. Pronamide and chlorpropham were tested against ryegrass only and at the concentrations examined were toxic only when localized in the shoot zone. Root exposure suppressed root growth, but the shoots were able to grow normally if the soil was kept sufficiently moist. Shoots contained more ¹⁴C-atrazine at emergence after shoot exposure compared with root exposure, but there was little subsequent uptake from the shoot zone. There was extensive uptake from the root zone after emergence. In the shoot-zone treatments, concentrations in the plant were high at emergence but were rapidly diluted by plant growth, whereas with root exposure, they increased throughout the experiments. The possible significance of these results to herbicide bebaviour under field conditions is discussed.
La distribution verticale des herbicides dans le sol et leur disponibilité pour les plantes: absorption comparée par la partie aèrienne et par les radnes     

A comparison of the tolerance by winter wheat of herbicide mixtures containing dicamba and 2,3,6-TBA, or ioxynil

The tolerance by winter wheat of two types of herbicide was examined in field experiments over 3 years. When overdosed or sprayed late, an ioxynil + mecoprop mixture had a greater margin of safety to the crop than mixtures containing dicamba or 2,3,6-TBA. Herbicide mixtures containing MCPA, applied before the leaf sheaths of the crop began to extend in the spring caused ear deformities. Spraying dicamba mixtures when the crop was jointing resulted in narrowed ears and shrivelled grain, In one experiment the latter effect was produced by a dicamba mixture, at twice the recommended dose, when sprayed on any of ten dates, covering crop development stages from 6-leaves through to jointing. Varieties differed in their tolerance of the dicamba mixtures; Maris Ranger was most sensitive but part of this difference could have been due to differences in the stage of development of the varieties at the time of spraying.     

A SPLIT-ROOT TETRAZOLIUM METHOD FOR EVALUATING EFFECTIVENESS AND PHYTOTOXICITY OF ROOT-ACTIVE HERBICIDES

Summary. Copper sulphate (GUSO₄.5H₂O) and sodium hydroxide (NaOH) were used to test the performance of a split-root method for evaluating the effectiveness and phytotoxicity of root-active herbicides. This method involves a vertical separation of the root system of weeping willow ( Salix babylonica L.) cuttings. One section is treated with the herbicide dissolved in half-strength Hewitt's medium while the other remains in medium without herbicide during the test period. Roots are assayed with 2,3,5-triphenyttetra-zolium chloride 24 h after treatment to determine their viability. Both herbicides satisfactorily reduced the viability of the treated roots as measured by the quantity of formazan extracted. Copper sulphate was systemic and injured untreated plant parts; sodium hydroxide affected only treated roots. The split-root tetrazolium method is relatively rapid and does not require sophisticated laboratory equipment.
Méthode au tétrazolium sur portions de racines pour l'évaluation de l'efficacité et de la phytotoxicité des herbicides agissant par les racines     

UNTERSUCHUNGEN ÜBER AUFNAHME UND TRANSLOKATION VON 14C-MARKIERTEN HERBIZIDEN IN AGROSTEMMA GITHAGO L. UND TUSSILAGO FARFARA L.

Studies of the uptake and translocation of ¹⁴C-labelled 2, 4-D, MCPA and aminotriazole in Agrostemma githago L. and Tussilago farfara L. clarified the behaviour of the herbicides in both species. In A. githago, MCPA was more freely mobile than 2,4-D after application to the leaf; it was distributed in the plant more rapidly and in greater quantity. Similarly, following root uptake MCPA was transported in the shoot in greater amounts than was 2,4-D. There is a clear relationship between the susceptibility of A. githago to MCPA and the mobility of the herbicide in the plant. In T. farfara, 2,4-D and aminotriazole applied to the leaves were equally well absorbed and relatively rapidly translocated. During the period up to 72 h the amounts of herbicide in the plant increased to similar levels; after that, ¹⁴C activity in plants treated with 2,4-D fell slightly whereas there was further accumulation of aminotriazole. Following uptake through the roots, translocation and accumulation in the leaves were considerably greater with aminotriazole than with 2,4-D. The lack of accumulation of 2,4-D could be a factor in the resistance of T. farfara to this herbicidie. Recherches sur l'absorption et la migration d'herbicides marqués au ¹⁴ C dans Agrostemma githago L. et Tussilago farfara L.     

The in vitro sensitivity of some species of Chlorophyceae to a selected range of herbicides

A survey was undertaken to establish the sensitivity of seventeen algal strains belonging to the Chlorophyceae to a selected range of herbicides up to a concentration maximum of 10 ppm under in vitro conditions. The herbicides were found to vary considerably in their toxicity to algae and a sequence was established based upon the magnitude of this toxicity: diuron > bromacil > paraquat> monuron> diquat> linuron> 2, 4-DB >barban> dicamba, dichlobenil, dalapon, picloram> bromoxynil, 2,3,6-TBA > 2.4-DP. 2,4,5-T and EPTAM. Of the algae, the most sensitive genera were Hormidium and Haematococcus, and the strain most sensitive to a wide range of herbicides was Hormidium barlowi (strain 335/la).     

THE EFFECTS OF SUB-LETHAL DOSES OF MCPA ON THE MORPHOLOGY AND YIELD OF VEGETABLE CROPS

Summary. The effect of sub-lethal doses of MCPA-sodium on the morphology, growth and yield of carrots and parsnips was studied in twenty replicated field experiments. Slight epinasty was produced in carrots by doses as low as 0.002 lb/ac a.e., but formative effects were not usually seen below 0.01 lb/ac. Crop weight was only reduced by doses greater than 0.1 lb/ac, but abnormalities rendered a proportion of the roots unmarketable at doses below 0.05 lb/ac. Small plants were sometimes killed by 0.3 lb/ac, but larger plants survived treatment with up to 0.5 lb/ac. Tests on carrots which were unmarketable on grounds of appearance failed to reveal any effect of MCPA on flavour, sweetness or texture of the roots. The effects of MCPA on parsnips were similar to those on carrots, but the expression of injury symptoms was generally less marked.
Les effets de doses sub-létales de MCPA sur la morphologie et le rendement des cultures légumières III. Carottes et panais     

THE EFFECTS OF SUB-LETHAL DOSES OF MCPA ON THE MORPHOLOGY AND YIELD OF VEGETABLE CROPS

Summary. The effects of sub-lethal doses of MCPA-sodium on the morphology, yield and marketability of cabbage, cauliflower and Brussels sprouts were studied in field and glasshouse experiments. The epinastic and formative effects in all three crops were similar. Cauliflower was the most susceptible and cabbage the least, but the marketability of these two crops was adversely affected at lower doses (less than 0.06 lb/ac a.e.) than that of Brussels sprouts (not below 0.1 Ib/ac a.e.), where the damage to the apex had less effect on the economically important portion of the plant. Brussels sprouts also produced lateral shoots on which normal sprouts developed even when the main stem above had been killed. In all three crops, characteristic galls were produced on the stems, and these, together with the effects on the stem apices, the leaves and the roots, are described and contrasted with abnormalities caused by other agencies.
Effets de doses sub-létales de MCPA sur la morphologie et le rendement des cultures tégumières IL Choux, choux-fleurs et choux de Bruxelles     

Ein Vergleich von Biotests mit chemisch—analytischen Methoden zum Nachweis von Atrazin, 2,4—D, DNOC und Napropamid im Boden

A comparison of bioassays with chemical methods of analysis for the determination of atrazine, 2,4-D, DNOC and napropamide in the soil The degradation of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), 2,4-D (2,4-dichlorophenoxyacetic acid), DNOC (4,6-dinitro-o-cresol) and napropamide (2-(α-naphthoxy)-NN-diethylpropionamide) in the soil was investigated using chemical methods of analysis and bioassays and the results were compared. For the biological determination, millet (a sorghum/Sudan grass hybrid) was used for atrazine in a glasshouse test, a cress (Lepidium sativum L.) root test for 2,4-D and DNOC. and an oat (Avena sativa L.) root test for napropamide. The instrumental determinations were carried out using gas chromatography for atrazine, 2,4-D and napropamide, and colorimetry for DNOC. The bioassays showed degradation kinetics typical for the herbicides but quantitative determination was unsatisfactory. Compared with the instrumental methods, bioassays indicated more rapid degradation in the case of atrazine and napropamide and lower initial concentrations in the case of 2,4-D and DNOC. Possible causes for these differences are discussed and from the results it is inferred that quantitative determinations with bioassays are only justifiable where their accuracy has been verified with established chemical methods.