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Abstract -84- Summary This carried out to study the behaviour of some herbicides in different Egyptian soi s for reducing their pollution to soil and ground water. The present . vestigation aimed to throw light on the behaviour of sethoxydim and pen imethalin herbicides in three different soils named clay soil (Banha), calcareo s soil (El-Nobaria) and sandy soil (Salheia) . The following oints considered the course of the present investigation:- 1- Adsorptio behaviour time, soil type and herbicide concentration on adsorption behavio of sethoxydim and pendimethalin in clay, calcareous and (a) Effect of e The equilibri m clay soil and 90 different soils increas osure time :- time for sethoxydim and pendimethalin were 120 min. in. in calcareous and sandy soil . The adsorption on as the time increased . (b) Effect of so I type :- The herbicid pendimethalin showed more adsorption capacity compared with setho dim at all tested concentrations on the three tested soils, i.e 92.67 , 82.40 an 69.15 %and 92, 78.87 and 65.75 % with pendimethalin and sethoxydim in cay, calcareous and sandy soil, with an intial cone. of 4 ug/mL, respectively On the other hand the clay soil adsorbed more herbicides than the calcareous d sandy soil i.e 92.67 and 92 %, 82.40 and 78.87 %, 69.15 and 65.75 % or pendimethalin and sethoxydim in the case of clay, calcareous and sandy oils respectively . The (K) and ) values obtained from pendimethalin and sethoxydim adsorption isotherm indicated that both of the initial adsorption and the subsequent adsorptio of clay soil were higher than the calcareous and sandy soils. -85- The distributi n Coefficient (Kd) values for pendimethalin were higher than . This mean that pendimethalin was adsorbed more strongly by the soils 2- Down-war movement and leaching of herbicides in soils. The effect of soil type, herbicides concentration and leaching water volume on the do -ward movement and leaching behaviour of sethoxydim and pendimethalin w e studied :- f both herbicides amounts were located in the upper layer of the different soils der investigation (0-5 em) were 40.93 ,36.79 and 30..41 sand sady soils respectively 33.97% of sethoxydim were detected in the leac ate from clay soil 41.36 and 50.44 %were found in the leachate of calcareo s and sandy soils respectively. On the other hand, 63.71 % of pendimethalin esidue were found in the first layer (0-5 em) in clay soil 60.91 and 45.05 0 in calcareous and sandy soils respectively . The pendimethalin was clay soil than the calcareous and sandy soils. By e concentration of sethoxydim and pendimethalin (500- 1000ug) their amo adsobed on soil surface increased while their residues in the leachate were de reased . On the other hand increaseing the leaching water volume decreased se oxydim and pendimethalin adsorption in the first layer the leachates . Increasing leaching volume ts of sethoxydim and pendimethalin recovered from the other layers . -86- 3- Persistence nd degradation of herbicides in soils :- The behaviour f herbicides are affected by several physicochemical and environmental factors (a) Type of soil and herbicide: The pendimeth in and sethoxydim were degraded rapidly in clay soil comparing with calc ous and sandy soils. The differences in sethoxydim and pendimethalin persist ce among the three tested soils was mainly due to the great variability in chemical and mechanical properties of these soils. Sethoxydim and pen methalin persistence were relatively high in sandy soil y soils. Its amounted 40.81,21.72 % sandy soil 30.42, 17.86 % calcareous d 27.59 , 11.27 %in clay soil of the original doses for sethoxydim and pen ethalin respectively (after 180 days) . It is interesting to note that the percent f sethoxydim and pendimethalin residues increased with increasing the herbici es concentration especially during the late periods after the application i.e 18 days, 62.61 , 49.46 % in sandy soil, 53.52, 37.75% in calcareous and 48.83 21.72% in clay soil for persistence of sethoydim and pendimethalin at conce tration 250 ug/50g respectively. (b)- Effect of p on sethoydim and pendimethalin degradation :- Sethoxydim bre down was accelerated in the alkaline medium but pendimethalin was les affected in this medium pH from 4.5 to 6.8 slightly affected sethoxydim d pendimethalin degradation increasing the pH above 7.5 increased the the ra of sethoxydim and pendimethalin degradation 4- Microbiologi al degradation of herbicides :- Autoclaving so’ s, suppressed the degradation of sethoxydim and pendimethalin as com ared with non autoclaved soils. Sethoxydim was more ---- ~~,----_.~~-- -87- persistence In tested oils than pendimethalin . Degradation of sethoxydim or pendimethalin was m re effective in clay soil than in calcareous and sandy soils according to its high organic content. Degradation of both sethoxydim and pendimethalin is p ysico-chemical and biochemical processes. |