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Effect of pre emergence weedicide on adsorption and desorption in inceptisol soil

           Effect of pre emergence weedicide on adsorption and desorption in

         inceptisol soil.

Megha V. Nagmote and  A. D. Kadlag

           Department of Soil Science and Agriculture Chemistry Mahatma Phule Krishi

                           Vidyapeeth, Rahuri-413722, Dist. Ahmednagar, Maharashtra.

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Abstract : The present investigation was carried out by conducting an experiment in laboratory at Department of Soil Science and Agricultural Chemistry, Post Graduate Institute, M.P.K.V., Rahuri during 2002-03 to asses the effect of pre emergence weedicide on adsorption and desorption in inceptisol soil. There are four pre emergence weedicide used viz., oxyfluorfen, alachlor, fluchloralin and pendimethalin, in different concentration. The adsorption of pre emergence weedicides were increased with an increased concentration. The mean adsorption of oxyfluorfen, alachlor, fluchloralin and pendimethalin were 9.440, 9.519, 9.560 and 9.652  µg mL-1, respectively. The Freundlich adsorption equation gave good fit for the R2 values. The desorption of pre emergence weedicide was more in fluchloralin (7.740 µg 10 g-1 soil). The per cent desorption were in the order of fluchloralin > alachlor > oxyfluorfen > pendimethalin (17.11 > 14.41 > 13.20 > 12.37per cent), respectively. (Key Words – Adsorption, desorption, Pre emergence weedicides, Freundlich  adsorption isotherm)

 

                The tendency of pesticides to be adsorbed by soil is determined largely by the characteristics of pesticides and soils. The adsorption is directly associated with the soil characteristics like organic matter, nature and amount of clay and sesquioxides contents. Once a weedicide finds its way into the environment, a major part of it comes in contact with soil. Adsorption influences almost all the reactions of pesticides in soil. The adsorbed compound persists in soil for a longer time. This phenomenon therefore, appear to be one of the major factors affecting weedicide soil interactions. The adsorption, desorption and interaction of weedicides are the most significant processes influencing persistence, movement and efficacy of weedicide in soils. These phenomenon lead to   physico-chemical charges, including nutrient availability and susptibility to microbial degradation in soils.

 

 

 

Material and Methods –

Adsorption of pre emergence weedicides

Concentration of weedicides viz., pendimethalin, alachlor, fluchloralin and Oxyfluorfen were used for adsorption study was 1 mg, 5 mg, 3.      10 mg,15 mg, 20 mg and 25 mg a.i. kg-1 Soil. For this the procedure was used as ten gram soil was shaken with 20 ml solution of pre emergence weedicide separately in plastic tubes on horizontal shaker for four hours. The working solution having 1, 5, 10 and 25 mg g-1 acetone solution concentration were prepared separately. To ten gram soil, 20 ml each of working solution were added to give concentration 1, 5, 10 and 25 mg g-1 soil. After four hours equilibration at room temperature the tubes were centrifuged for 5 minutes at 1000 rpm. Ten milliliter of aliquot was taken for pre emergence weedicide content.

               Pre emergence weedicide were estimated by gas chromatography method described in pesticide Analytical Manual Vol. II. The pre emergence weedicide adsorbed were calculated by difference in concentration of this herbicide originally present in equilibrating solution and that after equilibration. Per cent adsorbed of these weedicide were calculated.

Desorption studies of pre emergence weedicides

                   To determine pre emergence weedicide desorption characteristics of soil, the method suggested by Wahid and Sethunathan (1978) was followed. Soil samples equilibrated with 20 ml of 5 mg g-1      pre emergence weedicide were centrifuged and 10 ml aliquot were taken out separately for analysis. The samples were resuspended with the help of shaker and equilibrated for 2 hrs. Distilled water washing was analysed separately for pre emergence weedicide. The total and per cent pre emergence were calculated.

Adsorption isotherm

                   The adsorption isotherm was obtained by plotting data of pendimethalin, alachlor fluchloralin and Oxyfluorfen adsorbed against equilibrating concentrations.

The isotherm data were interpreted interms of Freundlich equation.

 

               X

          --------- = KC1/n

              m

where,

          X/m =         Amount of weedicide (X) adsorbed per gram of soil

         C      =         Weedicide concentration in equilibrium                                 solution

          K    =           Constant

                    In practical terms a straight line is obtained when data were plotted as log (X/m) Vs. log C. The intercept is equal to log K and slope to 1/n.

                   The constant K provides indication of the extent of adsorption and has sometimes been used in correlation studies for determining the relative importance of the various soil properties on adsorption.

                   Theoretically, Freundlich equation represents a situation in which the quantity of weedicide adsorbed increases indefinitely with increasing concentration. This was occur when multilayer are formed and also heterogeneous sorption sites are implied.

Results and Discussion

Adsorption of weedicide in inceptisol soil

                   The adsorption of pre emergence weedicide as influenced by weedicide concentration in equilibrating solution are reported in Table 1. The adsorption of pre emergence weedicide were varied considerably by the equilibrium concentration in solution. The adsorption of pre emergence weedicide did not varied by the type of weedicide. The adsorption of pre emergence weedicide was increased with an increased concentration in equilibrating solution irrespective of pre emergence weedicide. It was 0.780, 4.572, 9.672 and 23.272 µg mL-1 in 1, 5, 10 and 25 µg mL-1 concentration of pre emergence weedicides, respectively. The adsorption of pre emergence weedicide irrespective of concentration of equilibrating solution was numerically  more  in  pendimethalin  (9.652 µg mL-1) followed by fluchloralin (9.560 µg mL-1) and alachlor (9.519 µg mL-1). It was the least in oxyfluorfen (9.440 µg mL-1). The variation in adsorption phenomenon of pre emergence weedicide might be associated with the adsorption of pre emergence weedicide mainly depends on nature of weedicide, soil organic matter, soil reactions and type and clay content. Similarly mobility, solubility and polarity of the weedicides. Primarily the adsorption of weedicide occurred on humic substances. It was decided by polarity power of exchangeable cations and hydrogen bonding between C=O. Functional groups, carboxyl oxygen and amide nitrogen was assumed to contribute to bond formation between clay and herbicide (Weiping et al. 2000). The interaction between phenyl ring of weedicide and organic carbon (Nir et al. 2000).The higher adsorption of pendimethalin on soil might be because of their higher mobility.

Freundlich adsorption

                   The data obtained were used for fitting in Freundlich equation. The equation thus obtained and goodness of fit of data are given by coefficient of determination (R2) which are reported in Table 4. The R2 values of Freundlich equations were 1, 1, 1, and 1 for oxyfluorfen, alachlor, fluchloralin and pendimethalin indicated good fit of equation.

Quantity (K) and Intensity factor (1/n) -  The data on Freundlich adsorption constant i.e. quantity (K) and intensity factor (1/n) calculated by regression equation of Freundlich adsorption isotherm are reported in   Table 5. The K values which are the measure of the capacity factor were almost in order of combine adsorption capacities of pre emergence weedicide. The intensity factor measured by 1/n does not vary greatly in pre emergence weedicide oxyflurofen (0.83) and alachlor (0.84). Similarly, in fluchloralin (0.90) and pendimethalin (0.90). The difference in quantity and intensity factor of  pre emergence weedicides are might be due to soil system composed of silicate and organic matter which is highly heterogeneous and hence constant loosing their inherent meaning.

Desorption of pre emergence weedicide

The per cent desorption of pre emergence weedicide are presented in Table 6. The initial values of adsorbed weedicide were numerically higher in pendimethalin (48.95 mg 10 g-1 soil) followed by fluchloralin, alachlor and oxyfluorfen (45.25, 44.90 and 42.75 mg 10 g-1 soil respectively).  The desorption of weedicide at Ist, IInd and IIIrd washing revealed that the desorption were decreased with increased washings.  The total desorption of weedicides was the least in oxyfluorfen (5.646 mg 10 g-1 soil) followed by pendimethalin (6.658 mg 10 g-1 soil).  The highest desorption was observed in fluchloralin (7.740 mg 10 g-1 soil) followed by alachlor (6.470 mg 10 g-1 soil). The per cent desorption of pre emergence weedicide were in the order of fluchloralin > alachlor > oxyfluorfen > pendimethalin (17.11 > 14.41 > 13.20 > 12.37 per cent respectively).  These observations are conformity with Raj et al. (1999). The desorption studies of pre-emergence weedicide revealed that fluchloralin and alachlor weedicides were more beneficial than the pendimethalin and oxyfluorfen.

Conclusions

             The adsorption of pendimethalin and fluchloralin were higher

than alachlor and oxyfluorfen. The higher concentration equilibrium solution observed in Oxyfluorfen and alachlor. The Freundlich adsorption equation gave good fit for the R2 values. The desorption of fluchloralin was higher than the other pre emergence weedicide.

  References –

  *Nir, S., Undabeytia, T., yaron Marcovich, D., El-Nahhal, Y., Polubesova, T., Serban, C., Rytwo, C., Lagaly, C. and Rubin, B. 2000. Optimization of adsorption of hydrophobic herbicides on montmorillionite pre adsorbed by monovalent organic cations, interaction between phenyl ring. Envion. Sci. Techn. 34 (7) : 1269-1274.

 *Raj, M.F., Patel, B.K. and Shah, P.C. 1999. Adsorption, desorption of pendimethalin, fluchloralin and oxadiazon on soils. Pesti. Res. J. 11 (2) : 162-167.

 Wahid, P.A. and Sethunathan, N. 1978. Sorption desorption of    herbicide in soils. J. Agric. Fd. Chem. 26(1) : 101-105

 Weiping Liu, Jianying Gan, Sharon, K., Papiernik and Scott, R. Yates. 2000. Structural influences in relative sorptivity of chloroacetanilide herbicides in soils. J. Agric. Fd. Chem. 48 : 4320-4325.

                   * Originals are not seen.

 

 

Table 1.      Adsorption of weedicide in Inceptisol Soil

 

Sr. No.

Different weedicide

Weedicide adsorbed (µg g-1)

Weedicide Concentration in equilibrating solution (µg ml-1)

1

5

10

25

Mean

1

 

Oxyfluorfen

0.745

4.400

9.505

23.110

9.440

2

 

Alachlor

0.754

4.490

9.645

23.185

9.519

3

 

Fluchloralin

0.785

4.525

9.710

23.220

9.560

4

 

Pendimethalin

0.838

4.895

9.830

23.574

9.652

 

Mean

0.780

4.572

9.672

23.272

 

 

 

 

Table 2. Coefficient of determination (R2) indicating goodness of fit to freundlich equation

Sr. No.

 

Weedicide

R2

Freundlich Equation

1

 

Oxyfluorfen

1

X/m=0.62 C0.83

2

Alachlor

 

1

X/m=0.60 C0.84

 

3

 

Fluchloralin

1

X/m=0.67 C0.90

4

 

Pendimethalin

1

X/m=0.68 C0.90

 

 

Table 3. Freundlich adsorbtion constant of weedicide in inceptisol        soil

Sr.

No.

 

Different weedicide

Freundlich adsorption constant

K

1/n

1

 

Oxyfluorfen

0.62

0.83

2

Alachlor

 

0.60

0.84

3

 

Fluchloralin

0.67

0.90

4

 

Pendimethalin

0.68

0.90

 

 

Table 4.  Desorption of different weedicide in soil

Sr.

No.

 

Different weedicide

Weedicide adsorbed (µg 10 g-1 soil)

Weedicide desorbed (µg 10 g-1)

Per cent desorpt-ion

Washing I

Washing II

Washing III

Total

 

1

 

 

Oxyfluorfen

 

42.75

 

3.521

 

1.287

 

0.838

 

5.646

 

13.21

 

2

 

Alachlor

 

44.90

3.982

1.595

0.893

6.470

14.41

 

3

 

 

Fluchloralin

45.25

4.22

2.602

0.925

7.740

17.11

 

4

 

 

Pendimethalin

48.95

3.641

1.435

0.982

6.058

12.37

 

 

About the Author

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