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Assessment of 2, 4- D residues in the major rice soils of Kerala
An attempt was made to assess the behaviour of 2,4-D in the three major rice soils of Kerala viz., Palakkad, kuttanad and kole lands. Three laboratory experiments were conducted in sixteen soil samples (five each from Palakkad and kuttanad and six from kole lands) collected from the major rice gr...
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| Format: | Ph.D Thesis |
| Language: | Undetermined |
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Vellanikkara
Department of Soil Science and Agricultural Chemistry, College of Horticulture
2002
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| Summary: | An attempt was made to assess the behaviour of 2,4-D in the three major
rice soils of Kerala viz., Palakkad, kuttanad and kole lands. Three laboratory
experiments were conducted in sixteen soil samples (five each from Palakkad and
kuttanad and six from kole lands) collected from the major rice growing regions, so
as to understand the persistence, degradation, adsorption and leaching pattern of
2,4-D in these samples. A field experiment was also conducted in kole area of
Thrissur district (farmer's field at pidikkaparambu) for estimating the dissipation
pattern of 2,4-D from soil and plant uncier field condition.
In order to estimate 2,4-D residues in soil samples of the laboratory
experiments, a colorimetric procedure was standardized. . The standardization
process consisted of selection of estimation procedure, standardization of extractants
and soil extractant ratios and clean up method. Among the different extractants
tried, acetonitrile: distilled water: glacial acetic acid (80:20:2.5) in the soil extractant
ratio of I:4 was the best on in terms of recovery of applied 2,4-D from soil. Clean
up of the exctract with chemicals in sequence: NaOH (IN), concentrated HCl,
diethyl ether, sodium hydrogen phosphate buffer, concentrated HCl and carbon
tetrachloride was efficient to remove the co-extractives. Heating the residue with
chromotropic acid in concentrated sulphuric acid at 135 QC for 20 min. was
employed to develop the colour. Absorbance of the coloured extract was measured
at 565 mu in a Spectronic 20 spectrophotometer. Eighty to ninety percent of the
applied 2,4-D was recovered from most of the soils under study.
Persistence of 2,4-D in the soil samples was studied by fortifying the
samples with 2,4-D at 0.50, 1.00 and 2.00 ug g-l and incubating them under
submerged condition for varying periods. Residues of 2,4-D in the soil samples
were estimated at 0, 1, 3, 6, 9, 15, 30 and 60 days after incubation. Half life of
2,4-D in the soil samples was worked out by fitting first order kinetic equation of the
form C = Coe-kt. A variation of 3.44 to 10.76 days was noticed in the half life of
2,4-D in the major rice soils ofKerala. Lowest half life was recorded by Karumudy
sample of the kuttanad region and the highest by Moncompu 3 of the same region.
Extremely high organic carbon content in combination with partial decomposition of
organic matter in the karumady soil would have favoured the microbial growth and
enzymatic reactions and enhanced the rate of degradation in this soil.
Adsorption of 2,4-D in the three rice soils of Kerala was studied at three
different levels of equilibration (2,4 and 6 h) and three levels of 2,4-D (0.5,1.0 and
2.0 ug g-l soil). The results 'indicated that degree of adsorption increased with
concentration of 2,4-D in the soil solution i.e. adsorption of 2,4-D in the rice soils
of Kerala followed Freundlich isotherm of the form x!m = KCn. A comparison of
the strength of adsorption of 2,4-D in the soil samples was made by using the
isotherm constant K. It indicated that there was not much variation in the adsorptive
capacity of soils with high and low organic matter contents. This could be attributed
to the difference in the degree of humification of organic matter in the soils studied.
Organic matter present in karumady area is under varying stags of decomposition
and due to the lower degree ofhumification, the adsorption of 2,4-D on the clay
organic matter complex would not have taken place to a great magnitude.
Leaching and movement of2,4-D in the rice soils was studied by applying
2,4-D to the top of soil columns collected in PVC tubes, directly from the field. The
treatments consisted of 2,4-D @ 1.0,2.0 and 4.0 kg ha-I., 2,4-D residues retained in
0-10 cm, 10-20 cm and in the leachate (>20cm) were estimated. The result showed
that major part of the 2,4-D (more than 50% of the applied 2,4-D) remained in the
0-10 cm depth and less than 36 per cent remained in the 10-20 cm soil layer. Ten
per cent of the applied 2,4-D leached up to a depth of> 20 cm in some.soils ofkole
region (Cherpu and Anthikkad) which had higher rates of percolation of water.
However, no 2,4-D residue was available in the leachate of any soil at the lowest
level of application of herbicide i.e. 1.0 kg ha-I. 2,4-D residues in drinking water at
concentrations greater than 0.0001 ug r' is considered to be toxic to human beings
and animals. The findings of the study emphasizes the need for restricting the 2,4-D
application to 1.0 kg ha", particularly in the sandy soils ofKerala.
Studies on the dissipation of 2,4-D from soil and rice plant under field
condition consisted offive treatments viz., 2,4-D @ 0.0,0.5, 1.0,2.0 and 4.0 kg ha".
At the present recommended level of 1.0 kg ha" 2,4-D residues persisted in paddy
field for less than 30 days. Residues of2,4-D in the soil were not detectable at 60
days even if the rate of application was increased to 4.0 kg ha-I.
Effect of 2,4-D on microbial population was studied by soil plate dilution
method. The total number of fungal colonies in the soil samples were higher in the
treatments which received higher quantities of 2,4-D, while the bacterial population
was inhibited at higher rates of 2,4-D application. Major fungal species noticed on
the study was Aspergillus niger. The results of the study revealed the key role of
fungi in the degradation of2,4-D in the paddy field under investigation.
2,4-D residues in the grain and straw samples were very much lower than the
maximum residue limits. The findings of the present investigation made it clear that
the present recommendation of 2,4-D at the rate of 1.0 kgha" for weed control in
rice does not cause any adverse effect in the soil or crop produce.
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