Effect of Pesticides on Soil Health
Dr. Pramod Kumar Rai1, Dr. Hadi Husain Khan2, Dr. Huma Naz3, Surender Kumar4, Pushpendra Singh Sahu5 & Mohd. Danish6
1Director, ICAR-Directorate of Rapeseed-Mustard Research, Bharatpur – 321303 (Rajasthan), India.
2Research Associate, ICAR-DRMR-APART, Dhubri – 783324 (Assam), India.
3Assistant Professor, Plant Pathology, MANFDC, Hardoi – 241001(U.P.), India.
4Department of Plant Pathology, CSAUA&T, Kanpur – 208002 (U.P.), India.
5Department of Entomology, SHUATS, Naini- Prayagraj (Allahabad) – 211007 (U.P.), India.
6Research Scholar, Department of Entomology, SHUATS, Naini- Prayagraj – 211007 (U.P.), India.
The proportion of pesticides applied reaching the target pest is less than 1.0 per cent so 99.0 per cent go ‘somewhere else’ in the environment (Carrieger et al., 2006 and Pimentel, 1995). Pesticides controlled insect pests of the crops as well left residues in soil and thereby greatly reduced microbial populations there by disturbing soil health.
Exposure of Pesticides to Soil
Pesticides enter the soil via direct application to control soil pests, spray drift during foliage treatment, washoff from treated foliage, release from granules or from treated seeds in soil.
Pesticides penetrate into soil because of:
a. Application of pesticides to soil for control of soil infesting pests like termites etc.
b. Application of systemic insecticides in soil or seed treatment for protection against soil pests and sucking pests.
c. Pesticide also affects through surface run off to the untreated soil and wind-blown dust particles.
d. Falling of some insecticides on ground by run off are spraying on the crops.
Factors affecting fate of pesticides in soil:
a. Pesticide properties.
b. Soil properties.
c. Site conditions.
d. Application methods.
e. Management practices.
Pesticide properties which affect movement of pesticides in soil include structural stability, volatilization, solubility, adsorption and degradation.
1. Structural stability: Pesticides range in stability from instable to extremely stable. A readily metabolisable pesticide breaks down rapidly in plants and soil and forms stable chemical that persist for longer periods even in unsuitable environmental conditions. Eg: Heptachlor epoxide and dieldrin are stable than parent compounds heptachlor and aldrin.
2. Volatilization: Pesticides range in volatility from extremely low to high vapour pressure necessary for fumigant action. Highly volatile and water insoluble pesticides are lost in atmosphere.
3. Solubility: Highly soluble pesticides are known to readily dissolve in water and have a tendency to be leached from the soil to ground water.
4. Adsorption: Adsorption depends on pesticide, soil type and soil organic matter present. Eg: The sorption of neonicotinoids on the soils was generally low following the order thiacloprid > Imidacloprid ≈ Clothianidin and is influenced mainly by the soil organic carbon content. The percentage degradation rates of the pesticides in different soils ranged from 25.4% to 80.9%, all following the order thiacloprid > Imidacloprid ≈ Clothianidin (Zhang et al., 2018).
5. Degradation: Pesticides are degraded or broken down into other chemical forms by sunlight, micro organisms in the soil.
Soil properties affecting the movement of pesticides include soil texture, soil permeability, organic matter content and soil acidity. Two most important factors of soil properties that affect pesticide intake in soil are:
Texture and Organic Matter.
1. Soil Texture: The relative proportions of sand, silt and clay decide it. Clay and organic matter-rich soils retain water and dissolved chemicals for longer. These soils also have more surface area on which pesticides can be adsorbed. The pesticide has a higher risk of hitting ground water if the soil texture is coarse. Eg: The half life of thiamethoxam @ 12.5 and 25 µg/g in silty clay loam was 15.0 to 18.8 days whereas in loam soil it was 20.1 to 21.5 days. (Kumar et al., 2014)
2. Soil permeability: It’s a measurement of how quickly water can flow downward through a specific soil. Water travels easily across high-permeability soils. High-solubility pesticides can be washed away by the percolating spray. In highly permeable soils, therefore the timing and methods of application need to be carefully designed to minimize leaching losses. Eg: The dissipation of thiamethoxam in soil was faster at 10 mg kg-1 than at 1 mg kg-1 under dry conditions, but the opposite pattern was observed under field capacity moisture and submerged conditions. (Gupta et al., 2008).
3. Organic matter: The amount of water a soil can retain and how well it can adsorb pesticides are both influenced by soil organic matter. Increasing the organic content of the soil, whether by manure application or ploughing under cover crops, improves the soil’s capacity to retain both water and dissolved pesticides in the root zone, where they would be accessible to plants and eventually degraded.
4. Soil acidity: The acidity of the soil affects chemical properties of pesticides as the soil pH decreases (becomes more acidic), pesticides bind more to the clay in the soil making the pesticides less likely to reach the ground water.
The conditions of the site where a pesticide is applied include the depth to groundwater, geologic conditions and climate.
1. Depth to ground water: The shallower the depth to groundwater, the less soil there will be to act as a filter. Also, there will be a fewer chance for degradation or adsorption of pesticides. In humid regions, groundwater may be only a few feet below the surface of the soil. If rainfall is high and soils are permeable, water carrying dissolved pesticides may take only a few days to percolate downward to groundwater. In arid regions, ground water may be several feet below the soil surface and leaching of pesticides to groundwater may be a much slower process.
2. Geologic conditions: The highly permeable materials (gravel deposits) in the geologic layers between the soils and groundwater allow dissolved pesticides to freely percolate downward to groundwater. Clay layers are less permeable, preventing dissolved pesticides from moving around.
3. Climate: Areas with high rates of rainfall or irrigation are highly susceptible to leaching of pesticides, especially if the soils are highly permeable.
Method of application: On site application, method of application, formulation and pesticides rates and timing.
1. On site application: The total quantity and form of a pesticide reaching plants or soils depends on the type of equipment used and formulations of the pesticides. The adsorption and absorption of pesticide depends mainly on their molecular polarity for instance, polar aqueous cannot penetrate waxy hydrophobic layers of plants as easily as non-polar liquid waste compounds. Different parts of plants and soil fractions differ in their ability to absorb pesticides so this site to which a pesticide is applied influences its persistence very much.
2. Method of application: The persistence of a particular pesticide depends particularly on a method by which it is applied. The common methods of applying pesticides to plants are in order of decreasing particle size asdust > spray> mists> fog and smoke. At either end of dropet scale of size there is risk of loss of the pesticides, the larger droplets tend to runoff the surface of leaves and the smaller once to become lost going to air and wind movements; soil pesticide treatments have gradually progressed from a broadcast application to spot or localized treatment. The pesticides are most readily available for leaching when they are injected or incorporated into the soil, as in the case of nematicides. The majority of pesticides found in ground water are pesticides that are absorbed into the soil rather than sprayed on growing crops.
3. Formulation: The ways in which pesticide are formulated, considerably influence their persistence. The formulation in order of increase in persistence in plants and increase in size of particles are water soluble liquid > Emulsions > Miscible liquids > Wettable powder > dusts > Microcapsule granules.
Management practices include the method used to apply the pesticide and the rates and timing of application.
1. Application methods: Injection or incorporation into the soil, as in the case of nematicides, makes the pesticide most readily available for leaching. Most of the pesticides which have been detected in ground water are ones which are incorporated into the soil rather than being sprayed onto growing crops.
2. Pesticide rates and timing: The larger the amount used and the closer the time of application to a time of heavy rainfall or irrigation, the more likely that some pesticide will leach to ground water.
Harmful Effects of Pesticides on Soil
Effect of pesticides on soil biological properties:
1. Effects on arthropods: Soil act as sink or reservoir of pesticides whether applied directly/ indirectly. Earthworms enhance soil aeration and organic matter digestion, as well as increasing nutrient content in the top layer of soil. By ingesting decomposing litter and acting as a bio-indicator in the case of soil fertility, earthworms help to protect human health. Some pesticides kill earthworm and may decrease population indirectly by killing the vegetation on which worms feed. Imidacloprid, chlorpyrifos, and phorate showed negative impact on earthworm in rice- maize cropping system (Ghoshal and Hati, 2019). 2. Effect on snails and slugs: Snails and slugs have the ability to get insecticides and concentrated on their body Eg: Organophosphates and carbamates. High concentration of diazinon, phorate and carbofuran were found in their bodies as this are water soluble. The insecticides are not detrimental to them but harmful to predatory birds which prey on this snails and slugs will die.
3. Effect on soil microbial organisms: The microflora isimportant for soil fertility. The transformation of organic ‘N’ to inorganic forms by micro-organisms is the principal source of ‘N’ for plant growth and also the fixation of atmospheric ‘N’ by bacteria. The soil microflora helps in decomposition of carbamaceous organic matter. Pesticide affects the growth, activity and enzyme of soil microflora will be adversely affect soil fertility in turn soil health. Rynaxypyr, cartap hydrochloride, fipronil and chlorpyrifos showed no significant detrimental effect on the collembola population but carbofuran- and phorate-treated plot showed reduction of 27.65% and 13.47% in rice- maize cropping system (Ghoshal and Hati, 2019). Several minute soil arthropods include minute ants, beetles, and soil oribatid mites, pseudoscorpions showed no appreciable toxic effect. Predatory mites being negatively affected when treated with OC insecticides (e.g., DDT, endosulfan, aldrin, chordane and heptachlor), most OP insecticides and carbamate biocides (i.e., aldicarb, carbofuran). On the other hand, enhancement of pesticides degrading microbial population has practical implication in retaining soil health by allowing degradtion and consequential removal of the pesticides and toxic residues but the purpose for which the pesticide is applied may not solve. Development of resistance in soil pests.
Effect of Pesticides on Soil Chemical Properties
1. Elevation of total S in soil over the control, indicating that the extent of mineralization of organic S compounds was slower due to the detrimental effect of insecticide on proliferation of microorganism, particularly, thiosulphate oxidizing bacteria as well as their potentiality.
2. Inhibit biomethanation i.e., methane generation from rice paddy fields treated with dimethoate.
3. HCH, phorate and fenvalerate increased mineralization of organic C.
4. Pyriproxifen and fipronil decreased the root nitrogen, shoot nitrogen, root phosphorus, shoot phosphorus, seed yield and grain protein of pea plants. (Ahemad and Khan, 2010)
5. Chronic exposure to sublethal doses of thiamethoxam results in smaller colonies of black garden ant colonies, Lasius niger with fewer workers and larvae. Residue analysis of thiamethoxam and clothianidin further suggests that queens could have superior detoxification, with a possible trade-off scenario between reproduction and detoxification(Schlappi et al. 2020).
6. Insecticide seed dressing with imidacloprid and prothioconazole combined with the fungicide difenoconazol, fludioxonil increased surface activity of collembola. Fungicide seed dressing like fluoxastrobin, fluopyram, tebuconazole, prothioconazole increased resource availability for collembola. Seed dressings containing insecticides and fungicides increased the abundance of flagellate protozoa but reduced litter decomposition. Seed dressings had no effect on earthworm behaviour, but they did alter the response of collembola and soil microorganisms to seed dressing. (Zaller et al., 2016)
7. The potential for land destruction was shown by the overuse of pesticides.
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