In this article we will discuss about the chemicals used for plant protection. Also learn about how to minimise the residual effects of chemicals.
Chemicals for Plant Protection:
Use of insecticides, fungicides and herbicides in India is increasing at the rate of 2 to 5 per cent annually. A large fraction these plant protection chemicals eventually finds its way into soil which acts as reservoir of these residues.
From the soil the residues enter the bodies of invertebrates, gets transported into water and air or broken down to innocuous substances. These chemicals can have unforeseen effects on non-target organisms and can thus influence crop production to an extent to even more than the pests, diseases or weed infestation.
1. Insecticides:
In almost all the soils surveyed for insecticide residues in India, the most common chemical and the one that is found in large amounts is DDT followed by HCH and dieldrin. Highest level of 0.08 mg g-1 DDT-R is in cotton growing areas of Punjab, which is four times its permitted level of 0.02 mg g-1. Presence of cholinesterase inhibitors in soil samples indicated contamination with organophosphates and carbonate insecticides.
Organophosphates may persist in soil only for a few days. However, DDT and other chlorinated hydrocarbons may last from 3 to 15 years or even longer. Biochemical degradation by soil organisms is the single most important mechanism that can remove insecticide residues from the soil.
2. Fungicides:
Residues of fungicides based on inorganic compounds of sulphur, copper and mercury accumulate in soil as heavy metals contained in them are irreversibly adsorbed on soil colloids. Toxicity from accumulated copper based fungicides may render the soil useless for crop production. Fungicides like captan, carboxin benomyl etc. are decomposed rapidly through biochemical processes.
3. Herbicides:
The herbicides applied to one crop in the system may persist in soil at concentrations high enough to affect the subsequent sensitive crop. Atrazine is the most selective and widely used herbicide for weed control in several crops. Its repeated application leads to buildup of residues in soil.
Under Indian conditions, herbicide dose of 0.5 to 2.0 kg ha-1 results in build-up of residues in the range of 0.25 to 1.0 mg g-1, which is safely below the potential residual effect. At several places in India, herbicides including fluazifopbutyl, metolachlor, oxadiazon, nitrofen, 2, 4-D, metoxuron, isoproturon, oxyfluorfen and tribunil leave a little or no residual effect on succeeding crops.
However, residues of fluchloralin, metabenzthiazuron and atrazine were detected in amounts that could adversely affect not only succeeding crop but also several processes in soil leading to inefficient nutrient uptake by the crop. Herbicide 2, 4-D affects the growth of Azotobacter. Blue-green algae can tolerate endrine considerably but HCH is more toxic due to suppression of heterocyst frequency.
Minimising Residual Effects of Chemicals:
The biochemical degradation by soil microbes is effective for minimising agro-chemical residues in soil. The polar groups such as –OH, COO– and –NH2 on the plant protection molecules are the favourable points of attack by microbes. Application of easily decomposable organic matter can help reduce plant protection chemical levels in the soil.
Application of the same chemical repeatedly on the same field can lead to increased rates of microbial degradation of the chemical. This may be advantageous in the case of insecticides and fungicides but the rapid breakdown may result in reduced effect of herbicides.
Introduction of safer/reduced risk chemicals as in USA, reduced rate technology and low toxicity compounds (glyphosate and sulfonylureas) appears to be potential technologies for minising residue build up in soils. Organic farming giving importance to biological control (bio-pesticides and bio-herbicides) is the best way of pollution free environment.
It has been indicated that the following are the sources of plant nutrients:
1. Biological Nitrogen Fixation:
Some microorganisms are able to convert nitrogen in the air to ammonia for use as their nitrogen source. The conversion is made by bacteria living either on their own in the soil or on a considerably larger scale, in symbiosis with leguminous plants or trees (rhizobium) or other specific trees (actinomycetes) or with azolla in aquatic conditions (blue-green algae).
Biological nitrogen fixation can be enhanced by inoculation with efficient strains of nitrogen fixing microorganisms, part of the fixed nitrogen being directly assimilated by plants. The term bio-fertilisers is sometimes inappropriately used for these microorganisms; microbial inoculants is the preferred term.
Bio-fertilisers can be applied as indicated below:
a. Seed Treatment:
200 g of nitrogenous bio-fertiliser and 200 g of phosphotika are suspended in 300-400 ml of water and mixed thoroughly. Ten kg seeds are treated with this paste and dried in shade. Treated seeds have to be sown as soon as possible.
b. Seedling Root Dip:
For rice crop, a bed is made in the field and filled with water. Recommended bio-fertilisers are mixed in this water and the roots of seedlings are dipped for 8- 10 hrs.
c. Soil Application:
Four kg each of the recommended bio-fertilisers are mixed with 200 kg of compost and kept overnight. This mixture is incorporated in the soil at the time of sowing or planting.
2. Aerial Deposition:
Some nutrients are supplied in small quantities to the soil surface through aerial deposition. These include nitrates in rainwater, ammonia as gas or dissolved in rainwater, sulphur in acid rain, salts and chlorine in marine spray and calcium in the form of dust.
3. Irrigation Water, Floodwater and Groundwater:
These sources also supply plant nutrients, either naturally or because fertilisers have been added to the irrigation water. While water often contains small amounts of nutrients, irrigation can also result in a loss of nutrients from leaching. Some of the nutrients provided by surface and groundwater originate from plant nutrient losses within the watershed.
4. Soil Amendments:
Amendments are substances that are applied to the soil to correct a major constraint other than low nutrient content. Lime, for example, is used to remedy acidity; phosphates are used to reduce phosphorus fixation; gypsum is used to improve sodic (alkali) soils and peat can be added to the surface layers to increase organic matter content.