Crops can be injured when exposed to high concentrations of various air pollutants. Injury ranges from visible markings on crop leaves, to reduced growth and yield, to premature death. In case of animals, it may lead to infection as a result of eating polluted feed and water.
The development and severity of the injury depends not only on the concentration of the particular pollutant but also on a number of other factors. These include the length of exposure to the pollutant, the plant species and its stage of development as well as the environmental factors conducive to a buildup of the pollutant and to the preconditioning of the plant, which makes it either susceptible or resistant to injury.
Some of these pollutants may directly affect the soil leading to infertile and acidic soil content, if used for farming will lead to low yields.
Most common pollutants, which end up affecting agricultural activity include sulphur dioxide, fluorides, ammonia, chlorine and particulate matter. But experts say widespread pollutants consist primarily of oxidants. Ozone, the major component of oxidants, is produced in the atmosphere during a complex reaction involving nitrogen oxides and reactive hydrocarbons, components of automobile exhausts and fossil fuel burning. As this process proceeds only in sunlight, it is called a photo-chemical reaction leading to injury on crops
Air pollution injury to plants can be evident in several ways. Injury to the greenery of the leaf may be visible in a short time and appear as dead tissue, or it can develop slowly as a yellowing of the leaf. There may be a reduction in growth of various portions of a plant. Plants may be killed outright, but they usually do not succumb until they have suffered recurrent injury.
Prof Shem Wandiga, from University of Nairobi, while explaining the dangers to agriculture from air pollution pointed out that photo chemical smog (a chemical reaction of sunlight, nitrogen oxide and volatile organic compounds in the atmosphere) increases with the amount of sunshine. In the end, it is dangerous to plants, animal and human beings.
This was during the India-Africa Dialogue on Air Quality, held in Nairobi, Kenya, last week. It was organised by Media for Environment, Science, Health and Agriculture (Mesha).
Besides chemicals farmers use such as herbicides, fertilizer, dichloro-diphenyl-trichloro-ethane (DDT) and others end up having negative effects on the soil leading to polluted soil with no soil fertility. According to Prof Wandiga, considering how soil is the reason we are to sustain agriculture for our livelihoods, the contamination of it has major effects on our health because crops grown on polluted soil absorb much of the pollution. This is passed to humans and animals which feed on it.
Unable to adopt
The ecological balance of any system gets affected due to the widespread contamination of soil.
Most plants are unable to adopt when the composition of soil changes because fungi and bacteria found in soil that bind it together begin to decline which ends up creating another challenge of soil erosion.
The fertility diminishes making land unusable for agriculture and any vegetation to survive. He noted that emissions from motor vehicles via diesel and petrol use as one of the dangerous pollutant, which may end up accumulating on soils making it become infertile.
Air pollution from gases such as sulphur dixide are acidic in nature and become corrosive in the atmosphere. Once they drop on the soil, it also becomes acidic rendering the soil infertile for any agricultural activity.
Major sources are coal-burning operations, especially those providing electric power and space heating.
Sulphur dioxide emissions also come from the burning of petroleum, diesel and the smelting of sulphur containing ores.
Other disastrous gases to plants are fluorides, which are discharged into the atmosphere from the combustion of coal, the production of brick, tile, enamel frit, ceramics, and glass, the manufacture of aluminium and the production of hydrofluoric acid, phosphate chemicals and fertilisers.
Fluorides absorbed by leaves are conducted towards the margins of broad leaves of crops and to the tips of monocotyledonous leaves.
Mix of gases
Little injury takes place at the site of absorption, whereas the margins or the tips of the leaves build up injurious concentrations. Since plants have breathing system called stomata, the breathing hole will end up being clogged leading to the plants to get withered.
Once there is air pollution in the atmosphere, these will lead to gases such as methane, carbon dioxide and oxygen to get mixed up thereby changing the temperature. This causes changes in the rainfall partners thereby affecting farming system because farmers will not be sure as to when to plant their crops as a result of the changes.
Dr Kawuki Kayizzi, from the National Agricultural Research Laboratories (NARL) Kawanda, explains that air pollution arising from gas emissions and dust as challenge for agricultural land. He said many farmers do not realise it because they don’t pay attention to it but farmlands which are close to industries and cities where there is motor vehicles are more affected.
Dust blown from such places as those from Sahara desert come with chemical particles of sulphur dioxide, which is eventually combated to acidic rays.
Once it accumulates on soil it renders it infertile for any framing activity but farmers will not realise this. Instead they will go ahead to plant crops on such land leading to poor yields.
If such dust settles on soil, during rainy season it will be washed into water bodies thereby ending up destroying fish species.
Sometimes air contamination in the atmosphere ends up polluting the clouds which end up in acidic rain thereby causing destruction to plants as a result of absorption.
Putting figures to it
UN Economic Commission for Africa estimates the cost of air pollution in African cities can be as high as 2.7 per cent of the country’s Gross Domestic Product. In Kenya, 40 per cent of air pollution is caused by vehicles while the estimate for Nigeria is 43 per cent. East African countries have moved to reduce the sulphur content in diesel to curb this danger. It is recommended that countries should set standards for monitoring air quality alongside other solutions such as afforestation and zonal planning. Farmers are advised to ensure that they test soil which is used for any farming activity.