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Impacts of Air Pollution & Acid Rain on Farmland

Introduction

North America and Europe are the areas with the most problematic and noticeable overall acid precipitation levels. They both receive precipitation with pH as low as 4.7. Acid deposition endangers forests, lakes and agriculture. In addition, both Europe and North America are major emitters of air pollutants (which cause the acid rain in the first place). Such pollution can also directly affects crop species. This fact sheet looks at the impacts of air pollution and acid rain on farmland and crops.

Air Pollution

When fossil fuels are burnt, the carbon they contain is combined with oxygen in the air to release heat. However, the process also creates by-products that are potentially dangerous. In addition, the usual fuels used in transportation, such as petrol or diesel, aren't a single substance, but a chemical soup of ingredients such as butane, propane, xylene and benzene. Carbon-based petrochemical products are broken up in combustion to form, among many other products, carbon dioxide (CO₂), carbon monoxide (CO), volatile organic compounds (VOCs), nitrogen oxides (NOx), sulphur oxides (SOx) and very fine particulates. In addition, unburned hydrocarbons, some of which evaporate directly from the petrol tanks of cars and trucks, escape before and after combustion and join other VOCs in the air.

Nitrogen dioxide is a poisonous brown gas used as a catalyst and oxidising agent. Nitric acid is a transparent, fuming corrosive liquid that is a highly reactive oxidising agent used in the production of fertilisers, explosives and rocket fuels. Ozone is an unstable oxidising agent, poisonous in high concentrations, with a pungent, irritating odour. In weak concentrations, ozone is used as a bleach and to sterilise water.

When a sufficient concentration of sulphur and nitrogen oxides and hydrocarbons builds up in the atmosphere and is bombarded by sunlight, a complex series of chemical reactions takes place that creates more chemicals, including nitrogen dioxide (NO₂) and ozone (O₃). Also, very fine acidic particles are formed, such as sulphates and nitrates. When nitrogen oxides and reactive organic gases combine, especially on sunny, still days, a photochemical (ozone) smog is formed and this can also cause crop damage.

Sulphur dioxide

One of the main components of acid rain is sulphur dioxide (SO₂), another by-product of fossil fuel combustion. The effects of SO₂ on crops are influenced by other biological and environmental factors such as plant type, age, sunlight levels, temperature, humidity and the presence of other pollutants (ozone and nitrogen oxides). Thus, even though sulphur dioxide levels may be extremely high, the levels may not affect vegetation because of the surrounding environmental conditions. It is also possible that the plants and soils may temporarily store pollutants. By storing the pollutants they are preventing the pollutants from reacting with other substances in the plants or soil.

Experiments have shown that SO₂ is capable of interacting with other factors which influence crop yields, in some cases increasing them. Also, some reports state that airborne concentrations of SO₂ and NOx in rural areas are generally below the levels at which yield losses of crops have been shown to occur.

Any effects of acid deposition on agricultural soils may be dwarfed by the action of fertilisers and other soil treatments. Food crops are not usually seriously affected because the fertilisers replace any nutrients washed away. Farmers may also add crushed limestone to the soil. Limestone is a basic material and increases the ability of the soil to act as a buffer against acidity.

Impacts on Crop Species

Acid deposition directly reduces the yield of radishes, beets, carrots and broccoli. Scientists believe that acid rain damages the protective waxy coating of leaves and allows acids to diffuse into them, which interrupts the evaporation of water and gas exchange so that the plant can no longer breathe. This stops the plant's conversion of nutrients and water into a form useful for plant growth and affects crop yields. In addition, crops such as corn, potatoes, soy beans and lettuce are damaged by ozone that is created when nitrogen emissions combine with hydrocarbons in the air.

Nutrient deficiency

The problems caused by acid rain across Europe and North America over the last 100 years became so serious that, in 1985, most European countries agreed to reduce their sulphur emissions by at least 30% by 1993. The reduction in acid rain would seem to have been a good thing but, strangely, trees and crop plants in northern Europe are still dying, crop yields have not increased and outbreaks of several new plant diseases have occurred.

There is evidence that crops of several plants, especially one called 'oilseed rape' have produced lower yields, despite decreased sulphur fallout from the atmosphere. In some regions the crop yields are slightly better. In those regions there seems to be a higher level of soil sulphur.

The conclusion is that, in many regions, crops are suffering from too little sulphur. Before the Second World War farmers mainly used fertilisers based on nitrogen and phosphate which are the major nutrients required by plants. These also contained small amounts of sulphur.

Since the 1950s farmers switched to sulphur-free fertilisers, using ammonium nitrate instead of ammonium sulphate. This reduction in sulphur supplied by fertilisers was counterbalanced by the increased sulphur in the atmosphere, caused by the increased burning of fossil fuels.

Today, with the combination of lower levels of sulphur in the atmosphere and little or no sulphur in the fertilisers, the crops are showing signs of stress. It seems that these crop plants actually require small amounts of sulphur to remain healthy.

Agriculture - a Source of Pollution

In industrialised countries 80% to 90% of ammonia emissions are from livestock and artificial fertilisers. World-wide, animal wastes remain the largest single source of ammonium. Ammonium is produced when nitrogen in animal excreta is volatilised. Ammonia directly damages plants by disturbing the uptake of minerals. It can also be a component of acid rain, increasing the conversion rate of sulphur dioxide to sulphate ion. This reaction produces ammonium sulphate, which increases the acidity in the soil.