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They can result in ‘scorching of young growth’. Ammonium salts; mainly ammonium sulphate and ammonium nitrate induces a ‘scorch’ or ‘burn’. Careless application of ammonium sulphate to the roots of young coffee, tea, cocoa and other plants may produce scorching of the foliage…and result in death of part or all of the plant.
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The continuous application of ammonium sulphate (used commonly in the tropics and subtropics) may greatly reduce the soil pH (increasing soil acidity). This may result in the necessity of liming, which is almost impossible on a large scale. An increase in acidity of the soil can have dramatic effects on the environment. Acidification and a reduced pH on the more productive agricultural land will limit crop production and the range of crops which can be grown. Water draining from acid soils may contain elements, particularly aluminium, which adversely effect water quality and fish stocks.’
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Since nitrate and ammonium ions are very soluble, they do not remain in the soil for long and are quickly leached out, ending up in local rivers and lakes; causing eutrophication. Leaching is if the nutrients in fertilisers are not taken up by the plants there is a danger that they will be washed out of the soil by rainwater and that the run off will enter streams and rivers. From this Eutrophication occurs. This is the process that takes place when freshwater is ‘enriched’ by nutrients, especially nitrates and phosphates. Increasing the concentration of these mineral ions, results in a rapid increase in the growth of algae and water plants. The resulting thick surface layer of vegetation blocks light out and many algal cells die. Bacteria in the water bring decompose these cells and increase in number. As bacteria are living organisms, they respire; therefore increased bacterial activity leads to a fall in the concentration of oxygen. This in turn causes the death of other organisms.
Organic fertilisers, an alternative solution to inorganic (chemical) fertilisers, do less harm to the environment. It does cause eutrophication but much less so than inorganic fertilisers as there is less concentration of minerals.
Organic systems are characterised by a cyclical structure in which by-products of production processes are re-used to maintain further production. The main cycle is the use of straw, animal manure, farmyard slurry and composted crop residues to maintain soil fertility rather than artificial fertilisers from outside the farm system. They contain the main elements found in inorganic fertilisers (NPK), but in organic compounds such as urea, cellulose, lipids and organic acids. Plants are unable to make use of these organic materials in the soil as their roots can only take up inorganic mineral ions (i.e. NPK). However the organic compounds are digested by soil organisms (animals, fungi and bacteria) who then release inorganic ions that the plants can use. These ions act in the same way as inorganic fertilisers and they replace the nutrients and increase the yield.
Some advantages of organic fertilisers are that since compounds on organic fertilisers are less soluble than those in inorganic fertilisers, the inorganic minerals are released more slowly as they are decomposed. This prevents leaching and so last longer; and do not cause eutrophication as much. Also as organic wastes need to be disposed anyway, they are therefore cheap. Secondly, it means that the waste will not be dumped on landfill sites; maybe causing uncontrolled leaching. Finally, the organic material improves the soil structure by binding the soil particles together and provides food for soil organisms (i.e. earthworms). This improves drainage and aeration.
Some disadvantages though are that they are bulky and not very concentrated in minerals. Therefore they do not increase the yields as well as inorganic fertilisers (though this can result in undesirable effects). They also may contain unwanted substances such as weed seeds and fungal spores. The main problem though is that livestock slurries have the potential to pollute surface watercourses during storage and land application causing: eutrophication, de-oxygenation and contamination.
Fertilisers altogether contribute to the considerable increase in production of crops in agriculture. However there are some environmental consequences that need to be considered before increasing the use of fertilisers. Organic fertilisers though are more ‘environmentally friendly’ but these however are not as effective.
Pesticides:
In many ways crop plants are no different from plants growing under natural conditions. They compete with other species of plant for mineral ions and water in the soil and for light. Insects feed on their leaves and they suffer from diseases caused by fungi and viruses. All these things reduce the growth rate of the plant and are likely to affect of seeds they produce. When crop plants are involved, this represents an important loss. Controlling unwanted plants, insects and fungi will increase the crop yield significantly. In agriculture, plants are growing closely together, and in turn fungal diseases and insect pests can spread rapidly. Therefore it is important to control unwanted organisms, such as pests, which can cause disease; in order to obtain a good harvest.
These pests and weeds can be controlled using pesticides. Pesticides are any substance used to kill a pest. A pest in agricultural terms is a plant, animal or micro-organism which damages man, his crops or domestic animals, an organism in the wrong place, eg: a weed, an insect, a smut or rust’. The use of pesticides has enabled the production of a sufficient quantity of agricultural produce and raw materials of appropriate quality and acceptable cost. Chemical pest control has, therefore, won a central place in modern agriculture, contributing to a great increase in yields that has been achieved in recent decades for cereal, fruit and vegetable crops. The use of pesticides has also allowed growers to produce crops in otherwise unsuitable locations, extend growing seasons, maintain product quality and extend shelf life. In agriculture their uses are:
- Preventing damage to crops by destroying insects and weeds
- Improving animal welfare by controlling parasitic diseases (e.g. sheep-dip).
They maybe divided into, insecticides, which are used to combat insect pests, fungicides, which target the fungi that cause many plant diseases, and herbicides, which are used to kill weeds.
Pesticides can work in three different ways:
- Contact pesticides: these are sprayed directly onto the crop. They are absorbed by the insect through the ‘spiracles’ (tiny pores along the body). Contact herbicides and fungicides are absorbed directly through the surface. They have the advantage of being inexpensive but they often have to be reapplied because there would be some pests that avoid the pesticide.
- Systemic pesticides: are sprayed onto the crop. They are absorbed by the leaves and transported around the plant; and would poison a sap-sucking insect. These are effective because it is not necessary for the spray to come into contact with every insect. Systemic herbicides are absorbed by the leaves and kills all of the tissue.
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Residual pesticides: are sprayed onto the soil, and remain active in the soil and will kill fungal spores, insect eggs etc… as they germinate.
Insecticides are of three types. Firstly Organochlorines e.g. DDT, Aldrin and Dieldrin. They are stable and persistent, lasting over eighty years in the soil, and are non-selective, killing or reducing the fertility of most animals. Secondly Organophosphates e.g. Parathion, Malathion, Diazitol. They are highly toxic to most animals but are unstable and non-persistent, breaking down in the soil in a few days. They are also selective, so are effective against sap-sucking plants. Thirdly Carbamates e.g. Carbonyl. They are less toxic and are effective against both defoliating and sap-sucking insects.
Herbicides are of two types. The first is contact e.g. Paraquat kills all foliage on contact but breaks down rapidly in the soil. The second is systemic i.e. weedkillers which act after being absorbed through the roots.
Fungicides are of two types. Firstly contact fungicides e.g. chlorothalonil and vinclozolin. These are active outside the cells of crop plants, and either inhibit the germination of fungal spores or kill fungal hyphae in the soil or on the surface of plants. Secondly there are systemic fungicides e.g. propiconazole and benomyl. These have protective properties against fungal diseases. After absorption by plant roots, stems or leaves, systemic fungicides enter the xylem and phloem and are translocated to organs of the plant. As a result the fungicide is carried to all sites of fungal infection in the plants tissues.
However although pesticides contain properties able to diminish pests; and therefore resulting in higher yields and food productivity there are many problems that need to dealt with. Most chemicals used as pesticides are toxic and the major argument against their use is the health risk factor and the danger of environmental pollution:
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The chemical and physical characteristics of pesticides vary greatly. Some are highly persistent in the environment and others degrade rapidly, but their break down products may be toxic to some animals and plants. Pesticides usually target specific pests, but the chemicals may be toxic to non-target organisms; such as domestic animals and man…There are fears, too, that some pesticides can cause birth defects and cancer in humans
- Inaccuracy in application can result in beneficial organisms in the soil and waterways get killed as only a small amount of the spray actually reaches the target pest.
- Pesticides have the potential to pollute water.
- The use of a non-selective insecticide may cause an outbreak of the target pest it is trying to control. For example if the insecticide kills a larger proportion of natural enemies than of the pest, the pest will therefore increase in number; accentuating the problem.
- As pesticides are used often, pests soon become resistant and so therefore become ineffective.
- Predatory wildlife suffer from poisoning from pesticide residues. Also populations of farmland birds are declining significantly. This is due to the loss off food as more and more insects are killed by pesticides.
- Drinking water can become contaminated. If the pest is living in water the pesticide may be sprayed directly into the aquatic environment. DDT for example, has been widely used to control mosquito larvae for malaria. DDT is non-biodegradable so can persist in water and can contaminate sources of supply. Pesticides can also reach water supplies by runoff and leaching from the land. The pesticides are sprayed onto crops to kill insects which are reducing the yield, but if the pesticide does not degrade it can be washed into watercourses.
When water is abstracted to supply drinking water, pesticides which have not degraded are present as contaminants. The chemicals in pesticides are potentially very dangerous to man and may be toxic even at low concentrations.
There are though two possible solutions to avoid the contamination of water. This is either prevention or treatment. Prevention is to try and find a non- toxic or safer alternative. The treatment methods are Ozonation, which breaks down many organic compounds including many pesticides. The other treatment method is carbon filters. Here, their large surface area absorbs organic pollutants including pesticides.
Scientists are constantly striving towards the new production of a new ‘ideal’ pesticide; as prevention is better than treatment, due to high costs. This pesticide must be: effective at low dosage; harmless to humans and domestic animals; without adverse affects on the crops themselves; harmless to beneficial organisms i.e. bees; doesn’t leave chemical residues; bio-degradable; insoluble in water; cost-effective; no-flammable; and has a long shelf life.
Serious incidents of pollution due to pesticides are however rare, comprising less than 1% of incidents. However, when they do occur they can cause severe environmental damage.
Altogether though, fertilisers and pesticides have increased yields of food crops since the 1940’s. Farmers in the undeveloped countries, where food shortages are a problem, grow the most food they can and so agrochemicals (pesticides and fertilisers) are therefore an advantage. Having addressed earlier there are though several disadvantages and problems to the uses of fertilisers and pesticides in agriculture. They face problems with the environment and have several health issues (especially pesticides and drinking water). Where in the Western World, quality is often as important as quantity and so therefore these problems have to be addressed first before using certain fertilisers and pesticides.
The Scottish Agricultural College
Extracted from A New Introduction to Biology by Indge, Rowland and Baker
Extracted from a fat sheet article entitled ‘Maximising Crop Yield’
Extracted from an article ‘Problems with Chemical Fertilisers’ by John Phillips
From Haylett and Theron on the problem created by excessive and continuous use of ammonium sulphate.
The Scottish Agricultural College
extracted form an article on fertilisers in www.biologymad.com
Extracted from a fact sheet article entitled ‘Organic Farming’
The Scottish Agricultural College
Extract from a ‘New Introduction to Biology’ by Indge, Rowland and Baker
Definition from a fact sheet article entitled ‘pest control’
The Environment Agency on Pesticides
From a New Introduction to Biology by Indge, Rowland and Baker
Extract from Fact Sheet Article entitled Pest Control
Extract from Fact Sheet article entitled ‘pesticides in drinking water’
Extract from Fact Sheet article entitled ‘safe and effective pesticides’