Fertilisers

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Fertilisers

Farming breaks the mineral cycles, since minerals taken from the soil by crops or animals are not returned to the same field.

This break means that the soil is gradually depleted of minerals, so the crops don’t grow as well.

It applies to all minerals and is an inevitable problem of allfarming (not just intensive farming).

The rate of  plant growth is generally limited by the availability of  mineral ions in the soil, particularly nitrogen, phosphorus and potassium (NPK), so farm land has always neededto be fertilised in with these minerals some way.

There are three ways this fertilisation can be done.

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Nitrogen Fixing Crop.

  •  A traditional way to replace lost minerals is to  use a crop rotation that
    includes a legumecrop such as clover for one year in four.
  • During that crop’s growing season, the nitrogen-fixing bacteria in the clover’s root nodules make ammonia and organic nitrogen compounds from atmospheric nitrogen.
  •  Crucially, the clover is not harvested, but instead the whole crop (or sometimes just the roots) is simply ploughed back into the soil.
  • The nitrogen that was fixed by the symbiotic bacteria in the clover’s root nodules (together with the other minerals that were taken up) is thus made available to crops for the following three years.
  • A mixture of clover and grass, called ley,does the same job for grazing pasture.

Inorganic Fertilisers.

  • The most commonly used fertilisers are the soluble  inorganic fertilisers containing nitrate, phosphateand potassium ions (NPK).
  • Inorganic fertilisers arevery effective, easy to apply, and can be tailored to
    each crop’s individual mineral requirements, but they can also have undesirable effects on the environment.
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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 and causing eutrophication.
  • They are also expensive and their manufacture is very energy-intensive, requiring fossil fuels, so it contributes to the greenhouse effect.

Organic Fertilisers.

  • An alternative solution, which may do less harm to the environment, is the use of
    natural (or organic) fertilisers, such as animal manure (farmyard manure or FYM), composted vegetable matter, crop residues, and sewage sludge.
  •  Not surprisingly, organic fertilisers are commonly just referred to as muck.
  • They contain the main elementsfound in inorganic fertilisers (NPK), but contained
    in organic compounds such as urea, proteins, lipidsand organic acids.
  • Of course plants cannot make use of these organic materials in the soil: their roots can only take up inorganic mineral ions such as nitrate, phosphate and potassium.
  • But the organic compounds  can be digested by the soil decomposers, who
    then release inorganic ions that the plants can use(refer to the nitrogen cycle).
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  • Since the compounds in organic fertilisers are lesssoluble than those in inorganic fertilisers, the inorganic minerals are released more slowly as they are decomposed.
  • This prevents leaching and means they last longer.
  • Organic fertilisers are cheap, since the organic wastes need to be disposed of anyway.
  • Furthermore, spreading on to fields means the muck  will not be dumped in landfill sites, where it may cause uncontrolled leaching.
  • The organic material improves soil structure by binding soil particles together and provides food for soil organisms such as earthworms.
  • This improves drainage and aeration.
  • Some disadvantages of organic fertilisers are that  they are bulky and less concentrated in minerals than inorganic fertilisers, so more needs to be spread on a field to have a similar effect, and they need heavy machinery to spread, which can damage the soil.
  •  Organic fertilisers may contain unwanted substances such as weed seeds, fungal spores and heavy metals.They are also very smelly!
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Eutrophication

Refers to the effects of nutrients on aquatic ecosystems.

In particular it means a sudden and dramatic increase in nutrients due to human activity, which disturbs and eventually destroys the food web.

The main causes are fertilisers leachingoff farm fields into the surrounding water course,and sewage(liquid waste from houses and factories).

These both contain dissolved minerals, such as nitrates and phosphates, which enrich the water.

  • Since producer growth is generally limited by availability of minerals, a sudden increase in these causes a sudden increase in producer growth.
  •  Algae grow faster thanlarger plants, so they show a more obvious “bloom”.
  • Algae produce oxygen, so at this point the ecosystem is well oxygenated and fish will thrive.
  • The fast-growing algae will out-compete larger plants for light, causing the plants to die.
  • The algae also grow faster than their consumers, so many will die without being consumed, which is not normal.
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  • These both lead to a sudden increase in detritus.
  • Sewage  may also contain organic matter, which adds to the detritus.
  • Microbial saprobionts can multiply quickly in response to this increase in detritus, and being aerobic they use up oxygen faster than it can be replaced by photosynthesis or diffusion from the air.
  • The decreased oxygen concentration kills larger aerobic animals and encourages the growth of anaerobic bacteria, who release toxic waste products.
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