5-4-4-fertilisers

Natural & artificial fertilisers

• In natural ecosystems, decomposition by saprobionts recycles nutrients from waste and dead organisms back into the soil

• In agricultural ecosystems, crops and livestock are harvested and removed, so the nutrients contained in their biomass are not returned to the soil

  • This disrupts nutrient cycles and can lead to reduced soil fertility

• To maintain productivity, fertilisers are used to replace lost mineral ions

Natural fertilisers

• Natural fertilisers are made from organic matter, such as:

  • manure

  • compost

  • crop residues

  • sewage

Artificial fertilisers

• Artificial fertilisers are made up of inorganic matter in the form of powders or pellets that contain chemical compounds, e.g. ammonium nitrate

Environmental issues caused by fertilisers

• Fertilisers are often applied in larger quantities than crops require; as a result, excess mineral ions may remain dissolved in the soil water

• These mineral ions can be carried by rainwater into nearby rivers, lakes or streams; this is known as leaching

  • Leaching is more likely to occur:

    • after heavy rainfall

    • when using artificial fertilisers, as the inorganic ions are highly soluble

  • Leaching is less likely to occur with natural fertilisers as organic matter must be decomposed by microorganisms before minerals become water-soluble

Eutrophication

• Leaching can lead to a process called eutrophication, which occurs as follows:

  1. mineral ions enter water bodies, causing rapid growth of algae at the surface

     • This is known as an algal bloom

  2. algae block sunlight, which prevents aquatic plants below the surface from photosynthesising

  3. these plants, and eventually the algae, begin to die and dead organic matter accumulates

  4. bacteria decompose the dead matter, respiring aerobically and using up the oxygen dissolved in the water

  5. oxygen levels fall and aquatic animals such as fish and insects can no longer survive