5.4.4 Fertilisers

• When organisms in natural ecosystems die or produce waste, the waste and dead matter decompose
  • This decomposition is carried out by saprobionts

• This ensures that valuable nutrients in the waste and dead matter are continually recycled back into these natural ecosystems
  • For example, some bacteria in the soil convert the nitrogen in organic substances (such as the proteins and nucleic acids in dead organisms) into nitrate, which can then be taken up and used by producers

Agricultural Ecosystems

• In agricultural ecosystems, crops and livestock take in nutrients (e.g. inorganic 'mineral' ions) from the soil (or from the grass that grows in the soil) as they grow and use these nutrients to generate biomass
• However, agricultural ecosystems are not like natural ecosystems because the crops or livestock are eventually removed from the fields instead of dying and decomposing there naturally
  • As a result, the mineral ions (e.g. nitrates and phosphates) now contained in the biomass of these crops or livestock are not returned to the soil by microorganisms
  • This interrupts the crucial processes of nutrient recycling (e.g. the nitrogen and phosphorus cycles)
  • If the interruption of these nutrient cycles occurs over a long enough time period, the concentration of nutrients in the soil will decrease, eventually leading to a decrease in crop yields or meat and milk yields from livestock

• Adding fertilisers to fields is a way of replacing the minerals lost from agricultural ecosystems
  • This ensures crops and livestock can continue to grow and increase in biomass as normal, ensuring yields remain high

Fertilisers

• Fertilisers can be used to add important mineral ions, such as ions of nitrogen, phosphorus and potassium, back into the soil
• There are two types of fertilisers:
  • Natural fertilisers
  • Artificial fertilisers

Natural fertilisers

• Natural fertilisers are made up of organic matter in the form of the dead and decomposing remains of organisms and their waste products
  • This includes manure, composted vegetables, crops residues (crop parts left over after harvesting) and sewage

• They can also improve soil structure, which helps in reducing soil erosion and increase the water-holding ability of the soil
• Nutrients from natural fertilisers are released over long time periods
• The nutrients present are not very concentrated so relatively large amounts are needed

Artificial fertilisers

• Artificial fertilisers are made up of inorganic matter in the form of powders or pellets that contain pure chemical compounds (e.g. ammonium nitrate)
• As the exact chemical composition is known, it is easier to how much to apply and the effects they will have on crop yields
• The nutrients present are concentrated so smaller amounts are needed
  • This means transport costs are lower

• These fertilisers are easy to apply evenly and are clean, making them easy to handle

• As fertilisers are very effective in ensuring high crop yields, they are often applied to fields by farmers in greater quantities than are actually needed by the crop plants
• As the crop plants are unable to use all the fertiliser provided, the soluble nitrate and phosphate ions in the excess fertiliser are not taken up by the crop plants and remain in the soil water
• These mineral ions can then be transported by rainwater or the water from irrigation systems into nearby bodies of water (such as ponds and lakes) or waterways (such as streams and rivers)
  • This process is known as leaching
  • Leaching is more likely to occur if fertilisers are applied just before heavy rainfall

• In artificial fertilisers, the inorganic ions are readily soluble and if they are not used immediately by crop plants, they can quickly leach into waterways
• In natural fertilisers, the minerals (e.g. nitrogen and phosphorus) are contained within organic matter that must first be decomposed by microorganisms before the mineral ions can be absorbed by crop plants
  • This means the release of the mineral ions into the soil is slower and more controlled, making leaching less likely

• In general, phosphate leaching occurs to a lesser extent than nitrate leaching, as phosphates are less soluble in water
• Leaching can lead to a potentially damaging process known as eutrophication

Eutrophication

• When the mineral ions from excess fertiliser leach from farmland into waterways, they cause rapid growth of algae at the surface of the water
  • This is known as an algal bloom

• This blocks sunlight so aquatic plants below the surface of the water start to die as they can no longer photosynthesise
  • The algae also start to die when competition for nutrients becomes too intense

• As aquatic plants and algae die in increasing numbers, decomposing bacteria feed on the dead organic matter and also increase in number
  • As they respire aerobically, these bacteria use up the dissolved oxygen in the water

• As a result, the amount of dissolved oxygen in the water rapidly decreases, so aquatic organisms such as fish and insects may be unable to survive

The sequence of events leading to eutrophication of water bodies and waterways