Fertilisers and Eutrophication

• loss of nutrients:

  • crops take in minerals from the soil as they grow and use them to build their own tissues
  • when crops are harvested, they are removed from the field where they've grown rather than being allowed to die and decompose there
  • this means the mineral ions that they contain (e.g. phosphates and nitrates) are not returned to the soil by decomposers in the nitrogen or phosphorus cycles
• • phosphates and nitrates are also lost from the system when animals or animal products are removed from the land
  • animals eat grass and other plants, taking in their nutrients
  • when they are taken elsewhere for slaughter or transferred to a different field, the nutrients aren't replaced through their remians or waste products
• using fertilisers:

  • adding fertiliser replaces the lost minerals so more energy from the ecosystem can be used for growth, increasing the efficiency of energy transfer
  • can be artificial or natural
• • artificial fertilisers are inorganic - they contain pure chemicals (e.g. ammonium nitrate) as powder or pellets
• • natural fertilisers are organic matter - include manure, composted vegatables, crop residues (parts left over after the harvest) and sewage sludge
• environmental issues:

  • sometimes more fertiliser is applied than the plants need/ are able to use at a particular time
  • can lead to fertilisers leaching into waterways
• leaching is when water-soluble compounds in the soil are washed into nearby ponds and rivers - can lead to eutrophication
• • inorganic ions in chemical fertilisers are relatively soluble - this means that excess minerals that are not used immediately are more likely to leach into waterways
  • leaching is also more likely to occur if the fertiliser is applied just before heavy rainfall
• • leaching is less likely with natural fertilisers - bc the nitrogen and phosphorus are still contained in organic molecules that need to be decomposed by microorganisms before they can be absorbed by plants
  • this means that their release into the soil for uptake by plants is more controlled
• • the leaching of phosphates is less likely than the leaching of nitrates bc phosphates are less soluble in water
• • using fertilisers may also change the balance of nutrients in the soil - too much of a particular nutrient can cause crops and other plants to die
• eutrophication is caused by excess nutrients
• 1. mineral ions leached from fertilised fields stimulate the rapid growth of algae in ponds and rivers
  2. large amounts of algae block light from reaching the plants below
  3. eventually the plants die bc they're unable to photosynth enough
  4. bacteria feed on the dead plant matter - the increased no. of bacteria reduce the oxygen concentration in the water by carrying out aerobic respiration
  5. fish and other aquatic organisms die bc there isn't enough dissolved oxygen