Artificial fertilisers are chemically made and contain specific amounts of nutrients. They are mined from rocks and converted into different forms to give an appropriate balance of minerals for a specific crop.

Natural fertilisers are organic materials like compost or manure. They consist of the dead and decaying remains of plants and animals, as well as animal wastes. 

1. Reduced species diversity - nitrogen-rich soils favour the growth of rapidly growing species like grasses, which outcompete other species
2. Leaching - may lead to pollution of bodies of water
3. Eutrophication - caused by leaching of fertiliser into bodies of water

Leaching is the process by which nutrients, such as nitrate ions, are washed out from the soil by rainwater and carried deep into the soil and into water in lakes and rivers, beyond the reach of plant roots.

Leaching is dangerous for human health because it can lead to high nitrate ion concentrations in drinking water, affecting oxygen transport in babies and increasing the risk of stomach cancer.

Leaching is dangerous for the environment as it can cause eutrophication.

Eutrophication is the process by which nutrient concentrations increase in bodies of water, leading to excessive growth of algae, which can significantly disrupt the ecosystem.

1. Organic manures
2. Animal slurry
3. Human sewage
4. Ploughing old grassland
5. Natural leaching
6. Leaching of artificial fertilisers

The leaching of artificial fertilisers is the main cause of eutrophication.

An increase in nitrate ion concentration can lead to excessive surface algal growth, known as an 'algal bloom'.

This happens because nitrate is often a limiting factor for algal growth.

1. Increased surface algal growth blocks light from plants and algae at lower depths, killing them
2. Levels of saprobiontic bacteria increase as they decompose the dead material
3. Aerobic saprobiontic bacteria deplete oxygen levels, causing aerobic organisms like fish to die
4. Anaerobic organisms thrive, further decomposing dead material and releasing more nitrates and toxic wastes, making the water unsuitable for drinking
5. The loss of aerobic organisms disrupts the food supply for higher-level predators

Fertilisers are needed in agricultural ecosystems to replenish the soil with mineral ions, especially nitrates, that are continually taken up by crops and are not returned when the crop is harvested.

Without fertilisers, the reduced concentrations of these minerals would limit plant growth and reduce productivity.

Fertilisers increase productivity by providing plants with the essential minerals they need for growth.

For example, nitrogen in fertilisers is an essential component of amino acids and nucleotides in DNA, increases the rates of photosynthesis and growth, and improves crop productivity.