Fertilisers

This lesson covers: 

1. How harvesting crops removes nutrients from soils
2. How fertilisers can replace lost nutrients
3. Artificial and natural fertilisers
4. Environmental impacts of fertiliser use
5. Eutrophication

How harvesting crops removes nutrients from soils

As crops grow, they absorb essential mineral ions such as nitrates and phosphates from the soil. These nutrients are crucial for building plant tissues.

However, when crops are harvested, the parts of the plant that contain these vital nutrients are removed from the soil. Animals that consume crops also remove nutrients.

The implications of removing crops include:

• Mineral ions are not returned to the soil through decomposition.
• Nutrients are extracted from the nitrogen and phosphorus cycles.
• There is a net export of nutrients from agricultural land, depleting soil fertility over time.
• This limits plant growth and reduces productivity.

How fertilisers can replace lost nutrients

Fertilisers are used to reintroduce nutrients like mineral ions back into the soil to compensate for those lost during harvesting.

There are two primary types of fertilisers:

1. Artificial (inorganic) fertilisers - These are chemically made for a specific crop, and consist of an appropriate balance of mineral nutrients mined from rocks for that plant. They are applied in forms like powders or pellets.
2. Natural (organic) fertilisers - These are made from organic matter like manure, compost, or crop residues, which release nutrients gradually as the organic matter decomposes.

The application of either type of fertiliser can provide 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.

Environmental impacts of fertiliser use

There are several environmental impacts caused by the excessive or improper application of fertilisers, particularly those rich in nitrogen.

Potential negative environmental impacts of excessive nitrogen-rich fertiliser use:

1. Reduced species diversity - Nitrogen-rich soils favour the growth of rapidly growing species like grasses, which outcompete other species.
2. Leaching - Nutrients like nitrates are washed out of soil and may pollute bodies of water.
3. Eutrophication - This is caused by leaching of excess nutrients in fertilisers into bodies of water.

Leaching

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.

Problems caused by leaching:

• Harmful for the environment - Leaching of mineral nutrients into waterways can trigger eutrophication. 
• Harmful for human health - Nitrogen-rich water may enter drinking water and its consumption can increase the risk of oxygen-transport issues in babies and stomach cancer.

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. 

The process of eutrophication:

1. Excess soluble nitrates and phosphates leach into water sources like ponds from the soil.
2. This leads to the rapid growth of algae at the surface of the water source, sometimes called an 'algal bloom'.
3. The algae then block light to plants and algae at lower depths.
4. These plants and algae die due to insufficient light for photosynthesis.
5. Levels of aerobic saprobiontic bacteria increase as they decompose the dead plant material.
6. This depletes oxygen levels in the water, results in the death of fish and other aquatic life, disrupting food webs.
7. Anaerobic organisms increase in number, further decomposing dead material and releasing more nitrates and toxic wastes.

Artificial fertilisers are particularly prone to leaching because their mineral nutrients dissolve easily in water, and are the main cause of eutrophication. The risk of eutrophication may also be increased with increased use of organic manures or animal slurry, human sewage, ploughing old grassland, or natural leaching.