Fetilisers (AQA A-Level Biology): Revision Notes
Fertilisers
The need for fertilisers
Plants require mineral ions, particularly nitrates, from soil to support their growth and development.
In natural ecosystems, these minerals cycle back into the soil when plants decompose after death, maintaining soil fertility through a continuous recycling process.
However, intensive farming practices disrupt this natural cycle. When crops are harvested or animals are transported away from the growing area, the mineral ions they absorbed are permanently removed from the soil. This creates a significant problem - the soil becomes progressively depleted of essential nutrients.
Without intervention, mineral ion concentrations in agricultural soils will decline below the levels needed for optimal plant growth. This reduction becomes the limiting factor for productivity, meaning crop yields will fall dramatically unless the missing minerals are replaced through fertiliser application.
Types of fertilisers
Agricultural fertilisers fall into two main categories, each with distinct characteristics and sources:
- Natural (organic) fertilisers are derived from biological materials. These include decomposed plant and animal remains, along with animal waste products such as manure, slurry, and bone meal. These materials break down gradually in the soil, releasing nutrients over an extended period.
- Artificial (inorganic) fertilisers are manufactured products created from mined rocks and mineral deposits. These materials undergo processing and are blended to provide specific nutrient ratios tailored to particular crops. Most artificial fertilisers contain the three key elements: nitrogen, phosphorus, and potassium (NPK).
Research indicates that combining both natural and artificial fertilisers produces the most effective long-term productivity gains in agricultural systems.
How fertilisers increase productivity
Nitrogen serves as an excellent example of how fertiliser application enhances crop performance. This element is essential for synthesising vital biological molecules including amino acids, ATP, and nucleotides used in DNA construction.
When nitrogen-containing fertilisers make nitrate ions readily available in soil, plants can utilise these nutrients more effectively. This leads to several improvements:
Benefits of adequate nitrogen availability:
- Earlier plant development
- Increased plant height
- Greater leaf surface area
- Enhanced rate of photosynthesis
- Improved overall crop productivity
The availability of adequate nitrogen allows plants to maximise their growth potential rather than being constrained by nutrient deficiency.
Law of diminishing returns
Fertiliser application follows the law of diminishing returns. Initially, adding fertiliser produces substantial increases in crop yield. However, beyond an optimal point, additional fertiliser applications result in progressively smaller improvements in productivity.
Eventually, further increases in fertiliser quantity provide no additional benefit to crop yield. This occurs because other factors (such as water availability, temperature, or light) become the limiting factors for growth, rather than nutrient availability.
Key insight: More fertiliser is not always better - optimal application rates are crucial for both economic and environmental reasons.
Comparing different nitrogen fertiliser forms
Various forms of nitrogen-containing fertilisers show different effectiveness levels:
Fertiliser Effectiveness Comparison:
Highest effectiveness:
- Ammonium nitrate - produces the highest productivity gains
Moderate effectiveness:
- Manure and urea - provide moderate improvements
Lower but significant effectiveness:
- Bone meal and ammonium sulphate - show lower but still significant benefits
The variation in effectiveness relates to how quickly different fertiliser forms release nitrogen into forms plants can absorb. Manure requires additional time for decomposition before its nitrogen becomes available to plants, which explains its initially lower impact compared to readily soluble forms like ammonium nitrate.
Strategic Application Timing: Farmers can strategically time manure application several months before planting to allow adequate decomposition time, maximising the fertiliser's effectiveness.
Remember!
Key Points to Remember:
- Intensive farming removes mineral ions from soil permanently, requiring fertiliser replacement to maintain productivity
- Natural fertilisers consist of organic matter while artificial fertilisers are manufactured from mined materials
- Nitrogen is crucial for plant growth as it forms amino acids, ATP, and DNA components
- Fertiliser application follows diminishing returns - more is not always better
- Different fertiliser forms release nutrients at different rates, affecting their immediate effectiveness