Fertilisers and the Environment

Environmental Impact of Fertilisers

Fertilisers are essential for modern agriculture, but their use can harm the environment if not managed carefully.

Nitrate Leaching into Water

Excess nitrates from fertilisers can dissolve in rainwater and wash through the soil into rivers, lakes, and groundwater. This process is called leaching. High nitrate levels in drinking water can be harmful to human health, especially for babies.

Eutrophication Effects

When nitrates enter water bodies, they act as nutrients for algae, causing rapid growth known as algal blooms. These blooms block sunlight, killing underwater plants. When algae die, bacteria decompose them, using up oxygen in the water. This oxygen depletion kills fish and other aquatic life, damaging ecosystems.

Soil Nutrient Imbalance

Overuse of fertilisers can disrupt the natural balance of nutrients in soil. Some nutrients may build up while others are depleted, reducing soil fertility over time and harming beneficial soil organisms.

Sources of Fertilisers

Fertilisers can be natural or synthetic, each with different origins and impacts.

Natural vs Synthetic Fertilisers

• Natural fertilisers come from organic sources like manure, compost, or bone meal. They release nutrients slowly and improve soil structure.
• Synthetic fertilisers are man-made chemicals that provide nutrients quickly and in precise amounts. They are widely used in modern farming.

Role of Haber Process in Ammonia Production

The Haber process produces ammonia by combining nitrogen from the air with hydrogen, usually from natural gas, under high temperature and pressure with an iron catalyst. This ammonia is a key ingredient in many synthetic fertilisers, especially nitrogen-based ones.

Ammonia can be converted into ammonium salts like ammonium nitrate, which are used directly as fertilisers.

Types of NPK Fertilisers

NPK fertilisers contain three essential nutrients:

• N 013 Nitrogen: promotes leaf and stem growth
• P 013 Phosphorus: encourages root development and flowering
• K 013 Potassium: improves overall plant health and disease resistance

They are often sold as mixtures with different ratios, e.g. 10:10:10 or 15:5:20, depending on the crop
9s needs.

Nitrogen Cycle and Fertilisers

Fertilisers interact with the nitrogen cycle, which is vital for maintaining soil fertility and plant growth.

Nitrogen Fixation by Haber Process

Nitrogen gas (N 2) makes up about 78% of the air but is inert and unavailable directly to plants. The Haber process fixes nitrogen by converting N 2 into ammonia (NH 3), a form plants can use after further conversion.

Ammonium Salts in Soil

Ammonia produced industrially is often turned into ammonium salts like ammonium nitrate or ammonium sulfate. When added to soil, these salts dissolve and release ammonium ions (NH 4), which plants absorb as a nitrogen source.

Impact on Nitrogen Cycle Balance

Synthetic fertilisers increase the nitrogen available in soil, speeding up plant growth. However, excessive use can disrupt the natural nitrogen cycle by:

• Reducing nitrogen fixation by soil bacteria
• Increasing nitrogen losses through leaching and denitrification
• Altering microbial communities in the soil

This imbalance can reduce soil health and lead to environmental problems like pollution.

For instance, when ammonium nitrate fertiliser dissolves in soil water, it releases nitrogen ions plants can use:

$\mathrm{NH_4NO_3 \rightarrow NH_4^+ + NO_3^-}$

Reducing Environmental Damage

Farmers and scientists use several methods to reduce the negative effects of fertilisers on the environment.

Controlled Fertiliser Application

Applying fertilisers at the right time, in the correct amounts, and in suitable weather conditions helps reduce nitrate leaching and runoff. For example, avoiding fertiliser use before heavy rain prevents nutrients washing away.

Use of Slow-Release Fertilisers

Slow-release fertilisers release nutrients gradually over weeks or months. This matches plant nutrient uptake better and reduces excess nutrients in soil that could leach away.

Crop Rotation and Soil Management

Rotating crops, especially including legumes like peas or beans, helps maintain soil nitrogen naturally. Legumes have nitrogen-fixing bacteria in their roots that add nitrogen to the soil, reducing the need for synthetic fertilisers.

Good soil management, such as adding organic matter and avoiding over-tilling, also preserves soil health and nutrient balance.

For example, if a farmer applies 50 kg of nitrogen fertiliser per hectare but only 30 kg is absorbed by crops, the remaining 20 kg can leach into water or be lost as gases, causing pollution. Controlled application aims to minimise this loss.