Getting fertiliser application right is essential for maintaining soil health, achieving high crop yields, and protecting the environment. Over-application leads to waste and pollution, while under-application can limit productivity. So how do you know how much fertiliser to add to your farmland?

In this blog, we’ll break down the process into clear, practical steps to help UK farmers make informed fertiliser decisions and avoid common pitfalls.

Fertiliser is vital for crop nutrition, supplying key elements like nitrogen (N), phosphorus (P), and potassium (K). However, too much can:

• Damage soil structure
• Leach into waterways
• Cause nutrient imbalances
• Increase production costs

Knowing how much to apply ensures better crop performance and environmental stewardship.

1. Start with a Soil Test

A soil test is the most reliable way to understand your land’s nutrient needs. Testing gives insight into:

• Soil pH
• Available nutrients (N, P, K)
• Organic matter content
• Trace elements

Soil testing should be done every 2–3 years, or more frequently for intensive cropping systems.

Tip: Use an accredited lab and take samples from representative areas of the field to get accurate results.

2. Understand Crop Nutrient Requirements

Each crop has specific nutrient needs. For example:

• Wheat requires high nitrogen input
• Potatoes need more potassium
• Legumes may fix their own nitrogen and need less applied N

Refer to AHDB nutrient management guides (RB209) for reliable, UK-specific recommendations.

3. Calculate the Nutrient Deficit

Once you know how much nutrient your crop needs and how much your soil already contains, you can calculate the deficit—that’s the amount you need to add.

Example:
If your crop requires 180kg/ha of nitrogen and your soil provides 40kg/ha, your fertiliser application should supply the remaining 140kg/ha.

4. Choose the Right Fertiliser Type

Different fertilisers contain different nutrient concentrations. Use the label or product spec sheet to match the correct fertiliser to your deficit.

• Ammonium nitrate (34.5% N)
• Triple superphosphate (46% P)
• Muriate of potash (60% K)

Convert the required nutrient weight into product weight using simple calculations or online fertiliser calculators.

5. Apply at the Right Time and in the Right Way

Timing is crucial. Avoid applying:

• Nitrogen on frozen or waterlogged soils
• Before heavy rain
• Too close to watercourses

Use precision equipment like GPS-guided spreaders or variable rate applicators to reduce waste and improve accuracy.

6. Monitor and Adjust Over Time

No two seasons are the same. Keep records of applications, crop performance, and weather conditions to adjust rates and timing in future years. Tools like nutrient management plans can help streamline this process.

Knowing how much fertiliser to apply is a balancing act that depends on accurate data, good timing, and understanding your soil and crop needs. By following a science-based approach—including soil testing, crop planning, and precision application—UK farmers can increase profitability, reduce input waste, and protect the environment.

How often should I test my soil for nutrient levels?

At least every 2–3 years, or more frequently in high-value or intensive systems.

What tools can help me calculate fertiliser rates?

Use online fertiliser rate calculators or refer to the AHDB RB209 guide for UK-specific advice.

Can I use organic fertiliser instead of chemical fertiliser?

Yes. Organic options like compost or manure can supplement or replace synthetic fertilisers, depending on crop and soil needs.

Is it OK to guess the amount of fertiliser?

No. Guesswork often leads to over- or under-application. A simple soil test can prevent costly mistakes.

How do weather conditions affect fertiliser use?

Rain and temperature can influence nutrient uptake and risk of runoff. Always check forecasts before applying.