Beyond the index scale #3: how to manage excess nutrients

Author: Sophie Cath

16th December 2025

NRM

Agriculture

In our first two blogs (on the national index scale and on building soil indices), we discussed the soil index scale based on NRM’s recently published soil summary for 2024-2025.

Our third blog in the series continues this discussion. Today, we’re exploring the number of samples where the index for phosphorus (P), potassium (K), and magnesium (Mg) exceeded the target index 2, and how to manage excess nutrients.

High soil indices and how to manage excess nutrients

A soil index above 2 typically indicates high fertility resulting from excessive nutrient inputs and/or lower than expected crop offtake. The problem with managing soils with higher indices is the increased risk they pose to the environment, particularly to watercourses and water quality.

In England and Wales, approximately 25% of the phosphates in water come from agricultural land. At high levels, they can harm aquatic life and disrupt the biological balance. Elevated levels can also increase the cost of water treatment, which is ultimately passed onto consumers through their water bills.

Surpluses that do not improve productivity or crop quality represent an avoidable business cost, which can introduce margin uncertainty and result in poor returns on investment. Protecting soil nutrients and recognising them as valuable, finite assets (especially phosphate, with global estimates of only 70-130 years’ supply remaining) can help agriculture collectively manage resources better, supporting profitable and sustainable farming.

Why excess phosphate is harmful

Phosphate is a serious threat if it escapes into the water environment. It causes blue-green algal blooms, which deplete the water of oxygen, suffocating all other life. The impact of algal blooms on ecosystems can be devastating, resulting in:

• Increased oxygen demand, causing anoxic (low oxygen) conditions that are harmful to fish and other aquatic organisms
• Blocked sunlight due to algal blooms on the water’s surface, which can severely affect plant growth beneath the surface, leading to the death and loss of species from ecosystems
• Overall biodiversity loss and long-term habitat damage.

The first step in reducing risk is by reviewing soil and land management practices to minimise erosion.

Mitigating soil erosion to manage excess nutrients

More often than not, phosphate reaches water through soil surface runoff. When soil particles are eroded from fields, they enter the water system by being washed into surface drains, ditches, streams, and rivers. Phosphorus is attached to these soil particles, which is mainly how it enters water.

However, scientists are exploring and collecting evidence of phosphate leaching deep into the soil profile on sandier or silty soils, which is thought to be a secondary pathway to water. Reviewing how farmers and growers manage their soils to prevent conditions that lead to soil erosion is a good first step in reducing the risk of it occurring.

Mitigation strategies for soil erosion include:

• Maintaining ground cover over autumn and winter by establishing a crop or cover crops to protect the soil from rain and wind.
• Contour ploughing (across slopes) when cultivating land or minimum tillage. This is effective in minimising water flowing down slopes.
• Reducing the impact of wind and water erosion by using windbreaks and riparian buffers.

A major ongoing project in the River Wye catchment in Herefordshire, led by the Environment Agency, Lancaster University, farmers, and local stakeholders, shows how collaborative action can address phosphate pollution at landscape scale. Click here to watch two short videos on how the team is tackling phosphorus challenges in the Wye.

What NRM’s data shows

A significant proportion of soil samples analysed between June 2024 and May 2025 exceeded target Index 2. Approximately 40% of arable soils and grassland soils managed for silage and grazing exceeded the target.

For most broadacre crops and managed grassland, maintaining soil phosphorus above Index 2 is unnecessary. However, some do argue that establishment benefits can be realised when phosphorus is applied at planting, even when total soil supply is sufficient, but this should be balanced against long-term accumulation and environmental risk.

Phosphorus (P) levels in index 3

Table 1 below shows the proportion of analysis results that were at the bottom, middle, or top of index 3. In all three crop management systems, most samples analysed at index 3 fell at the bottom, with fewer found at the top of the index in areas where soils were grazed or a cereal crop was grown. Silage fields had a greater proportion at the top compared with grazing or cereals.

An application of phosphorus is typically recommended for first-cut silage when soil phosphorus is in the middle or lower part of Index 3. In this instance, 20 kg P₂O₅/ha is required. However, if levels are at the top of Index 3, consider omitting any further phosphorus inputs or verify that the crop is removing more than is applied to prevent further accumulation and to avoid continued accumulation.

Table 1 – The percentage of soil P concentrations at the bottom, middle or top of the index 3

Potassium (K) levels in Index 3

Table 2 below shows the distribution of soil potassium results across Index 3. The story followed a similar pattern to phosphorus: most results at the bottom of Index 3, fewer in the middle, and fewer at the top.

While there are no environmental pressures to keep the soil potassium index below 3, there is also no economic benefit in maintaining reserves above Index 2. It costs the farm business more money to manage an index above 2 for minimal, if not zero, productive gain. When every penny counts on all but the largest farms, this is an area where cost savings can be made.

Table 2 – The percentage of soil K concentrations at the bottom, middle or top of index 3

Magnesium (Mg) levels and their impacts

Soil magnesium results also showed a clustering at the bottom of Index 3, with fewer samples in the middle or top. Crop type did not significantly affect distribution.

When organic inputs are applied in excess and crop offtake is less than expected, indices can gradually increase, and the risk of soil nutrient antagonisms can arise. For example, in an intensive grassland system, high potassium and nitrogen inputs can result in reduced magnesium uptake by the herbage, especially if soil magnesium levels are low.

This can lead to livestock health issues, including hypomagnesemia (grass staggers), where the animal is unable to absorb sufficient magnesium through its diet. Analysing the herbage/forage to confirm that the magnesium content is at least 0.20% (DM basis) and ensuring the K:Mg ratio does not exceed 20:1 can help determine risk levels.

If soils continue to accumulate magnesium to very high levels (index 4 or 5 and above), soil structure can be negatively impacted. Some high-magnesium soils are described as “tight,” as magnesium strongly binds to clay particles. This can result in reduced aeration, infiltration, and root growth.

To improve soil structure in this case:

• Minimise magnesium-containing inputs
• Apply calcium-based liming products if the soil pH needs correction
• Introduce greater plant biodiversity into the cropping rotation. Growing plant species with varying rooting densities and types can help break up compaction at different soil depths, helping to ease and open up the soil structure.

Table 3 – The percentage of soil Mg at the bottom, middle or top of soil index 3

Managing nutrients for long-term sustainability

As an industry, growers are both encouraged and incentivised to manage the nutrients in their soils responsibly, and for very good reasons. Managing resources more effectively and protecting the environment from nutrient excesses ensures that our food system remains sustainable now and in the future.

Looking beyond the index when we review soil analysis results reveals deeper insights into how growers are managing their farming systems. Viewing analysis results through a more focused lens allows them to adapt their farming practices more effectively to benefit the soil, productivity, and the environment.

Our final blog in this series will examine how soil pH results vary across the soils analysed and highlights their significance for sustainable production. Click here to sign up to our blog mailing list to be notified when it’s published.

How NRM can help

NRM offers a range of services that can help you identify potential issues in your soils and crops, allowing you to take prompt action to rectify them. Contact us to learn about how analysis can help you manage your business.

References

Approaches to Agriculture Video Series | Engage Environment Agency

RB209 Section 1 Principles of nutrient management and fertiliser use | AHDB

RB209 Section 3 Grass and forage crops | AHDB