Beyond the index scale #2: building soil indices

Author: Sophie Cath

10th December 2025

NRM

Agriculture

In our first blog post (you can read it here), we began our deep dive into NRM’s soil summary dataset for 2024-2025. We reviewed analysis results for the key parameters (phosphorus (P), potassium (K), and magnesium (Mg)) for target soil index 2, examined the proportion of samples that fell at the bottom, middle, or top of the index, and suggested what farmers can do to maintain their soils at target.

This time, we’re looking below index 2, focusing on indices 0 and 1, and offering a practical guide on building soil indices back up to target.

Click here to read NRM’s soil summary for 2024-2025.

Managing soils below target

Some farmers and growers choose to manage soil indices below current national guidelines. This can be due to pressures such as cost management, confidence in recommendations, or precision nutrient placement to improve efficiency.

However, allowing soil nutrient indices to fall too low can create longer-term challenges. Once soils are significantly depleted, rebuilding nutrient stocks becomes costly and time-consuming and can lead to poor crop health and reduced productivity in the meantime.

Building soil indices

If you manage your farm at soil indices 1 or 0, your productivity is likely lower than could be achieved by maintaining a target index of 2. Despite this, some farmers question the rationale for maintaining indices at 2 and intentionally manage one or more nutrients below target.

This approach requires careful monitoring of crop offtakes and more frequent routine soil testing to avoid further decline. Building soil indices from a low base can be a significant challenge and can be costly, depending on the period you choose to build the index over. This is because of the way phosphorus, potassium, and magnesium behave in the soil.

At very low indices, most of what is applied becomes locked up (complexed) within the soil system and is unavailable for crop uptake.

For example:

• To increase the soil Olsen phosphorus level by 10 mg/l requires around 200-300 kg P₂O₅/ha (or 435-650kg triple superphosphate/ha).
• This is not an insignificant cost: based on £523/t (AHDB October 2025), this would range between £234-£351/ha.
• To move from a mid-soil index 0 to a mid-index 2, an additional application of 100 kg of P₂O₅ per hectare would be needed over 5-10 years

Experimental evidence also suggests that applying a maintenance application of phosphorus, along with extra for buildup, is unlikely to increase crop yields. Therefore, the most cost-effective recommendation is to avoid letting soil indices drop too low in the first place.

Balancing productivity and the environment

There are additional risks associated with increasing soil phosphorus indices, especially regarding the environment. Applying additional phosphorus in the wrong conditions can increase the risk of soil runoff into surface waters. Therefore, it is essential to take every measure possible to minimise soil erosion and prevent phosphorus from escaping into water sources and negatively impacting water quality.

When building soil indices isn’t possible

There are some circumstances where achieving target index 2 simply isn’t realistic. For example, on lighter lighter-textured soils, building and maintaining soil potassium can either be difficult or impossible.

• Sandy loam texture soils may only reach the bottom of index 2-
• Loamy sands often plateau at the top of index 1

This is because these textural types lack clay particles, which reduces their ability to hold potassium in the soil system. Understanding your soil texture is essential for effectively building and maintaining soil nutrients.

What our soil analysis reveals

Soil available phosphorus (P)

The tables below illustrate the percentage of samples at the bottom, middle, or top of indices 0 and 1. “Target” refers to the midpoint of each index, which is associated with the recommended input rate.

Overall, most cereal and managed grassland soils for available phosphorus were in the top of index 0. For index 1, the results were mixed:

• Grazing soils were evenly split across bottom and middle
• Silage soils had more samples at the bottom
• Cereal soils clustered in the middle, with fewer at the top.

Tables 1 – The percentage of soil phosphorus (P) concentrations at the bottom, middle or top of index 0

As mentioned, growing crops and managing indices at such a low level will impact crop productivity, and it’s interesting to see how many soils are being managed at such low soil phosphorus levels. Fortunately, very few soils are at the bottom of index 1, with the majority at the top. However, those that are will need a significant injection of phosphorus to improve their nutrient content and to build back soil health.

The most cost-effective way is to apply organic phosphorus sources such as biosolids, livestock manures, or any other organic waste rich in phosphorus. Close monitoring of crops during the index build period will ensure soil nutrient content remains on track and doesn’t become excessive over time.

Soil available potassium (K)

When soil indices are low, potassium behaves in a similar way to phosphorus.

As you can see in the tables below, most samples within index 0 were concentrated at the top, while index 1 samples were generally mid-index, with fewer samples below and above.

Tables 2 – The percentage of soil potassium (K) concentrations at the bottom, middle or top of index 0 and 1

 Soil magnesium (Mg)

Table 3 – The percentage of soil magnesium (Mg) at the bottom, middle or top of soil index 0 and 1

At index 1, most soils under grassland management (for grazing or cutting) were near the top of the index. However, cereal soils showed a wider distribution, with samples spread more evenly across the range.

Managing magnesium supply in grasslands should be simpler due to access to organic manure, but applying these inputs does not necessarily mean the forage grown contains enough to meet livestock dietary requirements. Low magnesium in animal diets can increase the risk of grass staggers, so maintaining sufficient soil magnesium and supplementing livestock diets where necessary is vital. For cereal systems, adding organic manures or magnesium fertilisers supports both soil health and crop performance.

How NRM can help

NRM offers a comprehensive range of soil and crop analysis services designed to help you:

• Identify potential nutrient issues before they impact yield
• Optimise fertiliser applications
• Maintain soil indices at the levels best suited to your land and crops.

Contact us to learn about how analysis can help you manage your business.

Keep an eye out for our next blog on how to manage excess nutrients, coming soon!

Reference links

GB fertiliser prices | AHDB

RB209 Section 2 Organic materials | AHDB

Start GrainCheck – NRM