Over the past four years, Terrafarmer has been working in partnership with Anglo American to better understand how POLY4 influences soil health and agronomic performance across a range of UK farming systems.
POLY4 is a naturally occurring, multi-nutrient fertiliser made from the mineral polyhalite, mined in the UK and produced by Anglo American’s Crop Nutrients business. It supplies four essential nutrients in one product: potassium (K), sulphur (S), magnesium (Mg) and calcium (Ca).
POLY4 is a low-chloride granule that can be applied on its own or blended with other fertilisers, with neutral impact on soil pH and suitability for chloride-sensitive crops. and supports steady nutrient release throughout the growing season. It also has a lower carbon footprint than many other conventional fertilisers due to lower processing requirements.
Terrafarmer was contracted to project manage an on-farm trial to generate robust, field-scale evidence under real farm conditions, helping farmers and advisers understand how POLY4 affects soil nutrient availability, soil carbon dynamics and wider soil health indicators.
At a glance
Duration: four-year programme (ongoing dataset build)
Scale: 22 farms (2025 sampling round)
Design: split-field comparison (farm standard programme vs POLY4-treated area)
Cropping: winter cereals, winter oilseed rape, forage crops (grass and maize), pulses and legumes
Sampling: spring sampling; repeatable timing to support year-on-year comparison
Depths: 0–10 cm and 10–30 cm, on both arable and grassland soils
Analysis: Solvita soil health assessments plus additional carbon metrics (including available carbon fractions) and grain nutrient analysis
Knowledge gap
Multi-nutrient fertilisers are widely used, but there is limited independent, farm-scale data showing how they influence soil function and nutrient availability over time, across different rotations, soil types and management approaches. This project sought to close that gap with a consistent protocol and a wide trial network.
What the project is testing
The project compares two approaches within each farm:
- the farm’s standard fertiliser programme
- a POLY4-treated area, using a split-field design.
By repeating measurements in a consistent way across many sites, the project builds a dataset that can highlight patterns across:
- crop type and rotation
- soil type and baseline fertility
- management system and seasonal conditions.
Trial design and sampling approach
In 2025, soils were sampled across 22 farms using a split-field design. Sampling was carried out at the same time in spring to allow repeat sampling and year-on-year tracking of change. A consistent sampling protocol was applied across all sites:
- double-depth soil sampling at 0–10 cm and 10–30 cm
- sampling undertaken on arable and grassland soils
- aligned sampling windows across farms to improve comparability
What we measured
Samples were analysed for a broad range of soil and crop parameters, including:
- Solvita soil health assessments
- soil carbon metrics, including available carbon fractions
- key indicators linked to nutrient availability and nutrient cycling
- grain nutrient analysis to track changes in crop nutrient uptake.
Early learnings and practical on-farm relevance
Alongside measured soil and crop data, growers have highlighted practical benefits that matter operationally, including:
- the ability to supply sulphur independently of nitrogen applications, supporting more consistent sulphur nutrition where nitrogen rates vary due to rotation, market requirements or seasonal conditions.
- increased attention to calcium as a functional nutrient, linked with crop quality and resilience, soil structure and aggregation, and root development.
Why this matters
This growing dataset is intended to support evidence-based decision-making by building a clearer picture of how POLY4 may contribute to:
- improved nutrient efficiency
- soil health and long-term soil function
- consistent performance across different UK systems and conditions.
Want to keep up with results?
If you’d like project updates as findings come in, please register your interest using our online Contact Form.
Interested in Hosting a Trial or Partnering with Us?
Whether you’re a research body, supply chain partner, or farmer exploring alternatives, we’d love to hear from you. Get in touch for a no-obligation chat.
Let’s build the next phase of on-farm innovation together.
