Where do these fertiliser recommendations come from?

Every number is calculated by AHDB's own RB209 Web API — the calculation engine behind the Nutrient Management Guide (RB209). This tool sends your field details to that engine and shows the result; it never re-implements or approximates the tables, so the output follows the current RB209 methodology.

What if I don't have a recent soil analysis?

Leave the analysis fields empty and the engine still calculates nitrogen using the field assessment method — soil nitrogen supply estimated from your previous crop, soil type and rainfall. Phosphate, potash and magnesium recommendations depend on soil indices, so without them those results assume you will confirm indices when you next sample. RB209 recommends soil analysis every 3 to 5 years.

Is this the same as hiring a FACTS qualified adviser?

No. This tool shows what the RB209 methodology recommends for the details you enter — it is a planning aid, not advice. Nutrient management plans, NVZ compliance and product choice should be confirmed with a FACTS qualified adviser who knows your farm.

Every recommendation on this page is calculated by the AHDB RB209 Web API — the engine behind the Nutrient Management Guide (RB209), the intellectual property of the Agriculture and Horticulture Development Board — and reproduced under licence with attribution.

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Fertiliser Recommendation Calculator

Field-specific N, P, K, magnesium, sulphur and lime recommendations for arable crops in England & Wales — calculated live by the official AHDB RB209 engine, not a copy of its tables.

The calculator is paused for now

On 24 August 2026, AHDB asked us to pause the calculator while a licence agreement for the RB209 Web API is put in place — the API may not power a public-facing tool until that paperwork is signed. We've agreed, and the agreement is being arranged now.

This is the honest notice we promised below: nothing about the tool changes — it stays free, with no signup and no paywall — and we'll switch it back on here the moment the licence is in place.

About this calculator

Recommendations are calculated live by the AHDB Nutrient Management Guide (RB209) Web API — AHDB's own calculation engine — and follow the current published methodology. Results are reproduced with attribution to AHDB.

What this tool does: shows the RB209 recommendation for the field details you enter, exactly as the engine returns it, including the advice notes that go with it.

What this tool does not do: give advice. Nutrient plans, NVZ compliance and product choice should be confirmed with a FACTS qualified adviser. Where the soil analysis fields are left empty, index-dependent recommendations assume typical values — soil analysis every 3–5 years is the RB209 baseline.

Current scope: arable crops, single crop per year, England & Wales. Grassland and Scotland are planned.

Free — honestly: this calculator is free to use, and it stays free for as long as AHDB provides us access to the RB209 API on terms a zero-budget initiative can sustain. If that ever changes, we'll say so here rather than quietly adding a paywall.

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Keep learning

New to data-driven farming? Read our complete guide to precision agriculture in the UK, see what precision farming actually costs, or take the free interactive introduction course — story-driven, narrated, and cited to AHDB and gov.uk.