

Free daily UK fertiliser price index across nine grades, CC BY 4.0 licensed.
Tilth API is a free Open Data API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Daily UK fertiliser price estimates for nine grades, free as JSON. Methodology, licence and attribution.
Details from tilth.uk.

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The Tilth API is provided by Tilth (tilth.uk). Daily UK fertiliser price estimates for nine grades, free as JSON. Methodology, licence and attribution.
Yes. The Tilth API is listed in the Open Data category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Tilth API requires no authentication, so you can start making requests without signing up.
Yes. The Tilth API supports CORS, so front-end JavaScript can call it directly.
Yes. The Tilth API is served over HTTPS, so it's safe to use in production.