It seems comparatively easy to locate data for the area of land which was unsuitable for large scale arable farming (lakes, mountains etc) but when it comes to the area of land in the British Isles which was 'Difficult', i.e. light sand and heavy clay, it is a lot harder to find data.
Does anybody know if anyone has ever analysed the percentage of arable land in the British Isles, or any country therein, that falls into the 'Difficult' category?