While walnuts will grow in a wide variety of soils they do prefer well-drained alluvial soils, brown earths, loams and loamy clays. They have been noted to prosper on banks and on the site of old bonfires, where presumably the soil has been enriched.
They will prosper in mildly acidic and neutral through to quite strongly alkaline soils, with a suggested pH range from 6.7 to 7.5
Above pH 7.5 there may be problems with chlorosis, although the English Walnut appears better able to grow in such soils than the Black Walnut and its close relatives.
Manse Organics Ltd, The Manse, Capel Isaac, Llandeilo, Carmarthenshire, SA19 7TN