Posted: 23/01/2014 at 22:34
Yes, grit, sharp sand and any kind of organic matter will help improve heavy clay. However, herbs don't need much in the way of fertilizer and they have a more concentrated flavour when grown in poorer soil so put most of the farmyard manure in the end where you'll be growing the fruit which will really benefit from it. Clay is naturally very fertile anyway so will provide everything the herbs need as long as you really improve the drainage - many are Mediterranean plants which naturally grow on poor stony soils on slopes etc. so good drainage is essential.