My soil is about the same ratio of yours, but bagged compost instead. I haven't had a problem at all this year, although i think not stressing the plants out by giving them plenty of space to grow in has helped. I grow using a 4 year rotation which would help greatly if i did get any probs.

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Vegmonkey and the Mrs. - vegetable gardening in a small space in Cheltenham at www.vegmonkey.co.uk