QI grow tomatoes, peppers and chillies in beds in a small greenhouse, replenishing it every year, yet the results never seem that nourishing. How do I keep the soil in my greenhouse healthy? Ann Bridgland, West Sussex

If you grow tomatoes in the same soil year after year – regardless of how diligently you top it up with compost – there will be a build-up of soil-borne viruses. To avoid this, you need to practise crop rotation. But this is impractical unless you have a very large greenhouse, so instead of changing the crop, the answer is to change the soil. Every three years, dig out the top 25cm of your beds and replace with diseasefree garden soil such as from a loam stack.