This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
For planting out it may safely be said that Dahlias will grow in almost any soil. For choice a deep loam, rather heavy, suits them best, but they will flourish in any well-cultivated soil. The better the preparation by cultivation the better will be the results; though acres are grown under plough culture, autumn digging is undoubtedly best for heavy soils, while those that are light should be worked in early spring.