This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The conditions required for the effective growth of a cereal crop are light but fertile soils, which are dry and warm. The soils that furnish these conditions are mainly sandy loams, clayey loams, and calcareous soils. Rocky, too sandy, or siliceous soils do not give a deep or rich soil. Heavy clays are also unsuitable. Marls are, however, well suited for cereals.
These conditions are not always found in the soil itself without cultivation or improvement, hence the plants that would naturally come up in a cornfield are not always to be found on the soil, but are replaced by others, that may be introduced from elsewhere. But in some cases the weeds that are common to the district are able to adapt themselves to the altered conditions, and where a sandy soil is improved by the addition of lime, plants that prefer the former may still linger, and where the latter is the principal natural soil the addition of loam may not affect the lime-loving plants.
As a rule, the cornfield plants are those that grow naturally upon a sandy soil or a sandy loam. A few such, as the Hemp Nettle, are lovers of clay as well. A fair proportion are characteristic of limestone or chalk areas, such as Larkspur, Candytuft, Flax, Venus's Looking Glass, Small Snapdragon. Some are able to subsist on either, as the Red Poppy,