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.
A cornfield or allied cultivated tract is unique amongst botanical habitats in the fact that the soil is enriched each year with some form of manure or dressing. Normally no other type of vegetation receives the same type of dressing. Humus accumulates in a wood or hedge, and as a thin layer in meadows, whilst peat and humus (thick) play a great part in moors, bogs, and heaths. But though these are organic soil renewers, they do not resemble in effect the artificial dressings in the cornfield.
The direct result of such manuring is to cause the plants (weeds included) in a cornfield to present generally a robust, well-matured appearance. They are usually luxuriant and in marked contrast to similar or allied species growing elsewhere. The peat or humus-loving plants are not represented as a whole in the cornfield flora, and most of them are fond of clay, sand, or lime, and these soils are improved by the addition of manure in one form or another. At the same time there are some plants that dislike dressing, and will not grow under such conditions. The discovery of the requirements of plants in this respect will be an object for inquiry here also.