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 preponderance of the sand plants in cornfields owing to the conditions required by cereals, such as lightness and dryness of soils, is a well-marked feature, which is explained by the processes to which a cornfield is subjected in fitting it for cultivation.
The transition from the damp conditions of the woodland to those of a meadow is less great than from those of a meadow to those of a cornfield. It is true that the cutting down of trees has a great effect upon a district in making it as a rule much drier, and a meadow also has usually been drained before it is used for pasturage, etc.
In a cornfield, however, these conditions have been already fulfilled before a further stage, that of cultivation and better drainage, with yearly ploughing, is attained. These last factors tend to make the soil very much drier, and owing to this a cornfield is the extreme stage towards dry-soil conditions, which in a meadow are not nearly so well marked. The shielding of the lowest zone of plants amid the corn in mid and later summer does not prevent the free access of the sun to the soil and the rapid evaporation of the surface moisture.
Consequently, as would be expected from the natural predilection of the great majority of the plants for sandy soils, the plants in a cornfield are mainly xerophilous or dry-soil types. There are, in fact, few if any of them, except perhaps Corn Sow Thistle and Mouse-tail, that will grow in a moist habitat. White Campion is intermediate in this respect.
The mode of examination of a cornfield flora differs very little from that of a meadow or pasture, except that the cereal itself supplies the dominant plant.
Proceeding to examine the cultivated area first of all, it may be pointed out that Grasses have been artificially eliminated, and therefore the best means of studying the flora is to take each tier or zone by itself, and to estimate the dominance of the several plants that make up each zone. In some fields Creeping Thistle will be dominant in the zone corresponding to the tree zone, with Corn Sow Thistle proportionally next so.
In the intermediate zone, if the Creeping Thistle does not occupy this, we may have Corn Marigold. Charlock and Corn Buttercup may come next. In the ground flora the dominant plant may be Ivy-leaved Speedwell, early in the year, and Scarlet Pimpernel later.
Mousetail grows sporadically in the furrows rather than in large societies. The vegetation of the borders, especially the cornfield plants, should next be studied as in a meadow, since Grasses are here allowed to grow. The ditch and hedge should be also treated as in the case of a meadow or pasture, the definite occurrence of cornfield plants being especially noted.