This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Marsh vegetation is distinctly hygrophilous, or made up of plants that require a considerable amount of moisture; in the marsh the soil is saturated with water, but owing to the close association of the plants and the absence of channels, the water is stagnant, and peat is formed on a small scale.
Relatively the same conditions are thus obtained by the marsh plants in the lowlands that are seized upon by bog or moorland plants in the uplands. Hut there is an essential difference in the resulting soil conditions or water content. In the marsh the soil may be peat or silt, the water if rich in mineral salts is alkaline. In the bog the water is also alkaline, but more so, and the peat thick, and in the moor the water is acid, whilst the water is derived from aerial sources.
A marsh is an aquatic formation of a closed character in which there is not a free circulation of water. In aquatic vegetation the plants are submerged beneath the water-table, and as a rule 80 per cent of their surface is covered. In a marsh the percentage may be between 5 and 20 per cent.