Physical FactorsThese are frequently classified as edaphic or soil factors and climatic factors; but there is no sharp line of demarcation between them.
Edaphic factors include all those relating to the soil.
The water content of the soil, its mineral content, its humus content, its temperature, and its physical characteristics, such as its depth and the size of its component particles are all edaphic factors.
The difficulty of sharply delimiting edaphic and climatic factors is seen in the case of temperature.
Again, the well-known action of earthworms may be said to be a biological work; but the resulting aeration of the soil causes edaphic differences; and earthworms are absent from certain soils, such as peat.
Such groups are interesting in that they are vegetation units whose physiognomy is, in a broad sense, related more to climatic than to edaphic conditions.
Finally, within any district of constant or fairly constant climatic conditions, it is possible to distinguish plant communities which are related chiefly to edaphic or soil conditions; and the vegetation units of these definite edaphic areas are the plant formations of some writers, and, in part, the edaphic formations of Schimper.
When a district like England is divided into edaphic areas, a general classification such as the following may be obtained:
On the other hand, adaptations, especially those evoked by climatic or edaphic conditions, may only, be shown by the seedling if grown under the appropriate external conditions; here what is hereditary is not the actual adaptation, but the capacity for responding in a particular way to a certain set of external conditions.