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 influence of soil is well shown in the case of woodland plants in the predilection of the several types of dominant tree for a particular kind of soil. But the ground flora is also made up of plants that prefer certain types of soil before others. Whilst most woodland plants live in a soil rich in humus, there are many that do not absolutely require it. Typical humus-loving plants are Wood Anemone, Goldielocks, Wood-sorrel, Enchanter's Nightshade, Angelica, Ivy, Woodruff, Small Periwinkle, Wood Forget-me-not, Betony, Dog's Mercury, Aspen, etc. A few are also especially addicted to a sandy soil, as the Lime, Wild Cherry, Strawberry (the two last need some humus), Wych Elm (also on clay), Oak (or on clay), Snowdrop, Bluebell (both the latter need some humus too). Clay is preferred by Sanicle, Honeysuckle, Wood Loosestrife, Yellow Archangel, Twayblade, Ramsons, which are damp-loving plants, and they need some humus. Chalk or limestone is required by Green Hellebore, Wayfaring Tree, Marjoram, Wood Spurge, Beech, Bee Orchis, Lily of the Valley, and here, again, there is some humus required. The Columbine, Holly, Mountain Ash, Foxglove, Wood Sage, as a rule, grow on more rocky shallow soils.
The first object to aim at in surveying a wood is to estimate the nature of the dominant tree type. This may be done by marking out squares, and numerically counting or mapping the trees in such a space. If an entire wood is done the most perfect results will be obtained. It is possible, however, to estimate this factor by taking one or two small squares in different parts. A further fact to be ascertained is whether the wood is open or close, whether it is coppiced or not, and whether the tree types are artificial or natural. The character of the soil must be ascertained, and the trees should in mapping be put down accurately on squared paper, ruled to a definite scale. What applies to the trees also applies to the scrub.
In surveying the ground flora it is not enough to make a list of the plants found in the wood in order of dominance, noting the relative frequency of each, but attention should be paid to the form of association, and to the relative position of certain types which occur in a definite connection with each other. As there are early and late flowering periods, surveys should be made at different seasons of the year, in order to get a full and accurate idea of the whole formation. The conditions of light, moisture, height, soil, etc, must also be noted in each case, so that a connected idea may be formed of the full nature of the environment and its influence upon the woodland flora.