A very important factor in determining the sorts of plants adapted to any specific location is the soil type. By soil type is meant the relative acidity or alkalinity of the soil elements, the relative fineness of the soil particles, and the relative state of dryness of the soil material. All of these conditions overlap and combine with one another and produce soil types which are favourable for certain groups of plants and unfavourable for other groups. The exact physiological reasons why a plant should succeed so much better on one soil type than on another is seldom exactly understood; but practical experience has helped to solve many problems and to bring out many interesting facts.
Plants adapted to boggy soils are likely to be shallow-rooted like the elm, and they are able to withstand a soil condition which is poorly aerated and almost invariably acid. In fact, a pond full of alkaline or limy water will generally be found practically devoid of plant life, while in an acid pond plants thrive so that they gradually displace the water as in sphagnum bogs, which were open ponds at one time but have lost that character through the excessive growth of plants. In a true bog there is practically no free drainage except during the spring thaw or flood time.
Plants adapted to bog gardens are called bog plants. Perhaps the most prominent of these perennials are the different varieties of the lady-slipper and the pitcher plant. One of the most essential things for the success of bog garden plants is to have a moist condition which does not vary to any marked degree. It is much better to have the ground surrounding the roots of these plants over-saturated than to have the degree of moisture become too much depleted,' as the water table throughout the entire area of the bog garden should be maintained at a constant level and at the same time the water should not be allowed to become stagnant. If the area which is being developed as a bog garden does-not naturally possess a growth of certain plants which one knows to be indigenous to bog areas then investigation should be made to be sure that the artificial bog garden can have moist soil conditions which will be congenial to the plants in this location.
Peat soils are not only naturally acid but it is believed they contain low forms of plant life also which are of great assistance in promoting the growth of ericaceous plants that especially thrive on peaty soils. Peat bog soils lack available nitrogen and therefore carnivorous plants occur, such as pitcher plants and sundew. Humic acid is present and low temperatures are the prevailing ones. The humic acid acts upon the roots of the plants. Plants adapted to peat are likely to be those requiring considerable humus also a cool, moist, deep, loamy soil and a position not in full sun when transplanted.
Since in a light, sandy soil the particles are relatively large such a soil is generally well aerated and not retentive of moisture. Thus, plants adapted to such a soil are as a rule very hardy and possessed of a large root system due to the fact that the roots generally have to extend considerable distances in search of water which may fail just when most needed, that is, during summer droughts and winter freeze-ups accompanied by a drying wind. Also since this soil type is well aerated it is not likely to contain much humus or to be acid in character as the aeration or oxidation of the humus removes one cause of the soil acidity.
On the other hand, clay soils are composed of very fine particles, poorly aerated, and are retentive of moisture and tend to be acid in reaction. Therefore, plants adapted to clay soils are generally moisture loving, free from large fibrous root systems, and are not nearly so likely to be hardy, especially if removed to another soil type.
Light soils are said to be warmer than heavy ones. What is really meant by that is that the period of growth from spring to autumn is longer on a light soil than on a heavy one. For this reason, plants which start growth early in the spring, like peaches, or bloom late in the autumn, like chrysanthemums, generally succeed better on a light soil. For the present, and until the underlying reasons are better understood, experience will prove the best guide in selecting plants for the different soil types.