In recent years it has come to be recognized that certain species of plants, including some of the most highly prized ornamental shrubs, are favored by acids in the soil and injured or killed by alkaline materials, such as lime and manure. Universities have made a special study of this and accordingly, there has arisen a new type of horticulture, which may be termed, for short, acid-soil gardening.
Perhaps you have tried to grow some plants and you have wondered why they have dwindled and died in spite of all the care lavished on them. The reason has probably been that the soil was not sufficiently acid. Set aside a part of your garden as a special acid-soil bed and try again. Unless the climatic con-tions are extremely unfavorable you will find that such plants can be suc-cessfully cultivated far from their native haunts in the woodlands..
A list of these acid-loving plants may be of interest here. Among the broad-leaf evergreens there are:
Andromeda (Pieris japonica and Pieris floribunda)
The deciduous shrubs of like soil preference include:
Even some of the hardy perennials belong in the acid class, as for instance:
Is Your Soil Acid?
How can it be told whether the soil in a certain spot is acid or not? There is a common tradition that the appearance of moss on the surface of the ground is a sure sign of acidity. But many traditions prove fallacious when examined by modern scientific methods. Hundreds of tests of soil on which moss has appeared have been made and it has been found that altho a few of the mosses do indeed indicate the presence of acid, many other species flourish on some of the most alkaline soil in the East. Moss does seem to show when a soil has become low in ordinary plant foods, but this does not prove that a soil is acid.
The appearance of sorrel (Rumex acetosellaj is also commonly taken as an indication that acid conditions have developed in a soil, but here again actual tests have demonstrated that this is not always true. Sorrel appears to grow wherever it is not subjected to competition on the part of other weeds. It may become conspicuous in acid places because there are few other plants to compete with it there, yet it is often present where the reaction is distinctly alkaline but others do not happen to have got in. Sorrel alone is, accordingly, valueless as an indicator of soil activity.
Yellowing of leaves is often due to lack of drainage or other improper soil. If the leaves turn yellow, especially in patches between the veins, and blossoming is sparse or poor, you may be reasonably certain that the acidity of the soil is insufficient.
Testing for Acidity. To find out whether a given soil is acid, neutral, or alkaline, it is necessary to resort to the methods of the chemist, whose simplest test is the use of an indicator. Indicator refers to a dye which changes its color when brought into contact with materials of different reactions, and a number of outfits are now on the market in which such dye are used. No previous experience is necessary to operate these tests. Anyone can obtain the desired information by following directions furnished with the outfit. In general, the dye turns one color in an alkaline soil, other colors in neutral, weakly acid, and strongly acid soils. For the successful cultivation of the plants mentioned, it is essential that the soil produce the color of the dye indicating high acidity.
When you make an indicator test it will occasionally be found that the bed where it is desired to grow acid-soil plants is already sufficiently acid, but in the majority of cases the acidity will prove too low, making it necessary to set to work to develop and maintain acid conditions in this particular bed.
Preparation of Beds. Whenever practicable it is best to construct an acid-soil bed by digging out the original, soil which tests have shown to be insufficiently acid and filling it with material that has the proper degree of acidity. The more alkaline this original soil, the deeper the excavation should be, and a depth of at least 1 foot is desirable in any case; 2 feet is better. If the bed is small it may be lined with peat, bark, or even thin strips of wood, to discourage subsequent intermixing soil with that brought in. The ordinary garden earthworm has a way of bringing lime up from the subsoil and mixing it with the upper layers, thereby neutralizing the acids these may contain. The earthworm, therefore, must be kept out of the bed if acidity is to be maintained. In places heavily infested with worms a 2-inch layer of soft-coal cinders may be placed in the bottom of the excavation.
Coming now to the consideration of materials to be used in filling the acid-soil bed, another tradition has to be given up at the start. This is to the effect that leafmold, humus, peat, and muck are always acid in reaction. Accordingly, these are suitable for use in the growing of acid-soil plants. Chemical examinations of hundreds of samples of these substances have shown that in the majority of cases they are neutral, or nearly so, and quite unsuited to this purpose. Peatmoss derived from the disintegration of sphagnum moss can usually be depended upon to be strongly acid. So also can the upland peat which accumulates under pine, spruce, and hemlock trees, or under rhododendron, mountain-laurel, and blueberry bushes; or, lacking these, crumbling treebark, rotted wood, and well-weathered sawdust may be used. To be on the safe side, however, tests should invariably be made with soil testers, and no material which does not show a color corresponding to a high degree of acidity should be used to fill in the beds.
Most acid-soil plants prefer a porous open-textured soil, which can best be obtained by mixing in with the organic substance just enumerated a considerable volume of sand as free as practicable from clay and from lime. White sand from a pine barren or bank sand is particularly desirable, but river sand, such as used in cement work, is usually satisfactory.
To Acidify Soil. In case it is inconvenient to prepare a special acid-soil bed by digging out and refilling, treatment of the native soil with acidifying agents may be used and often gives good results. If a heavy mulch of any of the strongly acid organic materials already discussed is spread over the bed, rain or sprinkling-water will leach more or less acid from them and gradually carry it down to the plant roots. Fresh leaves of oak and various other trees yield a certain amount of acid and may be used for mulching, but when they become soggy, black, and rotted, they lose their acidity and must be renewed.
Acidifying chemicals may also be used, especially when quick results are desired. One of the most desirable chemicals is commercial Miricle Soil Acid Spray. It may be scattered over the surface of the ground, then dissolved by soaking the bed thoroly. Also, applications should never be made to mixed plantings, because the increase in acidity which would help an azalea., for instance, would be fatal to a nearby lilac bush.
The amounts of these chemicals to be applied varies greatly with the conditions.
Even when a bed has been made acid enough for satisfactory growth of plants, it is not safe to assume that it will remain so indefinitely. The indicator tests should be made once or twice each year, and if high acidity is not being maintained, the application of acidifiers should be repeated.
Fertilizers that Keep Soil Acid. The continued growth of plants in one place will gradually exhaust the fertility of the soil and make addition of plant food desirable. My Grandfather, foremost acid-soil gardening authority, recommends this formula: cottonseed meal, 10 pounds; superphosphate, 4 pounds; and sulphate of potash, 2 pounds, to be thoroly mixed and broadcast at the rate of 1/4 pound to each square yard.
Acid-soil gardening is thus not especially difficult. The soil must be made strongly acid and kept so, and soil-testers should be used to ascertain when this is attained.