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Rock Garden Soils

By August 1, 2017Seed Bank
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A General Purpose Rock Garden Soil

The gardener knows that there is nothing like coarse, gritty sand for increasing the drainage properties of a soil. He uses it in his cutting bed, in the soil he mixes for potting. So sand will be one of the ingredients.

The roots of rock garden plants, as we have seen, like to cling around the moist surfaces of bits of stone buried in the soil, while the leaves rest upon those pieces which work their way to the surface, thus avoiding direct contact with the dirt.

An important ingredient will be stone chips. Ordinary crushed stone, such as is used for surfacing roads is suitable; this can be readily procured in most sections. If not, bank gravel, preferably not too fine or smooth, and not “washed,” will serve as a substitute.

For our third ingredient, we add humus or decayed vegetable matter, which is found almost invariably in soils in which rock plants grow. This material holds an additional supply of moisture, besides furnishing some plant food. For supplying humus, granulated peat moss is best. It is so slightly acidic that only the extreme lime-loving plants object to it, and it absorbs and holds more moisture than any similar material.

Moreover, it is both pure and absolutely free from weed seeds, an advantage which cannot be overemphasized in rock garden planting. Peat moss is now readily obtainable anywhere; but if you do not happen to have it, finely sifted leaf mold will serve. Commercial humus has more of a tendency than either of the above to get wet or soggy.

Fourthly, and lastly, to give additional body to the plant food, we add good light garden loam. This, however, should be wholly free from clay, which is the last thing, in the way of soils, to be used where rock plants are to go.

All this has required some time in the telling, but if you boil it down it comes to this. To make a satisfactory all around rock garden soil, mix thoroughly together the following:

1 part clean, gritty sand, 1 part stone chips, or clean, gritty gravel, 1 part granulated peat moss, or sifted leaf mold, 1 part clean, light garden loam.

You will have a soil in which 90% of the rock plants you are likely to try at the start will grow satisfactorily.

Special Soils for Special Purposes

Occasionally, however, you will find plants that require something different from the above in the matter of soil; these will grow in the above mixture, but will grow better if their tastes are catered to.

Some insist upon having an extremely acid soil, or a lime soil, in order to survive at all. Such plants as these may either be grouped by themselves, or may be started in pockets filled with a soil supplying their own special dietary requirements.

These special soil mixtures may be made up according to the following prescriptions, the chief changes being an increase in one part or another of the several ingredients. These proportions are approximate; there is no necessity for weighing out the ingredients on a jeweler’s scale.

Acid Soil

1 part sand, 1 part stone chips, 3 parts acid leaf mold (that is, leaf mold gathered from under evergreens, laurels, or the like), Lime or Sweet Soil 1 part sand or 3 parts old plaster, 1 part loan, 1 part peat moss

Dry Soil.

3 parts sand, 2 parts stone chips, 1 part loam, 1 part peat moss

Moist Soil.

1 part sand, 1 part chips, 1 part loam, 3 parts sphagnum moss or granulated peat moss, or both


For plants other than the true rock plants and alpines–such as garden perennials and annuals, shrubs, evergreens, and most bulbs–and also for such of the rock plants or alpines as take readily to a somewhat stronger diet, very often decayed manure and bone meal may well be added to the general soil mixtures suggested above.

For evergreens, shrubs, or other plants which are to be set around the rock garden, by way of a background or setting, such conditions as are usually provided for them should, of course, be given. Particularly if any specialty garden features have been added, such as those in the Williamsburg Collection, http://technorati.com/videos/youtube.com%2Fwatch%3Fv%3D9HrZ2bNaiGk.

Do not use any of the commercial fertilizers, except bone meal, and even with this great caution should be exercised.

Lime is not a fertilizer, but may occasionally be needed for the rock garden. If old lime rubbish, which is better for this purpose, is not to be had, ground limestone or gypsum may be used, to modify a soil otherwise too acidic.

Aluminum sulphate, now offered by most seed houses, works in the opposite direction. This may be utilized either to neutralize a lime soil, or, by applying it in larger quantities, to make the soil acid-reacting. Acidic leaf mold, which is to be found in most sections if trouble is taken to hunt for it.

Leaf mold gathered under oak trees is sufficiently acidic for most purposes, if decayed laurel leaves, conifer needles, or rhododendron leaves are not to be had.

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