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The majority of successful American rock gardens are basically artificial constructions. Both the position of the rocks themselves and the nature of the soils have been much altered to suit the tastes of the plants and of the garden maker.

Generally speaking, there are several groups of plants in use in rock gardens, and no one garden can hope to satisfy all the types without considerable forethought and care. In addition, there are many species in cultivation which are still wild flowers, seldom prospering, no matter what the care. The neophyte had best forget these until the garden be well founded and planted to dependable material for his part of the world. A very large proportion of the dependable material is not always very satisfactory, however, without proper soil and culture. And inquiry into the individual plant nature in question is imperative. The soil must be suited to the needs of each plant.

Usually the plants are rather tolerant of varying conditions within limits, as regards water supply, nutrition and exposure. Naturally, a plant subject to decay must be singled out to inhabit a section where surface and under drainage are perfect. This perfect drainage, of course, must be prepared in the construction of that section.

The other soil factors include the heaviness or lightness, the sweetness or acidity, the richness or leanness, the grittiness or fineness and the relative amount of root run or depth. For this one must needs determine the character of the roots themselves, whether shallow or deep, fibrous or bulbous, and so on. Take it for granted that a good many alpines will be nearly as deep rooted as young trees when well grown. The ground covers are rather shallow rooted, but sand and crevice plants are likely to strike far down into the soil. An acid soil, therefore, which is to be ameliorated with some form of crushed lime should be dug away so that the lime may also be mixed with the subsoil.

Moisture Retention

Moisture retentiveness of soils is extremely important. A few plants can grow in dry sand. Some want average conditions. Some pine for the steady refreshment of icy mountain springs and the clear, bitter mountain air. While the latter may seldom be wholly satisfied, their lot may be made easier in our lowland atmosphere by making up a scree for them with underground water supply.

The requirements of most species are known within reason, and the soils may be apportioned in a general way in advance, so that little revision need be necessary at a later date. To make a heavy clay soil drain well during and after a rainstorm or Winter-thaw, add gravel, very coarse sand, or coal ashes, to be mixed thoroughly (and deeply).

Underlying hardpan must be shattered with the pick. A surface mulch of fine gravel will help and, naturally, a slight pitch to the grade. To make the acid soil sweet, ground marble or crushed limestone is best. To make an alkaline soil more acid bring in crushed granite, with acid peat, or even acid-forming chemicals like the sulfates of weak bases (aluminum, for example). Oak leaf mold is an acid humus or tanbark. In altering the soil chemistry, do not err on the side of stinginess, because soils tend to revert quickly to their original condition, unless forcibly prevented.

Soil Fertility

Fertility means the presence or absence of nitrogen, phosphorus and potassium compounds in soluble (“available”) forms near the roots: in short, plant food. Sometimes these are released in the ferment and decay of organic matter, and sometimes from chemical action on the minerals present in the soil. Earthworm castings are very rich plant food, and complete fertilizers tend to be very rich. Go easy. But really starve some plants for best results. Avoid cow manure if possible, because of seeds of weeds, but very well-rotted horse manure is a good dressing as it contains fewer viable weed seeds. Weeds are bound to blow in anyway, and there is little sense in multiplying one’s maintenance problems. On rich soils the weeds are harder to control, so I repeat: fertilize lightly, and then, only when necessary.

For ordinary purposes the standard potting mixture (preferably screened) of ⅓ mellow garden loam, ⅓ leaf mold or other humus (as leaf mold from the woods) and ⅓ sand (coarse and sharp) should be made up and used throughout the rock garden as the stones are laid up, and used to a considerable depth in some places. This should be rather neutral for most things, but if emphasis is to be placed on calciphile (lime-loving) or calciphobe (lime hating) plants, the mixture should be supplemented by other ingredients (as the lime or the aluminum sulfate), and for some plants gravel and small stones added liberally.

Sticking doggedly to these formulae will certainly make it far easier in the years to come to grow the real treasures of rock gardens, when the charm of little weeds (alpine or other) has begun to wear a bit thin. Thus. one may look forward to a far more rewarding satisfaction from his efforts than when he slights the all-important subject of rock garden soils.

By R. J. Darling