Rock Gardens and Soil Prep
A rock garden needs to be in full light and air, away from the shade of trees and structures, but should not be placed in full sun either. A rock garden should be placed in a location where the important sides face north or west, to take advantage of the dew and cooling of every night, and to make the slopes facing the sun short and abrupt unless the vegetation is to be mostly Sedums and desert plants.
Creating hollows in the level reaches is another way to provide relief from the sun for part of the day. They should be lowered at one side for drainage (unless bog plants are placed here), so that the stones of the rim cast shadows across them, whereas big boulders or ridges of rock may be erected boldly for the shade which they afford. A clever approach is to slope the entire rock garden to the north at such an angle that the sun in winter will hardly hit the soil or at such a low angle that there will be no thawing.
Winter killing is largely controlled by the angle of slope and shade from protruding boulders. The requirements of many rock plants make it necessary to choose the proper stones for the garden. Some plants grow best in an alkaline or lime soil. For these a lime rock, as marble or coral, is best if you can get it, or else lime rubble should be mixed into the soil. For plants demanding acidity in the soil, a hard rock, is needed, with plenty of peat fiber in the soil.
Fortunately, many rock plants have none of these needed requirements. While the stones are being gathered and hauled to the site, you should prepare the soil too. Do not use regular garden soil, or even a good soil, as these will quickly kill alpine plants. The general soil that is needed for an alpine plant is one that is a mixture of one-third sharp sand, one-third vegetable fiber, and one-third garden loam.
These materials should be mixed together before planting. The ideal natural soil is the top sod of stony pastures, long rotted, and screened to take out roots of weeds. Sand should be sharp and gritty, even rather coarse, and never fine sand that has clay in it. Porosity is needed and sifted hard coal ashes can be added, especially for lime lovers, like Dianthus. The vegetable fiber may be from any source as long as it has properly decayed.
Vegetable decay includes old sod, leaf-mold, or turf from the woods. The best is peat from the bog piled a year ahead, pulverized by the frost of one winter, and then dried by the sun of one summer. This will easily break into small pieces and is free from weed seed or roots. For city gardens, it will be best to use any of the prepared peat fibers, as these are well ground and free from weeds.
Garden loam should not be fertile and needs to be free from clay. Loam adds body as it juxtaposed the porosity of the sand, being able to hold water. Roots and seeds of common weeds are dangerous to the mixture, adding much trouble if present. Screen the mixture through a sieve to remove roots of weeds, but the seeds will have to grow to their next stage. Bone meal is the only fertilizer that should ever be added, as other manures and fertilizers are much too rich for the diet of alpine plants.