Two parts humus—peat, leaf mold, compost—for lightness and fertility.
Two parts stone chips or crushed rock—perhaps the most important ingredient, to keep soil open and cool.
One part sharp builders’ sand, or the gravelly sand used in mixing concrete, to lighten heavy soil and promote drainage.
If your garden loam is sandy or gravelly, of course, you would increase the proportion of humus. If it’s rich in organic matter, you would decrease the quantity of humus. Very sticky, heavy clay soils are best replaced.
Rock and alpine plants are also particularly sensitive to the acidity or alkalinity of soil. This is measured by the pH scale, ranging from very acid pH 4.0, through 7.0 at neutral, to very alkaline 9.0. Rock-garden specialists usually indicate the preferred pH for the plants they sell.
By using a soil-test kit or, better still, sending a sample to your state agricultural service to be tested, you can easily find out whether the soil in your garden is acid or alkaline, and to what degree. Then, to make acid soil more alkaline for plants that need it, mix in small quantities of agricultural lime—the amount depends on the degree of acidity and on the plant itself. Don’t overdo it. Too much lime can do as much harm as good. Alkaline soils are not quite so readily converted to acid, but you can use oak-leaf compost (not completely rotted), acid peat, or pine “duff” (rotted pine needles). Or you can be satisfied with the tremendous number of rockery plants that really prefer alkaline soil. These preferences vary, not only between different plant families, but also between individual members of the same family.
For our small gardens, I’ve found it not too tiresome to provide soils of several types. For specially precious plants, I often mix special, small batches of soil. Then I clean out the planting pocket, make sure that it is tightly closed in with rocks so water won’t wash this soil out or other soil in, and refill it with the preferred mixture.
Watering
Except during rare droughts in spring, when the plants are in active growth, we seldom have to water our rock and wall garden. Our summer rains are usually sufficient for their needs. But whenever or wherever there are long weeks of extreme heat and dry winds in summer, water may be needed. Use a fine spray gently, so that the small plants won’t be washed loose. And let the water soak in long enough so the entire area is moist in depth.
Fertilizing