Do you know how to assess the suitability of rock garden soil... consider these factors...
If you are starting your garden from scratch the best thing to do is to pick a soil mix that is the best for the type of plants you want to grow in your rock garden. A simple way to achieve this is by building raised beds or using sink gardens.
If you will be working on an existing garden, you may find you need to introduce a mixture of soil closer to requirements of your desired plants. If you are building rock gardens or laying pavements or lawns on level ground this is not too difficult, but if you will be working with a slope it is more difficult to ensure you prevent types of soil from erosion with the result of the soil types mixing.
If you are building a lime soil over an acid garden you will not need to use the following method as the lime soil won’t do very much harm to the acid soil.
If you have a limestone garden and are introducing an acid soil, resolve soil erosion issues by doing the following. Make a mound towards the middle of the existing site using the existing lime soil.
Cover the mound and a slightly larger area than the extent of your site using a one thousand gauge liner of black plastic.
Lay your stone on the top of the plastic as you would normally and this will help prevent drainage and stagnation issues occurring, along with preventing the soils from mixing.
There are three main elements to rock garden soil mixtures: loam, humus, and grit or shingle.
The proportion of these elements depends on how much light is received in your garden and what the climate is in your area.
If you have a shade garden your soil mixture should contain up to fifty per cent of humus in combination with grit and loam in equal parts.
If you have a more sunny garden setting, the humus will be reduced by around twenty five per cent mixed with the loam and grit or shingle in equal parts.
If you have a site that is windy or has high rainfall you will need more grit or shingle in order to provide the site with a better drainage, the opposite is also true.
Humus can be considered leaf mould and peat, with the former being a preference.
Loam can vary in quality, an ideal loam can be considered as the soil layer immediately under meadow grass, the roots of the grass giving it a fibrous texture that can be crumbled, neither sand nor clay.
However, note that it will contain the seeds of perennial weeds and therefore before using it must be sterilized.
Grit and Shingle
Grit and shingle is coarser than sand, various textures and sizes, and the grains range from 2mm to 30mm in diameter, half to quarter inches.
When they are present in rock garden soil they enable an effective method of drainage with courser grains providing better drainage than finer grains.
A sufficient quantity of large grains will also hold in some moisture beneath the surface of the soil allowing you to maintain a wider variety of plants without having to worry about over watering.
These mixtures should be applied to the top 30-40cm, 12-15 inches, layer of soil not matter what ground you have. If you already have well drained underlying soil the depth of the layer can be less but must be at least 15cm, 6 inches.