Advice on soil mix wanted

[Henrik S]

Hi all!

I am new to rock gardening and could need some advice on soil composition. The thing is I have a dry stone wall made up of rounded granite stones. The wall acts as a support for the garden soil in the descent of a garage in the basement of my house. I thought it would be nice to use the wall as a rockery. As it is now the gaps between the stones are filled with the surrounding garden soil, which must be quite fertile since nettles and dandelions thrive. However, after killing of the weeds I will dig out the soil between the stones and fill the cavities with some new soil suitable for alpines. I know there are alpines with different needs and there is no soil that suits all. Nevertheless I want to make a soil that will be OK for as many species as possible, a sort of general soil. The components I consider to add are: Coco coir (I want to avoid peat), washed horticultural grit (1-3 mm), pumice (2-8 mm), pelleted horticultural clay (2-6 mm), crushed limestone (2-6 mm) and pelleted lime powder (for instant pH increase). If I will plant som acidohiles I'll skip the limestone/lime powder.
So, can anyone give some advice if I should add or remove some components and what proportions I should have?

Best regards/
Henrik

[Maggi Young]

Hello and welcome, Henrik!
 I would think that you would only really need to amend the growing soil in your wall by adding gravel and perhaps sand.

[Henrik S]

Hello Maggi!

Thanks for the reply! So all those components I mentioned may be an overkill then? It will be fine with the garden soil (rich in nutrients but poor in organic matter) amended with sand and gravel? But what about the proportions? Too little sand and the soil will turn to concrete and too much it will be too poor? And what about the pH? The soil where I live is rich in calcium so there may be no need for adding lime I guess, although I haven't tested the pH of the soil. And what about organic matter, shouldn't I add some?

Best/
Henrik

[Maggi Young]

I think you should experiment with around 33/33/33 soil and sand  and gravel to start with - you'll hopefully be pleasantly surprised at the good results with most plants. 

[Henrik S]

Hello again,

But 33/33/33? Soil, gravel and what is the "third third", sand? And what gravel and sand fractions? Should it be washed sand or 0-x grain size ?

Best/
Henrik

[Maggi Young]

Yes, sorry - typo -  so soil sand and gravel in equal proportions.  6mm gravel and sharp sand of mixed sizes.  Can be wahed if you think there is a lot of dusty matter that might clog up and be too solid.

[Jeffnz]

There was an item on our local news last night about removing peat from horticultural mixes, we have had bark based pot mixes for a long time, not sure how much peat is in commercial mixes but the move forward is to compost the timber as opposed to the bark as an alternative to peat.