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How to Make Topsoil

westes Zone 9b California SF Bay
last month
last modified: last month

I have a little side yard garden up against a fence that I want to develop into a conifer garden with a variety of herbaceous plants for decoration. I have California clay loam soil, and the area does NOT drain well in the Winter. During week-long rains, I would expect this area to be a thin lake on top of soil mass.

I have seen several people solve these kinds of problems by putting "topsoil" on top of hard clay loam, to build up about 18 inches of well-drained soil above the problem soil. An appropriate amount of organic material is added to that soil to make sure it holds water.

The typical garden materials wholesaler in my area with a "topsoil" product shows a data sheet that is sandy loam soil with an organic amendment. The reality is they quickly deplete their topsoil and end up selling you extremely clay-heavy clay-loam soil. So I need to manufacture my own topsoil for this application.

A formula that I think will work is:

2 parts clay loam

2 parts gritty washed sand

2 parts organic soil amendment

1 part turface (optional)

For a conifer planting, I might add a bit of peat moss to get a pH closer to 6 than 6.5.

The resulting soil does firm up over time, but it remains friable if you squeeze it. It drains well but two inches below the soil surface it holds some moisture for many days. It's definitely a sandy loam soil, and it is heavy on organic content, but it drains very well and holds moisture.

How would you change the soil mix, and how and why?

Just for your information, the washed sand I used is Olympia #2, which is a pure white, gritty, well-washed sand. It is used in masonry but about half the volume is larger grit. This sand is why the photo of the soil below is filled with white specs.

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