For a successful, long-term container planting, you need to use a proper container/potting soil. "sandy loam" or any other garden soil will not provide adequate drainage or porosity in a container.

I grow grevillea, which also shares a phosphorus sensitivity common to Australia native plants, in the same bark-based 5-1-1 mix I use on almost all of my containerized plants. A friend who owns a local, mostly mail order nursery that focuses on hardy plants originating from the Southern Hemisphere and includes many P sensitive species uses a similar mix.

So I take that to mean that redwood bark and peat moss do not contain much phosphorous, and in general, do not add many nutrients to the soil. That might account for why they are used successfully as components in inorganic container soil mixes.

When you mix 5-1-1 for Protea and related plants, are you removing dolomite lime from the mix, in order to keep the pH around 4.5?

There are virtually NO nutrients in any kind of potting soil unless it is a commercial mix that has been implanted with some sort of fertilizer.

I actually use 2 different 5-1-1 mixes - one that is pH buffered and the second not, to accommodate more acid loving plants. I would skip or significantly reduce the dolomite lime added for any Aussie natives.

@gardengal48 in 5-1-1, the reason that bark is preferred to the white sapwood is exclusively to get better absorption of water? I notice one person online uses the cheaper bark products that have significant sapwood contamination, but he made up for the loss of water retention by using one more part of peat moss. So he had a 5-2-1 mix. In the interests of lowering soil cost, and avoiding the effort involved in sorting out bark sizes, do you think that approach could work?

@gardengal48 How do you like 5-1-1 mix (without the lime pH adjustment) for climbing roses? Are you using 5-1-1 for roses, azaleas, and rhododendrons?

Every custom blend soil is going to be just a bit different from any other as the ingredients will vary from place to place/mix to mix, as will the plants being grown and the care required. With Aussie natives, fast drainage is the key. If altering the mix to be more moisture retentive works for you, great. In my climate I won't risk it.

I don't grow any roses.....too much shade. But roses do not require a low pH anyway. Just slightly acidic is fine, which should be just about what the buffered 5-1-1 mix pencils out to.

@gardengal48 What is the ideal pH for Leucadendron? I read they want pH below 6, but what about 4 to 4.5 pH, the level at which acid lovers like Azaleas and Rhododendron would thrive?

P.S., I appreciate all your comments on my threads. After three years of reading these I am about 70% of the way there to understanding the most important concepts in container soils. So I follow and agree with most of what you are posting. I probably do not say that often enough, so thank you.

Does anyone know the exact pH range that works best for most Protea and Leucadendrons?

I found a good article regarding desired growing conditions for Protea.