How I Make My Custom Protea Potting Soil.

One of the questions I get asked most often is: what potting soil do you use for your plants? Over the years, I’ve experimented with just about every off-the-shelf mix and DIY recipe I could find. What I discovered is that members of the Protea family do best in soil that is acidic, drains quickly, resists compaction, and provides just enough nutrition to get them established without overdoing it.

That’s why I started making my own custom potting mix. It takes a little extra work, but the results speak for themselves: healthier roots, stronger plants, and a soil tailored to the unique needs of these South African and Australian natives. Here’s my step-by-step process.

Step 1: Aging the Fir Bark

The backbone of my mix is aged fir bark. I use 1/4 to 3/8-inch bark, which provides the right texture and air space around the roots. To kickstart the aging (or composting) process, I combine the bark with feather meal, a slow-release source of nitrogen. This promotes microbial breakdown of the bark, reducing its initial nitrogen demand and preventing nutrient immobilization once plants are established. In other words, it stops the composting bark from “robbing” the soil of nitrogen once your plants are growing.

For larger batches, I use 3 yards of bark and 40 pounds of feather meal. For smaller home batches, the ratio works out to about 15 gallons of bark and 1 pound of feather meal.

I build the pile on a hard surface (avoid dirt - but it may discolor concrete and pavers), keep it evenly moist, and turn it once a week.

This aeration keeps the breakdown process moving without going sour. Depending on the weather, the bark is ready in 6–8 weeks—warm conditions speed things up, cool conditions slow them down. You’ll know it’s ready when it has a rich, earthy smell and the bark has softened but still holds its chunky texture.

Step 2: Mixing the Protea Potting Soil

Once the bark is aged, it’s time to blend the rest of the ingredients for the Protea Potting Soil. My standard recipe is:

• 6 parts aged fir bark

• 2 parts No. 3 perlite (or substitute with 3/8" pumice or red lava rock; each will make the mix heavier)

• 0.5 part sharp sand (#12 silica sand, to keep the mix open, prevent compaction, and add some anchor weight to the pot—it’s windy at the nursery)

• 1 part peat moss (adds slight water-holding capacity and helps with acidity. If the aged bark looks like it has a good amount of fines then I may skip the peat.

This combination creates a balanced mix: fast-draining yet slightly moisture-retentive, acidic, and lean enough for proteas, which are sensitive to overly rich soils.

Step 3: Fine-Tuning with Additives

To finish the mix, I add:

• 5 lbs ferrous sulfate (iron sulfate) – about 2.5 tablespoons per 15-gallon batch

• 4 lbs gypsum – about 2 tablespoons per 15-gallon batch

• 
  

The iron keeps foliage green, while gypsum helps manage salts, especially if you’re using municipal water. After mixing, I let the soil sit for 1–2 weeks to settle and balance before potting plants.

Ready to Grow

That’s it—my go-to recipe for potting proteas. It’s not the fastest process, but the payoff is worth it. Aging the bark creates a stable base, and the custom blend ensures your plants have the ideal environment to thrive. Once you try it, you’ll see why I rarely buy bagged potting soil anymore.

Tip: Start with a small batch to see how it works for you before investing in larger quantities like I do.