Soil pH for Proteas - The key to success

Iron deficiecy in a Leucopspermum

Members of the Protea family—like Leucospermums, Grevilleas, and Banksias—are native to South Africa and parts of Australia, where they grow in naturally acidic, sandy, low-nutrient soils. These soils are highly well-draining and generally hover around a pH of 5.5–6.0, which is the range these plants are adapted to for optimal nutrient uptake.

Compare that to most soils in California, which can be neutral to highly alkaline, especially in areas with heavy clay or alkaline irrigation water.

When soil pH is outside the preferred range, proteas struggle to absorb essential nutrients such as iron. The first signs of iron deficiency often show up in the leaves: yellowing between veins on new growth. Over time, poor nutrient uptake translates into reduced flowering and overall plant performance.

Unfortunately, there’s a lot of misleading advice online and on social media (shocking, I know) encouraging gardeners to reach for iron supplements as soon as they notice yellowing leaves—sometimes a symptom of other nutrient deficiencies or cultural issues. While iron can help temporarily, it doesn’t solve the underlying problem. If the soil pH is too high, plants simply cannot take up nutrients efficiently, and applying more fertilizer or iron is just a band-aid solution.

The real solution is to adjust the soil pH first. In my garden, I use granulated sulfur to gradually acidify the soil. Soil microbes convert the sulfur into sulfuric acid, which lowers pH over time. This is a slow, safe way to bring garden soil into the ideal 5.5–6.0 range without shocking your plants—but make sure to get a soil test first, so you can calculate the proper dosage and avoid under- or over-application. For container-grown proteas, using a potting mix that is already acidic or lightly acidified is key. Most high-quality mixes are acidic from

the start, unless lime or other alkaline amendments have been added to raise the pH.

Ultimately, the best potting soil is the one you craft yourself.

By focusing on pH first, you set the stage for healthy nutrient uptake. Only once the soil is in the proper range should you consider supplemental nutrients like iron or trace minerals. This approach ensures strong root development, lush foliage, and abundant flowering, rather than treating symptoms while leaving the root cause unresolved.

In short, don’t overlook soil pH for Proteas. Understanding the natural environment of proteas and replicating it in your garden or pots is the most effective way to grow healthy, happy plants. Addressing acidity first will save frustration and expense down the line and let your proteas thrive as they were meant to.