September 15, 2025, 2:27 am | Read time: 4 minutes
Special soil for hydrangeas? That sounds like a secret recipe or a marketing gimmick. Find out what hydrangea soil is all about here on myHOMEBOOK.
Follow myHOMEBOOK on WhatsApp now
What Makes Hydrangeas So Popular
Hydrangeas are very popular, and it’s no wonder. Their large blooms in white, pink, red, or blue create striking accents in beds and containers. Additionally, unlike many other plants, they thrive in semi-shaded or even shaded areas. When the location and soil are right, they are considered easy to care for.
Soil Requirements for Hydrangeas
Like rhododendrons, azaleas, camellias, heathers, or blueberries, hydrangeas are bog plants and prefer acidic soil with a pH value between 4.5 and 6. Not every garden soil meets this requirement. Soils with high lime content are usually neutral to alkaline and therefore not acidic. Besides a low pH value, hydrangeas have other soil requirements: It should be rich in humus, loose, permeable, and consistently moist.
What Hydrangea Soil Should Achieve
Hydrangea soils available in stores consist of humus-rich components like bark humus, wood fibers, or compost, supplemented by peat or peat-free alternatives. Clay minerals improve water retention. These mixtures have an acidic pH value, providing suitable conditions for hydrangeas.
Special soils like hydrangea or rhododendron soil usually cost more than regular potting soil. Therefore, it’s worth considering the actual benefits: While regular potting soil is mostly neutral to slightly alkaline and thus unsuitable for hydrangeas, the acidic special soil offers exactly the environment they need.
Some products also contain aluminum compounds, such as alum. Aluminum plays a special role in the color of the blooms, as only when aluminum is available can the blooms turn blue. However, these additives are not automatically present in hydrangea soil. Most soils on the market—often labeled as “rhododendron and hydrangea soil”—are essentially just acidic substrates.
Also interesting: 5 Common Mistakes When Watering Hydrangeas
Do Rusty Nails Really Help Hydrangeas Bloom Abundantly?
Which Plant Needs Which Specific Soil?
Bloom Colors and Aluminum
Whether a hydrangea gets blue blooms depends on three factors: the variety, the aluminum content in the soil, and the pH value. Only garden hydrangeas, also known as mophead hydrangeas, and lacecap hydrangeas can change their bloom color. However, this is only possible with pink varieties.
Although aluminum is abundant in almost all soils, it is mostly bound in minerals and not usable by plants. Only when the soil is very acidic—with a pH value below about 5—do aluminum ions dissolve and enter the soil solution, allowing the roots to absorb them. Many garden soils are neutral to alkaline and thus not acidic enough. In such soils, the aluminum remains bound and unavailable to plants. For intense blue coloring, natural release is usually insufficient.
If you want to see your hydrangeas bloom blue, you can use special preparations with aluminum sulfate, also known as alum. This salt provides the element immediately in a form available to plants. However, it is crucial that the soil is also acidic enough—otherwise, added aluminum will be bound and lose its effect. Only the combination of acidic soil and additional aluminum reliably ensures vibrant blue blooms.
This means: The acidic soil creates the condition, alum provides the necessary amount of plant-available aluminum. Only when both come together do the blooms turn blue.
Which Soil Is Appropriate When
For healthy growth, any good bog soil like rhododendron soil is completely sufficient. If you want to keep the natural bloom color, you can do so without additional additives. However, if you want to change the bloom color to blue, you need additional aluminum—either in special hydrangea soils with aluminum salts or through your own blue coloring agents like hydrangea fertilizer with alum.
Practical Tip: It’s best to water hydrangeas with rainwater. Tap water often contains a lot of lime, which raises the pH value in the soil and can negate the effect of acidic special soils.