Geraniums are frequently associated with bright, warm-weather gardens, leading many gardeners to wonder if they require the acidic soil conditions favored by species like rhododendrons and blueberries. Geraniums are generally not acid-loving plants. The flowers commonly known as florists’ geraniums (Pelargonium) prefer a more neutral soil environment for optimal health and blooming. Although the perennial “true” geranium belongs to the genus Geranium, both types share similar soil requirements. Neither the annual Pelargonium nor the hardy Geranium thrive in highly acidic soil.
The Preferred Soil pH Range for Geraniums
Geraniums flourish in a soil environment that is neither strongly acidic nor highly alkaline, performing best in a pH range of 6.0 to 7.0. This span represents soil conditions that are slightly acidic to completely neutral. A neutral pH of 7.0 is the point where acidity and alkalinity are perfectly balanced.
The upper limit for healthy growth often extends slightly into the alkaline side, up to a pH of around 7.5. Plants typically struggle when the soil becomes much more alkaline than that. Maintaining this narrow range is important because soil pH acts as a gatekeeper for nutrient availability. When the pH is within the ideal zone, essential nutrients remain soluble and readily absorbable by the roots. This promotes robust development and better flowering performance.
How Soil pH Affects Nutrient Uptake
Deviations from the neutral to slightly acidic pH range directly impact a geranium’s ability to absorb nutrients, a phenomenon known as nutrient lock-out. Maintaining the target pH range is important not only to prevent deficiencies in alkaline soil but also to prevent toxic overloads in overly acidic environments.
Alkaline Soil Issues
When the soil becomes too alkaline (above pH 7.0), micronutrients like iron and manganese become chemically bound and insoluble. This unavailability of iron causes interveinal chlorosis, where the newest leaves turn yellow while their veins remain green.
Acidic Soil Issues
If the soil becomes too acidic, dropping below pH 5.8, different problems occur. Geraniums are classified as “iron-efficient” plants, highly effective at absorbing iron and manganese. In highly acidic soil, these elements become excessively soluble, leading to over-accumulation and toxicity in the plant tissue. This toxicity manifests as bronze or brown speckling and necrosis, typically starting on the older leaves. Highly acidic conditions can also reduce the availability of macronutrients such as phosphorus, calcium, and magnesium.
Practical Steps for Soil pH Adjustment
Before attempting any soil amendment, conduct a soil test to determine the current pH level. This can be done using a simple home kit or a precise laboratory analysis. Amending the soil without knowing the exact starting point can lead to over-correction, which is often more damaging than the initial problem. Once the soil test provides the necessary data, amendments should be applied slowly and deliberately.
Raising Acidic Soil pH
If the soil is found to be too acidic (below 6.0), the pH needs to be raised by incorporating alkaline materials. Dolomitic lime is a common and effective choice, as it supplies both calcium and magnesium while gradually raising the pH. Finely sifted wood ash can serve as a faster-acting alternative, but it must be used sparingly.
Lowering Alkaline Soil pH
To lower the pH of overly alkaline soil, elemental sulfur is often used. Soil bacteria convert the sulfur into sulfuric acid, a process that takes several months to complete. Sphagnum peat moss is an organic option that can be mixed into the topsoil to introduce acidity, though it is a milder and more temporary solution. Any amendment requires patience, as it can take several months for the full chemical reaction to stabilize the soil’s pH level.