The common garden geranium (Pelargonium) is a popular flowering plant known for its vibrant blooms and durability. Geraniums are not acid-loving plants; they perform best in a soil environment that is slightly acidic or neutral. Understanding the precise pH requirements is the first step toward creating an optimal growing environment for consistent, robust growth and flowering.
The Ideal pH Range for Geraniums
Geraniums flourish when the soil pH is maintained within a narrow range of 6.0 to 7.0, preferring slightly acidic to neutral soil. Optimal growth occurs between pH 6.0 and 6.5. This range ensures that the chemical processes in the soil, which dictate nutrient uptake, are working correctly.
This slightly acidic environment maximizes the availability of essential macronutrients, such as phosphorus, and most micronutrients. If the soil becomes too acidic (below pH 5.8), geraniums, which are “iron-efficient” plants, may absorb excessive amounts of iron and manganese, leading to toxicity. Toxicity symptoms include brown spots, burnt edges on older foliage, and stunted growth.
Conversely, if the soil becomes too alkaline, rising above pH 7.5, other nutrients like iron, zinc, and phosphorus become chemically unavailable to the plant roots. Maintaining the correct pH balance allows the geranium to efficiently absorb all the necessary elements for healthy foliage and abundant blooms.
How to Measure Your Soil’s Acidity
Before adjusting your soil, accurately determine the current pH level. The most precise method is sending a soil sample to a professional laboratory, often operated by a local cooperative extension office. The lab provides an accurate reading and typically includes specific amendment recommendations based on your soil type and target plant.
Home gardeners can use self-testing tools, such as chemical test kits or electronic probes, for a quicker, more affordable alternative. Chemical kits mix a soil sample with a reagent solution and compare the resulting color to a reference chart, providing a general pH estimate. Electronic pH meters are probes inserted directly into moist soil, offering an instant digital reading.
Home test kits and electronic probes are generally less precise than laboratory analysis. While accuracy varies, they are useful for obtaining a ballpark estimate or monitoring changes after applying soil amendments. For the most accurate baseline measurement and detailed analysis, a professional soil test is the recommended first step.
Adjusting Soil pH to Suit Geraniums
Once you know your current soil pH, you can adjust it into the optimal 6.0 to 7.0 range. If the soil is too acidic (below 6.0), raise the pH by adding a liming material. Finely ground agricultural lime, which can be either calcitic or dolomitic, is the most common amendment.
Dolomitic lime is the preferred choice if your soil also shows a deficiency in magnesium, as it contains both calcium and magnesium carbonates. Liming materials neutralize soil acids but are slow-acting and require time—often several months—to fully integrate and change the soil chemistry. Wood ash is a faster but less long-lasting alternative that can be used sparingly to provide a quick, modest increase in pH.
If the soil is too alkaline (above 7.0), lower the pH by incorporating acidifying materials. Elemental sulfur is the most effective amendment, as soil bacteria slowly convert it into sulfuric acid, reducing alkalinity. This process is gradual, taking several weeks or months depending on soil temperature and microbial activity.
Sphagnum peat moss is another option for lowering pH due to its naturally acidic composition. When amending the soil in either direction, it is important to apply the materials gradually, following recommended rates for your soil type, and retest the area after a few months. Moderate, repeated applications are better than a single, heavy treatment, as drastic changes can harm the plants.