The appearance of yellow leaves on a blueberry bush, known as chlorosis, signals that the plant is struggling to produce chlorophyll, the pigment necessary for photosynthesis and growth. Blueberries evolved in highly acidic environments, making them sensitive to changes in growing conditions. Identifying the cause requires understanding the specific nutritional and environmental demands of this acid-loving shrub. The pattern of the chlorosis—which leaves are affected and the location of the yellowing—serves as a precise diagnostic tool, pointing toward the underlying problem.
Soil pH and Nutrient Lockout
The most frequent and defining cause of leaf yellowing in blueberries is a soil pH that is too high, a phenomenon that triggers nutrient lockout. Blueberries are acidophiles, thriving in highly acidic soil with an optimal pH range of 4.5 to 5.5. When the soil pH rises above this range, the chemical availability of certain micronutrients changes drastically.
This shift in soil chemistry makes elements like Iron (Fe) and Manganese (Mn) insoluble and inaccessible to the plant’s root system. The resulting Iron deficiency is visually distinctive: it causes interveinal chlorosis. The leaf tissue between the veins turns pale yellow, while the major veins themselves remain dark green.
This specific pattern appears first and most severely on the newest leaves at the tip of the branches. Iron is an immobile nutrient, meaning the plant cannot relocate it from older leaves to newer growth, so the youngest leaves suffer first. Correcting this issue requires addressing the alkalinity of the soil, not a lack of Iron.
Key Nutrient Deficiencies
While soil pH often leads to micronutrient lockout, the blueberry plant can also suffer from physical deficiencies of macronutrients, which present with different symptoms. Nitrogen (N) deficiency is common because blueberries prefer the ammonium form of nitrogen. A lack of nitrogen causes a uniform yellowing or pale green color (general chlorosis) across the entire leaf blade.
Because nitrogen is a mobile nutrient, the plant relocates it to support new growth, so the yellowing appears first on the older, lower leaves of the bush. A deficiency in Magnesium (Mg), also a mobile nutrient, starts as yellowing between the veins of older leaves. It soon progresses to a distinctive bright red or purplish-red color along the leaf margins.
Differentiating these symptoms is important: Iron deficiency affects new leaves with green veins, while Nitrogen and Magnesium deficiencies affect old leaves. Nitrogen causes uniform yellowing, and Magnesium causes red-tinged interveinal yellowing. These deficiencies indicate that the element is either insufficient in the soil or is being rapidly depleted.
Water Management and Environmental Stress
Beyond soil chemistry and nutrient levels, physical environmental factors, particularly water management, can induce leaf yellowing and stress. Blueberries require consistently moist, but not waterlogged, soil due to their shallow, fibrous root systems. Overwatering or poor soil drainage is particularly damaging because it leads to root suffocation.
When roots lack oxygen, they cannot function properly, leading to root rot and an inability to absorb nutrients, which mimics the symptoms of pH-induced chlorosis. Conversely, severe underwatering causes significant plant stress, resulting in wilting and the eventual browning and yellowing of the leaves as the plant sacrifices foliage to conserve moisture. A sudden change in leaf color can also be a reaction to abrupt environmental shifts, such as a heatwave or intense sun exposure.
Diagnostic Steps and Corrective Treatment
The first step in correcting leaf yellowing must be diagnosis, which is best achieved through a professional soil test. A comprehensive test will provide an accurate reading of the soil’s pH and the levels of all essential macro- and micronutrients. This information removes the guesswork and dictates the precise corrective action needed.
If the soil test confirms a high pH, the treatment involves acidification. Elemental sulfur is the most cost-effective long-term solution, as soil microbes slowly convert it to sulfuric acid, but this process can take many months to significantly lower the pH. For faster results, applying acid-forming fertilizers like ammonium sulfate provides immediate nitrogen in the preferred form while also contributing to soil acidification.
If the test reveals a specific nutrient deficiency, an appropriate fertilizer should be applied, ensuring it is formulated for acid-loving plants and contains nitrogen in the ammonium form. For example, a magnesium deficiency can often be corrected with an application of Epsom salts (magnesium sulfate). If drainage is the problem, the solution is to improve the soil structure by incorporating organic matter like peat moss or pine bark, and adjusting irrigation practices.