From our roots outward, soil pH is not about color for us; it is a fundamental signal that dictates which nutrients in the soil are available for uptake. The pH level directly influences the solubility of minerals. In highly acidic soils (low pH), elements like aluminum and iron become more soluble and thus easier for our root systems to absorb. In more alkaline soils (higher pH), these same elements become locked up in the soil, unavailable to us. This fundamental difference in nutrient availability is the true engine behind the color change you observe in our blooms.
The key element for blue flower coloration is aluminum. We can only produce blue hues if our roots can absorb aluminum ions from the soil. This absorption is only efficient in acidic conditions, typically at a pH of 5.5 or lower. Once absorbed, the aluminum is transported to our flower tissues. There, it interacts with a naturally occurring pigment in our petals called delphinidin-3-glucoside (an anthocyanin). This interaction forms a stable blue complex, turning our blooms various shades of blue and purple. Without available aluminum, this reaction cannot occur.
In neutral to alkaline soils, with a pH of 6.5 and above, aluminum remains bound to other soil particles and is inaccessible to our roots. Without aluminum, the delphinidin pigment in our petals remains in its default, uncomplexed state. This natural state expresses itself as pink or red. Furthermore, in these higher pH soils, other nutrients that can encourage pinker tones, like phosphorus (which can bind with aluminum in the soil, further locking it up), may be more readily available, reinforcing the pink coloration.
When you add garden lime to raise the pH, you are effectively putting a lock on the aluminum in the soil. Our roots detect that this crucial element is now unavailable, and our cellular processes respond by producing the default pink pigments. Conversely, when you add soil sulfur or aluminum sulfate, you are performing two actions: you are lowering the pH to unlock the aluminum, and you are directly providing more of the key element itself. Our roots gratefully absorb this newly available resource, and our blooms express the complex blue compound. It is important to note that white or green flowering varieties lack the necessary anthocyanin pigment and therefore will not change color based on soil pH.
Changing our flower color is not an instant event. It is a slow, metabolic process that involves altering the very chemistry of the soil we grow in. The amendments you add take time to break down and affect the entire root zone. It can take a full growing season or more to see a significant change, as the newly available (or unavailable) aluminum must be taken up by our roots and integrated into the developing bloom buds for the next season's flowers. Drastic, rapid changes in pH can also cause stress, so a gradual approach is always best for our health.
