From our perspective, rooted firmly in the soil, the issue you call "chlorosis" is a deeply frustrating and systemic problem. It begins not in our leaves, where the yellowing is so visible to you, but at our roots. We require iron to produce chlorophyll, the vital green pigment that captures sunlight for photosynthesis. When iron is unavailable, our food-production system breaks down. The specific challenge we face is that iron, while often abundant in the soil, is frequently in an insoluble form that our root systems cannot absorb. This is particularly true in alkaline soils (high pH), where the iron becomes chemically "locked up."
Within our vascular systems, iron is not a free agent; it must be carefully transported and managed. Once absorbed by our root hairs as ferrous iron (Fe²⁺), it is oxidized to ferric iron (Fe³⁺) for transport through our xylem. This form, however, is highly reactive and insoluble in the neutral pH of our sap. To solve this, we produce chelating agents like citrate. These molecules bind to the ferric iron, forming a soluble complex that can be safely transported to our growing points and young leaves. In azaleas, this internal regulatory system is especially sensitive. Any imbalance in soil pH or root health disrupts this precise chelation and transport process, leaving our newest growth starved first.
The pattern of chlorosis is our primary language for communicating this deficiency. You will notice that the yellowing starts on our newest, most apical leaves first. This is because iron is an immobile nutrient within our tissues. This means we cannot rob it from older leaves to redirect it to new growth. The young leaves emerge pale yellow or white, while the veins often remain green, creating a distinct interveinal chlorosis pattern. This network of green veins is a key sign that the problem is iron-related, as the vascular tissue within the leaf veins can sometimes still manage to transport tiny amounts of the scarce nutrient. Without intervention, these affected leaves will eventually become scorched and necrotic, significantly reducing our ability to feed ourselves.
Our struggle with iron is intrinsically linked to our evolutionary preferences. Azaleas, as ericaceous plants, evolved to thrive in acidic soils where iron is naturally more soluble and bioavailable. When planted in neutral or alkaline soils, our root systems are operating in a hostile environment. The high pH not only locks away iron but also disrupts the beneficial mycorrhizal fungi that form a symbiotic relationship with our roots. These fungi act as an extension of our root system, vastly improving our access to water and nutrients, including iron. In poor soil conditions, this critical partnership is compromised, directly leading to the chlorosis you observe.
