How does heavy metal availability relate to pH?
Uptake of metals is strongly affected by pH levels in the soil. A lower pH will create an environment suitable for heavy metals to dissolve. The result is more heavy metals available for uptake by plants. A higher pH is more likely to keep metals bound to soil particles.
The main driving forces for this relationship between pH and heavy metal availability are cation exchange (when the pH is low) and precipitation (when the pH is high). More hydrogen ions are present when the pH is lower. These positively charged ions will bind more readily to soil particles, taking up binding sites and displacing heavy metals. When the pH is higher, insoluable compounds such as hydroxides and carbonates are more likely to form. Being insoluble, the presence of these compounds can reduce the rate of heavy metal uptake in plants.
If you are using salt-based fertilizers and are concerned about heavy metals uptake, we strongly encourage you to maintain a growing system pH between 6.5 - 7.0.