The hydrangea is in new topsoil, but I don’t know how to identify what additional amendment it needs. Calcium? Mag? or ???
Thanks for contacting Toronto Master Gardeners. Chlorosis refers to the yellowing of leaves. There are two types: veinal and interveinal. Interveinal chlorosis refers to the yellowing of the leaf but not the veins. Causes of interveinal chlorosis include poor drainage, damaged roots, compacted roots and nutrient deficiencies in the plant. Nutrient deficiencies can occur when there is an insufficient amount in the soil or when the nutrients are unavailable in soil with a high pH (alkaline soil). Another possibility is that the nutrients may not be absorbed due to injured roots or poor root growth.
Leaves with veinal chlorosis have yellow veins. Veinal chlorosis is the more serious condition and can be caused by factors such as an overuse of herbicides or insecticides or long term nutrient deficiencies in the plant.
Although you haven’t described the yellowing pattern of the leaves, generally interveinal chlorosis is more common in hydrangeas. The most likely cause is an iron deficiency due to an alkaline soil. To assess the foliage for iron or other nutrient deficiencies click here for an assessment guide.
Hydrangeas generally prefer soil with an acid pH. To help identify the pH of your soil and any nutrient deficiencies consider testing your soil click here for some soil testing sources.
Once you have diagnosed the problem accurately, you will find products that lower the pH of alkaline soils and increase nutrients in the soil at reputable garden centers.
Good luck with your hydrangea.