Depending on the species of the tree, turning yellow could either be a sign that the tree is nearing a typical autumn shedding, the sign of an anemic tree or an indication of an imbalance in the PH level in the soil.
Some species of holly tree lose their leaves in the fall like many other types of trees. Winterberry holly is one such species. Although the leaves of the winterberry tree turn yellow prior to falling from the tree each fall, yellow leaves may be a symptom of an iron deficiency called iron chlorosis. Iron chlorosis is typically caused when the tree is exposed to too much moisture or when the soil surrounding the tree becomes overly saturated and cannot drain. Holly trees also require acidic soil in order to thrive. This is also related to the processing and distribution of iron. A high PH level interferes with a holly tree's ability to properly manage its iron levels. When iron levels are low, holly trees are unable to produce chlorophyll. Chlorophyll is what makes tree leaves green. To resolve issues with yellow leaves on your holly tree, test the PH level in the soil and make sure that the tree is not getting more than two inches of water per week.