Birch tree leaves often turn yellow because of chlorosis, which occurs for several reasons. According to the University of Illinois, chlorosis occurs because of an iron deficiency, poor drainage, damaged roots, high soil alkalinity, compacted roots and nutrient deficiencies. Nutrient deficiencies usually occur because the nutrients are unavailable due to a high soil alkalinity, or pH.
According to the Forest Service of the United States Department of Agriculture, most birch trees grow better in acidic, or low alkaline, soil. Ideally, the pH of the soil should be between five to 6.5. Some birch trees, such as the White-barked birch trees, grow well in alkaline, or high pH, soils. However, the River birch develops chlorosis in alkaline soils with a pH higher than 6.5. This is most likely the reason for yellowing leaves in river birch trees.
Other disturbances in the soil cause chlorosis as well. For instance, birch trees do not like compacted soil because they have shallow root systems. Compacted soil damages the root system, which prevents the tree from absorbing nutrients and turns the leaves yellow. Poor drainage has similar effects. According to the Forest Service, birch trees prefer moist soil that is not overly wet. Thus, if a pool of water remains around the tree after watering, poor drainage and wet soil is the cause of the chlorosis and yellowing leaves.