The leaves on my river birch are yellow-green. Why?

In Iowa, the foliage of the river birch (Betula nigra) often turns a sickly yellow-green. The yellow-green foliage is due to a deficiency of iron. The problem is referred to as iron chlorosis. (A close examination of chlorotic leaves will show that while most of the leaf is yellow-green, the tissue around the major veins is a darker green.)  Most soils in Iowa contain sufficient amounts of iron.  However, in alkaline soils (those with a pH above 7.0), the river birch is unable to absorb adequate amounts of iron because much of it is in an insoluble form.  Since many soils in Iowa are alkaline, chlorotic river birches are common in the state. 

Correcting an iron chlorosis problem is difficult.  Lowering the soil pH to 6.0 to 6.5 would allow the roots of the river birch to more readily absorb iron in the soil.  Unfortunately, lowering the soil pH is extremely difficult, if not impossible.  Applications of iron-containing compounds, particularly chelated iron, may help trees that are mildly chlorotic.  When using iron chelates, carefully follow label directions.  Applications should be made in early spring.