Ask Mr. Smarty Plants

The diagnosis is that the leaves on the smaller elms are chlorotic, or not having enough chlorophyll in the leaves. Chlorosis is caused by the failure of the tree roots to absorb enough iron from the soil for the creation of chlorophyll, with which the plant manufactures food. Ulmus crassifolia (cedar elm) prefers moist to dry, alkaline soil. In Boerne, in the Texas Hill Country, you have the alkaline soil, but not necessarily the moist soil. But even for a plant preferring a slightly alkaline soil, the Central Texas soils can be too alkaline to permit the plant to take up iron in the soil. In excessively wet or poorly drained soils, the chemistry of the soil changes and iron becomes unavailable. 

Here are some things to try to prevent chlorosis. Improve the soil-work compost in around the roots, this will also help make the soil drainage better. Apply iron sulfate, blending with organic matter like compost and concentrate the mix in soil near plant roots. Apply iron chelate-these organic compounds keep iron soluble and available to roots. Scatter the dry granules within plant's drip line, then water thoroughly. Leaves should start to green up in 2 to 3 weeks. 

The website Ulmus crassifolia (USDA Forest Service) mentions that Elm leaf beetles and aphids can infest the Cedar Elm. The elm leaf beetle can seriously defoliate a tree, but is not a cause of yellowing, and defoliation is usually not too severe. Aphids also may suck fluids out of the leaves, causing them to yellow; again, not a big threat. Aphids can usually be identified by the honeydew they deposit on the undersides of leaves, which often hosts sooty mold, turning the leaves dark gray or black. Another disease that has yellowing leaves as a symptom is Dutch Elm Disease (USDA Forest Service. This disease is usually fatal, and there are other more compelling symptoms on the tree besides yellowing leaves. 

In answer to your question, is this a soil problem, yes, we believe the alkalinity of the soil and/or poor drainage is probably the reason the leaves are turning yellow. If only the smaller trees are showing these symptoms, it is probably because their roots are not getting good enough drainage to permit them to draw on the minerals in the soil. See the first paragraph for compost around the roots and drainage improvement. 

Ulmus crassifolia

Ulmus crassifolia

Ulmus crassifolia