Although oak leaves do have the potential to acidify soil, they are highly unlikely to be causing problems in your yard in northeast Kansas. If you are having trouble growing grass (or anything else) under an oak tree, the real culprit is most likely root competition, low light and soil compaction, or some combination of all three.
As far as oak leaves and acidification, I know your neighbor or your friend or your relative who gardens may have told you otherwise. They probably heard about it from their neighbor who heard about it from their neighbor. In some parts of the country, some oak disease problems have been associated with oaks growing in acidic (low pH) soils. Here, however, there are more oak diseases and problems associated with basic (high pH) soils.
The simplest way to know for sure whether your soil pH is less than ideal is to test the soil. Take samples from several locations around the tree and mix them together to get a representative sample of the area. Use a knife or a trowel to take slices of soil four to six inches deep. Test the soil pH with a pH testing kit or through the K-State Research and Extension – Douglas County office at 2110 Harper St. Testing kits are available at most garden centers. (If you test through the Extension office, you will also get recommendations to correct the soil pH if necessary.)
If pH is not the problem, fight soil compaction by core aerating the lawn under the tree. Select grass seed that contains three to four varieties of turf-type tall fescue, as they perform the best in shade trials in this area. Grass seed labeled as “sun/shade mix” or “shade-loving” often contain grass species that are inappropriate for our area. Avoid overwatering as it will encourage shallow root growth.
Another good indication that soil is not too acidic for lawn grasses is when it is too basic for good oak tree growth. Ever see a pin oak with light green or yellow leaves? Chlorotic pin oaks are common in the Midwest. In oaks, chlorosis is caused by an iron deficiency that is almost always the result of the tree’s inability to take up iron. Iron is tied up in high pH soils. Adjusting the soil pH requires applications of sulfur and may take years.
Researchers in France who studied the relationship between oak leaf litter accumulation and soil acidity note that the trend was different in each of the 30 oak trees in the study. Soil clay content plays a role in soil acidification, as well as soil organisms, bark and the soil parent material. The only conclusion drawn from the study was that on average more acidification and litter accumulation occurred near the trunk base.
Pine needles carry the same reputation as oak leaves and acorns, but also have little effect on soil pH. White pines are a good indicator — they often suffer from iron chlorosis in high pH soils just like pin oaks.
Another option if you are having trouble growing grass under an oak or pine tree is to cover the area with mulch or a drought-resistant ground cover.