Installing lime stone class 5 on exsposed tree roots

Asked September 15, 2016, 4:49 PM EDT

I am doing a driveway project in St Paul. We are using limestone class 5 for a base for our driveway. After doing some research I found that using limestone is going to chance the alkaline in the soil. My concern is that it will kill a very large Elm tree the my customer is very fond of. Is there a better compact able gravel that I can use that will not change the soil. Thank you for your time. Nate 651-269-9801

Ramsey County Minnesota

1 Response

Hi Nate,

Limestone will cause the chemistry of the soil in the vicinity to change, due to an increase in pH. Crushed limestone, or lime, is commonly used when a soil's pH is too low in order to raise it to more appropriate levels.

With that said, the use of limestone in a driveway is not a guarantee of killing a tree. The first consideration would be how close the tree is to the driveway, and what other landscape features are in the area. If the tree is a good distance away and only a few roots are in the vicinity of the gravel, the harm to the tree would be expected to be slight, as the majority of roots would still be in desirable soil.

The primary concern with raising the soil's pH with limestone would be availability of micronutrients including manganese and iron, which are much more available at low pH than at high pH. If these nutrients are lacking, a general chlorosis (yellowing) of leaves may occur. Unless the deficiency was very severe (ie most of the soil in the area was affected), this chlorosis would not cause a rapid death of the tree.

In general, Elm's are fairly tolerant of elevated pH levels- at least up to 8.2. If deficiencies do occur, there are a variety of products that can help to alleviate the concerns.

I'm not sure of the engineering properties of these materials, but crushed granite or crushed basalt have been used by some people when large concerns about alkalinity are present. With limestone, larger aggregate sizes will react slower compared with smaller sizes or powder, and will lessen the pH changes over time.

It may also be helpful to get a soil test from the area and see what the current pH in the area is. If the pH is already alkaline (>7), the limestone could cause more severe problems than if the pH is below 7.0.