Marble Chip Mulch

Marble chips are composed of metamorphosed limestone, and like limestone, consist primarily of calcium. When calcium is added to soil in large enough quantities, it can raise the pH of the soil. Most plants grow well in a pH of 5.5 - 6.5 in our area. This is particularly true for woody plants. If the pH gets higher, then iron, a critical soil nutrient, becomes less available for uptake by the plant, and the plant shows the symptoms of iron deficiency, which are chlorotic leaves or needles.

In the first photo, a trio of mugo pines (Pinus mugo var. mugo) was planted in front of a house. Marble chip mulch was added. The pines show signs of dulling of color, and several months after the photo was taken, one of the pines is dead.

In the next two photos, a row of Rose of Sharon (Hibiscus syriacus) was planted along the wall of the house, and marble chip mulch was added. The first photo shows the shrubs with yellowing leaves, and the second, a close-up, reveals details of the leaf discoloration. The intervenal areas are yellow, and the veins are green, which is typical of iron chlorosis.

What can be done? First, the mulch should be removed. Second, the soil pH needs to be raised. This can be accomplished with the addition of soil amendments or fertilizers containing iron sulfate. These products are available in your home improvement stores or local garden centers. Follow the instructions on the product container for how to apply the product. Use it cautiously -- apply a little at a time and wait for the plants to respond before applying more. Note: One should first test the soil pH to verify that the higher pH is indeed causing the problem before applying soil amendments. The Cooperative Extension Service can do this for you.