Gardening with Allen: Meeting of two distinct layers of soil cause common problem
By Allen Wilson
Published: March 19, 2019, 6:00am
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I had a new sod lawn put in two years ago because I had several very thin spots. It was good for a while; however, the next year I noticed that there were even more thin spots! So I planted seed, which sprouted but then died. A neighbor’s lawn guy tested the soil. He said the sod they put down was in clay, which they put down on top of 3-way soil mix. Apparently, the seed I planted won’t grow through the clay and root into the 3-way soil.
You have a problem that is very common when two distinct layers of soil meet. It also often causes problems with trees and shrubs as well as lawns. Water movement and therefore root growth is restricted in its flow from one layer to the next. Water is trapped in the clay layer, and the 3-way soil mix below stays dry unless a lot of water is applied.
As a result, grass roots are very shallow causing the grass to thin. When you planted more seed it sprouted in the clay layer but roots were too shallow to survive.
The best way to solve this problem is to hire a landscape contractor with a core aerator or rent a core aerator with long tines that go 1 1/2 to 2 inches into the soil. Run the aerator two or three times in different directions across the lawn. This will cut through the half-inch clay layer and bring some soil from below.
Rake the plugs that are removed to scatter them evenly over the entire lawn. Water will flow readily through these core holes into the soil underneath.
Then apply a top dressing with a thin layer of 3-way soil mix (about 1 yard per 1,000 square feet). Broadcast seed and rake lightly so most of the seed is covered. Apply fertilizer at the same time you plant seed.
It is very important to keep the soil constantly wet on top for about 3 weeks. Tiny new roots are easily killed by drying until they grow into the soil below the old sod.
Soil in containers of trees and shrubs is a mixture of well drained ingredients. When a plant with this type of soil is planted in heavy clay soil the difference in soil texture makes it difficult for the roots to grow into the surrounding clay soil because of this same layering effect. The best way to alleviate this problem is to mix an amendment such as bark dust into an area at least 3 times the width of the root ball. When a group of shrubs is planted, mix amendment into the entire area. This provides a soil into which plant roots can grow and develop.
If a layer of top soil or soil mix is placed over native clay soil, the same kind of layering problem is created. Soil should be tilled so there is an intermixing of the two layers and a gradual transition so roots can grow readily to their full depth.
