Feeling the ground buckle or spongy when walking across your lawn indicates that the underlying structure of your turf is compromised. This phenomenon, which causes the surface to appear uneven or lifted, is a symptom of distinct problems affecting the balance between soil, roots, and organic material. The rising sensation signals a change in density beneath the grass blades, often due to organic buildup, soil displacement, or excessive moisture. Understanding the specific cause is the first step toward restoring a firm, healthy surface.
The Problem of Excessive Thatch
Thatch is a layer of dead and living organic matter that accumulates between the grass blades and the soil surface. A thin layer (less than a half-inch thick) is beneficial as it insulates roots and conserves moisture. The problem begins when this material decomposes slower than it accumulates, leading to an overly thick layer. When this layer exceeds 12 to 15 millimeters, it acts like a dense sponge, creating a buoyant or “spongy” feeling underfoot.
Excessive thatch physically lifts the grass crowns away from the soil, causing roots to grow shallowly within the thatch rather than penetrating the ground. This results in a weakly anchored lawn susceptible to drought and temperature stress. Rapid buildup is encouraged by over-fertilizing with nitrogen and improper mowing practices. Frequent, light watering also keeps the thatch moist, inhibiting the microbial activity necessary for decomposition.
Subsurface Tunneling Pests
The lawn surface can be raised by the displacement of soil due to the tunneling activity of small animals. Moles are the most common culprits, creating raised, volcano-shaped mounds of soil as they excavate deep tunnels. Their hunting tunnels, pushed up just beneath the surface to search for grubs, appear as noticeable ridges that can be easily compressed underfoot.
Voles, often called meadow mice, cause surface disruption by creating shallow, narrow, trench-like runways for foraging. These runways are typically two inches wide and run along the surface. A large infestation of white grubs (beetle larvae) also contributes by feeding on grass roots. When roots are severed, the turf loses its physical anchor, leading to a loose, spongy sensation that allows the grass layer to be lifted or rolled back.
Water Dynamics and Soil Swelling
Water saturation can cause the lawn surface to lift or become soft and unstable. Poor drainage is a major contributor, often caused by compacted soil or heavy clay, which prevents water from infiltrating and draining effectively. When topsoil is compacted, water pools beneath the surface, creating a waterlogged condition that feels soft. This can lead to hydrostatic pressure, where the trapped water exerts upward force, making the lawn feel springy.
The expansion of water during freezing temperatures can also cause the ground to lift in a process known as frost heaving. As water freezes in the soil profile, it expands, pushing the soil and turf surface upward. This effect is most noticeable when cycles of freezing and thawing cause the soil to become uneven. In clay-heavy soils, the molecular structure causes the soil to swell significantly when saturated, contributing to the lifted appearance.
Diagnosing and Resolving the Issue
Diagnosing the cause of a lifted or spongy lawn is the first step toward an effective solution. The simplest diagnostic is the “foot test,” where a consistently soft, bouncy feeling across a wide area points toward a thatch or drainage issue. To confirm thatch, cut out a small plug of turf and measure the brown organic layer between the soil and the green blades; a thickness over 12 millimeters requires mechanical removal.
If the problem is excessive thatch, the solution is dethatching or scarification, which physically removes the dense material to allow water and nutrients to reach the soil. If pest activity is suspected, conical mounds or raised ridges indicate moles, while surface-level trenches point to voles. Controlling these pests involves identifying the specific animal and applying targeted methods, such as treating the lawn for grubs if they are the primary food source.
For water-related issues, the key is improving the soil’s ability to drain and manage moisture. Core aeration is highly effective as it removes small plugs of soil, relieving compaction and allowing deeper penetration of air and water. If water pools due to poor grading, the surface may need to be regraded to create a gentle slope away from structures. Adding organic matter to clay-heavy soil can also improve structure, increasing permeability and reducing saturation and swelling.