MASTER GARDENER: The basics of compacted soil

lifestyle_homegardenResearch tells us that soil structure refers to the combination of the soil particles sand, silt, and clay into larger units called clods which are seen when soil is tilled. Well-structured soils promote root growth, have good drainage, high water holding capacity, and good aeration properties.

Compaction forces soil particles closer together, drives out pore space and reduces the volume of air and water in soil. The surface is sealed off reducing the amount of air and water that enters the soil.

Compaction increases the soil’s ability to reduce movement; therefore, the roots of plants must exert greater force to penetrate the soil. Reduced oxygen concentration in the root zone is common.

One way to visualize soil compaction is to think about the difference between living in a house with 10-foot ceilings and one with 5-foot ceilings. In the compacted house, there is less air and water, moving around is more difficult and neighbors (helpful earthworms) don’t want to visit.

Compaction is normally caused by repeated foot or vehicle (construction, maintenance or car) traffic or even raindrops over the same soil area. Any unprotected soil can become compacted as the forces of humans and nature take their toll.

Sparce growth or barren paths in turf areas are common indicators of soil compaction. If a soil probe, shovel or hollow pipe can be easily pushed into the soil there may be another problem.

To prevent compaction, avoid tilling, cultivating or walking on saturated soil as soil is weakest then. Use stones, gravel or wood chip mulch to create paths in ornamental plantings. Beds should be 3- to 4-feet wide and separated by narrow paths. Use the lightest and smallest equipment on the smallest area possible.

To reduce and treat compaction, the soil particles must be loosened. Try adding a 4-inch-deep layer of chunky organic mulch to the area. Chunky mulch will allow oxygen and water to penetrate and reduce the impact of compacting forces.

Add organic matter and till to 18 inches in the impacted area before replanting. Aerate once or twice a year or restrict traffic totally and wait for the soil to recover, which could take a full season or more. Removal of the soil and replacement with better soil is a drastic solution and feasible if the amount to be replaced is not too large.

Jennifer Caldwell, a Master Gardener, is a trained volunteer of the Mississippi State University Extension Service. For gardening questions, call the Help Center at (662) 620-8280 in Lee County or (866) 920-4678 outside Lee County and leave a message.