Weight from a single intense force or small repeated forces pushes soil particles together, causing them to compact. Compacted soils have reduced macro and micro pore space, which result in limited air and water movement, restricted root growth, reduced infiltration rates and decreased biological activity. Therefore, highly compacted soils greatly limit plant growth and the ability of the soils to absorb rainfall and filter pollutants.
Over-compaction is one of the most common characteristics of degraded soils. Simple changes in landscape use and maintenance practices can protect soils.
Common causes of soil compaction in urban landscapes include:
- Construction and maintenance equipment such as skid steer loaders used to remove vegetation and prepare the site for construction or riding lawnmowers used when the soil is wet.
- Parking or driving on portions of the site not designed for vehicular traffic.
- Repeated pedestrian and animal traffic.
- Repeated tillage.
- Walking on, compressing or digging in the soil while wet.
- Rainfall on bare soils, particularly in areas where rainfall is intense and concentrated such as runoff from rooftops.
- Removing organic matter such as leaves, grass clippings and branches will, over time, reduce the organic matter content of the soil and increase the likelihood of compaction.
Common signs of soil compaction can include:
- Water ponding and/or very slow infiltration rates.
- Surface water runoff from irrigation and rain.
- Soils that are bare and not supporting vegetation.
- Shallow tree rooting.
- Stunted vegetation.
- Strong resistance to penetration when compared to surrounding soils. Students can test soils using a garden shovel or steel rod and applying weight evenly. It is often helpful to begin by comparing obviously compacted areas with soil in other parts of the landscape.