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Soil Compaction Remedies

Monday January 23, 2012

In a previous article, general information about soil compaction was presented. This article aims to offer suggestions to remedy compacted soils and ways to prevent compaction in the future.

The purpose of reducing the level of compaction is to increase the amount of pore space in the soil structure. An increased amount of pore space elevates the amount of water and oxygen available to the plant and other beneficial microorganisms.  

Mediterranean Mounds: In many instances, the cost and effort of amending a severely compacted soil (especially true clay-like soils) may be too great. In these situations, consider importing well draining soils to be placed on top of the existing soil. These soil mounds, commonly referred to as Mediterranean mounds, should be a minimum of 18” tall. Before installing, break up the existing soil to the extent possible, then place the new layer of soil. By breaking up the existing soil, you increase the opportunities for plants and microorganisms to reach beyond the new soil down into the existing, compacted soil. Over time, the roots and microorganisms will likely reduce overall levels of compaction.

Suggestion: Your mounds will likely vary in height for aesthetic and grade-related reasons. So to make best use of the new soil depths, place larger shrubs in areas with deeper soil and your shallow rooted creepers and crawlers on the shallower soils.

  Incorporation of Compost and other Organic Materials: The best way to improve compacted soils is to incorporate aged compost, manure, or other organic materials into the soil. In this sense, incorporation to a 1) sufficient depth, and 2) fully-blended state is paramount. Ideally, cultivate the soil and incorporate the compost to a depth of 6-8”, or deeper if practical. A good rule of thumb is to consider a 4-6” depth a minimum and anything beyond a bonus to the plants. Also, ensure the compost and/or other organic material is blended to a homogenous state with the existing soil to prevent uneven settling, root growth abnormalities, and uneven percolation of water.

Suggestion: Add an even, 1” layer of compost over the area to improve then incorporate by means of double-digging, roto-tilling, or discing.

Install Thick Layer of Mulch: If the last suggestion is the best, the following suggestion is by far the easiest. Simply adding a 4-6” layer of tree chippings or mulch will, in time (many months to years), break down and add to the vitality of the soil. The added organic materials, working in tandem with microorganisms, work their way down the soil profile in a awe-inspiring process Take note: Adding typical walk-on bark, commonly found as redwood bark chips in plastic bags at garden centers, is not preferred. This bark takes much longer to decompose and further lengthens the amount of time required to improve the soil.

Aerating: Many times landscapers and gardeners find themselves struggling to green turf but no amount of fertilizer or water seems to remedy. The re-occurring root of this problem is a compacted soil. This happens because the moisture and nutrients are unable to penetrate into the soil where they can be absorbed and processed. To solve this issue, aerate the lawn. Aerating is most commonly done by means of a mechanical “corer” or “aerator”. This machine removes small, cylindrical tubes of soil at even intervals. The cored soil is deposited onto the turf where it is allowed to decompose naturally. These voids allow water, oxygen, and nutrients to penetrate deeper into the soil and encourage growth. For smaller areas, non-mechanical turf corers are available.

Soil Inoculum: Unseen to the naked eye are armies ofmicroorganisms that reside and work in healthy soils. All of their work is unseen yet it is critical to the health of all soil. In compacted soil, these microorganisms are often absent or in such small numbers that they are over-worked and fighting a losing battle. Soil inoculum comes in wide variety of applications but our favorites are a spray type. To apply, one basically mixes a potent concoction of microorganisms and water then sprays over compacted soils. This provides an immediate boost in soil activity and health. Microorganisms, mostly fungi, permeate the soil and work to create a network that connects roots to nutrients. These fungi provide the basic elements for the health of soil biology.  


Looking Ahead: What Can Be Done to Avoid Soil Compaction

New Construction Projects: During construction projects, the focus of the general contractor is compacting the soil to 90-95% relative compaction. In other words, their aim is to compact the subsoil into a concrete-like base to ensure stability for the structures. Although entirely understandable and appropriate for the built structure, these normal construction activities will no doubt spread the high levels of compaction to areas well beyond the building limits. As a landscaper or homeowner, consider relaying the following suggestions to your general contractor before and during a construction project. The construction schedule normally trumps other concerns but a creative and thoughtful contractor will want to accommodate your requests when he understands the full picture.

1) Utilize light-weight skid steer machinery around building perimeter when possible. The less weight placed on the soil the better.

2) Keep site activity to a minimum when the soil is moist.

3) Confine construction activities to specific areas with the use of temporary construction fencing.

4) When and if heavy equipment are needed on site, utilize the machinery to till/disc the soil they have compacted before leaving the site.

5) Attempt to confine building material storage / lay-down areas to locations that will be eventually paved such as walks, patios, driveways, etc.

6) Re-focus traffic patterns in areas less prone to compaction.

7)  If forced to work on soil in damp/wet conditions (it happens all the time), roll out old sections of carpet or jute netting to keep the area clean and as mud free during work hours.  During after work hours, roll up and allow to dry.  Using mulch for long periods of time may actually keep the soil wet for too long.


Existing Gardens: Time, neglect, and many other factors can contribute to compaction. Consider the following suggestions to prevent soil compaction in the future:

1) Keep foot traffic off wet or moist soils.

2) Keep a thick bed of mulch over planter areas.

3) If your site accommodates heavy foot traffic, consider formalizing the paths and installing a material that better tolerates constant use.

4) Aerate turf areas at regular intervals.

5) Annually add compost or mulch to planter beds.