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Introduction to Soil Compaction in the Landscape

Friday January 20, 2012

Think of the word “soil” and what image first pops into your head?  If you're like most people, you visualize a dark, fluffy, rich, nutrient-dense, compost-laden material.  But the reality landscapers and gardeners face is often a much different picture.  Arguably the most significant limiting factor in the success of a garden, soil compaction is a concept that few understand.

What is Soil Compaction?

Simply put, soil compaction refers to the deterioration of structure through the loss of open voids, also known as pore space, between soil particles.  These pores are critical to the health of a plant because they store and transport water and air to plant roots and other soil microorganisms.  Without proper pore space, water and air movement through the soil is restricted.  As a result, you have plants with a reduced capacity to thrive and increased quantities of stormwater run-off.

soil compaction graphic with different size pores

text low soil oxygen levels caused by soil compaction are the primary factor limiting plant growth in landscape soils

What Causes Soil Compaction?

As one might suspect, soil compaction often occurs under the weight of heavy equipment, intense foot traffic, over-saturation of soils, and light foot traffic on moist soils. In addition, soil may compact because of improper landscape maintenance.  Two common places this occurs is on lawns and planter beds.  Lawns will easily compact when mowed when still moist, are not cored/aerated at regular intervals, or if organic material isn’t allowed to decompose naturally over the lawn.  Similarly, if planter beds are left un-mulched for long, chemical and natural processes will slowly harden and compact the soil (especially when watered with overhead spray irrigation) to the point of compaction.

Do I have Compacted Soils?

For you to ask this question, you likely have some plants that look like they are struggling but you can’t seem to nail down the problem.  Here are a few easy tests to help you identify if soil compaction is the issue:

-Visual Test:  Do you dread digging holes in the garden?  Do you require an adze and way too much sweat?  Do you see lingering puddles or water run-off from the bare soil surface?  Do your plants look down-trodden and no amount of water (little to lots) solves the problem?  If so, odds are good that you have a compacted soil.  If you aren’t convinced or want to know how serious of a problem it is, conduct the following Dig Test.

-Dig Test:  Excavate a 12” x 12” x 12” hole and fill water infiltrating through soilwith water.  Wait for water to infiltrate through the soil.  Wait 1-2 hours, then fill again.  Take note of the time at filling and record how long it takes the water to infiltrate.  Results:  If your second infiltration takes over 45 minutes (and you don’t have a very clayey soil to begin with), you likely have a soil compaction problem.

-Contact a Landscaper:  Most experienced landscapers carry around a soil compaction tester in their trucks.  These probes measure soil compaction quite easily and really only require your body weight.  

Turns out I have Compacted Soils!  Now What?

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