Preventing Soil Compaction in the Right-of-Way

Roots respire — they absorb oxygen from pore spaces in surrounding soil and release carbon dioxide. Whether tree or tomato, roots flourish in loose, porous soil because it is more permeable to air and water.

In the flower and vegetable garden we promote porosity by amending the native, mineral soil with organic matter like compost or peat. Amending soils in the ROW isn’t practical, so the best plan is to prevent and if possible, alleviate compaction.

Soils in the right-of-way typically become compacted by pedestrian traffic and vehicles driving or parking off pavement. Compaction is made worse when the activity occurs on damp/wet soils. Even raindrops falling on bare soil cause compaction.

Chosen and applied correctly, mulches can prevent, minimize or even alleviate compaction.

In flower gardening and perhaps more so in vegetable gardening where soils may be highly amended, the general public often uses the terms mulch and compost interchangeably. In the world of tree care this is not the case. It is this context which frames the following material.

Mulch can be organic in origin (wood chips, shredded bark, recycled wood, cocoa hulls, etc.) or inorganic (river stones, lava rock, concrete rubble, shredded tires, plastic mats, etc.). The unifying factor is that all are structural — that is, when applied in a sufficient layer over bare soil, load applied to the surface of the mulch is distributed across a larger area of the soil beneath.

This structural feature of mulch prevents or minimizes soil compaction from foot and/or mower traffic. When soil solids are compressed, large pore spaces collapse. Without large pore spaces, gas exchange with air above ground and water infiltration are reduced, impairing root growth and thus plant growth.

Among the organic mulches, this structural strength comes from lignin — what you would generally perceive as wood fibers. Lignin resists decay longer than other tissue, sometimes taking years to fully break down (think of logs on the forest floor). Because organic mulches do decay, eventually improving soil, they are the only type recommended for use in the ROW.

As an example of how effective coarse, woody mulch is, consider this — home addition and remodeling projects often involve use of equipment like skid loaders. Simply running this relatively small device over turf three times will compress the soil to the same bulk density as concrete — not conducive to root proliferation. The application of a 12” layer of wood chips over the same soil will allow passage of full-size construction equipment without soil damage (and prevents muddy messes, too).

What’s needed for your ROW and landscape trees?
Choose coarse, woody/organic mulch, not inorganic materials. Initially, apply a 3-4” deep layer, extending at least 3’ from the trunk, but the greater the area of soil protected, the better — expand your thinking from circular to rectangular mulch zones. With time, the chips in contact with the soil will begin to degrade and worm activity will drag this organic matter down into the soil, improving soil structure and enhancing plant ability to mine essential elements from our nutrient-rich native clays. Over extended periods of time, attentive mulching can alleviate some degree of pre-existing soil compaction.

Annually, add another inch or two of mulch to replace that which has decayed below or been lost from the top. Never let the mulch get deeper than 4” and ALWAYS keep the mulch about 4” away from the trunk.

Composts are only organic in origin. They are farther along the continuum of decay than mulch and frequently derive from materials that didn’t have much structure to begin with — animal manures, leaves, grass, vegetable food waste, etc.

While variable in nutrient content, composts are consistently valuable as soil amendments. They can be incorporated into garden beds before planting or applied as a top-dressing. When incorporated, they quickly lighten soil texture (making it easy for roots to grow) and increase microbial activity that releases nutrients from the surrounding mineral soil for plant uptake. Use of compost amendments helps improve drainage in clay soils and helps retain moisture in sandy soils.

Unlike mulch, composts have little if any structural quality, so surface applications do not protect soil from compaction.

For your ROW trees:
At planting, the soil over the root ball of your new ROW tree is top-dressed with compost made from leaves collected each fall in Wyoming. For several reasons, it should be covered with mulch when you or your lawn crew gets to work in the spring.

1. Your tree needs a greater area of “de-turfing” than is achieved in the planting process. Grass is very competitive with tree roots for moisture. By providing a large mulched area you reduce stress on your young tree during dry spells.
2. Since compost is not structural, even repeated top-dressings will not prevent soil compaction from weekly mower passages.
3. Unprotected, that dose of compost will be fully decayed, oxidized and/or blown away by August. Bare soil dries faster than mulched soil, it is subject to compaction by pounding rains (not to mention, again, the friendly mowing crew), it begins to erode, and it becomes colonized by weeds that sprout in the fall.

By adding mulch, the compost remains in close contact with the soil and begins the process of passively amending it. To accelerate this soil improvement, pull back the mulch every year or two, add a couple inches of compost and reapply the mulch with a layer of fresh mulch to reestablish the structural protection discussed above — always keeping the maximum depth in the 4” range.

Further reading:
Introduction to Soils — ~2000 words. Nice readable piece. Ignore the section on amending pH — it is Mississippi-specific and wholly inappropriate to soils of SW Ohio. — ~650 words. Explains why porosity matters in commercial greenhouse production. — ~500 words. For plant geeks — new info on explaining how plants respond to saturated soils/ poor oxygen diffusion.

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Preventing Soil Compaction in the Right-of-Way
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