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Written by Jeff Shimonski, BT Contributor   
June 2017

The down and dirty of topsoil

LPix_YourGarden_6-17ast month I wrote about mulch use, so I thought now would be a good time to follow up with a discussion about what occurs underneath the mulch, in the ground around trees and shrubs. I’ve often discussed the structural health of tree branches and canopies, but at the end of the day, it’s what we don’t see, the life below ground, that’s the most critical area of tree and plant care.

The organisms that live below, in the ground around the roots, are critical in establishing the overall health of the landscape’s plants. These organisms decompose the organic matter that improves the nutrient status of the plants and the water-holding capacity of the soil. Many of the fungal species underground are responsible for decomposing woody plant material. Some also have symbiotic relationships with plant roots, providing them with water and nutrients. The fungi in return receive carbohydrates from the plants. Some fungi also attack or exclude pathogens from the root zone.

A very important component of the below-ground ecosystem is the structure of the soil. Does it enable good oxygen penetration and gas exchange with plant roots? Is it porous enough so that water can drain easily and the soil doesn’t become anaerobic, causing roots to die from lack of oxygen?

I’m always interested in seeing what’s happening below ground. For example, I’ll monitor the demolition of buildings to determine if tree roots have grown underneath structures. When a contractor is digging a trench, I’ll stop to look at the soil profile or the different layers of soil types that have been exposed by the excavation. This is easy enough to see; along the wall of the trench the layers are different colors, composed of soil and rock of different textures, giving the layers of soil different characteristics with respect to oxygen penetration and water drainage.

A normal soil profile has different layers. The first layer contains the most organic matter and decomposing mulch. This is the “O” horizon. The second is the “A” horizon, or topsoil. There are layers below these, but usually the first two go down about two feet and are the most important for plant roots. Remember plant roots need oxygen so don’t grow too deep and normally are not found below 18 to 24 inches deep.

The accompanying photo shows a structure being demolished. The machine operator is extracting the footer, or outer edge of the structure, and I’m able to look for roots from the adjacent trees. Normally in our area, tree roots do not pass underneath the structure because the soil is so compacted and there’s no oxygen or moisture the roots can access.

The only time I find roots underneath a structure is when there’s a leaky or broken pipe. The roots follow the path of the water, which has created a gap in the substrate that enables access to water and an oxygenated zone. This is how roots end up clogging pipes.

There’s already a crack and leak, often from poorly connected pipes or a weak pipe joint. On this site there were no roots underneath the structure, and I noticed the adjacent topsoil layer was about two feet deep. This is excellent for growing trees.

Getting back to looking into trenches and the layers of soil that can be seen on the side of the trench, I can see the history or sequence of the construction and landscaping. What is most noticeable are the layers closest to the surface. I can see where the compacted structural fill was laid to support the structure. This fill is compacted to a point where there are no soil pores large enough to allow roots to penetrate. This layer often extends well beyond the edge of the walls. Then above that is the “landscaping” soil, sometimes two or three different layers deep. This tells me that the owners of the property had been talked into adding more topsoil over time, to “improve” the landscaping. I see the results of this on many properties where the landscape or lawns are much higher that the adjacent sidewalks, patios, and driveways.

What does adding all of this extra soil accomplish? Have you ever seen the upturned root plates of trees that have been blown over in a storm? Were those root plates flat as pancakes? Many times there are no deep roots on trees because the interface between the different layers of soil have a layer of water accumulate between them causing anaerobic conditions that will not allow roots to pass through. This is a perched water table, something landscape architects and designers need to be aware of when designing landscapes.

 

Jeff Shimonski is an ISA-certified arborist municipal specialist, retired director of horticulture at Parrot Jungle and Jungle Island, and principal of Tropical Designs of Florida. Contact him at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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