How Long Can Soils “Live” Under Paving Without Trees?

Occasionally we work on Silva Cell projects where the installation of the product and the soil it contains happen far in advance of any trees being planted. In these instances, we often get asked by concerned designers whether the soil biology can be maintained under sidewalks in the temporary absence of trees — and if so, for how long.

Based on my experience and a search of peer reviewed literature I can find no issue with soil biology decay under pavement for at least a two year time span. The macropore rich soils inside the Silva Cells will continue to “live” under the pavement for the following reasons:

In horticultural parlance, soils become “dead” in extreme anaerobic environments. It is a biological fact that there are numerous anaerobic bacteria and viruses that survive even in low oxygen soil environments. However, these are “dead” soils from a horticultural standpoint. Typically, long periods of inundation (months) causes soils to “die,” again this is from a horticultural point of view. The key to “living” soils is the free movement of water and oxygen through the soil column. This oxygen-rich environment is made possible by macropores, and these soils are ideally comprised of 50% mineral, 25% water, and 25% air.

Silva Cell specifications call for these macropore rich soils to be installed inside the frames of the system. Beneath the deck of the Silva Cells is a 1 inch to 3 inch (2.5 to 7.5 cm) air gap. This air volume allows free exchange of oxygen through the soil column.

The primary microorganisms of concern for “living” soils are Mycorrhizae, filamentous fungi primarily of the Glomus genus. Of all the biological associations in the world of trees, Mycorrhizae provide the greatest and most dramatic benefit to trees for water and nutrient uptake (USNFS; Amaranthus; Shigo).

The largest supplier of laboratory cultured Mycorrhizae in the world has placed a minimum 2 year shelf life on its shipped Mycorrhizae product inside sealed plastic bags at 80 degrees  F/26.7 degrees C, stored out of direct sunlight (Amaranthus et al).

The growing conditions under the pavement will not reach those extremes. Therefore the macropore rich soils inside the Silva Cells with air gaps under the decks should continue to keep the soils “alive” for a minimum of two years.

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