Soil Compost Amendments

Soil Compost Amendments

Soil restoration is an ESD practice applied after construction to deeply till compacted soils and restore their porosity by amending them with compost. Soil restoration can reduce the generation of runoff from compacted urban lawns and enhance the runoff reduction performance of several non-structural practices at sites where soils are poor to start with.

Despite abundant research elsewhere, acceptance of soil restoration as an environmental site design practice has been exceptionally slow in the Bay watershed. To my knowledge, specifications for soil restoration are absent in all State stormwater manuals in the Bay watershed. A Baywide specification hopes to remedy the situation. The basic idea is to assemble a low cost practice that works and is easy to install after the site has been constructed.

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The proposed new spec does two things. First, it sets forth numerical runoff coefficients for undisturbed, disturbed, restored and reforested soils, organized by hydrologic soil group. This approach, for the first time, provides a real incentive to designers to prevent soil disturbance and/or use soil restoration practices. Second, the spec is intended to go beyond just reducing runoff from compacted lawns, and focuses on how to restore soils to treat runoff from adjacent impervious surfaces, particularly if they are in the tough C or D hydrologic soil group. Some examples of possible applications include:

  • Enhancing rooftop disconnections by creating a filter corridor for lawns with poorly infiltrating soils
  • Increasing runoff reduction rates within a grass channel or a filter strip that will be located on disturbed soils
  • Increasing the hydrologic function of a tree cluster or reforestation area before the trees grow into maturity

Since this is a new specification for a new practice, I would be grateful if you could give it a “hammer down” review, and perhaps think through some of the design issues I have outlined below:

  • The recipe: There are many kinds of compost out there. Ideally the spec would include an inexpensive and widely available type of compost that works. I have included a generic compost spec developed by Jim Lenhart of Contech several years ago that appears serviceable. There are other compost specifications out there that also could be used, but they may be too fancy for widespread use. The spec would require some sourcing, but not a lot of batch testing. I was also thinking that it might be good to cut the compost with sand…ala the bioretention spec…to improve its drainage properties. Any other ideas?
  • The decay and the pulse: Since compost is organic and decomposes over time, what implications does this have more the longevity of the practice? How much settlement or decay should be expected, and would there be an initial pulse of nutrients that leaches from the compost? The Composting Council, a group of fine decomposers, suggests a high initial nitrogen mineralization rate after the first year, followed by a much lower long term rate.
  • Amending the amendment: Grass cover will stabilize restored soils in most applications. Given the influence of compost on soil properties, such as pH, what kind of simple soil testing is needed to determine additional amendments to ensure that future grass growth will be vigorous? (e.g., gypsum and lime).
  • Driving the cost down: Soil restoration is not exactly cheap, and ranges from about 25 cents to a dollar per square foot installed. This doesn’t seem so bad, until you realize there are 43,560 square feet in an acre. So, while previous guidance suggested soil restoration for entire sites, it might be more cost effective to target specific areas of the site where boosting runoff reduction is important (grass channels, filter strips, tree clusters, and rooftop disconnection).
  • Simple spreaders: Runoff usually enters the soils at a point, while the restored soils occupy a large area. What simple flow spreaders can be used to maximize the use of the soil restoration sponge? I know that larger level spreaders have an awful rap, but they seem to be an important design element for grass channels, filter corridors and other applications.
  • Tilling versus subsoiling: Subsoilers are valuable since they can rip soils to a depth of 24 inches or more. They are pulled from tractors, and have been widely used on farms, but are not common at most construction sites. Personally, I am envisioning some monster tractor pulls here. Can anyone recommend a generic subsoiler or tiller equipment that could be specified?
  • Testing and inspection: Although soil restoration seems like a simple practice, it requires at least three kinds of testing—to understand soils to determine if they are worth amending, to test the amended soils to determine what, if any adjustments, are needed to support plant growth, and finally a post construction test to make sure it is done right (we all know there will be a lot of temptation among contractors to “hydroseed” a few inches of compost and call it a day). Any thoughts on simple and effective tests that could be used?
  • Terrain adaptations: I have done a lot of thought experiments, yet I can’t find any good reasons why the soil restoration spec could not be applied to karst, the coastal plain, linear highways, or the colder areas of the Bay watershed. Surely, I must be deluded. Obviously, it wouldn’t work on real steep slopes or intersect with groundwater, but let me know your thoughts.
  • Secondary erosion controls: What sort of erosion controls are needed to protect the restored areas during construction, and what kind of perimeter control would be needed during the soil restoration process itself? Is it worth putting together a standard ESC spec for soil amendments?
  • Maintenance: Once the grass gets established over the restored soils, it’s hard to come up with a lot of costly and onerous maintenance tasks (outside of regular mowing). Am I neglecting something here, or is this one of those “maintenance-free” stormwater practices we have been hoping for years?

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