Susan Day and her team at Virginia Tech published a paper earlier this year about a soil rehabilitation technique called Soil Profile Rebuilding, or SPR. They were looking for a way to rehabilitate soils damaged by land development in a way that would restore their biological function and be more supportive of plant growth. I had a chance to talk to Susan about their study, why they developed it, and how designers who want to implement this more sustainable approach can integrate it into their project documents. — LM
Can you briefly summarize what you looked at in this study?
We examined soil rehabilitation techniques in the context of improving tree growth, rooting depth, stormwater capture, and carbon sequestration in soils damaged by land development.
There’s a lot that’s known about soil restoration, but little of the research to date has addressed urban contexts. I would get a lot of questions from people working in the urban landscape who couldn’t convince their clients to invest any money in the soil. It would be a situation where you could spend a relatively modest amount of money improving soil to ensure that a half million dollar planting project succeeded and people wouldn’t want to do it!
Why did you want to examine urban soil rehabilitation? Why is this important?
We really wanted to help people doing landscape design and development projects have tools to help them address soil problems. Globally, plants are thought to contain three times the carbon of the plants they support. When you manipulate soil, you lose carbon – this happens even if you just scrape off topsoil. So we lose a lot of carbon, and at the same time, we create difficult conditions for storing carbon. We thought, perhaps there is a new frontier if we can store more carbon at greater depths.
Soil compaction and its effect on plant growth, of course, is a more immediate problem. In particular, we wanted to devise a way to deal with soil compaction and loss of structure so that natural processes can take it forward and sustain it over the long term, without further intervention. We also wanted to quantify the benefits—in terms of ecosystem services– that soil rehabilitation provides so that people can take it to the bank. That was the origin of the project.
What typically happens on an urban development site where the soil is NOT rehabilitated?
Typically – and this happens even when you have the best of intentions and high quality soils – the topsoil gets scraped off, the land is graded and the slopes get altered to direct water away from any buildings. Often new topsoil will be brought in (although not always). If limestone gravel has been used in construction and mixed into the soil, this can also affect pH levels. This all results in very difficult conditions for plant material to grow.
In your study, you examined the effects of something called SPR. What is SPR? Is this a new technique? How is it different from typical tree planting?
SPR stands for Soil Profile Rebuilding. It’s a technique that we developed to break up soil compaction, and provide the condition for the soil structure to re-form, in a way that’s going to last. It’s important to understand that it is not a tree planting technique, but a soil improvement technique that includes trees or other woody plants as part of the treatment.
Compaction in its essence breaks down soil structure – and it takes 15 or 20 years before you see formation of new aggregates (peds). In order to improve soil structure, you need some clay (fine particles), you need organic matter, and you need some physical action (freeze/thaw, earthworms, etc.) — those are the elements that need to be in place.
With SPR, rather than just putting some topsoil on top of highly compacted subsoil and then adding plant material – which is what is usually done on urban planting projects – you lay in 4” of compost and, using a backhoe, work backwards lifting large chunks and dropping them to break it up into cracks and 6” to 12” clumps. Then you add in topsoil in and till it really well. Then you can plant woody plants, and trees or shrubs.
The roots grow down into the cracks and then grow and die, adding more organic matter and helping to sustain the health of the soil. That’s how we create the condition to rebuild the structure. It’s not particularly expensive or time consuming; you just need compost and a backhoe.
Is it safe for designers to assume that all urban soils need to be modified or rehabilitated before being appropriate for plant material?
No. We often treat urban soil as junk and act as if it can’t do anything, but we think it can actually function at a high level with proper care and amendment. There are some bad soils, but they’re not all bad, and they can be bad in different ways. It’s really important to do a site assessment to understand what you have before you start deciding on how to remedy it.
As far as compaction is concerned, research shows that pretty much any compaction slows the growth of tree roots. So you need to understand the action threshold of compaction that is appropriate for your situation and your soil.
How did trees growing in SPR plots compare to trees growing in traditional plots that didn’t use rehabilitated soil?
The SPR trees established more quickly, they looked fuller, and they grew much faster. It was species-dependent, since some types of trees take much longer to grow and establish – with those types we saw less of a response, but they still seemed to respond faster than trees planted in traditional plots.
We were getting about double the canopy spread over seven years. Soil that starts off in really bad condition will show a much bigger response than soil that was already pretty OK to begin with.
Can SPR be used anywhere?
It can be done on most sites, although some are more suitable than others. If you have just a tiny planting pit, for example, this isn’t the ideal application. And of course you can’t use it around existing trees.
We saw it done in Arlington, VA on a site where the soil was worse than that of our control plots. In Arlington they wanted to use it because they were digging up parts of the street and turning it into planting areas – so they knew the soil they were working in would be very compacted. This was a way for them to achieve a good planting condition much more cheaply, and not worry about sustainability issues that may result from hauling off the soil and bringing in new soil. The tree response was really strong.
Can using rehabilitated soil help reduce overall project costs?
We didn’t do any economic studies or price the cost of operators. But in our work with Arlington, they found it very cost effective.
The value may depend on what you’re comparing it to – is it compared to replacing all the plants several times? Importing a special soil blend for an entire park? I would urge people to always consider the different options and think about the costs over a period of years and decades, not just the of the immediate site work, when thinking about the total expense of a technique like this.
How widely is SPR being used? Do you have any plans for trying to educate designers and builders about this technique?
It is being used, but we don’t know exactly how much – I only know if someone specifically tells me they’re using it! I don’t have any additional studies or partnerships planned right now, but if anyone is using it I’d love to know their results and see pictures.
What should specifiers who want to use this technique on their project know?
There is a specification that I wrote in conjunction with a number of other people, including some landscape architects. We did two versions – one is friendlier to contractors and really just explains what to do. That’s the short version. A full version goes into a lot more detail and is helpful in the bidding stage to ensure people are bidding on exactly the same procedures.
Designers can grab it, modify it as necessary, and stick it in their own project documents.
Any last thoughts?
We’ve become accustomed to urban trees that don’t grow that well, it almost doesn’t faze us to see the terrible state that many street trees are in. But when you give trees what they need, it doesn’t take that long for them to grow and fill in really beautifully.