Soil health is the foundation of a thriving ecosystem and integral to a successful project through its entire life cycle.
So what is the big deal with dirt? When it comes to construction, we tend to think of dirt in terms of earthwork. Soil isn’t typically the subject of much forethought, even though it could — and should — be on every project. Beyond mass grading and backfill activities, ensuring quality soils is critical to support revegetation on a building site. Existing topsoil is rarely tested to determine whether it is suitable to be reconditioned and reused as a horticultural soil. The status quo has become that manufactured planting soils are always imported and assumed to be superior to existing soils, when that is not necessarily true — it may just take a bit more analysis and effort to stage the revitalization of what is already on site. When we consider the impact that healthy soils can have on the environment over the life cycle of a facility, we begin to place more importance on the conservation and planning of soil as a limited natural resource.
Soils and Carbon Footprint
All plants in the landscape industry have some level of embodied energy: Their propagation and the nurturing process in preparing them for the marketplace, their transportation to a construction site, and the machinery used in placing them have an associated carbon footprint just like any other material. Therefore, keeping them healthy after planting and preventing the need for their replacement are important in reducing the carbon cost of a facility throughout its life cycle. Some plants can help store carbon, but trees in particular have to live a relatively long time just to reach carbon neutrality. Healthy soils are the foundation of any healthy ecosystem; therefore, providing healthy soils is one of the best ways to keep any landscape productive and capable of carbon sequestration for many decades.
The Challenge With Texas Soils
Topsoil across much of Texas is naturally calcareous, containing various forms of calcium carbonate. This results in extremely high pH — an immediate problem, considering that most ornamental plants in the landscape industry prefer slightly acidic soils. Clay particles (especially in soils with excessive amounts of sodium) can easily become dispersed, sealing the surface and limiting water infiltration and percolation through the soil profile. Compounding this problem, hard groundwater (and sometimes even hard surface water) with high levels of bicarbonates can exacerbate soil problems when used as an irrigation source. All of these can lead to a continuum of management challenges.
Designing for Living Organisms
Unfortunately, specifying high-quality soil is not as simple as calling out “Awesome Brand, Model No. 123,” as one would on a schedule for a lavatory fixture. A “BEAT TO FIT, PAINT TO MATCH” callout might work for decorative formed metals, but with soils we’re dealing with living — literally respirating — organisms that require nurturing and care. It’s almost like raising kids, except soils don’t throw tantrums; the plants just show you when soils aren’t happy. Breaking the cardinal rule of spec writing, we then proceed to specify means and methods that contractors must take to execute this type of work properly. Sometimes that even includes describing what not to do as much as what to do to effectively convey best practices.
Soil Is an Essential Investment
We all know communication and agreement on any project are key, and this applies equally to providing high-quality soils. Full team buy-in is crucial among the owner, the design team, and the builder. However, we often find projects in need of value engineering, even after initial construction budgets have been established and approved. Architectural cost overruns can send all disciplines — and contractors — looking for easy ways to cut costs, and premium quality soils quickly find themselves on the chopping block.
Healthy soils reduce the need for fertilizers, pesticides, herbicides, and fungicides; therefore, an investment in soils is an investment in the long-term health of a constructed project. If all parties understand the future value of the initial investment, then perhaps these assets will be better protected from being downgraded. When we understand total lifecycle costs of a facility, we soon arrive at the old adage: An ounce of prevention is worth a pound of cure. Unfortunately, property management groups too often end up chasing their tails trying to right the wrongs of a failed project delivery process.
Establishing Soil Health
The first step toward establishing long-term success requires utilizing high-quality materials, which might include resources already on site. Investigate the site, assess and analyze what you have, and minimize external inputs. Don’t just copy antiquated industry habits; know what you’re growing and exactly what its needs are, and tailor the soil to that planting palette.
What your soil has in it and where it came from matters; therefore, select sources carefully for high-quality ingredients generated by sustainable practices. Since the end result is only as good as the materials provided by the vendors, the design team must review (and approve) the product data and test results of soil components, as well as ensure that materials within range of the quality requirements are being delivered and installed.
Ways to Amend and Improve
There are numerous soil amendments available, with compost being the most prevalent because it is a dense source of organic matter (OM) for soils lacking adequate amounts of humus. Contrary to common assumptions, not all compost products are created equal. In a recent assessment of 10 compost products from three vendors in the Austin and San Antonio markets, some had barely over 20 percent OM while others were over 60 percent. When calculating the cost relative to the percentage of OM, the more “potent” compost products did not necessarily prove to be the most expensive ones. Attention to this level of detail can significantly reduce costs, especially on larger-scale projects.
When evaluating compost products, the property owner or governing agencies may determine whether biosolids (organic matter from wastewater treatment) may be used or if only food or landscape waste product types should be chosen. If animal manure is acceptable, a maximum threshold should be established to prevent detrimental concentrations of nutrients such as ammonium, calcium, magnesium, potassium, and sodium. It should also be considered that compost and soil stockpiles have a shelf life and must be maintained, as biology levels can eventually decline in the absence of plants or other food sources.
If space on the site allows and water is available to maintain soil moisture levels, the use of cover crops to start biological activity earlier within salvaged and stockpiled soils can pay huge dividends. This can occur while the building is being constructed and the site is awaiting the final installation of the landscape. These enriched soils can also be used to inoculate other imported soils, which will ensure only native, beneficial mycorrhizal fungi (types of fungi that form symbiotic relationships with plant roots) are being propagated. Starting soil biology as early as possible will help kick-start a self-sustaining system.
Public Enemy #1
If soil has a “kryptonite,” it’s compaction. This condition reduces the amount of pore space that is critical to properly functioning soil, essentially suffocating plants. Roots need access to water and air, so although it may seem solid to us, healthy soil should actually have approximately 50 percent void space to allow for moisture and gaseous interchange between the ground and the atmosphere. Unfortunately, construction site activities are notorious for contributing to soil compaction. Imagine standing on a loaf of bread, stomping your feet, and then stepping aside to assess the results. That is essentially how soil is treated on the job site. Just as the loaf of bread is partially, yet permanently, deflated, so is the pore space within the soil profile, which leaves less room for air and water to penetrate and move freely.
One strategy to address this situation is to concentrate haul routes for construction activities while prohibiting any vehicular access within established vegetation and soil protection zones (VSPZs) to minimize disturbance. While these routes become more heavily compacted, the extent of the damage is reduced, resulting in less area to decompact and remediate later.
The placement process of soil is equally important. Optimal soil moisture is required, and compaction by the machinery installing each lift, or layer, of soil must not exceed specified limits. As opposed to a minimum compaction rate required for a building slab’s subgrade, planting soils must not surpass a maximum rate. While careful supervision by the general contractor and the landscape subcontractor foreman are important, a skilled machinery operator is essential to meeting the soil placement specifications. Once placed, the soil is susceptible to unintentional compaction by workers during irrigation and plant installation, not to mention other trades such as site electrical and signage contractors; therefore, ongoing protection is critical throughout the construction process.
Monitoring for Long-Term Success
Once effort and care have been taken to procure, properly install, and protect the revered soils, this delicate harmony within the rhizosphere (the soil zone surrounding the plant that is influenced by root activity) can be easily disrupted by improper soil management. One way this happens is through the use of synthetic mineral fertilizers, which discourage the symbiotic relationship in which roots nurture microbes that in return provide nutrient availability to the plant. The roots will take nourishment directly from the synthetic fertilizers instead of feeding the microbes that feed them. The interruption of this natural relationship results in the decline of overall microbial biomass in the soil. The resulting dependence on more mineral fertilizer usually results in weaker plants that often require more pesticides, fungicides, and herbicides to control competing species.
Most boilerplate planting specification sections require the contractor to provide a maintenance plan, but that usually results in the typical “mow, blow, and go” type of service contract after construction. Having a conscientious consultant provide a detailed maintenance program to the facility manager can help facilitate competitive bidding, ensure that long-term expectations are met, and reduce unnecessary expenses.
Don’t let post-construction maintenance become an afterthought. Connect facility managers with the design consultants. Monitor soil health to maintain optimal performance into the future. Know what you have, how it’s being treated, and how well that may (or may not) be working. You can’t effectively manage what you don’t measure. Finally, make sure that the management program is updated from lessons learned annually.
The Recipe for Success
Engage a qualified consultant team. Whether you’re producing guidelines for large regional projects or producing construction documents for small development projects, push your specialty subconsultants to do more than the current industry norm. Today’s standards are not producing performative landscapes capable of providing the ecosystem services that the modern environment needs. Hire the experts that the project requires to achieve these goals. Only once we accept the challenge of designing soils as the critical foundation can we venture to build truly greener projects and a more sustainable future.
Jason Radcliff is a landscape architect and principal with dwg.