Proof Resistant Soil Specifications
By Howard Stenn, Stenn Design
Soil specifications with unnecessary, contradictory or unverifiable standards are an invitation for plant and drainage problems, project delays, and ensuing conflicts over responsibility. Problematic provisions often accumulate in office specifications—added for a particular project or not removed when a divergent section is added –and remain unnoticed until they cause a failure. Reviewing and editing specifications, and making sure that everyone in the office is using current versions can prevent many mix-ups.
The following strategies focus on creating enforceable specifications that facilitate delivery of the intended product, simplify testing and verification processes, and help minimize costly disputes. They apply to any mix, and don’t make (many) judgements about what the best soil mix should be.
1. Soil Mix Proportions and Names:
Specify soil qualities to be achieved by adjusting the proportions of desired ingredients, rather than with exact mix ratios. Due to variations in natural soil and compost properties, mixes using fixed proportions often will not yield the specified organic content or texture. The mismatch causes delays, added testing costs, and sometime approval of a lower quality substitute.
– A good approach is to call for desired properties of the final soil, and allow the proportions of approved ingredients to be varied as needed to achieve the goal. For example: “A blend containing a minimum of 60% naturally-occurring USDA loam or sandy-loam topsoil; plus compost and aggregate in amounts as needed to produce a USDA Sandy Loam containing 10-25% clay, 15-30% silt, and 5-8% organic matter dry weight.” Additional qualitative parameters to require in the mix should include pH, Cation Exchange Capacity, Sodium Absorption Ratio, Conductivity and selected salts.
– Don’t list pre-approved product names—or “equivalent”, which is often used to allow product to be accepted based on a name rather than tested performance. Common mixes like “Planting Soil” and “Three Way Topsoil” can include many variations of ingredients and proportions from different suppliers, or even from a particular supplier in different seasons. “Or equivalent” is even more open to interpretation.
– USDA Sandy Loam or Loam with a reasonable amount of silt and clay are adequate texture requirements for most plantings. (Many sandy-clay loams, silty loams and clay loams may also work if not screened to destroy existing structure). It is not productive to demand detailed particle size distribution requirements for the soil or aggregate, except for intensive-use sports /event turf or some green infrastructure applications. Achieving narrower ranges can add costs without necessarily enhancing performance. 1
– Add fertilizers and pH amendments only based on professional (Agronomist, Soil Scientist) interpretation of lab tests for the complete soil mix. Recommendations should be based on the planting type (turf, ornamentals, natives or acid-loving plants), and the depth of soil to be placed. Generic amendment rates can lead to unnecessary fertilization that conflicts with pH requirements of phosphorus limits, or causes excessive plant growth that is vulnerable to pests and storm damage.
-Avoid biological amendments and preparations lacking scientific evidence of benefits. Inoculants and proprietary testing associated with them add time and expenses, but side by side tests generally show they provide no benefits compared to natural topsoil with adequate organic matter. Their value is when used in sterile soil mixes or pure sand media.2 Regimes that require soils to be inoculated and stored for months prior to placement discourage reliable suppliers from bidding on projects. Beneficial mycorrhizae naturally inhabit most soils, and will proliferate in a good topsoil with ample organic matter when host plants are introduced.3
2. Testing: Soil testing is not as precise as commonly assumed. It is important to be specific about exact methods and which laboratories are used on a project in order to get consistent results and avoid unresolvable variations that can hold up approvals.
– Require all testing be performed by a single lab throughout submittal and acceptance processes. I have worked on several projects where multiple labs tested portions of a homogenized sample using the same ASTM reference method, and reported widely divergent results–even after detailed coordination on procedures.
– Specify exact test methods using ASTM or AASHTO reference numbers, and procedures in compliance with regional laboratory procedural manuals (such as the Soil, Plant and Water Reference Methods for the Western Region). Require methods to be listed on lab reports. There are significant differences in chemical tests based on regional soil properties, and in physical properties tests depending on if intended use is structural or horticultural.
– Whenever possible—certainly for large projects or resolving disputes—have a laboratory technician or soils consultant gather samples for testing. I have been in a project troubleshooting role many times where a sales representative or equipment operator gathered the wrong material or an unrepresentative sample, resulting in false alarms about quality. Or gather samples yourself using standard sampling and compositing procedures –most labs will provide instructions.
– Don’t require tests that are proprietary to a single provider so cannot be independently verified. Much microbiology testing is an example (see above): Many methods simply measure biological activity, but cannot reliably assess how it relates to specific plant needs.
3. Verification and Protection of Quality. Clearly defined processes and benchmarks for approvals are critical to getting good soil installed. Even good soil can be destroyed by poor placement or by compaction once in place, and product substitution and misrepresentation are commonplace—whether intentional to cut costs, or due to a lack of knowledge. Once installed and planted, tight schedules often make it difficult to turn back to adequately fix problems.
– Require a sample of the approved product be kept on site to compare to delivered product. Have the project manager gather and store a gallon sample from each load or 100 yards delivered, and match each sample to a delivery ticket number. On large projects mark the approximate location where the load is installed. These steps make inspections easier when site visits are limited, and can help explain any problems that emerge later.
– Train the project manager or construction administration staff to conduct basic visual and texture comparisons, and empower them to reject or test delivered loads prior to placement if they do not appear to match the approved product sample. Get a copy (or a few) of Jim Urban’s Up By Roots to help everyone develop these skills.
– Proper placement and consolidation of lifts, and protection of installed soil are critical to establishing good drainage and plant health. (see photos and caption at end of text). Specifications from the Urban Tree Foundation provide a good model for requiring construction and approval of mock-ups that can be used as benchmarks to evaluate compaction and appearance of the rest of the project.
– Develop clear measures to protect installed soil from compaction by construction traffic, such as fencing or barriers to traffic, wood ship paths for planting operations, or mulching first and planting through the mulch. Written specifications and even training sessions at project startup are not adequate.
These two planting areas were installed and planted at the same time using the same soil mix: 18” depth of imported 70% sand, 22% silt, 8% clay, with 13% organic matter content. The area with puddled soil and dead plants was used as a staging area for plants, and received extensive foot and tractor traffic during wet conditions during planting operations. It was free-draining just below the surface.
1. See Jim Urban’s DeepRoot blog posts on Urban Soil Reuse as Planting Soil: Current Science and Lessons Learned, and discussions in Up By Roots.
2. Chalker-Scott, L. A Gardener’s Primer to Mycorrhizae. Washington State Extension. FS296E. 2017
3. Kleczewski N, Lewandowski D, and Pieriuigi B. Mycorrhizae in Urban Landscapes, Ohio State University Extension. 2PLPATH-TREE-01016