Proper soil testing is vital for establishing successful revegetation. Critical components of proper soil testing include ensuring that you and the lab understand the scope of the project and its environment; identifying critical parameters to test using a detailed template; conducting soil biology tests on reference sites; determining when to sample and how many samples are needed; and, of course, crafting an educated specification.
Nutrients and structure tests typically are conducted for agricultural, landscaping, revegetation and restoration projects. A good template for a specifier includes testing of: percent saturation; soil texture; infiltration rate; pH; conductivity; total dissolved salts; cation-exchange capacity; potassium; sodium; calcium; magnesium; nitrate; and phosphate.
Additional items on the template may include sulfate; chloride, boron; zinc; copper; iron; manganese; sulfur or lime requirement; gypsum requirement; sodium absorption ratio; exchangeable sodium percent; and organic matter.
Soil biology tests can be an invaluable tool for determining a desired restoration target, and they should
be conducted on reference sites—undisturbed plant communities near the project site that are representative
of the desired plant community. These tests can be extensive, evaluating a full array of soil macroorganisms and microorganisms and their interaction. Other less complex evaluations can be conducted for several symbionts (e.g., mycorrhiza and rhizobia) by obtaining roots from a reference area. This process is particularly helpful in designing specifications and determining whether the correct inocula has been selected.
Sampling for soil biology in reference soils, including evaluation of mycorrhizae (spores, hyphae, inoculated roots) and rhizobia (nodulated roots), needs to be conducted in the spring or summer, depending on the local climate, when soils are warm and moist and microorganisms are active. Sampling also can be conducted post-construction if vegetation fails, but doing this raises issues regarding liability and cost for the tests as well as retreatment. Post-construction sampling usually is performed as a research project, as it can be beneficial to test both successful and failed projects to evaluate the effectiveness of soil treatments.
Soil testing of disturbed soils always is recommended. Reference area sampling can help avoid or limit the use of costly amendments. Even if topsoil is available and handled correctly during the salvage and stockpile process—and it rarely is—it should be tested in place after it has been reapplied.
Designers should be able to build soil sampling tasks and expenses into their scope of work when evaluating existing conditions. The number of samples to be taken depends on the size of the project and the variation in soil types, as well as the available budget. For nutrients and structures, generally at least four subsamples are obtained per soil type and combined, with one composite subsample sent to the lab.
Specify what you consider to be the best product(s) within the constraints of the bid process. Even if you are unable to obtain samples prior to bid, as would be the case in sampling topsoil on finished slopes, it is better to have a “placeholder” and later delete material or amend the material specified.
Most labs provide recommendations geared more toward agriculture or landscaping rather than revegetation, so be prepared to spend some time carefully digesting what is said.
Do not automatically assume that all recommendations should be implemented. Carefully compare the results of a disturbed soil with a reference soil and talk with colleagues.
