Center for Environmental Excellence by AASHTO - Environmental Issue Construction and Maintenance Practices Compendium

Sustainable, vigorous plant growth is difficult to achieve on degraded soils from which topsoil has been removed by construction or erosion. Studies have indicated that plant available levels of phosphorous (P), potassium (K). calcium (Ca), magnesium (Mg), sulphur (S), micronutrients, soil acidity or salt are unlikely to limit plant growth on barren materials. [N] Low plant available nitrogen (N) and poor soil physical characteristics that result in poor root development and low water holding capacity remain the most likely and common reasons for poor plant growth, aside from insufficient water. [N]

The observation that nutrient deficiency may be a cause of the decline of plant cover is thought to result from the absence of topsoil as a growth medium; during construction the topsoil is often buried beyond the reach of plant roots by fill material (crushed, unweathered, siltstones and metamorphic sediments). [N] The loss of topsoil and humus removes the major source of available plant nutrients and reduces soil structure, nutrient retention capacity and microbial activity. [N] Microbial activity is reduced because the loss of organic matter eliminates the food supply of plant decomposing microorganisms; with death, the microbial nitrogen is available for leaching from the plant-soil cycle. [N] A continuing supply of plant-supplied carbon prevents this loss. [N]

In nutrient deficient soils, mycorrhizal fungi typically function to increase nutrient acquisition by plants. This occurs when certain fungi colonize the plant root and form a mutual relationship called a mycorrhizal infection. In this beneficial infection the plant provides energy for the fungi while the fungi provide nutrients for the plant. The loss of the topsoil removes the fungal spores or hyphae which are required to begin the infection. [N] The plant is then left without either the original nutrient rich topsoil or the mycorrhizae necessary to improve uptake. [N]

Claassen et al. have performed a large percentage of the studies on topsoil usage and compost, with funding from FHWA and Caltrans. Their work and recommendations augment that of DOT handbooks, and are summarized in the remaining bullets in this section.

Stockpile topsoil. Topsoil harvest, stockpiling and reapplication is strongly recommended wherever possible as the best method for reestablishment of plant communities on disturbed soils. Equivalent levels of chemical fertilizer cannot substitute for the benefits provided by topsoil reapplication. Topsoil provides, in addition to available nutrients, slow release nutrient reserves, improved soil structure and water holding capacity, increased microbiological activity for nutrient cycling and retention, increased mycorrhizal infection, and a potential source of native seed. [N]
Because of its high soil organic matter content, topsoil is an excellent method of providing the slow release, high N content needed to regenerate barren slopes, as it contains the well stabilized, slow release N needed to reestablish plant communities, as well as plant seeds and microbial inoculum. [N]

Stockpiling of topsoils apparently has little or no negative impact on topsoil quality. Caltrans studies of stockpiled and reapplied topsoils found that storage of topsoil material in a stockpile for periods of up to five months is an acceptable method of handling these materials during construction; topsoil nutrient content and biological quality was not degraded. Infection potential of mycorrhizal fungi did not decrease during stockpiling. Topsoil reapplication improved plant growth by 250 percent after three years compared to fill slopes which had no topsoil, with equivalent application of all other nutrients, erosion control and seed materials. [N]
Topsoil fraction had to exceed 20 percent of soil volume before significant improvements in plant growth and soil characteristics occurred. Higher rates are recommended in more severe environments. Plant and mycorrhizal production peaked in the 60 percent treatments. The researchers extrapolated these greenhouse results to field situations by recommending 10-20 cm (4-8 in) topsoil application over fill material, if available. If a volume of topsoil equivalent to less than 2 cm (1 in) topsoil is available, it should be concentrated in smaller volumes such as in furrows or roughened surface, rather than being spread thinly over the slope surface. [N]
The soil material which should be harvested includes the "duff," including decomposed, broken or chipped plant material and the mineral soil material down to the color change from the darker topsoil to the redder or grayer subsoil. [N]
Use of moderate amounts of fertilizer can be used to increase the total amount of mychorrhizal infected plants roots. Moderate fertilization improved plant growth without decreasing mycorrhizal root production. Mineralizable nitrogen was shown to be predominantly derived from soil microbes. Chemical fertilizers cannot by themselves regenerate soils, but their moderate use in conjunction with topsoil application was shown to be beneficial in promoting both plant growth and increased total mycorrhizal infection. Rates of P amendment should be limited to the range of the 39 kg P/ha (35 lb P/ac) treatment because the mycorrhizal infection dropped off significantly when the P rates were doubled. [N]

Develop a plan for stockpiling and redistribution within the contract's order of work. Washington State DOT makes the following recommendations with regard to developing a plan for soil preservation; e.g. a plan to stockpile and redistribute existing topsoil within the contract's order of work. [N]

Perform a site analysis

Examine proposed planting areas for any apparent drainage problems. Note any underlying characteristics that might affect drainage (hardpan, compacted subsoil, clay layers, and so forth.). Plan to correct deficiencies or plant appropriate species.

Analyze soil for susceptibility to erosion from stormwater runoff.

Determine solar exposure of slopes (slope aspect) and its effect on soil and vegetation.

Conduct a plant inventory or a germination test to determine seed bank to decide if topsoil stockpiling is practical. An examination of the site with an inventory of existing vegetation is necessary prior to determining when to use existing topsoil. Stockpiling of topsoil might not be advisable when noxious weeds and their seeds are present. Consult with a Landscape Architect for assistance. Imported topsoil can be used to provide a medium for plant growth when native soil has been removed or is highly disturbed.

Determine where to stockpile soil on-site and the extent of clearing and grading.

Set clearing and grubbing limits to minimize soil disturbance. In some areas grubbing is unnecessary. Stumps and root systems may be left in the soil to provide stability. Decomposition of trees varies in time depending upon species and climate, but all decomposition provides nutrients, organic matter, and habitat for microorganisms.

Perform a soil analysis (type, compaction, and fertility) , including a soil test to determine nutrient content and pH of soil.

Obtain a soil sample bag or a plastic bag capable of holding approximately one quart of soil

Select a representative area for the sample. If the soil seems to vary in color and composition within the project area, sample those soils also.

Dig a hole 300 to 460 mm (12 to 18 inches) deep and set the material to the side. Scrape off a small amount of material from the top to the bottom of the side of the hole and place into plastic bag. Do not include any material taken from the hole initially. Refill the hole with the set aside material.

Locate the test pit on the site map. If more than one sample is taken from the site, number the test pits to correspond with the samples taken.

Seal the bag tightly and place in a manila envelope and write all the information on the paper surface: name, date of sampling, site location, and sample identification (such as test pit #1). Fill out Soil Test Form and include it with the sample; box or wrap sample for mailing; and send the soil sample to a soil chemistry lab.

Consult with the Landscape Architect for specific amendment recommendations when test results arrive, if necessary.

Analyze the soil for compaction. Appropriate soil treatment is crucial for the success of roadside restoration (including erosion prevention seeding). Soil compaction can be tested using the bulk density test. Test the soil to a depth of 0.6 m (2 feet). If the density is greater than 80 percent, take steps to break up the compacted soil. Contact the regional Materials Engineer for assistance.

Pay close attention to areas that have been, or will be, staging areas. These areas will have to be ripped to restore pore spaces between the soil particles. Rip compacted soils, ideally in two directions, to a minimum depth of 460 mm (18 inches) before planting. The roots of most plants are above this depth.

Specify in all contracts that the contractor has the responsibility to restore the soil to a less than 80 percent density in all staging areas. Higher compaction rates are allowed in areas that are critical for road or structure stability. Include the costs of these procedures as part of the contract. The contract should not be closed until this step is completed.

Revegetation success should not be based on short term growth increases in the first season or year, but performance and biomass production in the 3-5 year range. [N]
Maximum slope design for topsoil application should be 1½:1 for fill slopes and 2:l for cut slopes. Placement to topsoil on steep slopes can lead to sloughing. [N]
Where topsoil is not available other amendments can be used, but the quantity and quality of the N materials applied is critical. The N release should be slow enough to keep plant-available N at modest levels, but the total amount of N amended should be high enough so that the site does not run out of N before the plant community is well established. The N amendment should be able to support three to five years' plant growth, for example. Controlled release of N is important because excessive N availability promotes weedy annual grass growth, drying out the site and crowding slow growing perennials. While the maintenance of moderate, sustained nitrogen levels may be achieved from commercial, slow release fertilizer sources, the inclusion of organic matter in the amendment is also important to improve the hard setting and poor water holding capacity of low organic content materials. [N]
Biomass associated with compost has been more effective than N amendments that were evenly disturbed throughout the profile (0-30 cm) or applied deeply within the profile (20-30 cm). [N] Studies of plant communities established on "problem soils" amended with commercial fertilizers have shown vigorous initial growth, but that vegetative cover often becomes sparse or nonexistent within several years. [N] In addition to transportation related studies, those of fertilized mine reclamation spoil observed that revegetated areas tended to be highly productive for two to five years followed by a sharp decline in plant growth and nutrient availability. [N] Reapplication of topsoil to subsurface materials enhanced reestablishment of vegetation by increasing nutrient availability, water holding capacity, and microbial activity. [N] Compost can be used to replace the organic matter and nutrients and can act as a surface mulch to protect against erosion, extreme temperatures, and droughtiness. [N]

Long-term nitrogen release rates from most yard waste compost materials approached the N release rates of moderately fertile soils. Composts were shown to be able to regenerate the N availability characteristics of low-nutrient substrates that have been stripped of topsoil organic matter. Well-cured composts and co-composts (biosolids blends) approached the N release rates of highly fertile soils. Compost application provides longer N release duration compared to chemical fertilizer and also provides organic materials for improved infiltration and microbial activity.
Potential compost sources and soils at the site should be analyzed before amendment, as compost products and the soils that are to be revegetated vary in fertility and water availability. Even after adequate N fertility amendment, some sites may still support insufficient plant cover if water or other nutrient deficiencies restrict plant growth. Improved soil and compost tests can guide selection of appropriate amendments to harsh and variable site conditions.
As compost materials are variable from producer to producer and variability in source material, processing method and curing time have significant effects on field performance, an interim recommendation is to apply in the range of 72 Mg/ha (dry weight) compost to extremely low-nutrient sites and in the range of 36 Mg/ha compost to low- or moderate- nutrient sites, or sites with shallow soils. Incorporate into the top 15 cm if possible. Plant-available N on drastically disturbed sites (on which the majority of the topsoil and organic matter has been removed) can typically be regenerated with a 500 to 1000 kg N/ha application of typical, common yard waste compost. This N application rate is roughly equivalent to 36 to 72 Mg/ha dry weight of compost (32, 143 to 64,286 lb/ac), or a volume of 85 to 170 m 3 (45 to 90 cu yd/ac), or a thickness of 0.84 to 1.7 cm (3/8" to ¾"). This rate can be reduced for sites that are not as nutrient poor as drastically disturbed sites.
The compost material should be moderately to well cured, meaning 3 to 6 months curing after the thermophillic compost process is to support plant growth. Recognizing the variability of compost N release behavior, the site should be monitored to detect if plant growth is too slow so that supplemental N can be applied if needed.
Yard waste composts need to be aged in order to achieve desirable rates of nitrogen release. Caltrans research showed that nitrogen (N) release rates change with time with a long-term incubation experience, and that extended curing after thermophilic composting increases N release rate. Long-term N release rates were in the range of the reference topsoils. Finely screened (<9 mm) compost can be applied with hydroseeder equipment, but this application method benefits from the addition of other structural material (straw, coarse wood fibers) to improve erosion control on barren slopes. The findings support the use of compost as a primary erosion control and soil amendment. [N] In addition, there is an environmental and social benefit derived from using these waste-stream materials for erosion control.
Avoid poorly composted or poorly cured materials, which, will not be biologically stabilized and can have atypical effects. Information on checking compost processing is available at the Composting Council Research and Education Foundation and at website for the UC Workshop on Compost Use for Pest Management. Cautions regarding use of uncomposted materials, especially in coastal regions are also found online.

Give special consideration to certain categories of materials, for best utilization in field situations to avoid negative impacts on field sites.
- Fibrous or poorly cured yard waste composts can have an initial period of N immobilization when high carbon materials are being decomposed. This period may last from several months to several years. Additional available N may need to be added to support plant growth N during this period.
- Fibrous or poorly cured yard waste composts may benefit soils in other ways than just N availability. Composts are rich sources of other nutrients as well as organic materials that improve water infiltration into the soil and water retention within the soil. The continued decomposition of compost by soil microbes further helps build soil aggregates, which improves drainage and water retention. If weed seeds and pathogen propagules have been killed, uncured materials can be used as surface mulches, or incorporated if N immobilization is not a problem. Do not transport infested, uncomposted materials to uninfested areas.
- Co-composted materials (biosolids blends) have much larger N release rates than yard waste composts. Co-composts should be used at about one half to one quarter of the amount of yard waste compost or at sites with rapid plant growth to absorb the higher N release rates. Because of the slow rate of N release, most hard waste composts are expected to have small or non-existent potential to leach N to watercourses, even when using large amendment loadings.
- Sites with residual fertility (topsoils not completely removed, or some soil material has been re-applied to the site) may not need compost amendment. Additional N may accelerate weed growth. Surface applied wood chip mulches may provide erosion control, microbial activity and mulch effects (temperature and evaporation protection) without the additional fertility of a composts material.
- Non-composted materials may produce phytotoxic compounds during decomposition. Any unprocessed plant material amendments other than wood chips should be stabilized using EPA regulation (40 CFR, part 503c) thermophillic composting, which sterilizes against weed seed and pathogen propagules.
- While composts are shown to be able to replace the N release function of native soil organic matter the best method for revegetation is still to harvest, stockpile and reapply the native topsoil that was on the site before disturbance. The quality of the organic matter is better, the harvested soil has better aggregate structure, the soil contains microbial inocula and site adapted plant seeds, and the costs are often less than regeneration of soil fertility from component parts. Extra steps may be needed to eliminate weeds, such as spraying, tillage or incorporation of topsoil beneath the surface.

Plans, Specifications, and Estimate (PS&E ) for Soil Preparation

The challenge to the roadside designer is to specify the appropriate soil preparation for planting, to prevent soil erosion, and to achieve desired soil structure. Appropriate soil preparation, including possible amendments, is crucial for the success of desirable roadside revegetation .

Specify soil amendments to achieve revegetation and restoration requirements.
Specify structural soils if needed in urban environments. The Urban Horticulture Institute at Cornell University has developed a cost effective structural soil mix that can improve the survivability of street trees in urban environments. The mix is:
- 80 percent angular stones ¾ to 1¼ inch in diameter.
- 20 percent topsoil with organic matter content of 10 percent.
- Soil stabilizer per the manufacturer's specifications.
- Potable water – enough to cause soil to coat the stones without having water run off
Angular stones form a skeleton that provides the weight-holding capability for the mix. Specialized compaction tests are not needed with this mix. The water-storing polymers bind the stones together and stabilize the soil mix. In addition, this structural soil mix leaves a large volume of rooting space that allows the plants to get oxygen and water. More information can be found at the website for the Society of Municipal Arborists.

Specify wide-track construction equipment in contract documents when it is necessary to work in wet soils.
Specify stripping topsoil and stockpile for redistribution after completion of rough grading. This is the best source of native seeds but it is also a source of exotic invasive vegetation and noxious weeds. (The plant inventory and germination test performed during the site analysis determine what plants are growing in the soil.)
Assess the entire project for other places to use removed topsoil. Restoration sites are practical locations to place excess topsoil.