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Big Sur Slide
The compost bid item refer to two distinctly different types of work. Compost either be applied as a mulch-like blanket, typically 2" thick, onto disturbed soil areas, or compost can be applied to a slope and then mixed together with other materials (such as bark, straw or topsoil) when used along with the Incorporate Materials specification. The Incorporate Materials specification only pays for the labor and equipment involved in the incorporation work. Payment for the compost material is paid for as part of the compost bid item described here.
A compost blanket provides several benefits, including protecting bare soil surfaces from wind and water erosion, water conservation, weed control, and providing the nutrients required for long term, sustainable plant establishment.
By being closer to the plant root systems, compost used together with the Incorporate Materials specification provides a long-term source of nutrients required for the long term, establishment of plants.
Compost blanket is a great tool to reduce raindrop impact erosion and provide the nutrients required for long-term sustainable vegetation. However, sites subject to high winds or significant rain events may require additional erosion control treatments to hold the compost in place. The photos below highlight common erosion control combinations, together with their strengths and weaknesses.
This photo shows compost applied to a slope as a blanket, with no additional treatments layered on top. If compost is being applied with a pneumatic blower truck, the seed can be placed in a side bin in the blower truck and applied together with the compost in a single step. For compost to be applied via blower truck, all slope areas must be within 300 feet of a spot where the blower truck parks as the blower truck hose is just 300 feet long. If the slope to be treated is located below a roadway, compost can also be dumped from the roadway onto the top of the slope and drawn down the slope with rakes.
This photo shows a 2” Compost Blanket, topped with blown Straw, then Incorporated in place with a crimper. The crimp depth is not terribly deep, but adds an additional extra step for the Contractor with an additional piece of equipment, so it can be costly. For most sites, a more cost effective approach to hold the compost in place is shown directly below.
This photo shows a 2” Compost Blanket, topped with blown Straw, and held in place by applying a layer of Hydromulch (a combination of tackifier and fiber). This combination of treatments is very cost effective, and reliable for 2:1 (H:V) slopes up to 30 ft in length. The straw is excellent at protecting the soil and compost from raindrop erosion while also providing a mulch for effective seed germination. For very windy sites, it is recommended to consider the approach below.
This photo shows a 2” compost blanket covered by Coir Netting (Type A), a costly but effective method of preventing erosion and keeping compost and seed in place in areas subject to high winds and/or highly erosive rain storms. The Type A netting has the largest open area - therefore it is least likely to inhibit the germination of seed. While the seed can be placed either before of after the netting, the most common approach is to place the seed prior to the netting as this approach ensures that the seed has good contact with the soil. If the seed is placed after the coir netting is put in place, a higher rate of seeding will be necessary due to the coir blocking seed from reaching the surface of the soil.
Note also in the photo above - Compost berms can be used instead of fiber rolls. As this photo shows, it is recommended to apply the Coir Netting (Type A) on top of both the compost blanket and the compost berms. Not only is this method more cost effective (than placing the compost berms on top of the coir netting), but placing the coir netting on top of the compost berms also reduces the likelihood that the compost berm will biodegradable the coir netting.
While it is possible to purchase medium or coarse compost (the best for compost blankets) that has a low trash content, it is not uncommon to find the coarser compost materials to have a higher level of trash (plastic) than fine compost products. Because coarse compost is produced through a screening process, and because the screens for coarse compost are larger than the screens used for fine compost - coarse compost often has more trash than fine compost. Compost producers can remove trasn (plastic) during the production process, but these steps increase cost. If you are having problems locating medium or coarse compost that is free from film plastic, another approach is to specify an application of fine (3/8" minus) compost materials, and then place on top of that bark mulch, and then use the Incorporate Materials specification to combine the materials in place on the slope. This way you get the benefits of a coarser compost product with less likelyhood of problems due to trash.
Average Compost Weight = 800 lbs/cubic yard (1050 lbs/cubic meter).
The large quantities of compost required for a compost blanket can be applied to a flat project site using a dump truck and a bulldozer. Projects with steep slopes, or areas inaccessible by vehicles require alternate methods - the most common method being a pneumatic blower truck.
Working like a vacuum cleaner stuck in reverse, a pneumatic blower truck can rapidly applies compost to steep slopes or constrained areas that are unreachable by traditional equipment. A 40 cubic yard blower truck holds roughly 35 cubic yards of compost, and applies this material at a rate of about 35 cubic yards/hour. A single truck should be able to apply a 1" thick compost blanket over a 2-acre site (280 CY total) in a single working day. The typical truck has a hose that can reach 300 feet - defining the farthest extents of application from the roadway. A rough estimate of cost for labor and materials for this work would run from $5,000 - $8,000 per acre. This price estimate does not include seed.
To hold compost in place on steep slopes, or to protect from wind erosion, consider combining compost with the following:
To effectively treat sites with compacted, sterile or poorly draining soils, consider combining this treatment with
Model Guided Specification for Using Compost to Promote Establishment of Vegetation and Stormwater Quality Improvements, 2010
This study establishes parameters for compost use based on performance criteria including soil type, climate, slope length and steepness, aspect, and location. The research addresses how compost affects water quality and erosion, and if compost improves the establishment of permanent vegetation cover.
Regeneration of Nitrogen Fertility in Disturbed Soils Using CompostGraphs nitrogen release from various composts and compares compost release rates with two native topsoils.
Planning & Design
Improve Soil Health
Improve Soil Health & Provide Cover
Long Term Cover
Steep Slope Techniques
Low Impact Development