California Department of Transportation
 

Compost (Incorporate)

Incorporate Compost

What is This Treatment?

This work typically involves placing and mixing compost into the soil surface. Depth of incorporation varies by slope gradient - steeper slopes involve lesser incorporation depths. Compost is typically mixed at a ratio of 30% compost to 70% soil.

When to Use This Treatment:

  • Typically applied on slopes 2:1 (H:V) and flatter. For specific maximum slope gradients, consult your geotechnical engineer.

Benefits:

  • Reduced stormwater runoff volume and velocity.

  • Improved infiltration rate.

  • Improved soil water holding capacity.

  • Improved soil structural properties - soil structure, porosity, and texture.

  • Improved plant rooting depth.

  • Improved soil chemical properties - providing proper pH, carbon, nitrogen, potassium and phosphorus levels.

  • Improved soil biology - activity by bacteria, mycorrhizal fungi, nematodes, protozoa, microarthropod and earthworms.

  • Improved soil nutrient levels and nutrient cycling.

  • Improved potential for vigorous long term vegetation coverage.

Limitations:

  • May require temporary storage space within the project limits to stockpile materials.

  • Requires site accessibility by earthwork equipment.

  • Applying high levels of organic materials may not be appropriate in arid regions of the state.

Technical Considerations

Construction Tips

Select the equipment to incorporate compost and other based upon the steepness of slope, depth of incorporation, site accessibility, and equipment availability. The photos below highlight these factors with regard to typical equipment type.

 

Rototilling

Rototilling (shown above) can be used to incorporate compost to shallow depths (less than 8 inches) in flat project areas. Rototilling is very effective at mixing compost with topsoil or other materials, but is ineffective below 8 inches in depth.

 

Bulldozer with ripping tines

Construction equipment (such as the bulldozer-attached chisel plow shown above) is good at ripping, loosen or decompact the soil surface is not effective at incorporating or mixing materials such as compost with topsoil. In short, ripping times do not mix soil amendments.

 

Disking Soil Erosion Control

A tractor drawn disk set (shown above) is a cost-effective to incorporate compost for large flat areas, but is limited to an incorporation depth of 6" to 12" and does not mix materials as well as the tracked excavator shown below.

 

Tracked Excavator WIth Bucket

A tracked excavator with bucket attachment (as shown above) can be used to incorporate compost on steeper slopes to depths of up to 3 feet. This approach is much more effective at mixing materials than rototilling or ripping, however the equipment is more costly. An additional benefit is that the irregular nature of this technique reduces the likelihood of creating a slip plane between the amended and non-amended soil layers.

 

Video - Incorporating Compost With an Excavator

Specifications:

Application Rate Guidelines

The best way to determine the appropriate application rate for compost is to perform a soils test to calculate the amount of compost necessary to add to the site to provide the required total minimum Nitrogen/acre to provide for the establishment of sustainable vegetation.

If testing the soil is not possible, several rules-of-thumb may be used to determine compost application rates:

Total Soil Nitrogen/Acre

Target a Total N/acre based upon typical ecoregion vegetation minimum requirements:

Ecoregion Average Total Nitrogen
LBS/ Acre
Minimum Total N required to support vegetation as identified by Bradshaw and others (1982) 625
Drastically Disturbed Sites 700
Desert Grassland 800
Minimum Total N required to support vegetation in decomposed granite soils as identified by Claassen 1,100
Shrub -Steppe Prairies 4,500
Deep Forest Soils 20,000

Soil Organic Material (SOM)

Minimum required soil organic material requirements vary by particular ecoregion vegetative needs, but generally range between 8-13% soil organic material, which typically equates to 30% compost by volume in the soil.

Available Nitrogen (First Year)

Note that in the first year, disturbed soil areas that have been seeded have sparse vegetation and initially will have a very low available nitrogen requirement - typically only 50 pounds/acre. Available nitrogen applied beyond plant needs will not be captured and may leach and be unavailable for future vegetation needs.

The application rates below are based upon achieving a target Soil Organic Material (SOM) rate of 8-13%, a Total Nitrogen/Acre range of 1000 - 3,000 lbs/acre, and an available Nitrogen amount of 100-300 lbs/acre. Lower application rates are recommended in arid regions that support arid vegetation. Higher application rates are recommended in decomposed granite soils, or in ecoregions that must support dense vegetation such as coastal forests.

Treatment Compost
Rate
CY/AC
Compost
Rate
Tons/AC
Total
N / AC
Avail.
N / AC
First Year
SOM Typ.
Slope
(H:V)
1" Compost incorporated
in top 3" of soil
135 52 1,080 Lbs 108 Lbs 8-13% 2:1
2" Compost incorporated
in top 6" of soil
270 96 2,160 Lbs 216 Lbs 8-13% 3:1
3" Compost incorporated
in top 9" of soil
405 162 3,240 Lbs 324 Lbs 8-13% 4:1
4" Compost incorporated
in top 12" of soil
540 216 4,320 Lbs 432 Lbs 8-13% 4:1

Consider Using With:

To provide protection for the soil surface, consider combining this treatment with:

Plans and Details:

  • No Standard Plan is required for Compost (Incorporate).

Estimate Information:

  • Typically $12-$16/SY (2009).
  • Use BEES code 203025, Compost (Incorporate).

Research:

Model Guided Specification for Using Compost to Promote Establishment of Vegetation and Stormwater Quality Improvements, 2010 New

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 Compost
Graphs nitrogen release from various composts and compares compost release rates with two native topsoils.

Annual Sediment, Nitrogen and Phosphorus Losses From Bare And Compost Amended Fill Slopes

 

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