California Department of Transportation
 

Incorporate Materials

Incorporate Compost

What is This Treatment?

The Incorporate Materials specification involves tilling compost, mulch, and/or straw into disturbed slope surfaces. Incorporating organic materials into the slope surface improves soil health and provides the nutrients and biotic activity necessary to support long term, sustainable growth of vegetation. The required depth of tilling or incorporation varies by slope gradient - steeper slopes are limited to a shallower depth of incorporation. Compost is typically mixed at a rate of 1 part compost to 3 parts soil, or 1/3 by volume. The rate of incorporation for other organic materials (such as straw or bark mulch) varies.

Where to Use This Treatment:

  • Use where disturbed site soils are compacted and/or low in nutrients. See the Application Rate guidance section below.
  • Consider these maximum incorporation maximum depths:
    • Slopes 3:1 (H:V) and steeper - 2" deep maximum
    • Slopes 4:1 (H:V) and flatter - 4" deep maximum
  • For 2:1 slopes, limit the use of Incorporate Materials to areas readily accessible to rototillers or by tracked excavators.

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 slope accessibility by equipment necessary to incorporate the materials.

  • High application rates for organic materials may not be appropriate in arid portions of the state. See Application Rate Guidance below.

Design Considerations:

  • Verify availability for medium and coarse compost as these mixes may require a "custom blend". Call local compost producers on list at: US Composting Council (USCC) Seal of Testing Assurance (STA) Program Participants.

  • Determine if slopes will require incorporation of organics to support sustainable vegetation. For information on how much compost a sterile soil might require, see the Application Rate guidance section below.

Construction Tips

Depth of incorporation is constrained by slope steepness and accessibility - long, steep, inaccessible slopes limit the equipment used to smaller devices. The photos below highlight the strengths and weaknesses of different types of equipment.

 

Rototilling

Rototilling (shown above) can be used to incorporate materials to shallow depths (less than 8 inches) in flat project areas. Rototilling is very effective at mixing materials with topsoil, but is typically ineffective at depths greater than 6 inches.

 

Bulldozer with ripping tines

While good at ripping and decompacting the soil surface, construction equipment (such as the bulldozer-attached chisel plow shown above) is good at ripping, is not effective at "incorporating" or "mixing" materials with topsoil. In short, ripping tines do not effectively mix amendments together with soil.

 

Disking Soil Erosion Control

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

 

Tracked Excavator WIth Bucket

A tracked excavator with bucket attachment (as shown above) can be used to incorporate materials 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 to rent. An additional benefit of this approach to incorporation is that the irregular nature of this mixing technique reduces the likelihood of creating a uniform "slip plane" between the amended and non-amended soil layers.

 

Video - Incorporating Materials With an Excavator

 

Application Rate Guidance for Compost

Several methods can be used to determine the application rate for compost that will provide the total Nitrogen/acre necessary for the sustainable vegetative establishment.

1. Soils Test

The best way to determine the best application rate for compost is to perform a soils test. If a testing the soil is not feasible, the methods below may be used to estimate the ideal compost application rate.

2. Ecosystem Based Soil Nitrogen/Acre Requirement

A ballpark estimate for the proper amount of compost to add to the soil can be based upon the typical total soil nitrogen levels in found in each ecoregion. The table below identifies the amount of total Nitrogen required for sustainable plant growth in various ecoregions. Use the Average Total Nitrogen LBS/Acre values found in the table below to identify the compost application rate necessary to provide the Nitrogen required by the ecosystems identified below.

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

3. Minimum Soil Organic Material (SOM) Requirement

The ideal compost application rate may also be quickly determined by correlating it with the typical minimum soil organic material (SOM) required to sustain healthy vegetation. While the ideal organic material (SOM) content varies based upon the specific ecoregion vegetation and soil type, research indicates that a typically sound SOM range is between 8-13%, which typically equates to 30% compost by volume in the soil.

Keep in mind that during the first growing season, seeded disturbed soil areas typically exhibit sparse vegetative cover, thus requiring very low available nitrogen levels - often just 50 pounds/acre. Nitrogen applied that exceeds plant requirements will not be utilized for plant growth and may leach to lower soil horizons thus becoming unavailable for future plant growth.

The recommended application rates below are based upon a target Soil Organic Material (SOM) rate of 8-13%, a Total Nitrogen/Acre range of 1,000 - 3,000 lbs/acre, and an available Nitrogen amount of 100-300 lbs/acre. Lower application rates are recommended in arid regions, or areas that typically receive less than 10 inches of precipitation per year. Higher application rates are recommended in decomposed granite soils, or in ecoregions that must support dense vegetation such as coastal forests. Specific site conditions as well will modify these general recommendations. Coordinate the use of Incorporate Materials with Caltrans Division of Engineering Services (DES) Office of Geotechnical Design.

 

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

 

Consider Using With:

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

Plans and Details:

Estimate Information:

Research:

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 how 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

 

External Links:

Updated 03-09-2016