California Department of Transportation
 

Incorporate Materials

Incorporate Compost

What is This Treatment?

This work typically involves mixing compost, mulch, and/or straw 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, the rate of other materials varies. See the Application Rate Guidelines below for detailed

When to Use This Treatment:

  • Typically used for slopes 2:1 (H:V) and flatter. Guidance on rates and depths is provided in the table at teh bottom of this page. For specific maximum rates and 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.

Construction Tips

Select the equipment to incorporate compost and other materials 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 materials to shallow depths (less than 8 inches) in flat project areas. Rototilling is very effective at mixing materials with topsoil, but is ineffective at greater depths.

 

Bulldozer with ripping tines

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

 

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 a 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 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. 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 Materials With an Excavator

Specifications:

Compost Application Rate Guidelines

A number of methods can be used to determine the compost application rate that will provide the total Nitrogen/acre necessary for the sustainable establishment of vegetation.

1. Soils Test

The best way to determine the appropriate application rate for compost is to perform a soils test. If a soils test is not feasible, the following alternate methods may be used to quickly estimate the ideal compost application rate.

2. Ecosystem Based Soil Nitrogen/Acre Requirement

The compost application rate may also be quickly estimated based upon the typical total soil nitrogen requirement for vegetation in an ecoregion. See the table under #3 below to determine the compost application rate that correlates with the desired Average Total Nitrogen LBS/Acre.

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

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.

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. Stability requirements for slopes 2:1 (H:V) and steeper, for example, will constrain incorporation depths. Coordinate the use of Incorporate Materials with Caltrans Division of Engineering Services (DES) Office of Geotechnical Design.

 

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:

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 04-15-2014