Compost, an erosion control material that enhances soil and is made from readily available, inexpensive materials, is gaining ground as an accepted material for erosion control and roadside landscaping in California. Construction projects typically degrade and compact soil, causing breakdown of aggregates and loss of structure. When tilled into the surface 6 inches, compost modifies the physical soil properties, improving water-holding ability and resistance to compaction. The microorganisms in compost aid in the formation of soil aggregates, the building blocks of good soil structure and tilth. Bacteria and fungi attach themselves to soil particles and bind them together, forming aggregate crumbs. Spaces in the aggregates allow water to percolate the soil and air to reach plant roots. When soil particles aggregate together, they are less likely to erode during rainstorms.
In clay soils compost reduces the bulk density and increases the porosity, thus improving the exchange of air and water through the soil. In sandy soils compost increases the water-holding capacity and soil aggregation. Availability of greenwaste compost products has stimulated research on how compost best can be used in typical landscaping and erosion control projects.
Roadside Landscaping
The California Department of Transportation (Caltrans), which develops specifications for roadside landscaping that are used by all government agencies, has added compost to the materials it specifies for erosion control.
Jack Broadbent, supervising landscape architect in Caltrans’ Office of Roadside Management, is taking a closer look at different ways to use compost. “We use native plants, but we are not planting them in native soil,” he says. The native topsoil often is removed from the site or degraded and compacted from site work. Rainwater runs off because it can’t penetrate the soil. Compost has proven effective for erosion control when incorporated into site soil at high rates.
“In one area the site soil was mainly decomposed granite—it was like a big granite sand pile,” says Broadbent. “We incorporated 24% greenwaste compost and found that it increased water infiltration and almost eliminated sediment production.”
Compost quality has emerged as an important consideration for Caltrans. Non-biodegradable contaminants can be a problem, as can weed seeds, which compete with seedlings that Caltrans is trying to establish.
Broadbent is working with the California Integrated Waste Management Board (CIWMB) to develop a compost classification system. The system will include compost-quality specifications that incorporate the Seal of Testing Assurance (STA) compost testing and information disclosure procedures. “STA testing puts everybody on the same page,” says Broadbent. “We’ll be able to tell our contractors what we want, and they’ll be able to determine if a given compost product meets the specs.” The STA program was developed by the US Composting Council (www.compostingcouncil.org).
Caltrans is also working with the CIWMB to develop a best practices manual for compost applications that will tie into the compost classification system and Caltrans specifications.
Broadbent works with University of California researchers to determine the most effective way to use compost in roadside landscaping projects.
Vic Claassen, a soil scientist with the UC Davis Department of Land, Air and Water Resources, is researching the viability of incorporating high volumes of compost into the site soil prior to planting. “I look at how undisturbed native soils work and try to figure out how to make degraded roadside soils function the same way,” he says. For plant establishment to succeed, soils must retain their plant cover, gradually accumulate nutrients, support microbial and mycorrhizal activity, allow for moisture infiltration, and hold onto rainwater.
Claassen completed a survey of compost products available in California in 2002. In 2004 he evaluated nitrogen mineralization and release of six yardwaste compost products, two of which contained biosolids (Claassen and Carey 2004).
“The good news is that in the long run compost can greatly help regenerate barren soils,” he says. Compost works well in the physical sense, adding pore space and improving water-holding capacity. After a couple of years compost products generally release nitrogen at comparable rates to undisturbed native soils and are suitable for providing nitrogen for plant establishment on disturbed, low-nutrient soils.
“But in the short run it’s difficult to predict the nitrogen release of different compost products,” Claassen says. Nitrogen is essential for the establishment of landscape plants, but too much nitrogen can lead to weed invasion. Excess nitrogen can leach into waterways, upsetting the ecosystem. And too little nitrogen can slow growth of the erosion control plant materials.
Nitrogen availability of compost products is variable, depending on maturity and feedstocks. Composts that contain biosolids are fairly high in nitrogen and can release it too fast for native plant communities to absorb. Greenwaste compost products that are incorporated into the soil before they are fully cured typically have a nitrogen deficit and can compete with plants for scarce nitrogen, reducing initial plant establishment and growth.
“We can always add nitrogen if we know it’s going to be deficient,” says Claassen, “but we’ve got to know how much to add.”
Brent Hallock of the San Luis Obispo–based California Polytechnic State University’s Earth & Soil Sciences Department has done experiments in conjunction with Caltrans to test the potential effectiveness of compost in combination with other materials for establishing vegetation on disturbed roadside sites for short- and long-term erosion control.
One project completed at Cal Poly’s erosion control facility involved test boxes filled with topsoil compacted to 95% and set at a 2H:1V angle on a south-facing slope. A 0.25-inch application of compost was applied in combination with jute netting and crimped straw. Morro Bay compost (biosolids and yard trimmings) and Cal Poly compost (manure, straw, bedding, yard trimmings) were used in the experiments. Overall, treatments including compost reduced runoff and sediment without affecting water quality. Seeding establishment was best in the compost treatments in the sandy soil, indicating compost may have contributed fertility for plant growth.
Hallock says complex interactions between soil type, compost product, and vegetation type play into his results. “It’s not a linear relationship,” he says. “Some compost products did better on coarse-textured soil, and some did better on fine-textured soil.”
Erosion Control Research in Other States
Research in other states has yielded similar results. Researchers at Iowa State University cooperated with Department of Transportation officials to evaluate performance of compost as a surface mulch—called “compost blankets.” They found that 2-inch blankets were stable during intense simulated rainfall events and suppressed weeds. Runoff from compost-treated plots did not pose an increased environmental risk. Coarse composts were more effective than fine composts in controlling sheet erosion (Glanville, Richard, and Persyn 2003).
Washington State, recognizing that soil quality is directly related to water quality, includes soil quality and depth as best management practices in the Stormwater Management Manual for Western Washington. Developers are required to restore natural functions in disturbed soils by using organic amendments for erosion control. Soils must be amended to allow for moisture infiltration and retention (Guidelines and Resources Manual 2002, http://www.compostwashington.org/PDF/SOIL_MANUAL.pdf).
The Minnesota Department of Transportation has developed specifications for three grades of compost to be used in highway projects. For erosion control, compost can be applied in a 2-inch blanket or used as “grouting” to fill riprap voids. Specifications include applying compost with blower equipment to achieve a non-compacted layer. Compost can be blown into an 8-inch-diameter photodegradable geotextile knitted tube to create a “log” that can be used for temporary sediment control around inlets and ditch checks (Minnesota DOT Standard Specifications for Construction, 2000 Edition, http://www.dot.state.mn.us/tecsup/spec/2000/sp3861.pdf).