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February, 2009
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Marin Landscaping Run-Off Threatens Coho Salmon
By Karen Nakamura
A couple months ago, the CP explored the rights of homeowners vs. those of the Coho salmon in West Marin and how to resolve the differences. A big problem for fish viability is soil residue and fertilizers, etc. running into the San Geronimo Creek. Much of this runoff comes from construction and landscaping along the watersheds. But it isn't just Marin creeks that need attention. Fish and aquatic life in waterways around the world are under the same attack.
With planting season here, the question is how do we plant landscape and gardens to keep soil from damaging our creeks. SPAWN's (Salmon Protection and Watershed Protection) website has many good suggestions. An example is to cover exposed earth, especially on slopes, with biodegradable erosion control blankets, "to avoid topsoil loss and help establish vegetation." Quick growing grasses are another aid.
SPAWN also suggests building "dams" between exposed areas and creek banks or storm drains to stop finer sediment particles (especially lethal to fish eggs) from reaching the creek. "These can be made from bio-logs, coir rolls, or straw bales, or even woody plant materials, and if nothing else recycled cardboard."
This brings to mind the ancient use of terracing. The idea is to keep the soil in place and then make it more fertile. American agriculture has used contour furrowing for generations to stop topsoil erosion. Retaining walls, found everywhere in Marin and other hilly regions, can be used as a foundation for terrace/silt dam building.
Among the many cultures that have engaged extensively in hillside contour farming are the Balinese and Incas. In fact, agricultural departments in South American countries have been looking into Inca farming practices to help solve their food production needs.
The Philippine Council for Agriculture, Forestry and Natural Resources Research and Development (PCARRD), along with other Asian agricultural organizations, was pushing the technique back in 2003. Its website states: "Increasing population pressure is forcing people to cultivate fragile ecosystems in hills and mountains. As a result, soil erosion and nutrient losses occur, leading to low productivity, flooding, and environmental degradation."
Applied to farming techniques or your dreamy little patio garden, it's the same process. One solution is what the PCARRD calls "alley cropping". "Alley cropping involves establishing hedgerows planted with trees, shrubs and/or grasses horizontally along the outer contours of the hill. Not only is this permanent hedge used to fertilize the soil with its leaves, it adds to the life nurturing forest canopy.
The spaces between hedgerows are then used for gardening and classic landscaping. The alley is obviously filled with soil held in place between one hedgerow/retaining wall and the next. The crop is your Azalea and strawberries. Not only does alley cropping fit into SPAWN's vision of reducing silt runoff, (especially if the runoff is contained, it's an alternative to slash-and burn agriculture in Latin America.
This method is considered especially useful to degraded hilly lands with a 10-50% slope. Not only does it reduce soil losses considerably, it maintains the "productivity of fertile soils and enhances the productivity of degraded ones. It improves the physical properties of the soil, maintains soil organic matter and promotes nutrient recycling."
PCARRD recommends a closely planted hedge with double rows using fast-growing plants when first establishing the hedge. It also highly recommends using nitrogen-fixing species of plants for the hedge. A paper titled "How to farm your hilly land without losing your soil" from the Sloping Agricultural Land Technology (SALT) Farming System and the Asian Rural Life Development Foundation in the Philippines, states that corn yields at its research farm were 2.7 tons per hectare as opposed to yields of 1.3 tons in alleys not surrounded by leguminous hedges; "nitrogen fixing species" or leguminous biomass. With fertilizer added to the nitrogen compost, the yield shot to 3.7 tons.
"The ability of nitrogen fixing trees to grow on poor soils and in areas with long dry seasons makes them good plants for restoring forest cover to watersheds, slopes and other lands that been denuded of trees. Through natural leaf drop they enrich and fertilize the soil. In addition, they compete vigorously with coarse grasses, a common feature of many degraded areas that have been deforested or depleted by excessive agriculture."
The SALT site also gives extensive instructions on how to figure the natural angle of your slope by making an A-frame a simple yet effective tool, which locates the natural contours. Check it out at pcarrd.dost.gov.ph/cin/AFIN/salt1.htm and scroll to 10 steps to SALT 1.
