June 1, 2022
If you have a garden or grow a specialty crop, consider mulching to help suppress weeds, retain soil moisture, prevent erosion, regulate temperature and increase soil organic matter. Mulching can also release nutrients throughout the growing season as material decomposes.
There is a possibility of nitrogen tie-up if mulch decomposes too slowly due to a high carbon-to-nitrogen ratio. How do you manage this ratio to have just the right amount of residue left on the surface to protect the soil? You want enough decomposition to help build soil organic matter, feed soil biology and release nutrients.
The ratio is often written as C:N, which shows the mass of carbon to the mass of nitrogen. For example, a ratio of 10:1 means there are 10 units of carbon for every unit of nitrogen. Soil microorganisms have a C:N ratio of 8:1. To survive and stay active, they need a diet with a C:N ratio of 24:1.
This ties directly into residue decomposition and nutrient cycling. If we add mulch that has a C:N ratio higher than 24:1, microbes seek additional nitrogen from the soil to help consume carbon in residue. Nitrogen consumed by organisms is unavailable for plants until microbes die and decompose. Common high C:N mulches include straw, cardboard, wood chips and sawdust, plus mature small-grain cover crops such as cereal rye. Wood chips have a C:N of 400:1, while mature cereal rye or straw has a C:N closer to 80:1.
If you add residue with a C:N ratio lower than 24:1, microbes consume it very quickly. A flush of nitrogen will be plant-available. However, there will be little residue protecting the soil surface. Low C:N ratio materials include shredded leaves and grass clippings, with a ratio close to 15:1. Legume cover crops are also low, with ratios close to 11:1.
Managing the ratio
Materials of different ratios can be layered to produce a more balanced ratio. A mid-range ratio can be reached that will decompose slowly enough to leave residue on the surface and release nitrogen for crops over time.
For example, a layer of straw mulch could be placed on top of a nitrogen-rich compost. The lower ratio of the compost will provide nitrogen for decomposition and prevent tie-up of soil nitrogen, leaving more plant-available.
Some crops, like leafy greens, require nitrogen soon after planting. Others, like tomatoes and peppers, need more nitrogen later during flowering and fruiting.
There are several additional factors to consider. Cost and availability play a large role. Some mulches, such as grass clippings, could contain unwanted weed seed or herbicide residues. Know your source.
High C:N materials in mulch decay more slowly over time. Materials left from last year will still release some nutrients as they decompose. Areas such as pathways benefit from high C:N ratio materials. A more durable material there will prevent soil compaction and erosion.
By having a basic understanding of the C:N ratios of your materials, you can easily balance the ratio to work for your system. The Natural Resources Conservation Service, in partnership with the Indiana Association of Soil and Water Conservation Districts, recently established an Urban Soil Health Program.
Kautz is a district conservationist with NRCS. She writes on behalf of the Indiana Conservation Partnership.
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