Organic mulch, which includes materials like wood chips, shredded bark, and straw, is initially applied to a garden for several functional reasons. Its primary roles are to help the soil retain moisture by reducing evaporation and to moderate soil temperature, keeping roots cooler in the summer and warmer in the winter. This fresh layer also acts as a physical barrier, suppressing the germination and growth of weeds. Over time, however, this organic material undergoes a series of physical and biological transformations that change its function and appearance.
The Process of Decomposition
The transformation of organic mulch begins immediately through the action of soil organisms. Microscopic life, primarily bacteria and fungi, break down the complex carbon structures found in wood and bark. Fungi are efficient at decomposing the tough lignin and cellulose components, slowly converting the mulch into stable organic matter, or humus.
The rate of biological breakdown is governed by three primary factors: moisture, temperature, and the composition of the mulch itself. Wet, warm conditions accelerate microbial activity, causing the mulch to break down quickly, whereas dry or cold conditions slow the process significantly. Coarser mulches, such as large wood chips or bark nuggets, decompose slower than finer materials, which offer a greater surface area for microbes to colonize. As decomposition progresses, the bottom layer gradually integrates with the topsoil, improving structure and drainage.
Surface Changes and Physical Degradation
While biological decomposition occurs below, the exposed surface layer undergoes visible physical changes. UV radiation bleaches the surface, causing vibrant colors to fade to a uniform, dull gray. This is purely a cosmetic change and does not affect the mulch’s underlying benefits.
Exposure to rain and foot traffic can cause the upper layer of mulch to become compacted and flatten into a dense layer. This matting effect can be counterproductive, causing it to shed water rather than allowing it to soak through to the soil below. Lightly raking or turning the top two inches periodically helps restore its loose structure, break up matting, and maintain proper air and water penetration.
Nutritional Consequences of Aging Mulch
The chemical impact of decomposing mulch revolves around the carbon-to-nitrogen (C:N) ratio. Woody mulches are high in carbon and low in nitrogen, which affects nutrient availability. Microorganisms require nitrogen to break down the high-carbon material for energy and cell building.
To meet this demand, microbes temporarily pull inorganic nitrogen from the soil, a phenomenon called nitrogen immobilization. This temporarily reduces the nitrogen available for plants, potentially causing leaves to yellow. This effect is limited to the area beneath the mulch layer and is minimized when coarse mulch remains on the surface rather than being mixed into the soil. As microbes die, the nitrogen is eventually released back into the soil in a plant-available form, but this process takes considerable time.
Replenishing and Maintaining Mulch Layers
Effective mulch management requires periodically assessing the remaining depth. Functional benefits—like weed suppression and moisture retention—diminish when the layer thins to less than two inches. When this depth is reached, add a fresh layer.
When replenishing, refresh the top layer with a light application of new mulch, typically one to two inches deep. The decomposed material at the bottom should be left in place, as it benefits soil structure. Avoid piling mulch against the base of trees or shrubs—often termed a “mulch volcano”—as this traps moisture against the bark and encourages decay and pests. Maintaining an overall depth between two and four inches while keeping a small ring of bare soil around the plant crown ensures continued health.