Organic material applied to the soil surface, known as mulch, provides numerous benefits like moisture retention, temperature regulation, and weed suppression. While mulch is designed to break down slowly, improper storage or application can accelerate its decomposition or trigger detrimental chemical processes. Mulch can degrade or “go bad” when conditions favor harmful microbial activity or excessive breakdown.
Degradation During Storage
Anaerobic decomposition is the most common way bulk mulch degrades before application, resulting in “sour mulch.” This occurs when dense piles of wood-based material are stored without adequate oxygen, causing the microbial community to shift from aerobic to anaerobic bacteria.
These anaerobic microorganisms produce toxic byproducts. Primary compounds include organic acids like acetic acid (smelling like vinegar or sour apple cider) and methanol. Ammonia and hydrogen sulfide gas may also be produced, leading to a strong, pungent sulfur or rotten-egg odor. This chemical change is accelerated by excess moisture.
If mulch smells strongly sour, do not apply it immediately, as volatile compounds can damage sensitive plant tissue. Spread the affected mulch into a thin layer for thorough aeration. Exposure to oxygen causes the toxic compounds to dissipate, rendering the material safe for use within a few days.
Signs of Unusable Mulch
Degradation that is not chemically toxic often manifests as visual and textural changes. One common sign is the appearance of white or gray patches on the mulch surface, typically saprophytic fungi or slime molds. These organisms are merely breaking down the organic matter and are generally harmless to living plants.
A more problematic sign is when the mulch material becomes slimy, compacted, or matted together. This clumping indicates the material has been saturated with water for too long, leading to a loss of air pockets and a dense surface layer. This physical change prevents water and air from passing through the mulch to the soil below.
Mulch left in place for many seasons may break down into a fine, dirt-like consistency. Excessive breakdown means the material has lost its coarse structure. When mulch reaches this stage, it is no longer serving its primary purpose.
Consequences of Using Degraded Mulch
Using degraded mulch introduces problems, starting with phytotoxicity, where toxic compounds from sour mulch damage nearby plants. These volatile chemicals can cause symptoms like leaf scorching, bleaching of foliage, and the wilting or death of new seedlings, especially when applied in hot weather.
Another consequence, particularly with older or finely decomposed mulch, is nitrogen tie-up. Soil microorganisms require nitrogen to break down carbon-rich materials. If the mulch has a high Carbon-to-Nitrogen (C:N) ratio, microbes consume available nitrogen from the surrounding soil. This microbial activity effectively starves nearby plants, leading to yellowing leaves and stunted development.
When mulch becomes overly fine and dries out, it can develop a hydrophobic layer. This prevents water from penetrating the mulch surface. Instead of soaking into the soil, water beads up and runs off, leaving the soil beneath dry and depriving plant roots of necessary moisture.