Sour mulch results from anaerobic (without oxygen) decomposition of wood mulches that have been stored in large piles. Affected plants are often yellow and may have leaf burn. Symptoms look like fertilizer or pesticide burn or water stress. Damage can be severe enough to kill plants.
The pH of sour mulches are extremely acidic, reportedly 1.8 to 3.6. Normal mulch is closer to a pH of 7.0. The toxicity of this mulch is not caused directly by the acidity, but rather the accumulation of by-products of the decomposition process. These by-products include methane, alcohol, ammonia gas, and hydrogen sulfide gas. Sour mulch smells like vinegar, sulfur, or silage. These compounds are capable of injuring plants quickly, usually within one day and often within hours of application.
Depending upon the extent of the injury, plants are often able to recover. Water affected plants during hot, dry periods to prevent further stress.
Mulch that has soured can still be used if it is "mellowed" before application. Spread the mulch in shallow layers and allow it to air out for several days until it no longer smells. It may also be helpful to water the mulch before application to wash away toxic substances.
