​Preventing Sour Mulch

Sour mulch may result when stockpiled mulch becomes compacted — usually due to high piling. This prevents oxygen from reaching the inner regions of the pile, resulting in anaerobic fermentation of the wood — a process that produces organic acids.


Here's how to avoid this potentially plant-damaging mulch condition:

  • BASF recommends limiting the height of processed mulch piles to 10 feet or less; ideally configured in compost windrows less than 5 feet tall so mulch can be turned frequently to promote aerobic decomposition.
  • Correct the acidic condition by spreading the processed wood fiber in a shallow layer and watering heavily to leach away potentially toxic chemicals.
  • Add large amounts of limestone to also help neutralize acidity.
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