How long does it take mulch to decompose into soil?

A common question on mulch and wood chips is how long does it take mulch to decompose into soil?

Decomposing Mulch

Mulch slowly decomposing in a garden

The answer isn’t straight forward. It depends on climate, conditions, and the type of mulch. If the mulch has been treated then it will take longer to decompose than untreated mulch (the kind you get from your local tree arborist).

Why would you want to know how long it takes for mulch to decompose? Wood chip, bark mulch, and leaves consume nitrogen from the soil when it starts decomposing. It sucks out the nitrogen in order to facilitate the decomposition. What is actually happening is that the micro-organisms that decompose wood chips require nitrogen in amounts greater than are available in the wood chips alone and thus source it from the soil. This is why bark mulch and wood chip is so highly effective in keeping weeds down. However, if it takes too much nitrogen from the soil it will stunt the growth of the vegetation it surrounds. The good news is that as the wood chips and mulch materials decompose the nitrogen returns to the soil in addition to providing many other essential nutrients for your plants to grow.

The best gardening practice is to add nitrogen or ammonium sulphate to the soil to help the vegetation grow and speed up the decomposition of the mulch. This will prevent the nitrogen being removed from the soil and in fact provide enrichment of it. You should always refer to the product being used as to the quantity and distribution of it, as different concentrations will require different applications. If you over apply nitrogen or ammonium to your garden you can poison your plants.

A simple answer to the question is: untreated arbor mulch will take around 1 year in typical Sydney weather to start breaking down. Within 3 years it will be fully broken down and providing excellent nutrients to the soil. Treated woodchips will take longer, around 4 years to fully break down, with the start of decomposition occurring around 2 years after the mulch has been laid. The factors that directly influence the decomposition of mulch and wood chips are;

  • moisture of the soil (moisture promotes decomposition)
  • warmth of the soil (warmth promotes decomposition)
  • if the mulch has been treated (treated mulch slows decomposition)
  • any additives to the mulch (nitrogen supplements and ammonium sulphate speed up decomposition)

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