pH during mushroom composting

One of the questions I am most frequently asked is ” what is the optimal pH value of compost?” I always answer that it is much more important to ensure the pH falls during phase II and spawnrun. But which factors influence the pH value? The obvious answer is gypsum – but is that true?

Concerning the pH during composting, there are two parts to the question; first, what factors influence the pH value and, second – and more importantly- what causes the pH value to fall during composting.

During the composting process, the pH value must decrease by at least half a pH point during pasteurising\conditioning and one pH point from phase II to phase III. So what happens if this is not the case? A good question – and one with multiple answers. However, in 90% of the cases, the answer is in phase I.

Mixing chicken manure

If chicken manure is not mixed evenly through the compost, and there are too many lumps of manure left, the nitrogen will not be released properly as it is “trapped” in the lumps. And, of course, if there is no chicken manure in some parts of the straw, it cannot be “spread”.


The most used raw material is water. Many litres of water are poured over each ton of compost. The water quality therefore greatly influences the pH value of the compost. This water must be aerobic. In other words, when water is added to the compost it must not smell. The microorganisms in this water are active, and they ensure the biological processes in the compost take place.

Phase I process

Anaerobic conditions must not be allowed to occur at any time during the process.  I sometimes see temperatures in a bunker that are nicely above or around 80oC, so it is quickly assumed that the compost is 100% aerobic. But if you enter the bunker to see, feel and smell the compost, at a height of between 60 to 100 cm, for example, you can remove a complete zone of anaerobic compost. In patches of anaerobic compost nothing happens in the sense of actually decomposing and composting so the pH will not fall either.

In my next blog I will explain more about the influence of raw materials on pH values.

Mark den Ouden

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