The pH values of phase 3 compost are a regular topic of discussion. The quality of phase 2 and phase 3 is visually assessed based on colour, smell, structure, firmness and moisture content. Phase 2 and phase 3 compost also undergoes analysis to determine aspects such as nitrogen (N), exact moisture content, C/N ratio and the pH value in particular, also known as the acidity level.

During the composting process, compost is exposed to a variety of influences that contribute to changing pH levels. The pH changes depending on the blend of raw materials and how fermentation takes place. With a normal outdoor process the pH value of fresh compost should be 8 to 8.5. After a normal phase 2, the compost ready to be spawned will have a pH of 7 to 7.5

Since the introduction of indoor composting processes with the related use of ammonia scrubbers, ammonium sulphate - the end product of this process - has become an interesting item with a strong influence on the pH. In general the average pH value with an indoor system average is lower than with traditional outdoor systems. This is explained by more advanced process management and improved oxygen supplies. The possibility to add ammonium sulphate is an additional benefit. In practice the following happens; when the ammonia NH₃ -emission produced in the exhaust air during phase 1 and phase 2 (which occurs when fresh air is needed to supply O² or for temperature control), is guided through sulphuric acid H₂SO₄, ammonium sulphate (NH₄)₂SO₄ is formed.

This liquid ammonium sulphate is collected in special tanks and subsequently dosed over the compost during phase 1. The result is a lower pH value after phase 1. A common pH value for fresh compost if ammonium sulphate is used during phase 1 is 7.8 to 8.0. It’s obvious that the pH in similar compost after phase 2 will also be lower - a pH value of 7.0 to 7.2 is not unusual.

pH development during phase 3

If spawn is added to compost with a pH lower than 7.5, the mycelium starts colonizing the compost more rapidly while the extent of colonization will be more intense in the same time span. It is very important to ensure the compost is highly selective as the low pH in the compost will also stimulate competitor mould development. Good selective compost with a low pH has, with sufficient actinomycetes development during conditioning and a high enough nitrogen content N, all the ingredients to encourage spontaneous growth and highly productive compost.

The actinomycetes that develop during a successful conditioning process when ammonia NH₄+ is correctly converted to protein are a beneficial nutrient base for healthy mushroom mycelium. Compost with a low pH of 7.0 to 7.5 that is inoculated should be able to reach a pH value of 6.0 to 6.2 under ideal circumstances using a spawn run of 15 days. Experienced growers easily label indoor phase 3 compost with a pH value higher than 6.3 as inferior quality. The colour of incubated compost with too high pH levels is also normally darker. If the growing schedule permits, a slightly longer period of mycelium growth may help improve matters.

Certain companies try dosing nitric acid HNO₃ during inoculation in order to lower pH levels during mycelium growth. This option actually shuts the stable door after the horse has bolted – it’s far better to examine the causes of too high pH levels during inoculation. If a grower still decides to use the nitric acid option, it's a bad idea to dose at the same time as spawn is added as this increases the risk of trichoderma.  In the past a lack of oxygen during composting also caused too high pH values in compost ready for inoculation.

pH influence on mycelium growth in the casing soil

It's clear by now that a sufficiently low pH value of 6.0 tot 6.3 in phase 3 compost means mycelial growth will be quicker in the casing soil than if the pH value at that moment exceeds 6.3. If cac-ing is also applied, the speed of mycelium development in the casing soil will be considerably faster, depending of course on the amount of cac-ing material.

Heed a word of warning here – for better mycelium quality and to allow enough time to spray the casing soil, preferably use relatively little cac-ing compost with a low pH of 6.0 to 6.3, than copious amounts of cac-ing compost with a pH value higher than 6.3.

Don’t underestimate the influence pH values in casing soil have on good mycelium growth in the casing soil. The optimal pH for casing soil is between 7.0 and 7.5. If the casing soil pH is higher than 7.5 mycelium growth may be slower, on the other hand in casing soil with a pH lower than 7.0 there is more risk of competitor moulds developing.

The pH value of the casing soil is influenced by the immediate surroundings. Around compost fibres in the cac-ing compost with a pH of 6.0 to 6.3 the general pH will drop faster. However, the pH of the casing soil itself will influence the mycelium growing in the cac-ing compost.

If, following a normal colonization period of about 14-16 days, the compost still shows a too high pH value (above 6.8) for whatever reason, there will be a degree of retarded colonization of the compost. This means that after casing the mycelium lacked the optimal condition required for vigorous growth through the casing soil. Delayed growth through the casing soil is also often seen. As mentioned before, good selectivity of compost is vitally important to keep competitor moulds at bay.

Caution with additives is also recommended. Retarded growth in casing soil is a signal to the grower to pay extra attention to the spraying pattern and temperature management. It’s very important to allow enough time in a similar situation for the mycelium to develop strongly; water and CO₂ are crucial factors here.

This situation doesn’t necessarily mean disappointing production, but this naturally depends on the degree of delayed growth and how much care is spent on remedying the problem.

Summary

The pH value of compost has a major influence on mycelium growth in the compost and therefore on the quality of mycelium growth in the casing soil.

High quality mycelium is the foundation for high quality mushrooms. The pH of the casing soil itself is of not to be underestimated importance too. If cac-ing compost is used, the speed and quality of mycelium growth greatly depends on the condition and pH value of the casing soil, the condition of the compost with a certain pH value at casing, and the amount of cac-ing compost used.