The introduction of phase 2 and 3 in tunnels years ago gave rise to a series of stormy developments resulting in an entirely different peak heating and incubation process. This trend continued when phase 1 started to be composted indoors. When these developments are critically examined, one can only conclude that the experience built up over the years has lead to considerable fine-tuning in the technique and to the eradication of any teething troubles. However, there are still companies to be found making choices that in the light of current knowledge should not be made in the first place.

Various types of composting systems are common. In phase 1 indoor composting tunnels or bunkers are used. Phase 1 tunnels generally have open grating or slatted floors like phase 2 tunnels. These provide a ‘low pressure ’ aeration system, where circulation is relatively high and - depending on the temperature zone and corresponding oxygen demand - relatively little fresh air is provided. The supply air temperature is therefore higher and closer to the average compost temperature.

Phase 1 bunkers generally work using the well-known spigot floors. This is an aeration system where air pipes disperse the air over the bunker floor. The openings of a few millimetres (usually spaced 80 centimetres apart) ensure a directed airflow. Spigot floors provide a ‘high pressure’ aeration system, where aeration is pulsed, and fresh air is given depending on the temperature zone and corresponding oxygen demand. This means the supply air temperature is subject to more fluctuation and with its relatively low temperature differs more from the average compost temperature.

In phase 1, bunker systems are seen with a single central fan for several bunkers, but also systems where each bunker has an individual fan. The familiar spigot floors also can vary in size.

Phase 2 and 3 also house a variety of systems featuring open grating floor and drag matting. Spigot floors without nets are also used which are filled using cassette or trough filling machines or by overhead filling units, but are emptied by loaders.

A highly important and sometimes overlooked aspect is keeping the air pipes and air chambers under the flooring scrupulously clean. Dirt-free drag matting also contributes to good end results. 

Wear and tear on mats

If drag matting or gliding matting (netting permanently attached to the concrete floor) is used a commonly asked question is how often it should be replaced. The extent of wear and tear should be considered. When is a mat actually ripe for replacement?

The primary factor to consider is safe and reliable air permeability, but not unimportant is the cost factor. How expensive is each drag movement (pulling the compost out of the tunnel) per mat, and what are the effects on cost if a worn mat lowers production. Restricted aeration has a very negative influence on compost quality and accordingly on productivity.

On many farms mats are mainly inspected for mechanical damage, which appears at the edges of the mat. It's important to trace the cause. Worn matting can easily lead to holes forming as there is a certain resistance to the concrete floor when the mats are pulled out of the tunnel which has the effect of sand paper grating against the matting surface.

The load on the matting also influences its useful lifespan. The amount of compost per square metre plays a crucial role. There are companies that decide to use separate tunnels and mats for phase 2. Mats in similar tunnels are subjected to more wear caused by the higher temperatures and high ammonia concentrations. Cleaning drag matting with high-pressure cleaners can also result in extra wear if the jets of water are too intense.

Worn matting can be defined as a mat where the mesh structure in the fabric is less than perfect. Check this by shining a torch through the mat. It's also important to register the number of pulling movements per mat, in order to predict when a mat is approaching replacement time.

There are various theories about reversing mats from back to front or turning them around from top to bottom at intervals to try and lengthen the working life, but remain critical about this practice.

A final word of advice, the quality of the mat at purchase is also a contributing factor.

Gliding matting should be checked for wear like drag matting. It's also important to regularly inspect underneath the floor and spray clean if necessary.

Aeration

If a company works with tunnels containing amounts of compost approaching critical limits, negative effects are likely to occur sooner or later. When the compost structure is gradually getting shorter and the moisture content is on the high side, a much used drag mat could literally be the last straw. When compost is at an even more advanced stage of fermentation and softer, these risks are closer at hand.

If there's one thing bad for compost, it's bad aeration, which always manifests itself in the patches with the most resistance so also creating greater heterogeneity. The initial results of a shortage of oxygen could be disappointing production, but in many cases the connection is not immediately made to the preceding composting process where aeration was less than optimal.

Another problem that may be caused by bad aeration is possible ammonia emission during conditioning as the bacterial activity has been disrupted by the lack of oxygen leading to badly decomposed patches in the compost, or by too high temperatures at pasteurisation.

A bunker filled too high combined with brief pulsed aeration may just not be getting enough time to spread the oxygen uniformly in the upper compost layer. As the compost is mixed again at a later stage, the effect might not immediately be obvious but it will still have a negative influence on the yield. Not being aware of these kinds of problems will make trying to trace the causes afterwards like looking for the proverbial needle in a haystack.

The combination of a too brief aeration time and an oxygen value set too low can cause undesirable anaerobic situations at certain stages in the process. Not losing track of these details will help avoid disasters. 

A good start

It is essential that those responsible for compost production stay alert to a good aeration process to ensure successful continuity. Consistent, high quality mushroom production cannot be seen as a separate entity to a correctly functioning composting process, even though less than optimal production cannot always be traced back to this aspect. As we all know in mushroom growing, it's the combination of the various factors that adds up to the end result. Never forget the golden rule, good preparation is halfway to excellent results!