Mycelium incubation in casing soil

Mycelium incubation or spawn run in the casing soil is a crucial aspect of mushroom cultivation. At this stage the amount of mycelium and the thickness (relevant to mycelium vigour) is created. The mycelium threads act as supply lines from the compost to the mushrooms for the nutrient bearing water.

By Jeroen van Lier, C point

After compost, casing soil is the next medium to be colonized by the mycelium. In an ideal situation, thick clouds of fungal strands grow up towards the top layer of the casing soil, while still leaving enough ‘un-colonized’ casing soil over. This bare casing soil acts as a water buffer for the compost and the mushrooms, and later on during cultivation determines the extent and for how long evaporation in the casing soil can take place before it dries out. If the casing soil dehydrates, spraying will be necessary, even if the quality of the mushrooms doesn't allow it.

Wet casing soil impedes mycelium development, and is much less favourable for weak mycelium. A disadvantage of wet casing soil is the dependency on compost quality and the caccing material in the casing soil. A slightly drier casing soil provides more reliability concerning mycelium growth, although with drier casing soil there is an increased risk of much weaker mycelium.

As well as the moisture content of the casing soil, the structure is also important. This means that the casing soil should still be lumpy enough for the mycelium threads to grow around.

If the casing soil structure is too short and fine (sandy) the mycelium will colonize the entire area leaving no open spaces Mycelium-free areas retain water which is essential to increase the water retention properties of the casing soil.

Casing

The casing process also deserves close attention, as a good, well-structured casing soil that is treated mechanically during casing can loose a great deal of this beneficial structure. The speed of the revolving axles, the depth to which the soil is penetrated, the number and type of teeth and how these are attached to the axle all greatly influence the casing soil structure and the amount of compost mixed through the layer of casing soil.

The levelling process also affects the structure of the upper layer to a great extent and influences how compact the whole layer of casing soil finally is. If the levelling roller has to cope with too much casing soil, there is a risk of over-compacting the casing soil layer. However, the structure of the upper layer is not always indicative of the structure in the casing soil layer!

With heavy casing soil growers can try using two levelling rollers. Here too, the speed at which the roller revolves, the number and type of teeth (or tape), the configuration of the teeth on the axle and the height setting all play an important role in influencing casing soil condition and therefore quality.

The final treatment of many growers is more or less compacting the casing soil layer. This has many reasons. Sometimes it's necessary because of the well-known ‘dip’ that can appear in the centre of the bed, or to prevent too many lumps of casing soil clinging to the mushrooms during picking. There are also a number of growers who prefer to work with firmer casing soil in order to control the flushes better. Whatever you choose, avoid creating a smooth, even upper surface.

Spraying and climate

Mushroom mycelium grows best at a temperature of 25 degrees Celsius. If the compost temperature at incubation is 25 degrees Celsius with an air temperature of 17 degrees, then conditions are right for vigorous mycelium growth in the casing soil.

The problem in this situation is that the casing soil temperature will adapt to the temperature in the growing room, meaning that the mycelium will have to grow with a casing soil temperature of 17 degrees.

By increasing circulation, it's possible to control the compost temperature better and raise the air temperature to 19 degrees Celsius. This means the casing soil is warmer than in the previous situation so mycelium growth will be faster and more spontaneous. In this case, adjust the spraying schedule.

Strong mycelium

If as grower has a structural excess of mycelium in the casing soil, a few small adjustments should be able to improve conditions, for example by changing the spraying schedule. As soon as the mycelium in the top compost layer and the caccing material starts growing, it's vital to keep the moisture in the casing soil well under control.

In principle, mycelium growth stops every time spraying takes place, the weaker mycelium cannot cope with water and further development is impeded. On the other hand, the stronger mycelium has less trouble with the water and continues to develop. This method ensures that weaker mycelium is restricted in development and allows the stronger mycelium to dominate in the casing soil.

If the casing soil is ruffled, a maximum of 2 litres of water can be given afterwards to reduce the amount of weaker mycelium during the recovery period. You can also start blow down earlier, but the disadvantage here is that mycelium will not grow as high on the surface as normal.

Blow down

Nowadays many growers work with intermediate hybrids varieties. These types are quite spontaneous to fruit, so many growers apply a very slow blow down process.

This is not a problem on its own, but growers must realise that as long as the compost temperature exceeds 23 degrees Celsius, mycelium will continue growing in the casing soil.

So, either start blow down earlier or increase circulation to quickly bring the compost temperature below 23 degrees.

As soon as this has been done, reduce circulation again to avoid dehydrating the growing surface and causing too many, unevenly spread fruits. By keeping circulation low in the growing room at this stage, a start can be made encouraging a good spread of mushrooms and creating the right conditions for good water retention in the casing soil.

This method gives a certain amount of control over the amount of mycelium, without directly resulting in too many mushrooms or an uneven spread in the first flush. The number of pieces and how they are distributed over the beds can be steered during pinheading.