COMPOST 3
Photo 4:
5% higher filling weight in the pasteuristion tunnel leads to uncontrollable temperatures.
Photo 5: In compost with less structure, there is proportionally less air available.
process of expelling the ammonia. And that is the result of just a 5% higher filling weight. There is no such thing as optimal moisture con- tent or maximum filling weight. The moisture percentage and filling weight must always be seen in relation to the structure. This is one of the reasons that some composters have to fill the pasteurisation tunnels with compost with a moisture content of 76% and spray copious water during filling, while other composters fill at a maximum of 70% moisture and don’t add another drop of water in case this causes problems.
There are problems connected to a lack of structure, but too much is not what you want either, as in this case it will not be possible to fill the required tonnage in the tunnels. And air will pass through the compost too easily, blowing away the activity and ammonia and drying out the compost in the process.
Structure and oxygen
As stressed in previous articles, the carbohy- drates in the compost form the energy source for mushroom growth. To decompose these sugars in the compost requires oxygen. In simple terms: sugars + oxygen => energy + water + carbon dioxide,
Alternatively, a more complicated formula: C6H12O6 + O2 => H2O+ CO2 + energy.
If no oxygen is supplied, then no organic matter will be decomposed and there will be no activity, no growth and no evaporation. The volume of air in the compost depends on its structure and moisture content. The following example (see photo 5) will clarify this. Imagine
16 MUSHROOM BUSINESS
a package of normal compost with a thickness of 21 cm. In this case you assume that it con- sists of 1/3 air, 1/3 solid compost components and 1/3 water. This can be simply visualised as three layers, each of seven centimetres. A seven centimeter layer of solid compost components is then proportionate to a seven centimeter layer of air in a ratio of 1:1.
However, with a batch of soft compost that eas- ily absorbs water, more kilos often have to be filled in order to create sufficient volume. The thickness of this package is, for example, 18 cm, but in this case there are eight centimetres of water and eight of solid compost components, leaving just two centimetres for air. This results in a solids to air ratio of 1:0.25. So with the heavier compost there will be four times less air in the beds and therefore less activity, less evaporation and vigour. This can only be compensated for by filling the beds are airily as possible, avoiding compacting the layers and preferably using a drier compost. Good yields can be achieved from compost with a short structure, but the process is more sensitive to other parameters. The main thing to consider is not the volume and structure of the compost, but how you respond to these factors. Where there is water, there is no oxygen – but you do need oxygen. With shorter structures you should fill a drier compost and spray a little less water so enough air is retained in the beds to provide the necessary activity and vigour. Compost with a bit more volume is more forgiving for growers. There is no immediate punishment for spraying a little too much water.
The following article will discuss selectivity, among other topics.
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