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Abstract: Carbon dioxide has for many years been one of the favourite subjects of mushroom research workers. It is a well-defined compound, easy to measure and produce in certain quantities in order to investigate its influence within the various phases of mushroom growing. Most of the investigations published have lead to the conclusion that C02—in certain concentrations—has an inhibiting effect on the mycelium's ability to fructificate and, in excessive concentrations causes formation of abnormal growth of the fruit bodies. The investigations so far have generally dealt with the influence of C02 where fructification normally takes place, namely in the casing layer; it has now been proved that it is important to keep the concentration very low (by means of ventilation) to obtain a normal cropping yield (e.g. 1, 2, 3, 4,5, 6 and 7). Very little work has, however, been done upon the influence of the CO2, in the mushroom compost itself during the cropping period. In a number of experiments at the Danish Mushroom Research Station it was found that a certain speed of the air movement over the beds could ensure optimum yield (6). More recent extensive experiments were carried out in order to see whether the inhibiting effect of the C02 could be avoided by placing different amounts of KOH in closed growing chambers and with different depths of compost and air/bed-ratios. It was shown that with sufficient amounts of KOH maximum cropping yields could be obtained, but that the quantity of KOH was very great and, from an economical point of view, it is not a feasible practice to follow. The work was continued in order to investigate the quantitative amount of C02 produced. By means of a very simple technique, in which a given quantity of compost and spawn (with casing) was kept in closed chambers, it was shown that about 38,000 litres of C02 were produced from each ton of fresh manure (about 54% moisture and about 1.25% N in dry matter) during a period of 104 days (14 days to first mushrooms and 90 picking days). Half of this amount was due to the compost itself and the rest to the mushroom spawn's activity. Later it was shown, as was also expected, that when all C02 produced was removed in a closed chamber by means of KOH, the fructification stopped completely when no more oxygen was available. As all the above mentioned investigations (8) were based on more or less "artificial" conditions, it was decided to investigate whether naturally accumulated CO2, (determined by a Wösthoff Ultragas 3) in the compost itself had any effect on reducing fructification and yield, if the C02-accumulation in the casing layer could be kept at a harmless level.
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