The mushroom casing layer is a multi functional part of the growing bed. There are four main casing requirements in order to maximise the potential of the nutrients in the compost.
1 It should have the minimum level of nitrogen containing nutrients in contrast to the nitrogen nutrients present in the compost. This facilitates the eventual pinning and cropping mechanism.
2 It must be capable of absorbing and retaining the maximum amount of water. This water must be taken up without losing its granular structure thus facilitating gas exchange between the bed and the outside air. This granular structure also allows easy penetration of mycelium at casing run.
3 The PH of the casing layer is optimum at 8 – a low PH of 6.5 can lose the grower 5-10% yield. If the farm uses the new carboxylate Mycro Nutrients in the casing the PH effect is magnified. Very little benefit is shown with Mycro Nutrient products used at PH6.5. However Mycro Nutrient products used at PH8 can dramatically increase yield by 10-20% as well as increasing piece weight and speeding up case run.
4 Calcium Content (Reserve). A simple mixture of ground chalk and peat will not reach much beyond PH7. Ground chalk is however an excellent calcium reserve. The reason why we need a calcium reserve is threefold:
(A) The natural acidity of the peat can take weeks to seep out of the peat lumps, especially with dense deep dug peat.
(B) During cropping the metabolic activity of the crop produces oxalic acid as an end product. The mycelium can only dump this acid waste product in the presence of calcium and at a preferred PH above 7. As the crop progresses to second and third flushes more of the calcium carbonate is converted into highly insoluble calcium oxalate. At this point the mycelium is stressed to find higher PH and available calcium carbonate, hence the need for a calcium reserve (At least 10% of the casing by weight).
(C) A secondary mechanism takes place within the casing layer with excess calcium carbonate present. As the crop progresses and more insoluble calcium oxalate is formed a counter effect is taking place. The reserve of calcium carbonate is highly insoluble so is slow to react to oxalate. This is a hindrance to the mycelium function – chemistry comes to the rescue! As carbon dioxide streams through the casing from the compost catabolism a reaction occurs with the excess of chalk. Equilibrium develops between the C02 and calcium carbonate where a significant percentage is converted into soluble calcium bicarbonate. This solubility makes the conversion into insoluble calcium oxalate much easier for the mycelium.
Calcium carbonate as discussed is a good reserve. It is more effective if it is ground to 2mm to dust to be available later in cropping. The calcium carbonate source with the finest particle size is sugar beet lime. Being a micro precipitate it is very easily utilized. Dependant on the methodology of manufacture and the number of years of storage the Sugar Beet Lime can vary in PH from 7 to 9 so every delivery should be monitored. A consistent supply of high PH Sugar Beet Lime is gold dust. It is the only way to obtain a PH8 casing without resorting to slaked lime. In effect high PH Sugar Beet Lime containing its own supply of slaked lime and very evenly distributed within the calcium carbonate. In the absence of high PH Sugar Beet Lime the PH is best adjusted by approximately 0.25% slaked lime pre-mixed with the reserve calcium carbonate minimum 10% on casing weight. This calcium carbonate can best be a mixture of lower PH Sugar Beet Lime and or ground chalk or limestone. As always thorough mixing is of primary importance.
There is growing evidence that limestone or chalk with significant amounts of magnesium content helps the mushroom metabolism.
