Paul Stamets' and J.S Chilton: The Mushroom Cultivator
CHAPTER VIII: THE CASING LAYER

Covering the substrate surface with a layer of moist material having specific structual characteristics is called CASING. This practice was developed by Agaricus growers who found that mushroom formation was stimulated by covering their compost with such a layer. A casing layer encourages fruiting and enhances yield potential in many, but not all, cultivated mushrooms.

SPECIES

In all of the species where the use of a casing has been indicated as optional, yields are clearly enhanced with the application of one. The chart above refers to the practical cultivation of mushrooms in quantity. It excludes fruiting on nutrified agar media or on other substrates that produce but a few mushrooms. Consequently, casing has become an integral part of the mushroom growing methodology.

Functions

The basic functions of the casing layer are:
1. To protect the colonized substrate from drying out.
Mushroom mycelium is extremely sensitive to dry air. Although a fully colonized substrate is primarily protected from dehydration by its container (the tray, jar or plastic bag), the cropping surface remains exposed. Should the exposed surface dry out, the mycelium dies and forms a hardened mat of cells. By covering the surface with a moist casing layer, the mycelium is protected from the damaging effect of drying. Moisture loss from the substrate is also reduced.
2. To provide a humid microclimate for primordia formation and development.
The casing is a layer of material in which the mushroom mycelium can develope an extensive, healthy network. The mycelium within the casing zone becomes a platform that supports the formation of primordia and their consequent growth into mushrooms. It is the moist humid micro climate in the casing that sustains and nurtures mycelial growth and primordia formation
3. To provide a water reservoir for maturing mushrooms.
The enlargement ot a pinhead into a fully mature mushroom is strongly influenced by the available water, without which a mushroom remains small and stunted. With the casing layer functioning as a water reservoir, mushrooms can reach full size. This is particularly important for heavy flushes when mushrooms are competing for water reserves.
4. To support the growth of fructification enhancing microorganisms.
Many ecological factors influence the formation of mushroom primordia. One of these factors is the action of select groups of microorganisms present in the casing. A casing prepared with the correct materials and managed according to the guidelines outlined in this chapter supports the growth of beneficial microflora.

Properties

The casing layer must maintain mycelial growth, stimulate fruiting and support continual flushes of mushrooms. In preparing the casing, the materials must be carefully chosen according to their chemical and physical properties. These properties are:
1. Water Retention:
The casing must have the capacity to both absorb and release substantial quantities of water. Not only does the casing sustain vegetative growth, but it also must supply sufficient moisture for successive generations of fruitbodies.
2. Structure:
The structure of the casing surface must be porous and open, and remain so despite repeated waterings. Within this prous surface are small moist cavities that protect developing primordia and allow metabolic gases to diffuse from the substrate into the air. If this surface microclimate becomes closed, gases build up and inhibit primordia formation. A closed surface also reduces the structual cavities in which primordia form. For these reasons, the retention of surface structure directly affects a casing's capability to form primordia and sustain fruitbody production.
3. Microflora:
Recent studies have demonstrated the importance of beneificial bacteria in the casing layer. High levels of bacteria such as Pseudomonas putida result in increased primordia formation, earlier cropping and higher yields. During the casing colonization period these benificial bacteria are stimulated by the metabolic gases that build up in the substrate and diffuse through the casing. In fact, dense casing layer and deep casing layers generally yield more mushrooms because they allow slow diffusion. It is desirable therefore to build up carbon dioxide and other gases prior to primordia formation. (For futher discussion on the influence of bacteria on primordia formation, see Appendix II.)
The selection of specific microbial groups by mycelial metabolites is an excellent example of symbiosis. These same bacteria give the casing a natural resistance to competitors. In this respect, a sterilized casing lacks beneficial microorganisms and has little resistance to contaminants.
4. Nutritive Value:
The casing is not designed to provide nutrients to developing mushrooms and should have a low nutritional value compared to the substrate. A nutriative casing supports a broader range of competitor molds. Wood fragments and other undecomposed plant matter are prime sites for mold growth and should be carefully screened out of a well formulated casing.
5. pH:
The pH of the casing must be within certain limits for strong mycelial growth. An overly acidic or alkaline casing mixture depressed mycelial growth and supports competitors. Agaricus brunnescens prefers a casing with pH values between 7.0-7.5. Even though the casing has a pH of 7.5 when first applied, it gradually falls to a pH of nearly 6.0 by the end of cropping due to acids secreted by the mushroom mycelium. Buffering the casing with limestone flour is an effective means to counter this gradual acidification. The optimum pH range varies according to the species. (See the growing parameters for each species in Chapter XI.)
6. Hygienic Quality:
The casing must be free of pests, pathogens and extraneous debris. Of particular importance, the casing must not harbor nematodes or insect larvae.

Materials

To better understand how a casing layer functions requires a basic understanding of the soil components and their specific structual and textural characteristics. When combined properly, the soil components create a casing layer that is both water retentive and porous.
1. Sand:
Characterized by large individual particles with large air spaces in between, sandy soils are well aerated. Their structure is considered "open". Sandy soils are heavy, hold little water and release it quickly.
2. Clay:
Having minute particles bound together in aggregations, clay soils have few air pockets and are structurally "closed". Water is more easily bound by clay soils.
3. Loam:
Loam is a loose soil composed of varying proportions of sand and clay, and is characterized by a high humus content.

Agaricus grower found that the best type of soil for mushroom growing was a clay/loam. The humus and sand in a clay/loam soil open up the clay which is typically dense and closed. The casing's structure is improved while the property of particle aggregation is retained. The humus/clay combination holds moisture well and forms a crumbly, well aerated casing.
There are two basic problems with using soil for casing - the increased contamination risk from fungi and nematodes, and the loss of structure after repeated waterings. Cultivators can reduce the risk of contamination by pasturization, a process whereby the moistened casing soil is thoroughly and evenly steamed for two hours at 160 degrees F. An alternative method is to bake the moist soil in an oven for two hours at 160 degrees F.

The development of casings based on peat moss has practically eliminated the use of soil in mushroom cultivation. Peat is highly decomposed plant matter and has a pH on the 3.5-4.5 range. Since this acidic condition precludes many contaminants from colonizing it as a substrate, peat is considered to be a fairly "clean" starting material. Peat based casings rarely require pasturization. But because peat is too acidic for most mushrooms, the addition of some form of calcium buffering agent like limestone is essential. "Liming" also causes the aggregation of the peat particles, giving peat a structure similar to a clay/loam soil. A corse fiberous peat is preferred because it holds its structure better than a fine peat. In essence, the properties of sphagnum peat conform to all the guidelines of a good casing layer.

Buffering agents are used to counter act the acidic effect of peat and toher casing materials. Calcium carbonate is most commonly used and comes in different forms, some more desirable than others.
1. Chalk:
Used extensively in Europe, chalk is soft in texture and holds water well. Chunks of chalk, ranging from one inch thick to dust, improve casing structure and continuously leach into the casing, giving long lasting buffering action.
2. Limestone Flour:
Limestone flour is calcitic limestone mined from rock quarries and ground to a fine powder. It is the buffering agent most widely used by Agaricus growers in the United States. Limestone flour is 97% calcium carbonate with less than 2% magnesium.
3. Limestone Grit:
Produced in a fashion similar to limestone flour, limestone grit is rated according to particle size after being screened through varying meshes. Limestone grit is an excellent structural additive but has low buffering abilities. A number 9 grit is recommended.
4. Dolomitic Limestone:
This limestone is rarely used by Agaricus growers due to its high magnesium content. Some researchers have reported depressed mycelial growth is casings high in magnesium.
5. Marl:
Dredged from dry lake bottum, marl is a soft lime similar to chalk but has the consistency of clay. It is a composite of clay and calcium carbonate with good water holding capacity.
6. Oyster Shell:
Comprised of calcium carbonate, ground oyster sell is similar to limestone grit in its buffering action and its structural contributions to the casing layer. But oyster chell should not be used as the sole buffering agent because of its low solubility in water.

Table: Comparing Casing Soil Components

Values vary according to source and quality of material used. Tests run by the authors.

Casing Formulas and Preparation
The following casing formulas are widely used in Agaricus culture. With pH adjustments they can be used with most mushroom species that require a casing. Measurement of material is by volume.

One half to one part coarse vermiculite can be added to improve the water retaining capacity of these casing mixtures and can be an aid if fruiting on thinly laid substrates. When used, it must be presoaked to saturation before being mixed with the other listed ingredients.
An important reference point for cultivators is the moisture saturation level of the casing. To determine this level, completely saturate a sample of the casing and allow it to drain. Cover and wait for one half hour. Now weigh out 100 grams of it and dry in an oven at 200 degrees F. for two or three hours or until dry. Reweigh the sample and the difference in weight is the percent moisture at saturation. This percentage can be used to compare moisture levels at any point in the sropping cycle. Optimum moisture content is normally 2-4% below saturation. Typically, peat based casings are balanced to a 70-75% moisture content.

Application

To prepare a casing, assemble and mix the components while in a dry or semi-dry state. Even distribution of the limestone buffer is important with a thoroughly homogeneous mixture being the goal. When these materials have been sufficiently mixed, add water slowly and evenly, bringing the moisture content up to 90% of its saturation level. There is an easy method for preparing a casing of proper moisture content. Remove 10-20% of the volume of the dry mix and then saturate the remaining 80-90%. Then add the remaining dry material. This method brings the moisture content to the near optimum. (Some growers prefer to let the casing sit for 24 hours and fully absorb water. Prior to its application, the casing is then thoroughly mixed again for even moisture distribution).
At this point apply the casing to the fully run substrate. Use a pre-measured container to consistently add the same volume to each cropping unit.
1. Depth:
The correct depth to apply the casing layer is directly related to the depth of the substrate. Greater amounts of substrate increase yield potential which in turn puts more stress on the casing layer. Prolific first and second flushes can remove a thin casing or damage its surface structure, thereby limiting future mushroom production. A thin casing layer also lacks the body and moisture holding capacity to support large flushes.
AS A GENERAL RULE, THE MORE MUSHROOMS EXPECTED PER SQUARE FOOT OF SURFACE AREA THE DEEPER THE CASING LAYER.
Agaricus growers use a minimum of one inch and a maximum of two inches of casing ontheir beds. Substrate depths of six to eight inches are cased 1 1/4 to 1 1/2 inches deep. Substrates deeper than 8 inches are cased 1 1/2 to 2 inches deep. Nevertheless, experiments in Holland using casing depths of 1 inch and 2 inches demonstrated thatt the deep casing layer supported higher levels of microorganisms and produced more mushrooms. (See Visscher, 1975). To gain the full benefits of a casing layer, an absolute minimum depth on bulk substrates is one inch. For fruiting on sterilized grain, the casing need not be as deep as for fruiting on bulk substrates. Shallow layers of grain are commonly cased 3/4 to 1 inch deep.
2. Evenness:
The casing layer should be applied as evenly as possible on a level substrate surface. An uneven casing depth is undesirable for two reasons: shallower regions can easily be overwatered, thereby stifling mycelial growth; and secondly, the mycelium breaks through the surface at different times, resulting in irregular pinhead formation. When applying the casing to large areas, "depth rings" can be as effective means to insure evenness. These rings are fabricated out of flat metal or six inch PVC pipe, cut to any depth. They are placed on the substrate and covered with the casing, which is then leveled using the rings as a guide. Once the casing is level and even, the rings are removed
Although the casing layer must be even, the surface of the casing layer should remain rough and porous, with small "mountains and valleys". The surface structure is a key to optimum pinhead formation and will be discussed in more detail in the next chapter.

Casing Colonization
Environmental conditions after the casing should be the same as during spawn running. Substrate temperature are maintained within the optimum range for mycelial growth; relative humidity is 90-100%; fresh air is kept to a minimum. (Fresh air should only be introduced to offset overheating). The build-up of carbon dioxide in the room is beneficial to mycelial growth and is controlled by an airtight room and tightly sealed fresh air damper. If the entrance of fresh air cannot be controlled, a sheet of plastic should be placed over the casing. This plastic sheet also prevents moisture loss from the casing.

Soon after casing, substrate temperatures surge upward due to the hampered diffusion of metabolicgases which would normally conduct heat away. This surge is an indication of mycelial vitality and is a positive sign if the room temperature can be controlled. This temperature rise can be anticipated by lowering either the temperature of the substrate prior to casing or lowering the air temperature of the room after casing.

Within three days of application, the mycelium should be growing into the casing layer. Once mycelial growth is firmly established, the casing is gradually watered up to its optimum moisture holding capacity. This is accomplished by a series of light waterings with a misting nozzle over a two to four day period (depending upon the depth of the casing). Deeper casings require more waterings. Optimum moisture capacity should be achieved at least two days before the mycelium reaches the surface.
IT IS EXTREMELY IMPORTANT THAT THE WATERINGS DO NOT DAMAGE THE SURFACE STRUCTURE OF THE CASING.
Heavy direct watering can "pan" the casing surface, closing all the pore spaces and effectively sealing it. The growing mycelium is then trapped within the casing layer and may not break through it at all. The ultimate example of panning is a soil turned to mud.

To repair a casing surface damaged by watering, the top 1/4 inch can be reopened by a technique called "scratching". The tool used is simply a 1 x 2 x 24 inch board with parallel rows of nails (6 penny) slightly offset relative to one another. With this "scratching stick", the casing is lightly ruffled prior to the mycelium breaking through to the surface. After the surface has been scratched, the casing should be given its final waterings prior to pinning.
A modified application of this technique is "deep scratching". When the mycelium is midway through the casing the entire layer is thoroughly ruffled down to the bulk substrate. The agitated and broken mycelium rapidly reestablishes it self and within three to four days it completely colonizes the casing. The result is an early, even and prolific pinhead formation. Before using this technique, the grower must be certain that the substrate and casing are free of competitor molds and nematodes.

Casing Moisture and Mycelial Apperence
Moisture within the casing layer has a direct effect on the diameter and degree of branching in growing mycelium. These characteristics are indicators of moisture content and can be used as a guide to proper watering.
1. Optimum Casing Moisture:
Mushroom mycelium thrives in a moist humid casing, sending out minute branching networks. These networks expand and grow, absorbing water, carbon dioxide and oxygen from the near saturated casing. This mycelial growth is characterized by many thick, white rizomorphic strands that branch into mycelia of smaller diameter and correspondingly smaller, finer capillaries. The overall aspect is lush and dense. When a section of casing is examined, it is held firmly together by the mycelial network but will seperate with little effort. The casing itself remains soft and pliable.
2. Overly Dry Casing:
In a dry casing, mycelium is characterized by a lack of rhizomorphs and an abundance of fine capillary type mycelia. This fine growth can totally permeate the casing layer, which then becomes hard, compact and unreceptive to water. It is common for puddles to form on a dry casing that has just been watered. Also, a dry casing rarely permits primordia formation because of its arid microclimate and is susceptible to "overlay". Mushrooms, if they occur, frequently form along the edges of the tray.

Overlay is a dense mycelial growth that covers the casing surface and shows little or no inclination to form pinheads. Overlay directly results from a dry casing, high levels of carbon dioxide and/or low humidity. (See Chapter IX on pinhead initiation).
Overly Wet Casing: In a saturated casing, the mycelium grows coarse and stringly, with very little branching and few capillaries. Mycelial growth is slow and sparse which leaves the casing largely uncolonized. Often the saturated casing leaches onto the substrate surface which then becomes waterlogged, inhibiting further growth and promoting contamination. Subsequent drying may eventually reactivate the mycelium, but a reduction in yield is to be expected.