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
Casing | Optional | Required | Not Required | |
---|---|---|---|---|
1. |
Ag. Brunnescens |
X |
||
2. |
Ag. Bitorquis |
X |
||
3. |
C. comatus |
X |
||
4. |
Fl. velutipes |
X | ||
5. |
Lentinus Edodes |
X | ||
6. |
Lepista nuda |
X |
||
7. |
Pl. ostreatus |
X | ||
8. |
Pl. ostreatus (Florida variety) |
X | ||
9. |
Pan. cyanescens |
X |
||
10. |
Pan subbalteatus |
X |
||
11. |
Ps. cubensis |
X |
||
12. |
Ps. cyanescens |
X | ||
13. |
Ps. mexicana |
X |
||
14. |
Ps. tampanensis |
X |
||
15. |
S. rugoso-annulata |
X |
||
16. |
V. volvacea |
X |
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
Material |
Absorption Potential millileters water/gram | % Water at Saturation |
---|---|---|
Vermiculite |
5.0 |
84% |
Peat |
2.5 |
79% |
Potting Soil |
0.7 |
76% |
Loam |
0.3 |
25% |
Chalk |
0.6 |
37% |
Limestone Grit |
0.2 |
15% |
Sand |
0.2 |
18% |
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.
FORMULA 1 | FORMULA 2 |
Coarse peat: 4 parts |
Coarse peat: 2 parts Chalk or Marl: 1 part Water: Approximately 1-1 1/4 parts |
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.