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Volume 10, Part 2, May 1996

CULTIVATING MUSHROOMS: MAKING COMPOSTEDAND NON-COMPOSTED SUBSTRATES

RICHARD SCRASE'Mushroom makers', 38 Eastbourne Avenue, Bath BA1 6EN, UK

Tel./Fax: 01255338165/01225447962This article describes the range of materials usedfor mushroom cultivation and introduces the methods for their preparation. The production of sufficient mycelial biomass to support a usefulmushroom yield has traditionally taken place within large quantities of naturally occurring materials.These can be relatively unprocessed, ego freshly cutwood (logs), or transformed by procedures known as'composting'The compost (using the term loosely, as somesubstrates mentioned have not necessarily beenthrough a compost ing process of alteration bymicroorganisms) not only provides the nutritionalrequirements of the fungus but also the physicaland chemical conditions that give the fungus acompetitive edge overother microorganisms.

All saprotrophic basidiomycetes require carboncompounds which are usually obtained from plantpolymers. The enzymes that break down thesepolymers can together be effective against a widerange of cellular architectures, and so have anaction that is non-specific enough to allowthe grower to replace the natural diet with more readilyavailable alternatives, ego substituting straw forwood. Nitrogen compounds also have to be provided and sources containing the nitrate ion are usually acceptable. Essential elements and vitamins arenot usually growth-limiting except for calciumwhich is often added to regulate pH and to modifythe structure ofthe compost.

Almost any organic material can be composted,but it must be mixed to givea carbon/nitrogen ratioof between 80:1 and 10:1 (Charlesworth, 1995).Chemical analysis of wood shows a C/N ratio of500-1250:1 (Stamets & Chilton, 1983). Other natural substrates have nitrogen contents that aregrowth-limiting so nitrogen supplementationincreases mushroom yields in many compost systems. One example is Lepis ta nuda (Bull. :Fr.)Cooke, the Wood Blewit; this fungus utilisesleaf-litter in nature but has been grown semi-commercially on Agaricus bisporus (Lange)Imbachmushroom compost (Frische& VanLoone, 1989).

The fact that Lepista nuda can fruit on thisrange of composts demonstrates a fundamentalecological niche for the WoodBlewit that is widerthan its perceived ('realised') niche in the wild.This contrast between the perceived and fundamental niches may well be found in other species,in which case the example ofLepista nuda is a message of hope to the cultivator because it raises thepossibility that the composts listed below mayserve as suitable substrates for many differentspecies.

Those fungi which are currently under largescale cultivation are grown on a spectrum of composts; from non-sterile fresh lignocellulosic materialsuch as straw used for Pleurotus spp. (Zadrazil &Reiniger, 1988) and logs used for Lentinus edodes(Berk.)Sing. (Przybylowicz & Donoghue, 1990)through Volvariella volvacea (Bull. : Fr.)Singer(Chang & Hayes, 1978) on straw composted for afew days, to Agaricus bisporus on straw supplemented with nitrogen (manure) composted in twophases overthree weeks. The processes of compostpreparation will only be outlined below, fuller treatments are available in the references (Stamets &Chilton, 1993; Fermor et aI, 1985).Natural bulk substratesThe use of non-composted materials is commonamong commercial growers of Pleurotus,Volvariella, Flammulina and Stropharia. Thesematerials can be sterilised, pasteurised, or useduntreated in their natural state. For example I havegrownPleurotus ostreatus (Jacq. : Fr.)Kummer outdoors on a 'bale' ofunsterilised wheat straw, soakedin cold tap-water, with goodresults. The Pleurotusmycelium grewin all but the outer 5cm of the bale.When growing mushrooms indoors however, problems with fungal and other contaminants make itprudent to pasteurise or sterilise the compost.

Steineck (1984) l is ts many species he hasgrownon straw or wood-chips buried in damp andshady places in his garden and these materialscan form the basis of both garden and commercial

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scale cultivation.Cereal strawStraw, clean and unspoiled by any previous decomposition, is prepared by chopping it into 2-8 cmlengths and soaking inwater for about three days toachieve 75% moisture. Baled cereal straw is idealas it needs no further chopping. This is sufficientfor competitive species such as Pleurotus ostreatusbut to reduce losses from competition the straw canbe pasteurised by using hot water or sterilised bysteam. Straw is usually inoculated with grainspawn at the rate of 1-2%by weight (Stamets &Chilton, 1983).

Volume 10,Part 2, May 1996

growth . Because alder is high in sugar content ,without resins, and abundant, it has often beenselected (Stamets & Chilton, 1983).

In spring, fresh cut branches are chipped, mixedwith the inoculum (which itself can be grown onwood-chips) and made into a ridge bed on theground. Pieces 2-8em long give better results thansawdust in non-sterile conditions. The bed sizecommonly used is 60 em wide by 15 em deep.Incidentally, the beds of wood-chips and bark usedto cushion falling children in parks frequently support massive fruitings of mushrooms, egoHypholoma fasciculare (Huds.: Fr.)Kummer (sulphur tuft) and the false morel Gyromitra esculenta(Pers.)Fr. (personal observation).

The optimum moisture level is between 55% and70%by dry weight before heat treatment. This canbe established by oven-drying a sample; howevermany growersestimate the proper moisture contentby squeezing the mixture in one hand ; drops of freewater should be squeezed out.

Air exchange is enabled by using low densitypolyethylene bags orby inserting cotton wool, foam,

SawdustThe particulate nature of sawdust makes nutrientsmore readily available, consequently competingorganisms can rapidly invade these materials.Therefore the sawdust-based substrate is usuallysterilised or pasteurised prior to inoculation.Generally the sawdust type is matched to the fungus, but alternatives to the usual host species canbe substituted. Even softwoodsawdust can be substituted for hardwood if it is aged for about a yearto allow the reduction in the amount of resins andphenolics; more commonly growers use a blend.The time taken to degrade the substratum is related to particle sizeand with shiitake the optimum is2-3 mm (Nisikado et al, 1942). Smaller particlesreduce gas-flow and restrict growth. Sawdust isusually supplemented to increase the amount ofnitrogen and easily assimilated carbohydrates.While high levels of supplementation result in higher yields it also encourages the growth of competitors. The recipes below are a result of finding abalance between these two opposing considerations(Stamets & Chilton, 1983).

Deciduous leavesLeaves were used by Matruchot (1917) to growLepistanuda in caves left by limestone mining. Inthe process of replicating his experiments in a localdisused mine, I discovered that standard mushroom compost could be used as a substrate instead.The yields from mushroom compost are higher, butit is generally used in a less romantic setting. Leafmould has a longhistory of use in horticulture, traditionally by being stacked outdoors in piles 60-90em deep for two years to allow it to decompose(Larkcom, 1976). The resulting fibrous material isused as a mulch or as the water-holding componentof seed, cutting and potting composts.The need to replace peat (Pryce, 1991) may create a niche for leaf-mould. Preliminary experimentsusing leaves inoculated with Lepista nuda producedhorticulturally useful leaf-mould in six months(Scrase, 1993). Leaves are often available in theautumn from civic authorities and should not becollected from the wild.Wood-logsThe most notable commercial species grown onwood is Lentinus edodes or Shiitake. Logs (traditionally oak) 7-15 em in diameter and 90 em longare cut between autumn and spring when sap content is highest. Care is taken to leave the barkintact as this has consequences for subsequentfruiting. Holes 2-3 cm deep are drilled 20 em apartinto the log, then plugged with sawdust spawn ormycelium-colonised dowels (Przybylowicz &Donoghue, 1990).Wood-chipsFreshly cut chips of alder (Alnus glutinosus), maple(Acer spp.), and fir (Abies spp.) can support mycelial

standard formulasawdust 80%cereal bran 20%

lowcontamination formulasawdust 90%cereal bran 10%

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Volume 10, Part 2, May 1996etc. into the neck of the container holding the substrate.Other lignocellulosic wastesA tremendous range of materials has been used,especially for the growth ofPleurotus spp. ego sugarcane bagasse, pomace (apple waste), sugar beetpulp, coffee, tea, cotton wastes etc. Unless thewaste contains a toxic component, it is likely thatby adjusting moisture levels to around 70%, providing aeration, and optimising carbon/nitrogen ratios,any plant waste can be degraded by basidiomycetefungi. At the 1995 congress on the science and cultivation of edible fungi delegates reported on several examples of using basidiomycetes to transformwaste materials , egoAuricularia polytricha has beengrown on coffee wood in Mexico, and Pleurotusostreatusused to turn waste-paper into horticultural mulchingmaterial in the Seychelles.Agaricus bisporus compostFermor et al (1985) give a very full account of theprocesses involved in preparing compost forAgaricus bisporus while Genders (1969) gives amore colloquialdescription; the following howeverisdrawn primarily from Stamets & Chilton (1983). Ipersonally obtain mushroom compost from a commercial compost producer, complete except for theinoculation ofmushroom spawn.The commercial composting process is dividedinto two stages, commonly called Phase 1 andPhase 2, but the process, called 'Long composting'is more appropriate for the amateur grower, asthere is no need for the environmentally controlledrooms used for phase 2 processing. Both systemswillbe described here.

The raw materials for the compost are straw,manure, calcium sulphate (gypsum), water andnutritional supplements. Wheat straw is preferredbecause its resilience maintains an open well-ventilated compost structure. The straw is the primarysource of carbohydrates and in addition it provideslignin that is converted into the nitrogen rich ligninhumus-complex, which provides a source of proteinto the mushroom mycelium. Calcium sulphateimproves compost structure by aggregatingcolloidalparticles and it also provides a source ofmetaboliccalcium.

Horse-manure (usually ready mixed with bedding-straw) is the traditional source of nitrogen;while so called 'synthetic' composts are made withchicken manure, but other sources of organic and

inorganic nitrogen can be used. Elephant manurehas been found satisfactory (Genders, 1969). Theaim is to create an initial mixture with a nitrogencontent of 1.5-1.7%, a water content of 70% (+/-3%),and a carbon/nitrogen ratio of approximately 30:1.Many material s can be used as supplements tomodify the C/N ratio, egourea, cereal bran, or sugarbeet pulp. The following recipes are based on usingfresh materials but are given as dry weight ratios.Horse manure has proportions ofmanure and straw100 to gypsum 5 while synthetic compost is madeup in these proportions: wheat straw 52, chickenmanure 45, gypsum 3.5.

To make mushroom compost, fresh manure andstraw is mixed, soaked and then stacked. To prevent the development of anaerobic conditions within the stack the vertical cross-section is generallynot more that 2 x 2 metres. This allows th emicroorganisms present in the raw materials tobegin to degrade the stack. Their activity heats upthe stack which in turn makes conditions ideal forfurther decomposition by other microorganisms.The stack core is depleted of oxygen in 48 to 96hours and it is then rebuilt. This procedure isrepeated three times during the first phase to allowdecomposition to continue, typically during thethird, fifth and seventh day after initial mixing.The centre of the stack, should reach 65-80C. Atthe end of this first phase the compost should bedeep brown, be flecked with whitish colonies ofactinomycetes , smell ofammonia, have a pH of8.08.5,and release liquidwhen firmly squeezed.

In the second phase, the compost is filled intotrays , beds or bags and pasteurised (58-60C). Thiskills nematodes, fly eggs and larvae, mites andcompetitor fungal mycelium and spores. The compost is 'fermented' for a further 5-8 days. Duringthis time the internal temperature of the compostis held at about 50C by controlling ventilation.This is optimal for the actinomycetes that incorporate ammonia at this stage. The presence or ratherabsence of ammonia is usually detected by smell,but cresyl orange indicator or gas detection tubescan be used. After the ammonia concentration hasdropped to 10-20 [lgg-l, the temperature is allowedto drop to 25Cand the compost is inoculated.

In the process of 'long composting' the wholecomposting procedure takes place outdoors. Thestack is turned four times, on days 6, 10-12, 13-15and 15-17. The anaerobic centre zone is moved tothe outside on each occasion. The finished compost is pasteurised for four hours. I f this is not pos-

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sible, discard the outer shell of the stack, and usethe compost showing the white flecking indicatingstrong actinomycete activity.Inoculation and spawn runningMushroom spawn is mixed into the compost, usuallyas 1-2% by wet weight. Conditions then need tobe maintained for the strain in question. The primary requirement is to maintain the core composttemperature at the optimum temperature. This is25C for many commercial strains of Agaricus,Lepiota, Lepista and Coprinus. The compost usually contains sufficient oxygen and water to supportthe development of the fungal mycelium for theduration ofthe growthperiod. The success of themycelial growthor 'spawn-run' can be seen by thechange in colour of the compost. The whitishcolourofmyceliumis also evident. Similar changescan also be seen with other compost types, leafmould and s traw become paler as the fungalenzymes bleach the substrate. However, the apparanently successfulutilisation of the substrate is noguarantee of success with mushroom production.On several occasions I have had s traw balessmelling sweetly of fungal decomposition, all butthe outermost layer of straw charged with mycelium, ready to produce ..nothing!

In essence the mycelium is induced to gothrough a developmental switch from mycelialextension to the production of mushroom primordia. Conditions are then maintained that facilitatethe development of these primordia into mushrooms. The initial switch is usually achieved byradically altering the environmental conditions surrounding the mycelium; typically by reducing temperature and increasing oxygen supply. Subsquentdevelopment then requires higher humidity (Flegg& Wood, 1985). The understanding of the mechanisms of basidiomycete morphogenesis have yet tobe synthesised into a complete story; some of thecomponents can be found in Moore et al (1985), butthis area is undoubtedly a major problem in thescientific understanding ofmushroom growing. Wecannot yet definitely say why on some occasions

Volume 10,Part 2, May 1996

mushrooms fruit from a mycelium, and on otheroccasionsthey do not, it is another part oflife's richcomplexity. The methods for initiating and maintaining fruitingwillform the basis for a further article.

ReferencesCharlesworth, K. (1995)Life, the universe & (almost) every

thing. Muck and Magic.New Scientist1979:53.Chang, S.T. & Hayes, W.A. (eds) (1978) The Biology and

Cultivation of Edible Mushrooms. Academic Press, NewYork.

Fermor, TR, Randle, P.E. & Smith, J.F. (1985)Compost asa subst ra te and its preparation. In P.E. Flegg, D.M.Spencer, D.A.Wood (eds): The Biology and Technologyof the Cultivated Mushroom. John Wiley & Sons:Chichester & NewYork.pp. 81-110.

Flegg, P.E. & Wood, D.A. (1985) Growth and fruiting. InP.E. Flegg,D.M. Spencer, D.A. Wood (eds): The Biologyand Technology of the Cultivated Mushroom. John Wiley&Sons: Chichester&NewYork.pp. 141-178.

Frische, G. & vanLoone, P. (1989) Breeding experimentswith th e wood blewit (Lepista nuda). In MushroomScience XII (partl), Proceedings of the Twelf thInternational Congress on the Science and Cultivation ofEdible Fungi, Germany (1987).

Genders, R. (1969)Mushroom growing for everyone. Faber &Faber, London.

Larkcom, J. (1976)Vegetables from small gardens. Faber &Faber, London.

Matruchot, L. (1917) Variations experimentales dueTricholoma nudum. Revue Generale Botanique. 503.Moore, D., Casselton, L.A., Wood, D.A. & Frankland, J.C.

(eds) (1985) Developmental Biology of Higher Fungi.BMS Symposium 10,Cambridge University Press.

Nisikado, Y., Kimura, K. & Miyawaki, T. (1942) Themycelial growth in pure culture on the sawdust mediumprepared of var ious k inds of t ree. Ber. Ohara Inst.Landw. Forsch. 9:39-60.

Pryce, S. (1991)The PeatAlternatives Manual. Friends ofthe Earth, London.

Przybylowicz, P. & Donoghue, J. (1990) Shiitake GrowersHandbook, Kendall/Hunt, Iowa.

Scrase, R.J. (1993)The Cultivation ofBasidiomycetes. M Philthesis, Bath University.

Stamets, P. & Chilton, J.S. (1983). The MushroomCultivator, AgarikonPress, Olympia,Washington.Steineck, H. (1984)Mushrooms in the Garden. Mad River

Press, California.Zadrazil,F. & Reiniger, P. (eds) (1988)Treatment of lignocel

lulosics withwhiterot fungi. Elsevier, London.