Partial substitution of peat in mushroom casing with fine particle coal tailings

Partial substitution of peat in mushroom

casing with fine particle coal tailings

R. Noble*, A. Dobrovin-Pennington

Warwick HRI, University of Warwick, Wellesbourne, Warwick CV35 9EF, UK

Accepted 16 September 2004

Abstract

Casing prepared from different peat sources and sugar beet lime (SBL) was substituted by 25% (v/

v) with different materials: composted bark fines, coconut fibre (coir), or fine particle, dewatered coal

tailings (CT). Casing substituted with 25% (v/v) bark or coir had similar water retention near

saturation (matric potential �0.7 kPa) to casings prepared from three different peat sources with

SBL: brown surface and deep-dug peats and black deep-dug peat. However, casing partially

substituted with bark or coir retained less water under applied suction (matric potential

�15 kPa), and resulted in a lower mushroom yield than peat + SBL casing. By contrast, casing

substituted with 25% (v/v) CT retained less water than non-amended peat + SBL near saturation, but

the effect of partial substitution on water retention under applied suction was either small or not

significant. Partial substitution with CT did not affect mushroom yield. These results indicate that

water retention near saturation and under applied suction, i.e. desorption curve characteristics, are

important in determining the suitability of a casing for mushroom cropping. Partial substitution of

casings based on black peat and brown surface peat with CT resulted in an increase in mushroom dry

matter content and the proportion of marketable mushrooms respectively. Mushroom cleanness was

not affected by any of the treatments. The heavy metal contents of the mushrooms were not affected

by the inclusion of CT in the casing. The partial substitution of peat and SBL in casing with CT will

have both environmental and economic benefits.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Mushroom; Agaricus bisporus; Casing; Coal tailings; Peat substitution; Matric potential

www.elsevier.com/locate/scihorti

Scientia Horticulturae 104 (2005) 351–367

* Corresponding author. Tel.: +44 24 765 75053; fax: +44 765 74500.

E-mail address: [email protected] (R. Noble).

0304-4238/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.scienta.2004.09.004

1. Introduction

Casing material or ‘soil’ (casing) is used in mushroom (Agaricus bisporus) culture to

cover a nutritional composted substrate colonised with mycelium, and has an essential

function in stimulating and promoting the development of sporophores (fruit bodies). The

key physical requirements of casing have not been precisely defined but it must have a high

water retention to supply water for the growth of mycelium and sporophores, sufficient

porosity for mycelial respiration, and be able to resist structural breakdown following

repeated waterings (Flegg and Wood, 1985; Visscher, 1988; Rainey et al., 1986). Noble

et al. (1999) showed that mushroom yield was correlated with the matric potential of peat-

based casing, determined using desorption curves. The optimum potential was �7.9 to

�9.4 kPa, probably due to the availability of casing water and air. Subsequent work has

shown that a higher potential of �3 kPa is optimum for mushroom yield from bark and coir

casings, which have lower water holding capacities than peat (Noble and Dobrovin-

Pennington, 2001). The casing must also have suitable chemical and microbiological

characteristics for sporophore initiation. The ability of mushrooms to accumulate

heavy metals (Stelte et al., 1982) means than a low heavy metal content in the casing is

required.

For several decades, casing in many countries has been based on mixtures of peat and

chalk or lime (Flegg and Edwards, 1953; Visscher, 1988). The peats used can range from

decomposed, amorphous, black peats to less decomposed, loose textured and well-

structured brown peats (Visscher, 1988; Noble et al., 1999). There has been environmental

pressure against extracting peat for horticultural use (Pryce, 1991). In many mushroom-

growing areas of the world, there are no available sources of peat, so that local, but inferior,

soils are used for casing (Vedie, 1995). This has led to considerable research into possible

peat alternatives for casing (Poppe, 2000). These materials include bark (Allen, 1976;

Rainey et al., 1986), spent mushroom compost (Sinden, 1987; Szmidt, 1994), coconut fibre

(‘coir’) (Labuschagne et al., 1995), and paper waste by-products (Yeo and Hayes, 1978;

Dergham et al., 1991). However, none of these materials has replaced peat in casings on the

market, although coir is now used as ingredient in some commercial peat-based blends

(Noble, 2000).

Originally, lump or ground chalk was used to neutralise peat for use in casing (Edwards,

1966). Visscher (1988) and Noble et al. (1999) have shown that sugar beet lime (SBL), a

precipitated form of CaCO3, produces better mushroom yields and quality than chalk,

possibly due to the smaller particle size and the effects it has on the texture and water

retention of the casing. This material must be ‘matured’ for at least 6 months before it is

suitable for use in casing (Visscher, 1988). Fine particle coal tailings (CT), a predominantly

rock particle by-product obtained from dewatered suspensions in coal washing plants, with

similar clay-like texture to SBL, have been found that may be used in casing (Noble and

Bareham, 2002). More than 300,000 t of CT are produced annually in a single coal mine in

England (UK Coal, Kellingley, North Yorks.) where the material also poses a significant

disposal problem. Similar CT by-products are also produced in the USA, Australia and

South Africa.

The aims of this work were: (1) to examine the effect of partial substitution of peat-

based casing on the mushroom yield and quality produced, including heavy metal content

R. Noble, A. Dobrovin-Pennington / Scientia Horticulturae 104 (2005) 351–367352

and (2) to determine whether the desorption curve characteristics of the casings related to

their mushroom cropping performance.

2. Materials and methods

2.1. Preparation of casings and mushroom culture

Mushroom culture was conducted in a controlled environment cropping room using

wooden trays (internal dimensions 0.9 m � 0.6 m � 0.2 m (deep)). Each tray was filled

with 50 kg substrate, i.e. a composted and pasteurised mixture of wetted straw, poultry

manure and gypsum (Noble et al., 1999), and inoculated (‘spawned’) with the mushroom

strain A15 (Sylvan Spawn Ltd., Peterborough, UK). Colonised (‘spawn-run’) substrate was

covered (‘cased’) with a 45 mm depth casing layer, which contained Sylvan casing spawn

(CI) at an inclusion rate of 0.5% (v/v). Casings were mixed before application in a

mechanical mixer for 100 s, during which water was added to achieve the desired moisture

content. The casing spawn was added during the final 15 s of mixing. Waterings to the trays

were adjusted to maintain a moisture content of 10 (�3)% (v/v) below the water retention

of the casing after drainage from saturation (see Section 2.3). This resulted in an average

matric potential of �3.3 to �7.9 kPa during cropping (Noble et al., 1999; Noble and

Dobrovin-Pennington, 2001). This was achieved by determining the moisture content of

4 � 100 g casing samples taken from each treatment at 3-day intervals, according to

Anonymous (1999a). The average moisture level during cropping of the black peat + SBL

casing was higher (6%, v/v below water retention after drainage from saturation) due to the

higher initial moisture content of the black peat. The compost temperature was maintained

at 25–26 8C to promote mycelial colonisation of the casing layer; the relative humidity and

CO2 concentration of the air were 95–98% and 0.35–0.45% (v/v). When mycelial growth

in the casing layer had been established after 6–7 days, fresh air was introduced to obtain an

air temperature of 17.5 8C, CO2 concentration of 0.10% (v/v) and relative humidity of 89%

to induce sporophore initiation.

Mushrooms at stage 2 (Hammond and Nichols, 1976) of diameter 30–40 mm, were

picked over a 23 day period (three ‘flushes’) with the first flush being picked c. 17 days after

the application of the casing. Percentage dry matter content of 10 mushrooms from the first

two flushes from each tray was determined according to Burton and Noble (1993).

Mushroom cleanness was assessed by scoring a 1-kg container of mushrooms from each

plot and flush on a 0 (cleanest)–5 (dirtiest) scale (Noble and Gaze, 1995).

2.2. Experimental treatments

Preliminary tests with partial substitution of peat + SBL casing with bark, coir (Noble

and Dobrovin-Pennington, 2001) or CT (Noble and Dobrovin-Pennington, 2004) showed

declining mushroom yields at inclusion rates in excess of 25% (v/v). These experiments

were therefore conducted at an inclusion rate for bark, coir or CT of 25% (v/v).

The ingredients and the proportions used in the 10 casing treatments are shown in

Tables 1 and 2. Three types of peat were used: a deep-dug Irish brown peat, a deep-dug

R. Noble, A. Dobrovin-Pennington / Scientia Horticulturae 104 (2005) 351–367 353

Scottish black peat, and an English brown peat dug from the surface of the bog. These peat

sources and a 1:1 (v/v) blend of the black peat and brown surface peat were used with 16%

(v/v) SBL (treatments 1, 3, 5 and 7). The latter blend is used commercially in the British

mushroom industry, and is used in these experiments as a standard control treatment. In

these four mixes, 25% (v/v) was also substituted with CT (treatments 2, 4, 6 and 8). The CT

were a by-product of coal washings, from which most (>85%, w/w) of the coal particles

had been extracted by flotation, and the residual CT dewatered with the polyacrylamide

flocculants Alcosorb AB1 and Magnafloc 1011 (Ciba Speciality Chemicals, Bradford,

West Yorks, UK), each added at 0.005% w/v, and filter presses. The CT consisted of clay-

like particles (82%, w/w silicates; 8%, w/w quartz; 10%, w/w coal) (Noble and Bareham,

2002). The black and brown peat blend was also substituted with 25% (v/v) composted pine

bark fines (bark) or coconut matting fibre waste (coir) (treatments 9 and 10). Water

additions to the mixes (Table 2) depended on the original moisture content of the

ingredients (Table 1) and the quantity required to achieve an initial moisture content of 7

(�2)% (v/v) below the water retention of the casing after drainage from saturation (see

Section 2.3). The moisture level of the casing materials prepared with black peat were

higher at application (1–2%, v/v below water retention after drainage from saturation) due

to higher initial moisture content of this peat source (Table 1).

The experiment was conducted as a series of six replicate crops with four trays of each

of the 10 casing treatments in each crop. The 40 trays in each crop were arranged in four


Table 1

Suppliers of casing ingredients and original moisture content and bulk density (n = 6)

Ingredient Moisture (%, w/w) Bulk density (g l�1) Supplier (all UK)

Brown deep-dug peat 88.2 631 Prunty Peat Ltd, Fivemiletown, Co. Tyrone

Black deep-dug peat 89.2 480 Humax Horticulture, Carlisle, Cumbria

Brown surface peat 63.8 334 Scotts UK Professional, Ipswich, Suffolk

Sugar beet lime 23.5 916 The Potter Group Ltd., Ely, Cambs

Coal tailings 32.7 1173 UK Coal plc, Doncaster, S. Yorks

Bark fines 61.6 303 Melcourt Industries Ltd., Tetbury, Glocs

Coir 15.6 169 Rawtex Ltd, Wakefield, W. Yorks

Table 2

Percentage by volume of ingredients (excluding added water), and the percentage water additions based on a 100%

volume of the other ingredients, in different casing treatments

Ingredient Casing treatment

1 2 3 4 5 6 7 8 9 10

Brown deep-dug peat 84 63 – – – – – – – –

Black deep-dug peat – – 84 63 – – 42 31.5 31.5 31.5

Brown surface peat – – – – 84 63 42 31.5 31.5 31.5

Coal tailings – 25 – 25 – 25 – 25 – –

Bark fines – – – – – – – – 25 –

Coir – – – – – – – – – 25

Sugar beet lime 16 12 16 12 16 12 16 12 12 12

Water added (%, v/v) 9.0 3.8 2.4 0.0 12.3 14.1 1.1 0.3 2.3 7.1

layers, with the four trays of the same treatment arranged in a stack and treated as a single

plot. The positions of the 10 treatments in the cropping room were randomised for each

crop.

2.3. Properties of casing materials

The particle size distribution of two replicate samples of SBL and CT were determined

using a hydrometer method (Anonymous, 1990a). Samples of SBL and CT were also

observed using a scanning electron microscope using preparatory methods described in

Noble et al. (1999). The water retention and bulk density after drainage from saturation,

and compacted dry bulk density of casing ingredients and mixed casing materials used for

each replicate crop were determined according to methods in Anonymous (1990b) and

Noble et al. (1999). This involved draining a saturated sample, held in a 1-l test cylinder

with eight 8-mm diameter holes in the base, for 30 min. Pore space was determined

according to methods in Anonymous (1999b) and Anonymous (2000a). Electrical

conductivity (EC) and pH of samples were determined according to methods in

Anonymous (2000b, 2000c). Water retention at decreasing matric potential (desorption)

was determined using a method modified from Montagne et al. (1992) and Noble et al.

(1999), based on a water tension table constructed from Buchner funnels. Casing

ingredients or mixes at near saturation were filled into metal rings (depth 50 mm, internal

diameter 69 mm) which had a muslin base attached. Each ring was placed on to a 10 mm

depth layer of silica fine powder (grade HPF-3, Hepworth Minerals and Chemicals Ltd,

Sandbach, Cheshire, UK), which covered a filter paper disc, Whatman No. 1, in the base of

a 250-ml Buchner funnel. Pores in the silica and filter paper retained water against hanging

water column potentials of <�25 kPa. Water-filled flexible plastic tubing forming a ‘U’

reservoir was attached to the funnel. A horizontal drain spout was positioned 20 mm from

the free end of the tube. By lowering the free end of the tube and the height of the water

level at the drain spout, matric potentials from 0 to �20 kPa were created in the silica layer

and the sample in the funnel. The volume of water flowing from the drain spout following

each lowering of the tube was measured to determine the water released from the sample at

each potential. Before each determination, the level of the water reservoir was first adjusted

to the top of the sample for 12 h and then to the top of the silica layer for 12 h. The quantity

of water in the sample at this matric potential near 0 kPa was taken as saturation. The

matric potential was decreased in about 1 kPa decrements; equilibrium at each decrement

was obtained in 40–60 min. At the end of each determination, the volume and fresh and dry

weights of the sample in the ring were measured. The relationship between the matric

potential and volumetric moisture content (desorption curve) was determined from the

initial and final moisture contents of the sample and the water released at each potential.

2.4. Analyses for heavy metals

Four replicate batches (1 kg) of CT and SBL, and of first and second flush mushrooms

grown on casings 1 and 7 (peat + SBL), and casings 2 and 8 (peat + SBL + CT) were

analysed for heavy metals: As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Zn. All the samples were freeze-

dried before analysis. Extractions were made from the dried samples using a hot mixture of


hydrochloric and nitric acids using a closed system microwave oven technique (L.J.

Church Laboratory Services Ltd, Castleford, West Yorks, UK), followed by quantification

of metal concentrations using inductively coupled plasma-optical emission spectrometry

(ICP-OES).

To test for the presence of polymer flocculants, 1 kg mushroom samples grown on

casing treatment 1, 2, 7 and 8 were each washed with 100 ml de-ionised water. The

washings were then analysed by Ciba Speciality Chemicals for the presence of polymers by

using an acid/bleach precipitation, and then measuring the turbidity of the solution with a

spectrophotometer. A turbidity/polymer concentration calibration curve was prepared with

solutions containing Magnafloc 1011 at concentrations from 20 to 180 ppm.

3. Results

3.1. Properties of casing ingredients

For SBL, 90% (w/w) of particles passed a 20 mm sieve, and 24% (w/w) passed a 2 mm

sieve. The values for CT were 58 and 35% (w/w), respectively. These values were

confirmed by scanning electron micrographs (Fig. 1) that showed that CT was composed of

irregularly shaped crystals of varying size, whereas the particles of SBL were more

uniform.

The volumetric water contents of the samples after drainage from saturation in the test

cylinder were similar to those in the Buchner funnel apparatus at a matric potential

�0.7 kPa. This is similar to the matric potential of �0.98 kPa for the ‘container capacity’

of potting composts described by Bunt (1976). At a matric potential of �0.7 kPa,

volumetric water retention and bulk density of the brown surface peat was slightly greater

than that of the deep-dug peats (Fig. 2; Table 3), and bark or coir had slightly lower water

retentions than peat, but bark had a lower bulk density. The water retention at �0.7 kPa of

SBL and CT were lower, and the bulk densities higher than those of the other ingredients

(Fig. 3, Table 3). Under applied suction (matric potential �15 kPa), the water retention of

the black peat was higher than that of brown peats, which retained more water than the

other materials (Fig. 2, Table 3). Of the other materials, SBL retained the most water and

bark and coir retained the least water under applied suction (Fig. 3, Table 3).

The pH of the brown surface peat was lower than those of the deep-dug peats, which had

similar pH values to the bark and coir. CT was less alkaline than SBL (Table 3). The brown

peats had lower ECs than the black peat and bark. SBL, CT and coir had higher ECs than

the other ingredients (Table 3).

3.2. Properties of casing materials

After drainage from saturation, at a matric potential of �0.7 kPa, casing prepared from

brown surface peat had a greater water retention than casing prepared from deep-dug peats

(Table 4). However, under applied suction (matric potential �15 kPa), casing prepared

with black peat had a higher volumetric water content than casing prepared with the brown

peats (Fig. 4, Table 4). Substitution of 25% (v/v) of the casing mix with CT reduced the


pore space and the water retention near saturation (�0.7 kPa). However, under applied

suction (matric potential �15 kPa), water retention was only reduced by CT with the black

peat (Figs. 4 and 5, Table 4). The volumetric water content of the casing during cropping

was also reduced by the inclusion of CT, with a corresponding reduction in matric potential


Fig. 1. Scanning electron micrographs of (a) sugar beet lime and (b) coal tailings.

(i.e. more negative value). Substitution with 25% (v/v) bark or coir did not affect the pore

space or water retention at �0.7 kPa (Table 4) but the casing retained less water under

applied suction (matric potential �15 kPa). However, inclusion of CT had a smaller effect

on water retention of the casing under applied suction than substitution with bark or coir

(Fig. 5). Inclusion of bark or coir also reduced the volumetric water content of the casing

during cropping, but the matric potential was not affected (Table 4).

Substitution of 25% (v/v) of the casing mix with CT increased the bulk density at

application and at �0.7 kPa, whereas a similar substitution with bark or coir reduced

bulk density (Table 4). Substitution with 25% (v/v) CT, bark or coir did not significantly

affect the pH of the casing, but EC was increased by the inclusion of CT or coir (Table 4).

Using previous results to relate the effects of EC on the osmotic potential of peat-based

casings (Noble et al., 1999), the range in EC values in Table 4 would correspond to a

range in osmotic potentials of about �90 to �140 kPa. This range would have a

negligible effect on mushroom mycelial growth (Magan et al., 1995) or crop yield

(Kalberer, 1990).


Fig. 2. Desorption curves for peat types (n = 6).

Table 3

Physical and chemical properties of casing ingredients (n = 6)

Property Peat Sugar

beet lime

Coal

tailings

Bark fines Coir LSD

(P = 0.05)Brown

deep-dug

Black

deep-dug

Brown

surface

Water retention at

�0.7 kPa (%, v/v) 79.3 79.0 82.4 55.1 52.0 75.8 75.9 1.48

�15 kPa (%, v/v) 55.8 60.8 56.2 44.8 40.7 32.9 38.4 1.39

Pore space (%, v/v) 89.7 84.7 85.6 74.1 62.0 83.9 89.4 1.22

Bulk density

dry (g l�1) 161 241 225 760 837 271 171 32.6

at �0.7 kPa (g l�1) 848 850 900 1349 1257 752 858 25.3

pH 5.30 4.94 4.21 8.47 7.90 5.67 5.93 0.195

Electrical conductivity

(mS cm�1)

44 89 55 847 938 101 1116 57.6

3.3. Mushroom yield and dry matter content

Total mushroom yield was significantly higher from the brown surface peat casing than

from casing prepared with the deep-dug peats (Fig. 6). However, the additional mushrooms

were unmarketable due to premature opening and misshapes. The inclusion of 25% (v/v)

CT to any of the peat + SBL mixes did not affect mushroom yield, but the proportion of

waste from the brown surface peat casing was reduced. The proportion of quality grades

was not affected by the other treatments. Substitution of 25% (v/v) of the black and brown

peat blend + SBL casing mix with bark or coir reduced mushroom yield (Fig. 6).

Mushrooms grown on the brown surface peat casing had a lower dry matter content than

mushrooms grown on the deep-dug peat casings (Fig. 7). Mushroom dry matter content

was increased by the inclusion of 25% (v/v) CT to casings containing black peat, or by

inclusion of bark or coir to the black/brown peat + SBL casing (Fig. 7). The effects of

casing treatments on mushroom yield and dry matter content were consistent in the six

replicate crops. The treatments did not significantly affect mushroom cleanness (mean

cleanness scores were 2.6–3.2).


Fig. 4. Desorption curves for peat + SBL casings, and casings substituted by 25% (v/v) with coal tailings (+CT)

(n = 6).

Fig. 3. Desorption curves for non-peat casing ingredients (n = 6).

R.

No

ble,

A.

Do

bro

vin-P

enn

ing

ton

/Scien

tiaH

orticu

ltura

e1

04

(20

05

)3

51

–3

67

36

0

Table 4

Physical and chemical properties of casing materials at application and during cropping

Peat type LSD

Brown deep-dug Black deep-dug Brown surface Black deep + brown surface P = 0.05

None (1)a CT (2)a None (3)a CT (4)a None (5)a CT (6)a None (7)a CT (8)a Bark (9)a Coir (10)a

At application

Water retention at

�0.7 kPa (%, v/v) 74.0 71.3 73.6 71.3 78.9 75.4 77.6 74.0 75.9 76.0 2.08

�15 kPa (%, v/v) 56.7 56.4 61.5 59.5 57.8 57.3 60.9 59.5 54.5 56.2 1.35

Pore space (%, v/v) 87.2 81.0 83.0 77.8 83.8 78.4 83.2 78.0 83.5 84.9 1.89

Bulk density

as applied (g l�1) 662 760 751 820 685 843 703 835 638 628 64.4

at �0.7 kPa (g l�1) 898 979 940 1027 990 1023 964 988 931 926 21.0

pH 7.50 7.67 7.54 7.62 7.53 7.58 7.62 7.72 7.47 7.49 0.192

Electrical conductivity (mS cm�1) 322 579 436 640 330 584 366 631 455 698 89.3

Mean during cropping

Moisture (%, v/v) 65.5 62.0 67.7 64.7 68.1 62.2 69.2 63.2 63.9 64.4 2.06

Matric potential (�kPa) 3.60 4.31 6.38 6.72 4.13 7.24 3.86 7.89 3.27 3.81 0.69

CT, coal tailings; n = 6.a Substitute (casing treatment number).


Fig. 6. Mushroom yield per tonne of pasteurised compost from peat + SBL casings, and the same casings

substituted by 25% (v/v) with coal tailings (+CT), bark or coir. Casing treatments are numbered on the bars. LSDs

are for P = 0.05 (n = 6).

Fig. 5. Desorption curve for black peat, brown surface peat + SBL casing (control), and this casing substituted by

25% (v/v) with coal tailings (+CT), bark or coir (n = 6).

Fig. 7. Dry matter content of mushrooms grown on peat + SBL casings, and the same casings substituted by 25%

(v/v) with coal tailings (+CT), bark or coir. Casing treatments are numbered on the bars. LSD is for P = 0.05

(n = 6).

3.4. Analyses for heavy metals

The level of Cd of CT was lower than that of SBL, but the levels of As, Pb, Ni, and Se

were higher in CT than in SBL (Table 5). There were no significant differences in the levels

of Hg, Cr, or Zn between these materials.

There were no detectable levels of As or Hg in any of the mushroom samples (Table 6).

There were no significant differences in heavy metal contents between first and second

flush mushrooms, or between mushrooms grown with or without the inclusion of CT in the

casing. Cd could only be detected in mushrooms grown on the brown deep-dug peat

casings, where levels of Pb were also higher. No polymer flocculants were detected in any

of the mushroom washings.


Table 6

Concentrations (mg kg�1 dry weight) of heavy metals in mushrooms grown on peat + SBL casings, without

(treatments 1 and 7) and with (treatments 2 and 8) 25% v/v fine particle coal tailings (+CT)

Element Casing treatment LSD (P = 0.05)

1 2 (+CT) 7 8 (+CT)

As n.d.a n.d. n.d. n.d. –

Cd 1.0 0.7 n.d. n.d. 0.5

Cr 5.1 5.2 5.8 6.0 1.2

Cu 32 33 46 32 15

Pb 2.0 2.1 1.1 1.8 0.8

Hg n.d. n.d. n.d. n.d. –

Ni 2.5 1.9 2.2 2.6 0.7

Se 1.3 1.7 1.8 1.4 0.6

Zn 54 52 59 51 12

Values are the mean of first and second flush mushrooms and four replicate analyses.a n.d.: not detected, i.e. values were below the detection limits in Table 5.

Table 5

Concentrations (mg kg�1 dry weight) of heavy metals in fine particle coal tailings (CT) and sugar beet lime (SBL),

and detection and regulatory limits

Element Detection limit CT SBL Regulatory limita

As 1.0 7.7b 4.5 25

Cd 0.2 0.3 0.8b 1.25

Cr 0.5 13.9 15.5 75

Cu 0.5 49b 26 75

Pb 0.5 42b 3 78

Hg 0.02 0.04 0.05 0.75

Ni 1.0 29b 6 30

Se 0.05 1.5b 0.2 –

Zn 0.5 47 43 300

CT and SBL values are the mean of four replicate analyses.a Limits in the Netherlands for organic materials applied to agricultural land (Walker, 2001).b Value significantly higher (P = 0.05) using a Students t-test in comparisons between CTand SBL for the same

element.

4. Discussion

Measurements in this work have shown that although bark and coir have only slightly

lower water retentions to peat at near saturation, under applied suctions of 5 kPa or greater

they hold much less water. This may explain the poorer mushroom yields obtained with

bark and coir when compared with peat-based casing in previous experiments (Allen,

1976; Labuschagne et al., 1995). Tests with the same sources of bark and coir used in the

present work, with SBL added at 16%, showed that mushroom yields from these casings

were 72 and 59% of the yield from the black/brown peat + SBL casing (Noble, unpublished

data). The difference in desorption properties between bark or coir and peat may also

explain the yield reduction resulting from 25% (v/v) substitution of peat-based casing in

these experiments. However, partial substitution with CT did not affect mushroom yield,

and the effect on water retention under applied suction was either small (with black peat) or

not significant.

Previous work examining the effect of peat-based casing matric potentials of �1 to

�16 kPa on mushroom yield has shown an optimum potential of �7.9 to �9.4 kPa (Noble

et al., 1999). However, results here show no differences in marketable mushroom yield

between casing treatments with mean matric potentials during cropping of �3.6 to

�7.9 kPa; the range in potentials examined was not sufficient to test the previous

relationship. Previous work has shown that casing treatments that produced a higher yield

tended to produce mushrooms with a lower dry matter content and vice versa (Noble et al.,

1999) and that mushrooms with a higher dry matter content were firmer and of better

quality (Noble et al., 1997). In the present experiments, substitution with 25% (v/v) CT in

black peat-based casings resulted in an increase in mushroom dry matter content without a

corresponding yield loss. This may have been due to the slightly lower matric potential and

EC (or osmotic potential) during cropping of the casings substituted with CT (Noble et al.,

1999).

Experiments with different rates of SBL, a material with similar particle size

distribution to CT, between 10 and 25% (v/v) have shown little effect on mushroom yield

(Noble et al., 1999; Staunton, 1985) although higher rates of lime have produced fewer but

larger mushrooms (Visscher, 1988). However, the cost of SBL, which is currently about

Euro 30 t�1 due to processing costs and other agricultural applications, is significantly

higher than CT, which has no processing requirements or alternative uses. Flegg and

Edwards (1953) used clay as a casing ingredient, and Overstijns et al. (1988) found that a

chalk clay produced a similar mushroom yield to SBL when added at 30% (v/v). However,

clay soils are likely to be more variable, as well as having a greater risk of containing

mushroom pathogens, than CT. Bentonite clays have also been used to produce a dense

casing, but are more expensive than CT due to alternative industrial uses (Buchanan and

Barnes, 2003). Proximity to the supply of CT, which has a high bulk density (Table 1), will

have a significant effect on the transportation costs of the material. Similar fine particle,

dewatered tailings are also produced from the washing plants of sand quarries and other

mineral extractive industries that use flocculating polymers (Noble and Bareham, 2002).

Mushrooms from the first two flushes of the 25% (v/v) CT casing treatments felt firmer

than mushrooms from non-amended peat + SBL casings, although no firmness

measurements (Noble et al., 1997) were made. Comparisons of mushrooms grown on


other casing materials have shown only small effects on firmness (Rama, 1998), although

mushrooms grown on shallower casing have been shown to be much firmer than those

grown on deeper casing (Noble et al., 1997). The results for mushroom cleanness confirm

earlier results (Noble and Gaze, 1995), which showed no significant effect of peat type,

within the range used here, on cleanness. Casing moisture has been shown to have a greater

effect on mushroom cleanness than different peat sources or substitution with bark or coir

(Noble and Dobrovin-Pennington, 2001).

The results show that there was no significant effect of adding 25% (w/w) CT to

casing on the contents of heavy metals of the mushrooms produced. Levels of Hg and Zn

in CT and SBL, and Cd in CT were lower, and values for As in CT and SBL, and Cu and

Pb in CT were higher than those reported for mushroom compost by Amsing (1983) and

Fleckenstein and Grabbe (1983). However, all of the values for CT and SBL were below

the regulatory limits for heavy metals in organic materials applied to agricultural land in

the Netherlands (Table 5). These limits are lower than those in the equivalent regulations

of the United States, Canada, the European Union, Germany and Sweden (Walker,

2001). The values for Pb in mushrooms were higher and the levels of As, Cu and Hg were

lower than corresponding values reported for cultivated mushrooms by Amsing (1983),

Fleckenstein and Grabbe (1983) and Stelte et al. (1982, 1983a,b). Levels for Cd and Pb

were below the EU regulatory limits for cultivated mushrooms (0.2 mg Cd kg�1 wet

weight and 0.3 mg Pb kg�1 wet weight, equivalent to 2.6 mg Cd kg�1 dry weight and

4 mg Pb kg�1 dry weight, assuming a mushroom dry matter content of 7.5%, w/w)

(Anonymous, 2001).

Although low concentrations of polyacrylamide flocculants are used in the dewatering

of CT, they could not be detected in the washings from mushrooms grown in casing

containing CT. This confirms measurements by Castle (1993) who found no uptake of

acrylamide monomer by mushrooms, following the inclusion of a polyacrylamide gel to

casing.

5. Conclusion

These results indicate that the desorption characteristics of a casing are important in

determining its suitability for mushroom casing. Water retention of a casing, not only near

saturation (matric potential �0.7 kPa), but also under applied suction (matric potential

�15 kPa) must be considered. Substitution of peat + SBL casing with bark or coir at 25%

(v/v) had little effect on water retention near saturation but resulted in significantly lower

water retention under applied suction and in a lower mushroom yield. Conversely,

substitution of peat + SBL casing with 25% CT reduced water retention near saturation, but

had no effect on water retention under applied suction (matric potential �15 kPa) or on

yield. Mushroom dry matter content was increased by the inclusion of bark, coir or CT in

black and brown peat blend + SBL casing. The proportion of marketable mushrooms was

increased by the inclusion of CT in brown surface peat + SBL casing. The partial

substitution of peat and SBL in casing with CT, and possibly other sources of fine

particle tailings from mining and quarrying, will have both environmental and economic

benefits.


Acknowledgements

The contributions of L. Bareham in identifying suitable coal tailings, Ms. C. Evered for

scanning electron microscopy, and N. Willoughby and M. Price of the HRI Mushroom Unit

for technical assistance, are acknowledged. This work was supported financially by the

Department for Environment, Food and Rural Affairs (DEFRA) and UK Coal.

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Partial substitution of peat in mushroom casing with fine particle coal tailings

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