Bioresource Technology 99 (2008) 287–296

Use of municipal solid waste compost (MSWC) as a growing mediumin the nursery production of tomato plants

F. Herrera a, J.E. Castillo a, A.F. Chica b, L. López Bellido a,¤

a Departamento de Ciencias y Recursos Agrícolas y Forestales, University of Córdoba, Campus de Rabanales, EdiWcio C-4 “Celestino Mutis”,Ctra. de Madrid, km 396, 14071 Córdoba, Spain

b Departamento de Ingeniería Química y Química Inorgánica, University of Córdoba, Spain

Received 28 July 2006; received in revised form 21 November 2006; accepted 20 December 2006Available online 6 March 2007

Abstract

Five media prepared from old peat (OP), white peat (WP) and municipal solid waste compost (MSWC) were used to determine opti-mum growing media for tomatoes (Lycopersicum esculentum Mill. cv “Atletico”). The mixtures of substrates used were: OP (65%) + WP(30%) + perlite (5%), OP (65%) + MSWC (30%) + perlite (5%), WP (65%) + OP (30%) + perlite (5%), WP (65%) + MSWC (30%) + perlite(5%), MSWC (65%) + WP (30%) + perlite (5%). Various seedling indices were measured in order to assess the quality of the nursery-pro-duced plant. Nursery-produced tomato seedlings grown in WP (65%) + MSWC (30%) displayed quality indices similar to those recordedfor conventional mixtures of old and white peat sphagnum, due to a correct balance between the compost nutrient supply and the poro-sity and aeration provided by white peat.© 2007 Elsevier Ltd. All rights reserved.

Keywords: MSW compost; Substrates; Old peat; White peat; Seedling indices

1. Introduction

Compared with direct sowing, transplants are a morereliable method of ensuring the proper establishment of arange of commercial horticultural crops of considerableeconomic value. The production of container-grown vege-tables is a highly-competitive business; fast, uniform seed-ling emergence and rapid growth are essential for eYcientproduction. Use of good crop substrates is therefore critical(Sterrett, 2001).

For a long time, peat was a virtually-irreplaceable com-ponent of crop substrates, old peat and white peat generallybeing mixed to achieve the desired porosity and consis-tency. However, peat is a non-renewable resource, anddiminishing availability is prompting price increases. Mas-sive use of peat as a substrate has led vegetable-growers to

* Corresponding author. Tel.: +34 957 218495; fax: +34 957 218440.E-mail address: cr1lobel@uco.es (L. López Bellido).

0960-8524/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2006.12.042

consider its replacement in the medium to long term (Ravivet al. 1998; Granberry et al., 2001; Sterrett, 2001).

Recent research has sought to identify alternatives totraditional peat, focussing on reusable, recyclable materialsnot derived from non-renewable sources such as peat bogs(Hadar et al., 1985; Raviv et al., 1986; Verdock, 1988). Anumber of studies have shown that organic residues such asurban solid wastes, sewage sludge, pruning waste, spentmushroom and even green wastes, after proper composting,can be used with very good results as growth media insteadof peat (Siminis and Manios, 1990; Pryce, 1991; García-Gómez et al., 2002; Benito et al., 2005).

Over 500 kg of municipal solid waste (MSW) per inhabi-tant and year are generated in the European Union (Euro-stat Yearbook, 2004). Worldwide residue generation hasincreased considerably over the last 30 years, entailing notonly the loss of materials and energy, but also negativeenvironmental impacts. Composting could turn large vol-umes of MSW into material to be used as fertilizer, organicsoil additive and crop substrate.

288 F. Herrera et al. / Bioresource Technology 99 (2008) 287–296

Field trials applying municipal solid waste compost(MSWC) as an organic soil additive suggest that it can beused in agricultural production, improving soil physicalproperties and increasing both water retention and the sup-ply of essential nutrients (McConnell et al., 1993; Rosenet al., 1993, Raviv, 1998).

Numerous studies have addressed the use of compost innursery plant production, and have analyzed its chemical,physical and biological properties (Sanderson, 1980; Shiral-ipour et al., 1992; Inbar et al., 1993; Fitzpatrick andMcConnell, 1998; Fitzpatrick, 2001; Sterrett, 2001). It hasbeen found that mixtures of compost with perlite (20–50%MSWC) may be used as substrates without the need foradditional mineral fertilizer (Bugbee, 1996; Chong andCline, 1996; Kostov et al., 1996; Pinamonti et al., 1997;Castillo et al., 2004). There are, however, certain limitationson the use of some composts: increase in salt content to lev-els which might aVect the growth of sensitive crops; heavymetal toxicity; low overall porosity and a marked variationin physical/chemical properties (Raviv, 1998; Spiers andFietje, 2000; Vavrina, 1995).

Although a wide range of wastes have been investigatedas potential peat substitutes, little information is availableregarding the use of municipal solid waste compost as apeat alternative for nursery production of horticulturalcrops. According to Shiralipour et al. (1992), Fitzpatrickand McConnell (1998) and Fitzpatrick et al. (1998), moststudies have focussed on ornamental potted plants, woodyshrubs and trees. While there is no one standard growingmedium recommended for all container crops under allgrowing conditions, and as a result of the variable qualityand composition of the MWC, each particular compost hasto Wnd the best amounts for growth of particular plants.

A previous study (Castillo et al., 2004) showed that thegrowth and development of nursery-produced tomato seed-lings using a peat + MSWC mixture was similar to thatobtained with the standard peat mixture. The present studysought to evaluate the eVect of varying the proportion ofMSWC mixed with conventional peat substrates, as agrowth medium in the nursery production of tomatoplants.

2. Methods

Three experiments were conducted with tomato nurser-ies (Lycopersicum esculentum Mill. cv “Atletico”) over athree-year period. All experiments used polystyrene trayswith 150 compartments, which were Wlled with the follow-ing substrate mixtures (% v/v): OP + WP (old peat65% + white peat 30% + perlite 5%); OP + MSWC (old peat65% + municipal solid waste compost 30% + perlite 5%);WP + OP (white peat 65% + old peat 30% + perlite 5%);WP + MSWC (white peat 65% + municipal solid wastecompost 30% + perlite 5%) and MSWC + WP (municipalsolid waste compost 65% + white peat 30% + perlite 5%).

White peats are formed in high-lying areas, are base-poor, display a low degree of decomposition (H1–H3 on the

scale of Von Post, 1922), and are thus lighter in colour; oldpeats are formed in low-lying areas, are base-rich, display ahigher degree of decomposition (H3–H5 and H6–H8) andare therefore darker in colour. The latter are treated priorto processing, by natural freezing, to improve physicalproperties.

The compost used was made from the organic fractionof selectively-collected urban waste (MSW). Followingelectromagnetic separation, manual sorting and use of an80 mm trommel screen to remove as many bulking agentsas possible, organic material was arranged in piles of 3.5 mwide£ 2 m high£ 50 m long, which were regularly turnedand watered over a 140-day period to ensure appropriatecomposting conditions (turned windrow system). Thismaterial was then passed through a densimetric table and a15 mm trommel screen to remove the largest particles.Compost maturity and stability were also determined. Theinitial C/N ratio of the selected organic fraction rangedbetween 14.1 and 15.3; by the end of the 140-day compo-sting period, values had fallen to between 7.1 and 8.2. Meanrespirometric values by the end of the composting periodwere below 1 (maximal speciWc oxygen consumption rate,expressed as mg O2/g SV h). These parameters indicated asatisfactory degree of maturity and stability of the compostused.

The chemical characteristics of the peats used were simi-lar in the three study years, unlike the MSW compost,which displayed year-on-year diVerences (Table 1).

Nursery sowing dates were: 15 January 2001, 15 January2002 and 12 January 2003. Trays were sown manually; oneseed was placed in each compartment and covered with ver-miculite. After sowing, trays were placed in a controlled-temperature greenhouse, programmed at a temperatureranging from 17 °C to 28 °C. Nursery trays were wateredmanually every day, using a sprinkler nozzle connected to ahose, in order to maintain the substrate at Weld capacity.

Over the 40-day growth-period in the nursery, no fertil-izer was applied; seedling nutritional requirements werethus met entirely by the substrates.

Emergence was noted on alternate days. Seedling emer-gences (deWned as a seedling with cotyledons visible abovethe medium surface) were counted 25 days after sowing.Rates of emergence were calculated using a modiWed Tim-son’s emergence velocity index: �G/t, where G is the num-ber of seeds emerged at 2-day intervals, and t is the totaltime of emergence (Khan and Ungar, 1998).

At the end of the nursery seedling growth period, the fol-lowing were recorded: seedling height, measured from theroot ball; stem diameter, measured below the cotyledonnode; stem height/diameter (H/D) ratio; height of the Wrstinternode; number of leaves per seedling, excluding cotyle-dons; leaf area per seedling; total dry matter per seedlingand its weight distribution among diVerent parts of theseedling; speciWc leaf area (SLA); and leaf area ratio(LAR). The SLA index, i.e. the ratio of leaf area to dry leafweight, reXects the relative proportion of assimilator tissuesto conductor and mechanical tissues. This value (cm2 g¡1)

F. Herrera et al. / Bioresource Technology 99 (2008) 287–296 289

serves to assess transplant stress resistance, which increasesas SLA decreases. The LAR index is deWned as the ratio ofleaf area to seedling dry matter, and represents the relation-ship between photosynthetic material and respiratorymaterial in the plant. It is also used to evaluate seedlingresistance at transplant.

The pH and electrical conductivity (EC) of all substratemixtures and treatments were measured at the start of theexperiment (Table 3).

Methods used for the analysis of chemical and nutrientcharacteristics were as follows: pH was determined on wetmaterial using a CRISON micropH 2002 ph-m (Chapmanand Pratt, 1961), and salinity was measured from the elec-trical conductivity of a saturated paste extract (Bower andWilcox, 1965) using a CRISON MicroCM2200 conductim-eter; nitrogen content was measured on dry matter usingthe Kjeldahl method (Black et al., 1979); organic matterusing the “chemical oxygen demand” method (Ciavattaet al., 1990); phosphorus by colorimetric spectrophotome-try (AOAC, 1975) using a BECKMAN DU 640 spectro-photometer; and calcium, potassium and metal content(HCl digestion) was determined by Xame photometry(Standard Operating Procedure Manual, 1995) with a Per-kin–Elmer A Analyst 300 atomic absorption spectro-photometer.

Seedling leaf area was determined by passing all thegreen leaves from the three plants harvested at each repli-cation through a LiCOR LI-3000 portable leaf areameter.

Annual data for each parameter over the whole 3-yearperiod were subjected to analysis of variance (ANOVA),using a year-combined randomized complete block designfollowing McIntosh (1983). Treatment means were com-pared using Fisher’s protected least signiWcant diVerence(LSD) test at P 6 0.05. LSDs for diVerent main eVect andinteraction comparison were calculated using the appropri-ate standard error term following Gómez and Gómez(1984). Various correlations were also calculated. The Sta-

tistix v 7.0 (Analytical Software, 2000) package was usedfor this purpose.

3. Results and discussion

3.1. Growth-media properties

The main chemical characteristics of substrates are listedin Table 1. Values for pH in the saturation extract rangedfrom 6.0 to 8.8, peat pH being the lowest. The characteris-tics of the MSW composts used displayed certain year-on-year diVerences (Table 1). Compost pH values were in allcases over 8.0.

MSW composts displayed high electrical conductivity(EC). The highest initial pH and EC values were recorded inthe last year of the study, and the lowest in the Wrst (Table 1).

Organic matter content was over 90% in peats, andbelow 40% in the MSW composts used (Table 1); by con-trast, total N in composts more than doubled that of peats,though never exceeding 3% (Table 1). MSWC phosphorus(P) content also diVered signiWcantly between study years(Table 1).

The C/N ratio is widely used as an indicator of the matu-rity and stability of organic matter. The low valuesrecorded here for the C/N ratio in MSWC in all study yearssuggest that composts were stable and mature. Davidsonet al. (1994) report that composts with a C/N ratio of lessthan 20 are ideal for nursery plant production. Ratiosabove 30 may be toxic, causing plant death (Zucconi et al.,1981). Wilson et al. (2001) found that an increased propor-tion of compost in crop substrates prompted a decline inthe C/N ratio compared to peats, although this will dependon the proportion of each ingredient in the mixture.

MSWC heavy-metal content (Table 1) was in all casesbelow the maximum levels permitted by Spanish regula-tions (Table 2). However, values mostly exceeded the maxi-mum limits established for composts for use with organicproducts (Ecolabel 2001/688/CE).

Table 1Chemical characteristics of the substrates

For MSWC, values in each index followed by the same letter are not statistically diVerent at P < 0.05 according to LSD.

Index Old Peat (OP) White Peat (WP) Municipal Solid Waste Compost (MSWC)

2001 2002 2003

Organic matter (%) 90 93 33a 36a 37apH 6.5 6.0 8.1a 8.4a 8.8aElectrical conductivity (dSm¡1) 1.5 1.4 11.4c 13.2b 19.8aTotal nitrogen (%) 1.3 1.1 2.7ab 2.6b 2.9aCalcium (%) 10 – 39a 40a 41aPhosphorous (mg kg¡1) 880 – 616c 1672b 1012aPotasium (mg kg¡1) 166 – 448a 415a 432aC/N ratio – – 7.1b 8.2a 7.2bCd (mg kg¡1) – – 2 3 10Cu (mg kg¡1) – – 373 252 280Cr (mg kg¡1) – – 84 57 30Ni (mg kg¡1) – – 64 57 50Pb (mg kg¡1) – – 144 165 170Zn (mg kg¡1) – – 603 415 420

290 F. Herrera et al. / Bioresource Technology 99 (2008) 287–296

3.2. Chemical characteristics (pH and EC) of substrate mixtures

The pH of OP + WP mixtures was slightly acid, whilethat of all other substrate mixtures was basic in all studyyears; the MSWC + WP mixture displayed the highest pHvalues, which diVered signiWcantly from those of all othersubstrates in 2003, and from those of all substrates exceptWP + MSWC in 2001 and 2002 (Table 3). Substrate pH val-ues in MSWC-containing mixtures fell as the proportion ofpeat increased (Table 3).

Although there is no single, ideal growth medium fornursery-produced horticultural crops (Poole et al., 1981;Raviv et al., 1986; Bugbee, 1996), most greenhouse-grownspecies display better growth at slight acid pH values (5.2–7.0); peat mixtures approach these values.

By the end of the experiment, in each study year, pH val-ues for the various substrate mixtures tended to converge;in 2001 and 2003, no signiWcant diVerences were recorded(Table 4). In 2002, by contrast, MSWC mixtures had ahigher Wnal pH value (Table 4). The highest Wnal pH wasfound for MSWC + WP and WP + MSWC (Table 4). Onaverage, Wnal pH values for the various substrate mixturesin 2001 were similar to, or even lower than, initial values.However, in 2002 and 2003 Wnal pH values were higherthan initial values for all substrates. These variations maybe due to year-on-year diVerences in maturity of the MSWcomposts used.

Table 2Metals content in MSW compost and proposed limits for diVerent regula-tions

a Ordinance on Fertilizers and Related Products, 28 May 1998 (Order28/V/1998 on fertilizers and related). Spanish regulations.

b European Commission (2001/688/EC). Establishing ecological criteriafor the award of the Communityeco-label to soil improvers and growingmedia (notiWed under document number C(2001)2597).

Metals content(mg kg¡1)

Spanish Lawa Ecolabelb

Cd 10 1.5Cu 450 75Cr 400 140Ni 120 50Pb 300 140Zn 1100 300

Table 3pH and EC values in the diVerent mixtures of substrates at the beginningof the period of nursery growth

a OP: old peat; WP: white peat; MSWC: municipal solid waste compost.b Within substrate and parameter (pH or EC) means followed by the

same letter are not signiWcantly diVerent at P < 0.05 according to LSD.

Substratea pH Electrical conductivity (dSm¡1)

2001 2002 2003 2001 2002 2003

OP + WP 6.7cb 6.1c 6.5d 2.0d 1.6d 1.3dOP + MSWC 7.4b 7.8b 7.7c 9.7b 8.9b 12.9cWP + OP 6.7c 6.2c 6.9d 2.3c 1.6d 1.0dWP + MSWC 7.8a 7.9ab 7.9b 10.2b 8.0c 13.4bMSWC + WP 7.9a 8.0a 8.2a 15.8a 12.6a 22.0a

Initial substrate EC values were always higher than Wnalvalues, perhaps due to leaching of salts by watering. Asdemonstrated by Noguera et al. (1997), the excess of solublesalts is easily and eVectively leached from the materialunder customary irrigation regimes when used for orna-mental plants of for tomatoes, Xowers, etc., in garden green-house. Like pH levels, the highest initial substrate ECvalues were recorded for mixtures containing MSWC(Table 3). The MSWC + WP mixture displayed signiWcantlyhigher EC values than all other substrates in all three studyyears (Table 3). The only substrates displaying the opti-mum EC values (<3.5 dSm¡1) recommended by Lemaireet al. (1985) and Wright (1986) for the growth and develop-ment of healthy, vigorous seedlings were those containingonly peat, which diVered signiWcantly from the rest. Finalsubstrate EC values presented similar behavior. Valueswere highest for the MSWC + WP mixture (Table 4).

3.3. Seedling emergence

Increased pH and EC in substrates containing MSWCprompted a decline in the seedling establishment rate at 25days (Table 5); the highest seedling emergence rate wasrecorded in 2001 (Table 5), when compost pH and EC val-ues were lowest (Table 1); signiWcant diVerences were foundwith respect to 2003, when compost displayed the highestpH and EC values (Table 1). Ribeiro and e Santos (1997)report that substrates with high EC values reduce waterretention, negatively aVecting the imbibing process.

The same behavior was recorded for all substrate mix-tures studied, i.e. higher seedling emergence was found forsubstrates with lower pH and EC values. Thus, peat-onlysubstrates (OP + WP and WP + OP) displayed signiWcantlyhigher seedling emergence rates than those containingMSWC (OP + MSWC and MSWC + WP) (Table 5). Burgeret al. (1997) and Pinamonti et al. (1997) report that low pHand EC values improve conditions for seedling emergencein nurseries.

3.4. Rate of emergence

Tomato seedling emergence rates (number of plantsemerging per day) were signiWcantly greater in 2001 than in

Table 4pH and EC values in the diVerent mixtures of substrates at the end of theperiod of tomato nursery growth

a OP: old peat; WP: white peat; MSWC: municipal solid waste compost.b Within substrate and parameter (pH or EC) means followed by the

same letter are not signiWcantly diVerent at P < 0.05 according to LSD.

Substratesa pH Electrical conductivity (dSm¡1)

2001 2002 2003 2001 2002 2003

OP + WP 6.9ab 7.5c 7.7a 0.5c 0.3c 1.1dOP + MSWC 7.1a 8.2b 8.0a 0.7bc 0.7b 2.5cWP + OP 7.0a 7.5c 7.6a 0.5c 0.3c 1.0dWP + MSWC 7.0a 8.6a 8.2a 1.0ab 0.9a 3.8bMSWC + WP 7.1a 8.6a 8.2a 1.3a 0.9a 5.4a

F. Herrera et al. / Bioresource Technology 99 (2008) 287–296 291

the other two years, between which there was no signiWcantdiVerence (Table 5). Better compost properties in the Wrstyear (lower pH and EC) were undoubtedly the main causeof improved seedling establishment.

The fastest emergence rates were recorded for peat-onlysubstrates (OP + WP and WP + OP), whose pH and ECwere signiWcantly lower than those of MSWC mixtures(Table 5).

3.5. Accumulated emergence

Emergence commenced on the ninth day after sowingin all substrates, with a signiWcantly higher proportionin peat-only mixtures (WP + OP and OP + WP) (Fig. 1).Delayed emergence was evident over the 25-day emergenceperiod in all MSWC-containing mixtures (Fig. 1).

From day 21 onwards, no signiWcant diVerence wasnoted between OP + MSWC and standard peat-only mix-

Fig. 1. Accumulated emergence of tomato seedling as aVected by mixtureof substrates (OP: old peat; WP: white peat; MSWC: municipal solidwaste compost).

0

20

40

60

80

100

9 11 13 15 17 19 21 23 25Days after sowing

Em

erge

nce

(%)

OP+WPOP+MSWCWP+OPMSWC+WPWP+MSWC

5

tures, emergence rates of over 90% being recorded for both(Fig. 1). Similar behavior was observed for the otherMSWC-containing mixtures towards the end of the 25-dayevaluation period. This may be due to the leaching of saltscaused by successive waterings, which gradually improvedseedling emergence conditions.

The worst overall emergence rates were recorded forMSWC + WP: only 3% emergence by day 9 and a maxi-mum Wnal emergence of 85% at 21 days, after which emer-gence ceased (Fig. 1).

3.6. Seedling growth

Tomato seedling heights were signiWcantly greater in2003 than in the other two years, between which there wasno signiWcant diVerence (Table 5). SigniWcant year-on-yeardiVerences in stem diameter, stem height/diameter ratio andheight of Wrst internode (Table 5), were attributable tochanges in the characteristics of the MSWC used.

The highest values for all growth parameters wererecorded for WP + MSWC, which in most cases diVered sig-niWcantly from the other substrates (Table 5). Strikingly,the mixture of 30% MSWC and 65% white peat performedbetter than standard peat mixtures, old peat mixtures andWP + MSWC mixtures with higher proportions of MSWC.Independently of the diVerential eVect of peat physicalcharacteristics and of compost proportions on tomatoseedling growth, diVerences in substrate salinity clearly hada decisive eVect on growth.

3.7. Seedling dry matter

SigniWcant year-on-year diVerences were also found fordry matter by seedling organ and total dry matter (Table 6),higher values being recorded in 2002 and 2003 (Table 6). Aswas the case for growth parameters, higher values wereobserved for both dry-matter parameters in seedlingsgrown on the WP + MSWC substrate; the lowest values

Table 5InXuence of diVerent mixtures of substrates on tomato seedling growth

¤,¤¤,¤¤¤ SigniWcant at the 0.05, 0.01 and 0.001 probability level, respectively.a OP: old peat; WP: white peat; MSWC: municipal solid waste compost.b Within treatment (year or substrate) and parameter means followed by the same letter are not signiWcantly diVerent at P < 0.05 according to LSD.

Treatmenta Rate of emergence Emergence (%) Height (mm) Stem diameter (mm) Height/diameter ratio Height 1st node (mm)

Year:2001 3.7ab 96a 156b 2.5c 61a 42.1a2002 3.0b 94ab 170b 3.7a 46b 31.8b2003 3.2b 92b 211a 3.2b 65a 40.6a

Substrate:OP + WP 3.8a 98a 182b 3.2b 58bc 39.4abOP + MSWC 3.0b 94b 175b 3.3ab 54bc 34.1cWP + OP 4.0a 98a 139c 2.7c 52c 35.8bcWP + MSWC 2.7c 95ab 222a 3.5a 65a 41.5aMSWC + WP 2.9b 85c 178b 3.1b 59b 39.9ab

Mean Interaction: 3.3 94 179 3.1 58 38.1Year £ Substrate *** *** *** *** * **

292 F. Herrera et al. / Bioresource Technology 99 (2008) 287–296

were recorded for the peat-only substrate (WP + OP). Theonly exception was root dry matter, which did not diVer sig-niWcantly for any of the substrates studied (Table 6).

Seedlings grown in the WP + MSWC mixtures displayedbetter quality and suitability for transplanting. Seedlingresistance to transplant stress is directly related to dry mat-ter content, which improves seedling establishment in thesoil or growth substrate (Pimpini and Gianquinto, 1991).

3.8. Seedling leaf indices

SigniWcant year-on-year diVerences were recorded for alltomato seedling leaf indices (Table 6). In nursery-producedtomatoes, seedling leaf area appears to be directly related tofruit production per plant, although Leskovar et al. (1991)report that beyond a certain maximum leaf area per seed-ling there is no appreciable increase in fruit production, andthat increasing this index may therefore not be appropriate.

Number of leaves/seedling and leaf area were signiW-cantly higher in compost-containing mixtures WP +MSWC and OP + MSWC and in the standard peat mixtureOP + WP (Table 6). By contrast, no signiWcant diVerenceswere found for speciWc leaf area (SLA) or leaf area ratio(LAR) between the substrates studied (Table 6). This sug-gests similar seedling behavior at transplant for all sub-strates, since SLA and LAR ratios are used to assessseedling resistance to transplant stress.

3.9. Relationships between parameters

Analysis of correlations between indices and parametersdeWning tomato seedling quality showed the diVerentialeVect of the substrates tested (Table 7). In overall terms, thelowest correlations – mostly non-signiWcant or negative –were recorded for the MSWC + WP substrate (Table 7).This serves, once again, to conWrm the poor quality of thismixture compared to the other substrates tested, due most

likely to the adverse eVects on seedling growth of high ECand pH values resulting from the larger proportion ofMSWC (65%) in the mixture with WP.

A highly-signiWcant direct correlation was observedbetween seedling leaf area and both height and total drymatter, for all substrates except MSWC + WP (Table 7).With the same exception, the correlations between leaf areaand both stem dry matter and leaf dry matter were also sig-niWcant (Table 7). Correlation coeYcients for leaf area androot dry matter were lower, attaining statistical signiWcanceonly in OP + MSWC, WP + OP and MSWC + WP (Table7). A strong direct correlation between leaf area and seed-ling dry matter is indicative of an intense and eYcient pho-tosynthesis, giving rise to greater carbohydrate productionand the accumulation of dry matter; this Wnding conWrmsthe good quality of the substrate for seedling production.There was no signiWcant correlation between leaf area andstem diameter for any of the substrates studied; substratesdisplayed disparate behavior, with low correlation coeY-cients, both positive and negative (Table 7).

Plant height also correlated signiWcantly with both totaldry matter and leaf and stem dry matter in all substrates,again with the exception of MSWC + WP, where correla-tions were negative, and only the height-leaf dry matter cor-relation was signiWcant (Table 7). The stem height/diameterratio is, as suggested earlier, an important indicator of qual-ity in nursery-grown seedlings. However, the correlationbetween the two parameters was signiWcant only in threesubstrates; in one case negative (OP + MSWC) and in theother two positive (WP + OP and MSWC + WP) (Table 7).A negative height-stem diameter correlation would give riseto unbalanced development and etiolation of seedlings, andthus to poor transplant quality.

The remaining correlations, particularly those involvingstem diameter and total dry matter, organ dry matter andleaves/seedling, were not statistically signiWcant in conven-tional peat substrates (Table 7). By contrast, in MSWC-

Table 6InXuence of diVerent substrate mixtures on dry matter production and on leaf indices of tomato seedlings

¤,¤¤,¤¤¤ SigniWcant at the 0.05, 0.01 and 0.001 probability level, respectively.a OP: old peat; WP: white peat; MSWC: municipal solid waste compost.b Within treatment (year or substrate) and parameter means followed by the same letter are not signiWcantly diVerent at P < 0.05 according to LSD.

Treatmenta Dry matter production Leaf indices

Leaves dry matter(mg/plant)

Stem dry matter(mg/plant)

Root dry matter(mg/plant)

Total dry matter(mg/plant)

Leaves/seedling Leaf area(cm2)

SLA(cm2/g)

LAR(cm2/g)

Year:2001 140bb 76b 62c 279b 3.8b 26b 185ab 92ab2002 239a 145a 121a 504a 4.8a 38a 178b 80b2003 240a 152a 92b 484a 5.1a 52a 213a 102a

Substrate:OP + WP 192b 130b 97a 419ab 4.6ab 41ab 204a 91aOP + MSWC 249ab 129b 101a 479ab 4.8a 45ab 181a 95aWP + OP 111c 71c 57a 240c 4.1b 23c 207a 94aWP + MSWC 279a 175a 111a 564a 5.0a 53a 185a 90aMSWC + WP 200b 116ab 93a 409b 4.3b 35bc 182a 87a

Mean Interaction: 206 124 27 422 4.6 39 192 92Year £ Substrate *** *** *** *** *** ** *** **

F. Herrera et al. / Bioresource Technology 99 (2008) 287–296 293

containing mixtures – and especially in WP + MSWC – cor-relations were negative and attained statistical signiWcance

(Table 7). This suggests that the presence of reduced pro-portions of MSW compost in substrates negatively aVects

Table 7Correlation coeYcients (r) between growth indices in tomato seedlings as aVected by mixture of substrates

¤,¤¤,¤¤¤ SigniWcant at the 0.05, 0.01 and 0.001 probability level, respectively.a OP: old peat; WP: white peat; MSWC: municipal solid waste compost.

Parameters Substratea

OP + WP OP + MSWC WP + OP WP + MSWC MSWC + WP

Leaf area-stem diameter 0.16 ¡0.63 0.43 ¡0.57 ¡0.39Leaf area-height 0.89¤¤ 0.95¤¤¤ 0.93¤¤¤ 0.83¤¤ 0.11Leaf area-stem dry matter 0.94¤¤¤ 0.72¤ 0.78¤ 0.81¤¤ ¡0.49Leaf area-root dry matter 0.53 0.69¤ 0.74¤ 0.63 0.68¤

Leaf area-leaves dry matter 0.93¤¤¤ 0.89¤¤ 0.89¤¤ 0.91¤¤¤ 0.40Leaf area-total dry matter 0.97¤¤ 0.82¤¤ 0.90¤¤¤ 0.87¤¤ 0.25Height-stem diameter 0.52 ¡0.71¤ 0.69¤ ¡0.37 0.81¤¤

Height-leaves dry matter 0.76¤ 0.82¤¤ 0.92¤¤¤ 0.74¤ ¡0.70¤

Height-stem dry matter 0.89¤¤ 0.69¤ 0.85¤¤ 0.80¤¤ ¡0.33Height-root dry matter 0.47 0.67¤ 0.72¤ 0.62 ¡0.10Height-total dry matter 0.85¤¤ 0.77¤ 0.94¤¤¤ 0.77¤ ¡0.64Stem diameter-leaves dry matter 0.17 ¡0.59 0.46 ¡0.82¤¤ ¡0.85¤¤

Stem diameter-stem dry matter 0.13 ¡0.63 0.64 ¡0.79¤ 0.03Stem diameter-root dry matter ¡0.30 ¡0.65 0.41 ¡0.77¤ ¡0.42Stem diameter-total dry matter 0.03 ¡0.63 0.54 ¡0.84¤¤ ¡0.69¤

Stem diameter-leaves/seedling 0.32 ¡0.63 0.03 ¡0.65 ¡0.58Leaves dry matter-root dry matter 0.24 0.87¤¤ 0.75¤ 0.80¤¤ 0.60Stem dry matter-root dry matter 0.64 0.95¤¤¤ 0.55 0.89¤¤ 0.15

Fig. 2. Relationship between height and leaf area in tomato seedling as aVected by mixture of substrates (OP: old peat; WP: white peat; MSWC: municipalsolid waste compost).

0

50

100

150

200

250

300

350

Hei

ght

y = -0.01x²+ 2.8x + 113; r²= 0.75* 0

50

100

150

200

250

300

0 20 40 60 80 100 120Leaf area

Hei

ght

y = -0.04x²+ 5.3x + 49; r²= 0.88**

y = -0.006x²+ 2.9x + 75; r²= 0.87**

0 20 40 60 80 100 120 140Leaf area

OP+WP

WP+MSWCWP+OP

y = -0.02x²+ 3.2x + 76; r²= 0.96***

OP+MSWC

294 F. Herrera et al. / Bioresource Technology 99 (2008) 287–296

seedling stem thickening but not height. This may be due tothe excessive salt content of the substrate.

Regression lines for the indices leaf area-height and leafarea-total dry matter displayed a strong coeYcient of deter-mination, and accounted for diVerences in the behavior ofthe various substrates tested (Figs. 2 and 3). Greater seed-ling leaf development with WP + MWSC led to seedlingsbeing taller than those grown on other substrates; leaf areatended to stabilize from 120 cm2 per seedling and height ataround 30 cm, the point at which the maximum curve wasattained and the slope was zero (Fig. 2). Similar behaviorwas observed for OP + WP and OP + MSWC, although leafarea averaged less than 100 cm2 and seedling height around25 cm (Fig. 2). The leaf area-height relationship displayedan almost-linear response in WP + OP, due to extremelylow values for leaf area per plant, which gave rise toreduced seeding height (Fig. 2). The WP + OP substratewould therefore appear unsuitable for tomato seedlings inthe conditions used in this study, as would theMWSC + WP mixture, although for diVerent reasons:MWSC + WP because of its high pH and EC values, andWP + OP due to nutrient deWciency resulting from the smallproportion of old peat in the mixture (30%), since OP isknown to contain more nutrients than WP (65% in the mix-

ture tested), whose main attribute is porosity rather thannutrient supply.

The behavior of the leaf area-total dry matter relation-ship for diVerent substrates is shown in Fig. 3. Seedlingsgrown on WP + MSWC displayed greater leaf area andhigher total dry matter content than the rest (Fig. 3).WP + OP again proved unsuitable, producing poorly-devel-oped seedlings with low dry matter content. The leaf area-total dry matter relationship for standard peats (OP + WP)also displayed an almost-linear response, but with highervalues for both leaf area and dry matter; however, valuesremained lower than those recorded for WP + MSWC(Fig. 3). This may be largely attributed to greater nutrientcontent in the WP + MSWC mixture compared withOP + WP, which may require additional fertilizer applica-tion to ensure optimum seedling development.

4. Conclusions

MSW compost was found to be an ideal component ofmixed-peat substrates for tomato seedlings, provided that itaccounts for less than half the mixture (30% MSWC and65% peat). These proportions reduce the negative eVects ofhigh pH and EC on seedling growth, and provide a seedling

Fig. 3. Relationship between total dry matter and leaf area in tomato seedling as aVected by mixture of substrates (OP: old peat; WP: white peat; MSWC:municipal solid waste compost).

y = -0.02x²+ 9.7x + 68; r²= 0.94***

0

100

200

300

400

500

600

700

800

900

1000

Tot

al D

ry M

atte

r

00 20 40 60 80 100 120

Leaf area

Tot

al D

ry M

atte

r

0 20 40 60 80 100 120 140Leaf area

OP+WP

WP+MSWCWP+OP

OP+MSWC

100

200

300

400

500

600

700

800

900

y = -0.11x²+ 18.7x –84; r²= 0.72*

y = 0.03x²+ 5.5x + 92; r²= 0.81** y = -0.09x²+ 19x –104; r²= 0.96***

F. Herrera et al. / Bioresource Technology 99 (2008) 287–296 295

comparable to that obtained using standard peat-basedmixtures.

SpeciWcally, the mixture of white peat (65%) and MSWcompost (30%) provides an ideal substrate for nursery pro-duction of tomato seedlings, yielding quality indices similarto those provided by conventional old peat + white peatsphagnum mixtures. This is in all probability due to a cor-rect balance between nutrient supply from the MSW com-post and the physical characteristics of white peat,particularly substrate porosity and aeration.

Improved methods of selective waste collection andcompost processing will enable increasingly widespread useof this renewable organic compost, as an alternative tohigh-quality sphagnum peats, which – because they arenon-renewable – are both less available and more expensivefor vegetable-growers.

Acknowledgement

This work was funded by the Spain’s Plan Nacional I +D (Proyect 1FD-1037-CO2-01).

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