- Email: [email protected]
Evaluation of fly ash as a component of potting substrates
SCIENTUI
HOmlCULluM Scientia Horticulturae67 (1996)87-99
Evaluation of fly ash as a component of potting substrates N.W. Menzies ap*, R.L. Aitken b aDepartment of Agriculture, The University of Queensland, St Lucia, 4072, Australia b Resource Management Institute, Department of Primary Industries, Meiers Rd, Indooroopilly, 4068, Australia Accepted 15 May 1996
Abstract A series of laboratory and glasshouse experiments were undertaken to assess the potentid for incorporation of fly ash in soilless potting substrates. The physical and chemical properties of a commercially available bark based substrate, the University of California (UC) 1:l peat:sand mix and a range of test substrates containing fly ash were characterised. In test mixtures, fly ash was substituted for a portion of either the peat or sand component of the UC mix, at rates of 10,20, 30 and 50% of the mix volume. Incorporation of fly ash greatly increased the plant available water capacity (10-1500 Pa) of the substrate. However, high pH, increased substrate strength and reduced air-filled porosity were considered adverse effects, particularly at ash rates > 20%. The growth of tomato (Lycopersicon esculentum), petunia (Petunia x hybrida grandifloru) and Boston fern (Nephrolepis exaltutu) in the substrates was assessed. Two watering regimes, capillary watering and irregular hosing, were used to identify effects of available water capacity on plant growth, but no effect was identified. Test mixtures containing fly ash as 20% of the substrate volume produced growth equal to that in the UC mix, with substrates containing 10% ash producing significantly greater growth of tomato and petunia. At rates of incorporation > 20% reduced plant growth was attributed to both adverse physical and chemical characteristics of the substrate. As fly ash is available at low cost and can be successfully substituted for a considerable portion of the expensive peat component, its use at low application rates in potting substrates may be desirable from an economic viewpoint. Keywords: Potting
* Corresponding
substrates; Fly ash; Available water capacity
author. Fax: 3365
0304-4238/%/$15.00 Copyright PN SO304-4238(96)00935-
1177;e-mail: [email protected].
0 1996 Published
1
by Elsevier Science B.V. All rights reserved.
88
N.W. Menzies. R.L. Aitken/Scientia Horticulturae 67 (1996) 87-99
1. Introduction The potted plant industry has undergone dramatic expansion during the last 2 decades with potted plants now being readily available in retail stores, as well as nurseries. This has increased the need for a regular supply of a uniform growth medium that has the ability to support vigorous plant growth. Owing to the difficulties of getting soil of a consistent quality, and the possibility of adverse physical problems when soils are potted, most nurseries use soilless potting substrates. As the rooting volume of a potted plant is very restricted, one important requirement of soilless potting substrates is that they possess considerable water holding capacity. Soilless potting substrates are frequently based on one component with a high plant available water capacity (frequently peat), which is combined with relatively inert materials such as sand, bark or polystyrene foam which provide aeration and act as ballast. The University of California (UC) mixes are of this type, consisting of peat and sand in a variety of ratios. The I:1 peat:sand mix is commonly used throughout the world for potted plant production (de Boodt and Verdonck, 1972). The supply of peat is finite, thus its cost has risen in response to increased demand as the nursery trade has grown, and, in addition, the quality of peat has fallen as the high quality peat supplies are exhausted. These problems are accentuated for Australian nurserymen as Australian peats are unsuitable for incorporation in potting substrates (Handreck and Black, 1994) and additional cost must be incurred importing peat. Thus, for Australian nurseries using UC type mixes, peat represents a considerable portion of production cost. Ply ash, produced by the combustion of pulverised coal in power generating stations, is the residue which is small enough to be entrained in the flue gas and carried from the site of combustion. This material is removed from the flue gas to lessen atmospheric pollution. Australian fly ash production has been estimated to be in excess of 10 million t per annum (A&ken, 1994). Approximately 10% of the fly ash produced is utilized in premixed concrete while the remainder represents a considerable disposal problem. The agronomic use of fly ash has been widely studied (Page et al., 1979; Adriano et al., 1980) and its use as a source of nutrients for plants considered (Martens et al., 1970; Elseewi et al., 1978; Hill and Lamp, 1980). Ply ash has also been shown to considerably increase the plant available water capacity of sands (Roberts, 1966; Campbell et al., 1983) and coarse textured soils (Chang et al., 1977). The possibility therefore exists that this waste product could be utilised in potting substrates.
2. Materials and method A series of glasshouse trials were conducted to compare potting substrates containing fly ash with those of the type generally used for potted plant production. A UC mix, 1:l peat:sand (by volume), was used as the standard comparison mix. In test mixtures, fly ash replaced a portion of either the sand or the peat component of the UC mix (Table 1). A second comparison mix, a commercially available bark based mixture, marketed through retail stores (Retail mix) was also included.
N.W. Menzies, R.L. Aitken/
Scientia Horticulturae 67 (1996) 87-99
89
Table 1 The proportions (by volume)of peat, fly ash and sand in each of the potting mixes, the mix volume following settling and the oven dry mass Substrate
Peat:fly ash:sand
Volume (cm31
Mass (g)
Retail mix UC mix Substitution of peat with fly ash
Not determined 5:0:5 4:1:5 3:2:5 2:3:5 0:5:5 5:1:4 5:2:3 5:3:2 5:5:0
800 800 800 800 700 630 800 800 680 400
620 800 790 770 760 690 640 590 340
Substitution of sand with fly ash
3.50
The addition of fly ash to sands and coarse textured soils can markedly increase the available water capacity (Chang et al., 1977; Campbell et al., 1983). Thus, incorporation of fly ash in a potting substrate was expected to increase plant available water capacity. To evaluate the importance of this effect with respect to plant growth, and to distinguish between the effects of water availability and other properties of the mix, two watering regimes were used. The watering regime, ‘randomly watered’ was intended to accentuate the effects of increased water availability, whilst ‘capillary watered’ was intended to minimise these differences. Three test species, tomato (Lycopersicon esculentum cv. Gross Lisse), Petunia (Petunia x hybria’a grundifloru cv. Cascade) and Boston fern (Nephrolepis emltutu) were grown to represent a cross-section of the type of plants grown in pots. The use of the differing watering regimes necessitated the separation of these treatments on different benches, thus the experiment was conducted as a split plot design. Four replications were made of each potting mix X plant species X watering regime treatment (240 pots). Data for each of the three test species was analysed as a separate completely randomised design experiment. Analysis of variance calculations were performed and least significant difference (LSD) values (P < 0.05) calculated using Statistical Analysis System @AS) programs. 2.1. Potting substrates Test mixtures were constituted from the following components: (i) West German sphagnum peat moss (Eurotorf @>, wet and allowed to expand fully before use, (ii) washed river sand of predominantly medium (0.2-0.5 mm, 42%), coarse (0.5-l mm, 38%) and very coarse (l-2 mm, 12%) sand fractions (Soil Survey Staff, 19751, (iii> fly ash from the Swanbank power station, Queensland was used as its properties are representative of most of the ashes from Australian power stations burning black coal (Table 2). Freshly precipitated ash was used as this was considered most readily
90
N. W. Menzies, R.L. Airken/ Scienria Horticulturae
67 (1996) 87-99
Table 2 Selected properties of fly ash from Swanbank power station and the range and mean on these parameters for flv ash from 11Australian Dower stations a Range for all stations pH (15 soil:water) &ctrical conductivity ’ (mS cm- ‘1 Particle size distribution (%o) Coarse sand (0.2-2.0 mm) Fine sand (0.02-0.2 mm) Silt (0.002-0.02 mm) Clay ( < 0.002 mm) Gravimetric moisture content d (%) 10 kPa 1500 kPa Available water capacity (%o) Total elemental concentration (%I ’ Si Al Fe Ca Mg K Ti P Mn Na
4.5--12.4 0.63-7.00
Mean b 3.14
Value for Swanbank 11.5 1.73
o-3 28-54 43-56 2-22
41 48 10
42 48 9
29- 108 2-19 27- 105
62 5 57
69 3 66
23.4 11.7 6.49 4.99 4.74 1.00 1.24 0.48 0.08 1.24
27.3 16.2 2.81 1.64 0.97 0.67 2.11 0.25 0.03 0.12
8-34 2-16 3-15 0.2-32 0.7-21 0.2--2.4 0.3--2.5 0.01-1.6 0.04 0.03-5.9
a Data from Campbell et al. (1983). b Of the 11 stations, nine had pH values > 8, and seven had values of > 10.5. ’ Detetmined on a saturation extract. d Determined on unconsolidated ashes. ’ Data from Aitken et al. (1984).
available to commercial producers. Substrate proportions were determined on the basis of component volume as measured in the loose state. Plastic pots (12.5 cm diameter) were bottom lined with blotting paper to prevent loss of potting mix and restrict root growth through the drainage holes. Pots were initially filled to a volume of 800 cm3. The potted substrate was then air-dried, and average pot weight determined for each mix, and the respective pots adjusted to this value (Table 1). Settling during the course of the experiment reduced the volume of several mixes from the initial standard (Table 1). Slow release fertiliser (Osmocote @, 6 month, 17% N, 4.4% P, 10% K, 4.1% S) was applied to each pot at a rate of 10 g per pot. In the capillary watered treatment the fertiliser was incorporated in the bottom half of the potting substrate whilst in the randomly watered treatment it was surface applied. Tomato plants displayed P deficiency symptoms soon after transplanting and an application of P (370 mg P per pot) as KH,PO, in solution form was made 10 days after planting to correct this deficiency.
N. W. Menzies, R.L. Aitken / Scientia Horticulturae 67 (1996) 87-99
91
2.2. Watering 2.2.1. Randomly watered
Pots in this treatment were watered (by hand held hose) when water stress signs were apparent on some plants. Thus, it was anticipated that the available water capacity of the mix would affect plant growth, as it determined the frequency and degree to which water stress was experienced, emphasising differences between substrates. 2.2.2. Capillary watered
In this treatment the water status in the potting mixtures was maintained at 0.3-1.5 kPa by use of capillary-watering-mat covered benches. Thus, growth of plants in this treatment would not be limited by water stress, minimising the differences between mixes attributable to available water content. 2.3. Species selection, establishment and growth The three plant species were chosen to represent a range of potted plant types; tomato a plant commonly grown in artificial media to produce fruit, petunia a potted flowering plant, and Boston fern a foliage plant. Tomato and petunia plants were established from seed in UC mix and then transplanted to the various treatment mixes when 5-7 cm high. Boston fern cuttings were taken from well rooted stolon plantlets and placed in gravel in a mist propagating unit until healthy shoots were produced. Cuttings of similar size and vigour were selected and transferred to the various mixes. Boston fern was maintained under 60% shade throughout the growing period. Observations of plant growth and foliar symptoms of disorders were recorded weekly. Although fungal disease was apparent on the Boston fern in treatments with high fly ash contents, no remedial action was taken as this problem was considered an effect attributable to the potting substrate used. As aesthetic quality is important in potted plants, the number and duration of flowers on petunia plants were recorded over a period of 10 days. 2.4. Harvest After growing periods of 5 weeks for tomato, 9 weeks for petunia and 18 weeks for Boston fern, plant tops were harvested by cutting at the substrate surface. The harvested material was dried to a constant weight at 60°C in a forced draught oven and its weight recorded. Tomato tissue concentrations of P, Ca, Mg, Zn, Cu, Fe, Mn, MO, B, Co and Ni were determined by inductively coupled plasma atomic emission spectroscopy (ICPAFS) following nitric/perchloric acid digestion. 2.5.
Physical
characterisation
of mixes
The unconfined compressive strength of the mixes was determined after three wetting/drying cycles had resulted in the substrates packing to a constant volume.
N. W. Me&es,
92
R.L. Aitken / Scientia Horticulturae
67 (1996) 87-99
Measurements were made on mixtures at both a moisture content equivalent to 10 kPa suction and in the air dry state. The electrical conductivity (EC) and pH of 15 solid:water suspensions after 1 h equilibration were determined for each mix before planting and following harvest. Bulk density was calculated from the oven dry weight and total volume of mix per pot. The gravimetric moisture contents at manic suctions of 0.2 (assumed to be saturation), 1, 10, 100 and 1500 kPa were determined for each mixture. These were converted to volumetric water contents using the bulk density. A volumetric basis was considered more appropriate for comparison of potting substrates as the mass of substrate in a pot changed markedly between treatments (Table 1) whereas the volume remained relatively constant. Air-filled porosity was calculated as the difference in volumetric water content between saturation and 10 kPa suction.
3. Results 3.1. Potting mix characteristics The incorporation of fly ash in the potting substrates raised the pH to values considerably higher than that of the UC mix (Table 3). During the growth period the pH of mixtures containing predominantly sand and fly ash decreased, whilst that in predominantly peat and fly ash mixtures increased. Electrical conductivity increased with fly ash addition and there was a general trend of further increase with equilibration over time (Table 3). Unconfined compressive strength was increased by fly ash additions in excess of 20%. Moisture content had little effect on the values determined for most mixtures;
Table 3 The mean pH and electrical conductivity (EC) of substrates prior to planting (initial) and following harvest (fmal), and bulk density (BD) and unconfmed compressive strength WCS) at field capacity (10 kPa) and after 10 days drying following settling to constant volume Substrate
Retail UC F:P I:4 a F:P 23 F:P 3:2 F:P 50 F:S 1:4b F:S 23 F:S 3~2 F:S 50
pH
EC (mS cm-‘)
Initial
Final
Initial
Final
6.28 4.91 7.84 8.29 8.76 9.44 7.04 7.32 7.92 8.66
6.60 5.25 7.16 8.27 8.54 8.63 7.00 7.93 8.36 8.91
0.36 0.12 0.18 0.24 0.29 0.25 0.23 0.30 0.33 0.37
0.51 0.15 0.25 0.25 0.27 0.26 0.21 0.32 0.47 0.74
a Fly ash used to replace peat at the ratio indicated. b F’ly ash used to replace sand at the ratio indicated.
BD (g cme3)
UCS (kg cm-*) 10 kPa
Air dry
0.38 0.77 0.98 1.00 1.12 1.27 0.87 0.79 0.77 0.76
0.16 0.12 0.32 1.16 1.36 2.66 0.60 0.78 1.32 1.28
0.20 0.14 0.16 0.39 0.34 0.62 0.08 0.54 1.22 1.32
N. W. Menties. R.L. Aitken / Scientia Horticulturae
67 (1996) 87-99
93
(b) 8420_
1
Rot
00 [I 0
dill UC
,
L
I
1:4 23 -__F:s---
32
I
5:O
I
k
1:4 2:3 ---F.p-__
I
,
32
$0
Potting sub&rota Fig. 1. The (a) available water capacity in the range IO- 1500 kPa (untilled) and I- 1500 kPa (cross-hatched), and (b) the air-filled porosity of Retail and UC mixes and a range of test substrates containing fly ash. (P, peat; F, fly ash; S, sand.)
however, where fly ash was used to replace peat at rates in excess of 20% the unconfined compressive strength at 10 kPa was considerably greater than in the air dry state. The effect of fly ash on bulk density was dependent on the component for which fly ash was substituted (Table 3). When used in place of a portion of the sand component, the bulk density was reduced, whilst replacement of a portion of the peat component caused an increased bulk density. Fly ash addition produced large increases in the amount of water held between 10 and 1500 kPa suction, the range normally considered as plant available water (Fig. 1). However, within the suction range 1 to 1500 kPa, a measure more appropriate to the drainage characteristics of potted material, differences between control and test mixtures were relatively smaller (Fig. 1). Air-filled porosity values for the test mixtures, calculated from the water holding characteristics, show a marked decrease with increasing fly ash content (Fig. 1). 3.2. Plant yields Mean dry matter yield of tomato, petunia and Boston fern plants grown under capillary and random watering regimes are presented in Fig. 2. Tomato and petunia plant
94
N.W. Menzies, R.L. Aitken / Scientia Horriculturae 67 (1996) 87-99
I
Ret
UC
I
I
I
I
,
I
,
,
1:4 253 J:Z
I:0 1:4 2rS $2 &O -__F:s--___F:p---
POTTING
SUBSTRATE
Fig. 2. The dry matter yield of (a) tomato (Lycopersicon esculentum), (b) petunia (Petunia x hybrida grandif7ora) and (c) Boston fern (Nephrolepis exdrata) tops grown in Retail and UC mixes and a range of test substrates containing fly ash when random (untilled) or capillary (cross-hatched) watered. The bar in each figure indicates the LSD (P < 0.05). (P, peat; F, fly ash; S, sand.)
growth was significantly greater (P > 0.05) in mixtures containing 10% fly ash than in the UC mix. Where fly ash constituted 20% of the mix volume replacing a portion of the peat component, or 20 to 30% when replacing the sand component, growth was not significantly different (P > 0.05) to that in the UC mix. Incorporating greater fly ash contents 60%) dramatically reduced yields (Fig. 2). Where Boston fern was the test species variability was high and many plants were lost due to fungal infections, thus few treatment effects were significant (Fig. 2(c)). Fungal infections occurred predominantly on plants growing in high fly ash substrates and are attributed to the near waterlogged
14.0 8.10 11.7 13.4 12.7 11.8 10.2 12.8 14.0 13.3 IS-30
5.50 4.35 3.57 3.65 4.63 3.46 3.71 3.42 3.75 5.03 4-7 s
a
(pgg-I)
4.67 4.61 6.21 7.88 9.21 7.59 6.41 8.86 9.11 11.0 4-8 a
10.0 4.03 4.88 5.86 5.94 5.99 2.17 3.27 14.7 35.6 5-200
cu
Zgg-‘1
Zgg-I)
(mpg-‘)
=
of selected elements in the tops of tomato
P
a Weir and Cresswell ( 1993). b Kabata-Pendias and Pendias (1984).
Retail UC F:P 1:4 F:P 2:3 F:P 3:2 F:P 50 F:S 1:4 F:S 23 F:S 3:2 F:S 5:0 Normal
Substrate
me concentration
Table 4
39.9 47.7 22.9 23.1 22.1 25.3 26.8 20.7 25.8 42.3 20-200
(pgg-‘1
zn
*
87.0 134 117 77.8 196 84.0 129 214 429 489 100-300
(j.hgg-I)
Fe
a
Mn
109 184 52.7 38.2 32.0 99.4 44.5 34.6 48.0 184 25-500
(pgg-‘)
a
B
44.2 33.3 34.8 35.5 48.0 51.1 38.8 36.6 40.0 69.0 25-100
(CLgg-‘1
a
MO
37.0 29.6 42.6 61.8 78.7 116 49.0 61.9 81.2 110 0.02-40
(LLgg-‘1
b
b
7.26 6.27 7.42 9.54 9.85 5.92 6.27 4.90 14.9 27.4 0.02-1.7
(cLgg_‘) 7.58 5.06 8.25 9.72 8.86 3.95 4.71 5.75 17.9 39.6 0.5-6.2
co (CLgg-‘1
Ni
b
%
N.W. Menries, R.L. A&ken / Scientia Horticulrutae 67 (1996) 87-99
y-
10.00 +
I
I
1
I
I
0
2
4
6
a
Air
flllsd
porosity
106
x
I
(X)
Fig. 3. Relationship between air-tilled porosity and dry matter yield of tops for tomato (Lycopersicon esculentum) CO*), petunia (Petunia x hybrida grandijbra) (v 7) and Boston fern (Nephrolepis exalrara) (A A ) tops grown in Retail and UC mixes and a range of test substratescontaining fly ash when random (unfilled) or capillary (filled) watered.
conditions prevailing in these pots; fungal damage was greatest where wet conditions were maintained continuously by capillary watering. The retail mix resulted in poor growth, significantly less than the UC control or test mixtures containing less than 20% ash. A reasonably strong correlation (r2 = 0.67) was found between yield and air-filled porosity (Fig. 3), with stronger correlations for individual species (tomato 0.90; petunia 0.74; Boston fern 0.90). Capillary watering produced significantly higher yields than the random watering regime in the UC and test mixtures containing s 20% ash (Fig. 2). Yield in the retail mix and test mixtures containing > 20% ash was not significantly (P < 0.05) affected by watering regime. No significant interaction between watering regime and the fly ash content of the substrates was observed. Flower production and duration data for petunia showed no consistent trends other than the expected relationship between the rate of flower production and plant size. Tissue concentrations of selected elements in tomato tops are presented in Table 4. Tissue MO concentration increased with increasing substrate fly ash content; however, the highest concentrations attained (110 pg g- ‘) are not considered toxic.
4. Discussion The plant available water contents determined for the various substrates (Fig. 1) support the original premise that incorporation of fly ash in potting mixes would reduce the requirement for peat as a water holding component. Addition of 10% ash, the lowest rate used, increased the available water capacity by a factor of about 5. Incorporating additional quantities of fly ash further increased plant available water content (lo-1500 kPa), with the addition of 50% ash resulting in an available water capacity approximately ten times that of the UC mix.
N.W. Menzies, R.L. Aitken/
Scientia Horticulturae
67 (1996) 87-99
97
Despite these increases in water holding capacity, the experiment has been unable to clearly identify the effects of differing available water capacities on plant yield. The use of two watering regimes was intended to isolate available water effects from other factors. One treatment, random watering, was designed to accentuate growth effects caused by differences in available water content while the second, capillary watering, eliminated differences by preventing water stress. Although yields achieved by capillary watering were greater than those with random watering, the relative differences between mixes were about the same in each watering regime (Fig. 2). Since it was assumed that the growth of plants in capillary watering treatments was not limited by lack of available water, this suggests that some factor other than plant available water content was responsible for yield differences between mixes. It is suggested that as all water held at potentials > 1500 kPa is available to plants, the upper limit of 10 kPa (established where free drainage can occur) may be too low when considering the near saturated condition remaining in a pot after free drainage is complete. Thus, the water held between 10 and 1500 kPa may not represent the total amount of available water present in the pot. When the volume of water held between 1 and 1500 kPa is used (Fig. I), the difference in water content between the substrates is greatly reduced; an addition of 10% ash increasing water content by 1.3. This supports the contention that yield differences between substrates are not primarily attributable to variation in plant available water content. These experiments have indicated that small amounts of fly ash can be incorporated in potting substrates without reducing yield (Fig. 2) or plant quality, and that increased yields are attainable provided that the addition of ash is not excessive. Incorporation of 10% fly ash significantly increased the yields of tomato and petunia compared with that in the UC mix. This increased yield may be attributable to increased nutrient availability, increased pH per se or removal of Al toxicity as a result of the pH increase, or a more favourable physical environment for root growth. However, no visual symptoms of disorders were identified on UC plants, nor were deficient or toxic concentrations of nutrients identified in these plants by tissue analysis (Table 4). Addition of 10% fly ash raised the pH from 5.0 (UC mix) to about 7 (Table 3) and this may have resulted in better plant growth. Incorporation of fly ash in the potting substrate at rates of > 20% resulted in a large reduction in yield (Fig. 2) which may have been attributable to a number of factors including, reduced aeration (Fig. l), high substrate strength, high pH (Table 3) and possible toxicities of elements not determined in the plant analysis. Of these factors, the effect of reduced aeration is considered to have the principal limitation to plant growth. Fly ash, when incorporated at rates of > 20%, reduces the air-filled porosity (Fig. I) by occupying voids in the coarse matrix of the UC mix which would otherwise have acted as drainage pores. While air-filled porosity values determined for the potting substrates (Fig. 1) indicate that none of these substrates have > lo-15% air-filled porosity, the level considered necessary for maximum plant growth (Bunt, 1974; Paul and Lee, 19761, it is considered that the values calculated underestimate the true air-filled porosity. The air-filled porosity calculation used is based on the assumption that the volume of water which drains between saturation and the potential under consideration (10 kPa) is replaced by air. However, saturation is assumed at 0.2 kPa, pores which drain at
98
N.W. Menzies, R.L. Aitken / Scientia Horticultwae
67 (1996) 87-99
potentials greater than 0.2 kPa will not be included in the estimate of air-filled porosity. In coarse textured materials, such as potting substrates, the error may be large. While the degree to which air-filled porosity is underestimated is greatest in the UC and Retail mixes, and progressively decreases with fly ash addition, thus partially masking the effect of aeration, a good correlation still existed between the air-filled porosity values calculated and plant dry matter yield (Fig. 3). The effect of fly ash on mix pH is complicated by its interaction with the peat component. Incorporation of alkaline fly ash in the substrate increased the pH over that of the UC mix in which the acidic nature of the peat produced a low pH (Table 3). Over time the pH of the predominantly ash/peat mixtures increased, dissolution of material from the fly ash being sufficient to overcome the high cation exchange capacity (CEC; Handreck and Black, 1994) and hence large buffering capacity of the peat. This finding is in contrast to that of Aitken et al. (1984) who concluded that as ashes are poorly buffered they should have little effect on soil PH. Where fly ash and sand were the predominant components an initially high pH was produced which decreased during the growth period, probably as a result of leaching in the random watered pots and the acidifying effect of the fertiliser. The equilibrium pH values attained are not considered sufficiently high to limit plant growth directly, and do not appear to have resulted in induced nutrient deficiencies (Table 4). At high rates of incorporation fly ash increased the substrate strength (Table 31, an effect attributed to the void filling by the fly ash. The poor plant growth observed in high fly ash mixtures may have been partly attributable to this effect. However, the influence of other adverse physical and chemical characteristics of these substrates, such as poor aeration and high pH, cannot be ignored. The scope of this experiment was limited to replacement of a portion of either the peat or sand component with an equal volume of fly ash. As it has been shown that the water holding capacity of fly ash in potting substrates is many times that of the peat it is used to replace, it may be more appropriate to replace several units of peat with a single unit of fly ash, the total volume being maintained by an increase in the inert component. Thus future experimentation should be directed toward replacement of a greater proportion of the peat component with both fly ash and inert materials. As reduced aeration and increased bulk density are both considered undesirable, incorporation of coarse materials with low bulk density, such as polystyrene foam or clinker may be beneficial.
5. Conclusions
This experiment has indicated that fly ash can be successfully incorporated in potting substrates at low rates ( s 20% of the mix volume) without loss of plant yield or quality. However, at higher rates, the adverse physical and chemical characteristics of the fly ash restrict growth. Thus the rate of incorporation of fly ash is limited to a maximum fly ash content and not by a minimum requirement for peat. As fly ash has the potential to replace the water holding capacity of several times its own volume of peat, it may be possible to reduce the peat requirement to a greater extent than that achieved here without exceeding the maximum fly ash limit.
N. W. Menties, R.L. Aitken/
Scientia Horticulturae
67 (1996) 87-99
99
While the economic justification for the replacement of peat with fly ash has not been investigated, as the cost of peat is high and that of fly ash is very low, the concept appears to have considerable merit.
References Adriano, D.C., Page, A.L., Elseewi, A.A., Chang, A.C. and Straughan, I., 1980. Utilization and disposal of fly ash and other coal residues in terrestrial ecosystems: a review. J. Environ. Qua]., 9: 333-344. Aitken, R.L., 1994. Characterization and phytotoxicity of boron in Australian fly ashes. Ph.D. Thesis, The University of Queensland, St Lucia. A&en, R.L., CampbeII, D.J. and Bell, L.C., 1984. Properties of some Australian fly ashes relevant to their agronomic utilization. Aust. J. Soil Res., 22: 443,453. Bunt, A.C., 1974. Some physical and chemical characteristics of loamless pot-plant substrates and their relation to plant growth. Acta. Hort., 37: 1954-1965. Campbell, D.J., Fox, W.E., Aitken, R.L. and Bell, L.C., 1983. Physical characteristics of sands amended with fly ash. Aust. J. Soil Res., 21: 147-154. Chang, A.C., Lund, L.J. and Wameke, J.E., 1977. Physical properties of fly ash-amended soils. J. Environ. Qual., 6: 267-270. de Boodt, M. and Verdonck, 0.. 1972. Some physical properties of the substrates in horticulture. Acta Hort., 26: 37-44. Elseewi, A.A., Bingham, F.T. and Page, A.L., 1978. Availability of sulphur in fly ash to plants. J. Environ. Qual., 7: 69-73. Handreck, K.A. and Black, N.D., 1994. Growing media for ornamental plants and turf. N.S.W. University Press, Randwick. HiII, M.F. and Lamp, C.A., 1980. Use of pulverised fuel ash from Victorian brown coal as a source of nutrients for pasture species. Aust. I. Exp. Agric. Animal Hush., 20: 377-384. Kabata-Pendias, A. and Pendias, H., 1984. Trace Elements in Soils and Plants. CRC Press Inc., FL. Martens, D.C., Schnappinger, M.G., Jnr. and Zelanzy, L.W., 1970. The plant availability of potassium in fly ash. Soil Sci. Sot. Am. Prcc., 34: 453-456. Page, A.L., Elseewi, A.A. and Straughan, I.R., 1979. Physical and chemical properties of fly ash from coal-tired power plants with reference to environmental impacts. Residue Rev., 71: 83- 120. Paul, J.L. and Lee, C.I., 1976. Relation between growth of chrysanthemums and aeration of various container media. J. Am. Sot. Hort. Sci., 101: 500-503. Roberts, TJ., 1966. The effect of land type and fme particle amendments on the emergence and growth of subterranean clover (Trifolium subterraneum L.) with particular reference to water relations. Aust. J. Agric. Res., 17: 657-672. Soil Survey Staff, 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA-SCS Agricultural Handbook 436, US Government Printing Office, Washington, DC. Weir, R.G. and Cresswell, G.C., 1993. Plant Nutrient Disorders 3. Vegetable Crops. lnkata Press, Melbourne.
HOmlCULluM Scientia Horticulturae67 (1996)87-99
Evaluation of fly ash as a component of potting substrates N.W. Menzies ap*, R.L. Aitken b aDepartment of Agriculture, The University of Queensland, St Lucia, 4072, Australia b Resource Management Institute, Department of Primary Industries, Meiers Rd, Indooroopilly, 4068, Australia Accepted 15 May 1996
Abstract A series of laboratory and glasshouse experiments were undertaken to assess the potentid for incorporation of fly ash in soilless potting substrates. The physical and chemical properties of a commercially available bark based substrate, the University of California (UC) 1:l peat:sand mix and a range of test substrates containing fly ash were characterised. In test mixtures, fly ash was substituted for a portion of either the peat or sand component of the UC mix, at rates of 10,20, 30 and 50% of the mix volume. Incorporation of fly ash greatly increased the plant available water capacity (10-1500 Pa) of the substrate. However, high pH, increased substrate strength and reduced air-filled porosity were considered adverse effects, particularly at ash rates > 20%. The growth of tomato (Lycopersicon esculentum), petunia (Petunia x hybrida grandifloru) and Boston fern (Nephrolepis exaltutu) in the substrates was assessed. Two watering regimes, capillary watering and irregular hosing, were used to identify effects of available water capacity on plant growth, but no effect was identified. Test mixtures containing fly ash as 20% of the substrate volume produced growth equal to that in the UC mix, with substrates containing 10% ash producing significantly greater growth of tomato and petunia. At rates of incorporation > 20% reduced plant growth was attributed to both adverse physical and chemical characteristics of the substrate. As fly ash is available at low cost and can be successfully substituted for a considerable portion of the expensive peat component, its use at low application rates in potting substrates may be desirable from an economic viewpoint. Keywords: Potting
* Corresponding
substrates; Fly ash; Available water capacity
author. Fax: 3365
0304-4238/%/$15.00 Copyright PN SO304-4238(96)00935-
1177;e-mail: [email protected].
0 1996 Published
1
by Elsevier Science B.V. All rights reserved.
88
N.W. Menzies. R.L. Aitken/Scientia Horticulturae 67 (1996) 87-99
1. Introduction The potted plant industry has undergone dramatic expansion during the last 2 decades with potted plants now being readily available in retail stores, as well as nurseries. This has increased the need for a regular supply of a uniform growth medium that has the ability to support vigorous plant growth. Owing to the difficulties of getting soil of a consistent quality, and the possibility of adverse physical problems when soils are potted, most nurseries use soilless potting substrates. As the rooting volume of a potted plant is very restricted, one important requirement of soilless potting substrates is that they possess considerable water holding capacity. Soilless potting substrates are frequently based on one component with a high plant available water capacity (frequently peat), which is combined with relatively inert materials such as sand, bark or polystyrene foam which provide aeration and act as ballast. The University of California (UC) mixes are of this type, consisting of peat and sand in a variety of ratios. The I:1 peat:sand mix is commonly used throughout the world for potted plant production (de Boodt and Verdonck, 1972). The supply of peat is finite, thus its cost has risen in response to increased demand as the nursery trade has grown, and, in addition, the quality of peat has fallen as the high quality peat supplies are exhausted. These problems are accentuated for Australian nurserymen as Australian peats are unsuitable for incorporation in potting substrates (Handreck and Black, 1994) and additional cost must be incurred importing peat. Thus, for Australian nurseries using UC type mixes, peat represents a considerable portion of production cost. Ply ash, produced by the combustion of pulverised coal in power generating stations, is the residue which is small enough to be entrained in the flue gas and carried from the site of combustion. This material is removed from the flue gas to lessen atmospheric pollution. Australian fly ash production has been estimated to be in excess of 10 million t per annum (A&ken, 1994). Approximately 10% of the fly ash produced is utilized in premixed concrete while the remainder represents a considerable disposal problem. The agronomic use of fly ash has been widely studied (Page et al., 1979; Adriano et al., 1980) and its use as a source of nutrients for plants considered (Martens et al., 1970; Elseewi et al., 1978; Hill and Lamp, 1980). Ply ash has also been shown to considerably increase the plant available water capacity of sands (Roberts, 1966; Campbell et al., 1983) and coarse textured soils (Chang et al., 1977). The possibility therefore exists that this waste product could be utilised in potting substrates.
2. Materials and method A series of glasshouse trials were conducted to compare potting substrates containing fly ash with those of the type generally used for potted plant production. A UC mix, 1:l peat:sand (by volume), was used as the standard comparison mix. In test mixtures, fly ash replaced a portion of either the sand or the peat component of the UC mix (Table 1). A second comparison mix, a commercially available bark based mixture, marketed through retail stores (Retail mix) was also included.
N.W. Menzies, R.L. Aitken/
Scientia Horticulturae 67 (1996) 87-99
89
Table 1 The proportions (by volume)of peat, fly ash and sand in each of the potting mixes, the mix volume following settling and the oven dry mass Substrate
Peat:fly ash:sand
Volume (cm31
Mass (g)
Retail mix UC mix Substitution of peat with fly ash
Not determined 5:0:5 4:1:5 3:2:5 2:3:5 0:5:5 5:1:4 5:2:3 5:3:2 5:5:0
800 800 800 800 700 630 800 800 680 400
620 800 790 770 760 690 640 590 340
Substitution of sand with fly ash
3.50
The addition of fly ash to sands and coarse textured soils can markedly increase the available water capacity (Chang et al., 1977; Campbell et al., 1983). Thus, incorporation of fly ash in a potting substrate was expected to increase plant available water capacity. To evaluate the importance of this effect with respect to plant growth, and to distinguish between the effects of water availability and other properties of the mix, two watering regimes were used. The watering regime, ‘randomly watered’ was intended to accentuate the effects of increased water availability, whilst ‘capillary watered’ was intended to minimise these differences. Three test species, tomato (Lycopersicon esculentum cv. Gross Lisse), Petunia (Petunia x hybria’a grundifloru cv. Cascade) and Boston fern (Nephrolepis emltutu) were grown to represent a cross-section of the type of plants grown in pots. The use of the differing watering regimes necessitated the separation of these treatments on different benches, thus the experiment was conducted as a split plot design. Four replications were made of each potting mix X plant species X watering regime treatment (240 pots). Data for each of the three test species was analysed as a separate completely randomised design experiment. Analysis of variance calculations were performed and least significant difference (LSD) values (P < 0.05) calculated using Statistical Analysis System @AS) programs. 2.1. Potting substrates Test mixtures were constituted from the following components: (i) West German sphagnum peat moss (Eurotorf @>, wet and allowed to expand fully before use, (ii) washed river sand of predominantly medium (0.2-0.5 mm, 42%), coarse (0.5-l mm, 38%) and very coarse (l-2 mm, 12%) sand fractions (Soil Survey Staff, 19751, (iii> fly ash from the Swanbank power station, Queensland was used as its properties are representative of most of the ashes from Australian power stations burning black coal (Table 2). Freshly precipitated ash was used as this was considered most readily
90
N. W. Menzies, R.L. Airken/ Scienria Horticulturae
67 (1996) 87-99
Table 2 Selected properties of fly ash from Swanbank power station and the range and mean on these parameters for flv ash from 11Australian Dower stations a Range for all stations pH (15 soil:water) &ctrical conductivity ’ (mS cm- ‘1 Particle size distribution (%o) Coarse sand (0.2-2.0 mm) Fine sand (0.02-0.2 mm) Silt (0.002-0.02 mm) Clay ( < 0.002 mm) Gravimetric moisture content d (%) 10 kPa 1500 kPa Available water capacity (%o) Total elemental concentration (%I ’ Si Al Fe Ca Mg K Ti P Mn Na
4.5--12.4 0.63-7.00
Mean b 3.14
Value for Swanbank 11.5 1.73
o-3 28-54 43-56 2-22
41 48 10
42 48 9
29- 108 2-19 27- 105
62 5 57
69 3 66
23.4 11.7 6.49 4.99 4.74 1.00 1.24 0.48 0.08 1.24
27.3 16.2 2.81 1.64 0.97 0.67 2.11 0.25 0.03 0.12
8-34 2-16 3-15 0.2-32 0.7-21 0.2--2.4 0.3--2.5 0.01-1.6 0.04 0.03-5.9
a Data from Campbell et al. (1983). b Of the 11 stations, nine had pH values > 8, and seven had values of > 10.5. ’ Detetmined on a saturation extract. d Determined on unconsolidated ashes. ’ Data from Aitken et al. (1984).
available to commercial producers. Substrate proportions were determined on the basis of component volume as measured in the loose state. Plastic pots (12.5 cm diameter) were bottom lined with blotting paper to prevent loss of potting mix and restrict root growth through the drainage holes. Pots were initially filled to a volume of 800 cm3. The potted substrate was then air-dried, and average pot weight determined for each mix, and the respective pots adjusted to this value (Table 1). Settling during the course of the experiment reduced the volume of several mixes from the initial standard (Table 1). Slow release fertiliser (Osmocote @, 6 month, 17% N, 4.4% P, 10% K, 4.1% S) was applied to each pot at a rate of 10 g per pot. In the capillary watered treatment the fertiliser was incorporated in the bottom half of the potting substrate whilst in the randomly watered treatment it was surface applied. Tomato plants displayed P deficiency symptoms soon after transplanting and an application of P (370 mg P per pot) as KH,PO, in solution form was made 10 days after planting to correct this deficiency.
N. W. Menzies, R.L. Aitken / Scientia Horticulturae 67 (1996) 87-99
91
2.2. Watering 2.2.1. Randomly watered
Pots in this treatment were watered (by hand held hose) when water stress signs were apparent on some plants. Thus, it was anticipated that the available water capacity of the mix would affect plant growth, as it determined the frequency and degree to which water stress was experienced, emphasising differences between substrates. 2.2.2. Capillary watered
In this treatment the water status in the potting mixtures was maintained at 0.3-1.5 kPa by use of capillary-watering-mat covered benches. Thus, growth of plants in this treatment would not be limited by water stress, minimising the differences between mixes attributable to available water content. 2.3. Species selection, establishment and growth The three plant species were chosen to represent a range of potted plant types; tomato a plant commonly grown in artificial media to produce fruit, petunia a potted flowering plant, and Boston fern a foliage plant. Tomato and petunia plants were established from seed in UC mix and then transplanted to the various treatment mixes when 5-7 cm high. Boston fern cuttings were taken from well rooted stolon plantlets and placed in gravel in a mist propagating unit until healthy shoots were produced. Cuttings of similar size and vigour were selected and transferred to the various mixes. Boston fern was maintained under 60% shade throughout the growing period. Observations of plant growth and foliar symptoms of disorders were recorded weekly. Although fungal disease was apparent on the Boston fern in treatments with high fly ash contents, no remedial action was taken as this problem was considered an effect attributable to the potting substrate used. As aesthetic quality is important in potted plants, the number and duration of flowers on petunia plants were recorded over a period of 10 days. 2.4. Harvest After growing periods of 5 weeks for tomato, 9 weeks for petunia and 18 weeks for Boston fern, plant tops were harvested by cutting at the substrate surface. The harvested material was dried to a constant weight at 60°C in a forced draught oven and its weight recorded. Tomato tissue concentrations of P, Ca, Mg, Zn, Cu, Fe, Mn, MO, B, Co and Ni were determined by inductively coupled plasma atomic emission spectroscopy (ICPAFS) following nitric/perchloric acid digestion. 2.5.
Physical
characterisation
of mixes
The unconfined compressive strength of the mixes was determined after three wetting/drying cycles had resulted in the substrates packing to a constant volume.
N. W. Me&es,
92
R.L. Aitken / Scientia Horticulturae
67 (1996) 87-99
Measurements were made on mixtures at both a moisture content equivalent to 10 kPa suction and in the air dry state. The electrical conductivity (EC) and pH of 15 solid:water suspensions after 1 h equilibration were determined for each mix before planting and following harvest. Bulk density was calculated from the oven dry weight and total volume of mix per pot. The gravimetric moisture contents at manic suctions of 0.2 (assumed to be saturation), 1, 10, 100 and 1500 kPa were determined for each mixture. These were converted to volumetric water contents using the bulk density. A volumetric basis was considered more appropriate for comparison of potting substrates as the mass of substrate in a pot changed markedly between treatments (Table 1) whereas the volume remained relatively constant. Air-filled porosity was calculated as the difference in volumetric water content between saturation and 10 kPa suction.
3. Results 3.1. Potting mix characteristics The incorporation of fly ash in the potting substrates raised the pH to values considerably higher than that of the UC mix (Table 3). During the growth period the pH of mixtures containing predominantly sand and fly ash decreased, whilst that in predominantly peat and fly ash mixtures increased. Electrical conductivity increased with fly ash addition and there was a general trend of further increase with equilibration over time (Table 3). Unconfined compressive strength was increased by fly ash additions in excess of 20%. Moisture content had little effect on the values determined for most mixtures;
Table 3 The mean pH and electrical conductivity (EC) of substrates prior to planting (initial) and following harvest (fmal), and bulk density (BD) and unconfmed compressive strength WCS) at field capacity (10 kPa) and after 10 days drying following settling to constant volume Substrate
Retail UC F:P I:4 a F:P 23 F:P 3:2 F:P 50 F:S 1:4b F:S 23 F:S 3~2 F:S 50
pH
EC (mS cm-‘)
Initial
Final
Initial
Final
6.28 4.91 7.84 8.29 8.76 9.44 7.04 7.32 7.92 8.66
6.60 5.25 7.16 8.27 8.54 8.63 7.00 7.93 8.36 8.91
0.36 0.12 0.18 0.24 0.29 0.25 0.23 0.30 0.33 0.37
0.51 0.15 0.25 0.25 0.27 0.26 0.21 0.32 0.47 0.74
a Fly ash used to replace peat at the ratio indicated. b F’ly ash used to replace sand at the ratio indicated.
BD (g cme3)
UCS (kg cm-*) 10 kPa
Air dry
0.38 0.77 0.98 1.00 1.12 1.27 0.87 0.79 0.77 0.76
0.16 0.12 0.32 1.16 1.36 2.66 0.60 0.78 1.32 1.28
0.20 0.14 0.16 0.39 0.34 0.62 0.08 0.54 1.22 1.32
N. W. Menties. R.L. Aitken / Scientia Horticulturae
67 (1996) 87-99
93
(b) 8420_
1
Rot
00 [I 0
dill UC
,
L
I
1:4 23 -__F:s---
32
I
5:O
I
k
1:4 2:3 ---F.p-__
I
,
32
$0
Potting sub&rota Fig. 1. The (a) available water capacity in the range IO- 1500 kPa (untilled) and I- 1500 kPa (cross-hatched), and (b) the air-filled porosity of Retail and UC mixes and a range of test substrates containing fly ash. (P, peat; F, fly ash; S, sand.)
however, where fly ash was used to replace peat at rates in excess of 20% the unconfined compressive strength at 10 kPa was considerably greater than in the air dry state. The effect of fly ash on bulk density was dependent on the component for which fly ash was substituted (Table 3). When used in place of a portion of the sand component, the bulk density was reduced, whilst replacement of a portion of the peat component caused an increased bulk density. Fly ash addition produced large increases in the amount of water held between 10 and 1500 kPa suction, the range normally considered as plant available water (Fig. 1). However, within the suction range 1 to 1500 kPa, a measure more appropriate to the drainage characteristics of potted material, differences between control and test mixtures were relatively smaller (Fig. 1). Air-filled porosity values for the test mixtures, calculated from the water holding characteristics, show a marked decrease with increasing fly ash content (Fig. 1). 3.2. Plant yields Mean dry matter yield of tomato, petunia and Boston fern plants grown under capillary and random watering regimes are presented in Fig. 2. Tomato and petunia plant
94
N.W. Menzies, R.L. Aitken / Scientia Horriculturae 67 (1996) 87-99
I
Ret
UC
I
I
I
I
,
I
,
,
1:4 253 J:Z
I:0 1:4 2rS $2 &O -__F:s--___F:p---
POTTING
SUBSTRATE
Fig. 2. The dry matter yield of (a) tomato (Lycopersicon esculentum), (b) petunia (Petunia x hybrida grandif7ora) and (c) Boston fern (Nephrolepis exdrata) tops grown in Retail and UC mixes and a range of test substrates containing fly ash when random (untilled) or capillary (cross-hatched) watered. The bar in each figure indicates the LSD (P < 0.05). (P, peat; F, fly ash; S, sand.)
growth was significantly greater (P > 0.05) in mixtures containing 10% fly ash than in the UC mix. Where fly ash constituted 20% of the mix volume replacing a portion of the peat component, or 20 to 30% when replacing the sand component, growth was not significantly different (P > 0.05) to that in the UC mix. Incorporating greater fly ash contents 60%) dramatically reduced yields (Fig. 2). Where Boston fern was the test species variability was high and many plants were lost due to fungal infections, thus few treatment effects were significant (Fig. 2(c)). Fungal infections occurred predominantly on plants growing in high fly ash substrates and are attributed to the near waterlogged
14.0 8.10 11.7 13.4 12.7 11.8 10.2 12.8 14.0 13.3 IS-30
5.50 4.35 3.57 3.65 4.63 3.46 3.71 3.42 3.75 5.03 4-7 s
a
(pgg-I)
4.67 4.61 6.21 7.88 9.21 7.59 6.41 8.86 9.11 11.0 4-8 a
10.0 4.03 4.88 5.86 5.94 5.99 2.17 3.27 14.7 35.6 5-200
cu
Zgg-‘1
Zgg-I)
(mpg-‘)
=
of selected elements in the tops of tomato
P
a Weir and Cresswell ( 1993). b Kabata-Pendias and Pendias (1984).
Retail UC F:P 1:4 F:P 2:3 F:P 3:2 F:P 50 F:S 1:4 F:S 23 F:S 3:2 F:S 5:0 Normal
Substrate
me concentration
Table 4
39.9 47.7 22.9 23.1 22.1 25.3 26.8 20.7 25.8 42.3 20-200
(pgg-‘1
zn
*
87.0 134 117 77.8 196 84.0 129 214 429 489 100-300
(j.hgg-I)
Fe
a
Mn
109 184 52.7 38.2 32.0 99.4 44.5 34.6 48.0 184 25-500
(pgg-‘)
a
B
44.2 33.3 34.8 35.5 48.0 51.1 38.8 36.6 40.0 69.0 25-100
(CLgg-‘1
a
MO
37.0 29.6 42.6 61.8 78.7 116 49.0 61.9 81.2 110 0.02-40
(LLgg-‘1
b
b
7.26 6.27 7.42 9.54 9.85 5.92 6.27 4.90 14.9 27.4 0.02-1.7
(cLgg_‘) 7.58 5.06 8.25 9.72 8.86 3.95 4.71 5.75 17.9 39.6 0.5-6.2
co (CLgg-‘1
Ni
b
%
N.W. Menries, R.L. A&ken / Scientia Horticulrutae 67 (1996) 87-99
y-
10.00 +
I
I
1
I
I
0
2
4
6
a
Air
flllsd
porosity
106
x
I
(X)
Fig. 3. Relationship between air-tilled porosity and dry matter yield of tops for tomato (Lycopersicon esculentum) CO*), petunia (Petunia x hybrida grandijbra) (v 7) and Boston fern (Nephrolepis exalrara) (A A ) tops grown in Retail and UC mixes and a range of test substratescontaining fly ash when random (unfilled) or capillary (filled) watered.
conditions prevailing in these pots; fungal damage was greatest where wet conditions were maintained continuously by capillary watering. The retail mix resulted in poor growth, significantly less than the UC control or test mixtures containing less than 20% ash. A reasonably strong correlation (r2 = 0.67) was found between yield and air-filled porosity (Fig. 3), with stronger correlations for individual species (tomato 0.90; petunia 0.74; Boston fern 0.90). Capillary watering produced significantly higher yields than the random watering regime in the UC and test mixtures containing s 20% ash (Fig. 2). Yield in the retail mix and test mixtures containing > 20% ash was not significantly (P < 0.05) affected by watering regime. No significant interaction between watering regime and the fly ash content of the substrates was observed. Flower production and duration data for petunia showed no consistent trends other than the expected relationship between the rate of flower production and plant size. Tissue concentrations of selected elements in tomato tops are presented in Table 4. Tissue MO concentration increased with increasing substrate fly ash content; however, the highest concentrations attained (110 pg g- ‘) are not considered toxic.
4. Discussion The plant available water contents determined for the various substrates (Fig. 1) support the original premise that incorporation of fly ash in potting mixes would reduce the requirement for peat as a water holding component. Addition of 10% ash, the lowest rate used, increased the available water capacity by a factor of about 5. Incorporating additional quantities of fly ash further increased plant available water content (lo-1500 kPa), with the addition of 50% ash resulting in an available water capacity approximately ten times that of the UC mix.
N.W. Menzies, R.L. Aitken/
Scientia Horticulturae
67 (1996) 87-99
97
Despite these increases in water holding capacity, the experiment has been unable to clearly identify the effects of differing available water capacities on plant yield. The use of two watering regimes was intended to isolate available water effects from other factors. One treatment, random watering, was designed to accentuate growth effects caused by differences in available water content while the second, capillary watering, eliminated differences by preventing water stress. Although yields achieved by capillary watering were greater than those with random watering, the relative differences between mixes were about the same in each watering regime (Fig. 2). Since it was assumed that the growth of plants in capillary watering treatments was not limited by lack of available water, this suggests that some factor other than plant available water content was responsible for yield differences between mixes. It is suggested that as all water held at potentials > 1500 kPa is available to plants, the upper limit of 10 kPa (established where free drainage can occur) may be too low when considering the near saturated condition remaining in a pot after free drainage is complete. Thus, the water held between 10 and 1500 kPa may not represent the total amount of available water present in the pot. When the volume of water held between 1 and 1500 kPa is used (Fig. I), the difference in water content between the substrates is greatly reduced; an addition of 10% ash increasing water content by 1.3. This supports the contention that yield differences between substrates are not primarily attributable to variation in plant available water content. These experiments have indicated that small amounts of fly ash can be incorporated in potting substrates without reducing yield (Fig. 2) or plant quality, and that increased yields are attainable provided that the addition of ash is not excessive. Incorporation of 10% fly ash significantly increased the yields of tomato and petunia compared with that in the UC mix. This increased yield may be attributable to increased nutrient availability, increased pH per se or removal of Al toxicity as a result of the pH increase, or a more favourable physical environment for root growth. However, no visual symptoms of disorders were identified on UC plants, nor were deficient or toxic concentrations of nutrients identified in these plants by tissue analysis (Table 4). Addition of 10% fly ash raised the pH from 5.0 (UC mix) to about 7 (Table 3) and this may have resulted in better plant growth. Incorporation of fly ash in the potting substrate at rates of > 20% resulted in a large reduction in yield (Fig. 2) which may have been attributable to a number of factors including, reduced aeration (Fig. l), high substrate strength, high pH (Table 3) and possible toxicities of elements not determined in the plant analysis. Of these factors, the effect of reduced aeration is considered to have the principal limitation to plant growth. Fly ash, when incorporated at rates of > 20%, reduces the air-filled porosity (Fig. I) by occupying voids in the coarse matrix of the UC mix which would otherwise have acted as drainage pores. While air-filled porosity values determined for the potting substrates (Fig. 1) indicate that none of these substrates have > lo-15% air-filled porosity, the level considered necessary for maximum plant growth (Bunt, 1974; Paul and Lee, 19761, it is considered that the values calculated underestimate the true air-filled porosity. The air-filled porosity calculation used is based on the assumption that the volume of water which drains between saturation and the potential under consideration (10 kPa) is replaced by air. However, saturation is assumed at 0.2 kPa, pores which drain at
98
N.W. Menzies, R.L. Aitken / Scientia Horticultwae
67 (1996) 87-99
potentials greater than 0.2 kPa will not be included in the estimate of air-filled porosity. In coarse textured materials, such as potting substrates, the error may be large. While the degree to which air-filled porosity is underestimated is greatest in the UC and Retail mixes, and progressively decreases with fly ash addition, thus partially masking the effect of aeration, a good correlation still existed between the air-filled porosity values calculated and plant dry matter yield (Fig. 3). The effect of fly ash on mix pH is complicated by its interaction with the peat component. Incorporation of alkaline fly ash in the substrate increased the pH over that of the UC mix in which the acidic nature of the peat produced a low pH (Table 3). Over time the pH of the predominantly ash/peat mixtures increased, dissolution of material from the fly ash being sufficient to overcome the high cation exchange capacity (CEC; Handreck and Black, 1994) and hence large buffering capacity of the peat. This finding is in contrast to that of Aitken et al. (1984) who concluded that as ashes are poorly buffered they should have little effect on soil PH. Where fly ash and sand were the predominant components an initially high pH was produced which decreased during the growth period, probably as a result of leaching in the random watered pots and the acidifying effect of the fertiliser. The equilibrium pH values attained are not considered sufficiently high to limit plant growth directly, and do not appear to have resulted in induced nutrient deficiencies (Table 4). At high rates of incorporation fly ash increased the substrate strength (Table 31, an effect attributed to the void filling by the fly ash. The poor plant growth observed in high fly ash mixtures may have been partly attributable to this effect. However, the influence of other adverse physical and chemical characteristics of these substrates, such as poor aeration and high pH, cannot be ignored. The scope of this experiment was limited to replacement of a portion of either the peat or sand component with an equal volume of fly ash. As it has been shown that the water holding capacity of fly ash in potting substrates is many times that of the peat it is used to replace, it may be more appropriate to replace several units of peat with a single unit of fly ash, the total volume being maintained by an increase in the inert component. Thus future experimentation should be directed toward replacement of a greater proportion of the peat component with both fly ash and inert materials. As reduced aeration and increased bulk density are both considered undesirable, incorporation of coarse materials with low bulk density, such as polystyrene foam or clinker may be beneficial.
5. Conclusions
This experiment has indicated that fly ash can be successfully incorporated in potting substrates at low rates ( s 20% of the mix volume) without loss of plant yield or quality. However, at higher rates, the adverse physical and chemical characteristics of the fly ash restrict growth. Thus the rate of incorporation of fly ash is limited to a maximum fly ash content and not by a minimum requirement for peat. As fly ash has the potential to replace the water holding capacity of several times its own volume of peat, it may be possible to reduce the peat requirement to a greater extent than that achieved here without exceeding the maximum fly ash limit.
N. W. Menties, R.L. Aitken/
Scientia Horticulturae
67 (1996) 87-99
99
While the economic justification for the replacement of peat with fly ash has not been investigated, as the cost of peat is high and that of fly ash is very low, the concept appears to have considerable merit.
References Adriano, D.C., Page, A.L., Elseewi, A.A., Chang, A.C. and Straughan, I., 1980. Utilization and disposal of fly ash and other coal residues in terrestrial ecosystems: a review. J. Environ. Qua]., 9: 333-344. Aitken, R.L., 1994. Characterization and phytotoxicity of boron in Australian fly ashes. Ph.D. Thesis, The University of Queensland, St Lucia. A&en, R.L., CampbeII, D.J. and Bell, L.C., 1984. Properties of some Australian fly ashes relevant to their agronomic utilization. Aust. J. Soil Res., 22: 443,453. Bunt, A.C., 1974. Some physical and chemical characteristics of loamless pot-plant substrates and their relation to plant growth. Acta. Hort., 37: 1954-1965. Campbell, D.J., Fox, W.E., Aitken, R.L. and Bell, L.C., 1983. Physical characteristics of sands amended with fly ash. Aust. J. Soil Res., 21: 147-154. Chang, A.C., Lund, L.J. and Wameke, J.E., 1977. Physical properties of fly ash-amended soils. J. Environ. Qual., 6: 267-270. de Boodt, M. and Verdonck, 0.. 1972. Some physical properties of the substrates in horticulture. Acta Hort., 26: 37-44. Elseewi, A.A., Bingham, F.T. and Page, A.L., 1978. Availability of sulphur in fly ash to plants. J. Environ. Qual., 7: 69-73. Handreck, K.A. and Black, N.D., 1994. Growing media for ornamental plants and turf. N.S.W. University Press, Randwick. HiII, M.F. and Lamp, C.A., 1980. Use of pulverised fuel ash from Victorian brown coal as a source of nutrients for pasture species. Aust. I. Exp. Agric. Animal Hush., 20: 377-384. Kabata-Pendias, A. and Pendias, H., 1984. Trace Elements in Soils and Plants. CRC Press Inc., FL. Martens, D.C., Schnappinger, M.G., Jnr. and Zelanzy, L.W., 1970. The plant availability of potassium in fly ash. Soil Sci. Sot. Am. Prcc., 34: 453-456. Page, A.L., Elseewi, A.A. and Straughan, I.R., 1979. Physical and chemical properties of fly ash from coal-tired power plants with reference to environmental impacts. Residue Rev., 71: 83- 120. Paul, J.L. and Lee, C.I., 1976. Relation between growth of chrysanthemums and aeration of various container media. J. Am. Sot. Hort. Sci., 101: 500-503. Roberts, TJ., 1966. The effect of land type and fme particle amendments on the emergence and growth of subterranean clover (Trifolium subterraneum L.) with particular reference to water relations. Aust. J. Agric. Res., 17: 657-672. Soil Survey Staff, 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA-SCS Agricultural Handbook 436, US Government Printing Office, Washington, DC. Weir, R.G. and Cresswell, G.C., 1993. Plant Nutrient Disorders 3. Vegetable Crops. lnkata Press, Melbourne.