Scientia Horticulturae, 29 (1986) 1--14 1 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE I N F L U E N C E OF P A R T I C L E - S I Z E D I S T R I B U T I O N IN P I N E B A R K : S A N D : B R O W N C O A L P O T T I N G MIXES ON W A T E R SUPPLY, A E R A T I O N A N D P L A N T G R O W T H

D. RICHARDS, M. LANE and D.V. BEARDSELL

Horticultural Research Institute, Knoxfield, Department of Agriculture and Rural Affairs, P.O. Box 174, Ferntree Gully, Vic. 3156 (Australia)

(Accepted for publication 12 February 1986) "

ABSTRACT

Richards, D., Lane, M. and Beardsell, D.V., 1986. The influence of particle-size distribu- tion in pinebark :sand :brown coal potting mixes on water supply, aeration and plant growth. Scientia Hortic., 29: 1--14.

In formulating soil-less mixes, high water-holding capacity must he coupled with good aeration. In pinebark-based mixes, the inclusion of a sand component to improve re-wettability properties may, however, result in an unacceptably low-water-holding capacity. Manipulating the particle-size distribution, particularly of the pinebark component, offers a means of improving water-holding properties. In composite mixes of pinehark, sand and brown coal, removal of all pinebark particles greater than 2 mm diameter improved total water, available water and days to wilting, generally without creating unfavourable levels of aeration. Manipulation of two sand sources resulted in only marginal changes in physical properties. The brown coal (lignite) component, despite increasing the total water supply, did not increase the water available for plant use. How- ever, brown coal, probably due to its high cation exchange capacity, increased the growth of all plant species tested. The development of a Coarseness Index, defined as the percen- tage by volume of particles > 1.0 mm diameter, allowed particle-size distribution of mixes to be characterized and correlated with physical properties. Alterations in physical properties due to manipulation of pinebark and sand components appeared to be directly related to subsequent effects on particle-size distribution. It is suggested that suppliers and nurserymen can readily alter performance of their potting mixes by sieving-off the coarse pinebark fractions which usually represent a high proportion of composite mixes.

Keywords: air porosity; available water; container plants; lignite; soU-less media; water- holding capacity.

Abbreviations: AP, air porosity after drainage from saturation; AWHC, available water- holding capacity; BC, brown coal (lignite); CI, coarseness index; CM, coarse--medium pinebark component; CMF, coarse--medium--fine pinebark component; M, medium pinebark component; MF, medium--fine pinebark component; RS, coarse river sand; SL, sandy loam; TWHC, total water-holding capacity.

0304-4238/86/$03.50 © 1986 Elsevier Science Publishers B.V.

INTRODUCTION

Pinebark-based mixes are now widely used in Australia, and many other countries, for growing containerized plants. In many instances a mixture of pinebark and peat, typically in a volume ratio of 1:1, has been recommended. Such mixtures have been shown to raise the water-holding capacity above that of pure pinebark whilst keeping costs substantially lower than for pure peat media (Bik, 1982, and references therein). Although pinebark:peat mixes adequately satisfy the principal physical requirements of high water-holding capacity coupled with good aeration, other factors need to be considered in formulating mixes. One important feature of a good mix, identified and quantified by Beardsell and Nichols (1982), is its re- wettability properties. Due to the hydrophobic nature of dried pinebark (and peat), it may be desirable to incorporate a proportion of sand into container mixes to aid water penetration and wettability (Beardsell and Nichols, 1982). In Australia, this recommendation of including sand is closely followed and, due to the high cost and variable quality of peat, many nurserymen exclusively utilize pinebark:sand mixes at ratios of about 4:1 by volume. However, by foregoing the high water-holding properties of peat and at the same time adding sand, more frequent watering is required. One method of improving the water-holding properties of pinebark-based mixes (without amendment with peat) is to control the particle-size distribution of the ingredients (Handreck, 1983).

In the present study, the influence of particle-size distribution of pinebark and sand components on physical properties and plant growth is determined. An additional component, brown coal (lignite), was included in the study because it is a readily available local product that might be important in terms of its chemical, rather than physical, effects on a composite mix. Other ingredients, such as coal cinders, have been shown to improve the nutrient status of mixes either by increasing the cation exchange capacity or by acting directly as a micronutrient source (Neal and Wagner, 1983).

The outcome of such studies may help in developing some simple guide- lines on how suppliers and nurserymen can readily manipulate pinebark- based mixes to improve their water-holding properties.

MATERIALS AND METHODS

Pot t ing mix ingredients. -- In each potting mix, the ingredients were selected from:

(1) pinebark -- hammer-milled and appropriately aged (Nichols, 1981) Pinus radiata bark obtained as a by-product from timber and paper industries;

(2) river sand --coarse sand dredged from river beds at various localities; (3) sandy loam -- an acidic, podzolic soil type described as Cranbourne

sand (Leeper, 1957); (4) brown coal -- a mined mixture of powdered and granulated lignite.

F o r m u l a t i o n o f p o t t i n g mixe=. - - Individual potting-mix ingredients were purchased from three different commercial suppliers. To characterize the particle-size distribution, each air-dried ingredient was separated by me- chanicaUy agitated sieves into 8 fractions (Table I). The volume of each fraction was determined. For all ingredients there was considerable variation in fraction volumes, particularly in the coarse and fine fractions. For this reason, it was decided to formulate experimental ingredients based on the average particle-size distribution. Thus, for each ingredient, the 8 sieved fractions (see Table I) were re-combined in proport ions that represented the average particle-size distribution of the 3 supplies. The resultant particle- size distributions of all these reconst i tuted ingredients are shown in Fig. 1. In addition to a fully reconst i tuted pinebark (CMF) using Fractions 1--8 (see Table I), a coarse--medium component (CM) using Fractions 1--6, a medium component (M) using Fractions 4-~3 and a medium--fine component (MF) using Fractions 4--8 were also prepared (Fig. 1D--G).

TABLE I

The particle-size range, designation and fraction numbers used when sieving all pinebark, sand and brown coal supplies in order to characterize their particle-size distributions

Fraction Designation Particle-size range number (mm)

1 Coarse > 9.50 2 Coarse 4.75--9.50 3 Coarse 2.00--4.75 4 Medium 1.00--2.00 5 Medium 0.50--1.00 6 Medium 0.25--0.50 7 Fine 0.10--0.25 8 Fine > 0.10

The formulation of experimental mixes was based on a commonly used volume-ratio of 4 pine ba rk : l sand: l brown coal. In mixes wi thout brown coal, the ratio was 4 p inebark: l sand. From these ratios, and using the re- const i tuted ingredients, 16 different pott ing mixes, resulting from the factorial combinat ion of 4 pinebark components × 2 sand components × 2 brown coal components , were prepared. Table II shows the composi t ion and particle-size distribution of these experimental mixes. In Table II, the term "Coarseness Index" has been used to quantify the effect of the different ratios and ingredients on the particle-size distribution. This index is the cumulative percentage, by volume, for all particles > 1.0 mm diameter (i.e. the sum of Fractions 1--4).

As is general nursery practice , dolomite at a rate of 2 kg m -3 was added to each experimental mix to adjust the initial pH to 5.7 and to provide a calcium and magnesium supply. Mixes were then dispensed into 12-cm

50-

40-

30-

20-

~10 - >. ~< 50-, z

40,

~. ao .

20"

10

A. BROWN COAL

E. PINEBARK- CM

1'2'3 '4 '5 '6 '7 '8 '

B. SANDY LOAM

'1°2'3'4'5'6w7'8 '

F. PINEBARK - M

1 '2'31 1 7 ' 8 ' FRACTION NUMBER

C. RIVER SAND

1 ~ 7 , 8 ,

G. PINEBARK - MF

,2,3~

D. PINEBARK-CMF

Fig. 1. The particle-size distribution of all experimental ingredients expressed as percentage by volume. The formulation of each ingredient was based on mixing together each of the 8 sieved fractions (see Table I). The volume of each fraction used was based on the average volume determined by sieving the ingredients from the 3 different suppliers. In addition to a fully re-constituted pinebark (CMF), a coarse--medium com- ponent (CM), a medium component (M) and a medium--fine component (MF) were prepared.

(830 cm 3) po ts to a s tandard dep th so tha t the mix initially occup ied app rox ima te ly 700 cm 3. A fine gauze covered the drainage holes to p reven t loss o f media f r om pots .

P l a n t e s t a b l i s h m e n t . - - T o m a t o ( L y c o p e r s i c o n e s c u l e n t u m Mill. cult ivar 'Grosse Lisse') seedlings were t ransplanted in to each p o t on 3 November . F o r each mix there were 5 repl icat ions. The pots were loca ted at r a n d o m in a vent i la ted glasshouse and were irr igated daily. Supp lemen ta ry feeding with nu t r i en t so lut ion (Aquasol R at 1 g 1-1) was appl ied 3 t imes per week. Af te r 4 weeks, supp lemen ta ry feeding was al tered to a so lu t ion o f 0.5 g 1-1 KNO3, 0.5 g 1-1 Ca(NO3)2 and 0 .15 g 1-1 Na2H2PO4, and also appl ied 3 t imes per week instead o f irr igation water .

P h y s i c a l p r o p e r t i e s of m i x e s . - - When the t o m a t o seedlings had reached app rox ima te ly 50 cm in height , a f te r 10 weeks growth , the wilting trial was begun. Pots were t ho rough ly wate red to sa tura t ion and al lowed to drain for 2 h before being weighed to de t e rmine TWHC. Wi thou t any fu r t h e r water ing, po ts were weighed at daffy intervals fo r 12 days, af ter which d ry weights o f

TABLE II

Descr ip t ion d i s t r ibu t ion

(percentage by volume) of exper imenta l mixes ~ and their particle-size

Pinebark: Brown Fraction number sand coal

1 2 3 4 5 6 7 8

Coarseness index 2

1 CMF:RS + < 1 21 30 21 14 7 6 3 71.4 2 - - 0 23 31 21 11 7 4 2 74.7

3 CMF :SL + <1 21 24 16 13 15 9 3 61.0 4 - - <1 23 24 16 13 15 8 2 62.1

5 CM:RS + <1 22 31 21 14 8 2 1 75.1 6 - - 0 24 33 22 12 7 <1 <1 79.1

7 CM:SL + <1 22 25 17 14 15 5 1 64.6 8 - - <1 24 25 17 14 16 4 <1 66.5

9 M F : R S + <1 2 11 36 23 13 10 4 49.4 10 - - 0 <1 8 40 23 14 9 4 48.4

11 M F : S L + <1 2 5 32 23 21 13 5 39.0 12 - - <1 <1 <1 34 25 23 13 4 35.8

13 M:RS + <1 2 11 42 27 16 2 1 55.0 14 - - 1 1 8 46 27 17 <1 <1 54.7

15 M:SL + <1 2 5 37 26 23 5 1 44.3 16 - - <1 <1 <1 41 29 25 4 <1 42.1

1 Mixes c o m p o s e d of: p i n e b a r k : s a n d : b r o w n coal in rat io 4 :1 :1 ; p inebark : sand in rat io 4:1. P inebark c o m p o n e n t s : CMF, Frac t ions 1--8; CM, Frac t ions 1--6; MF, Frac t ions 4--8 ; M, Frac t ions 4--6. Sand c o m p o n e n t s : RS, coarse river sand; SL, sandy loam. Brown coal c o m p o n e n t s : +, b r o w n coal: -- , no b r o w n coal. 2Coarseness Index is def ined as the cumulat ive percentage of part icles > 1.0 m m diameter , and thus is the sum of Frac t ions 1--4 descr ibed in Table I.

the media were determined by oven<lrying at 65 ° C. For each mix, 3 samples of known volume were used to determine bulk density. AP was then calculated according to the method and regression lines described by BeardseU et al. (1979a).

W i l t i n g t i m e . - - The time taken for the tips and young leaves to show the first signs of wilting was defined as "Stage I" wilting. The time for all leaves to wilt was defined as "Stage II", and the time at which subsequent stem wilting occurred was defined as "Stage III". Stage III wilting had occurred in all pots 12 days after the irrigation, and at this point plants were harvested for shoot dry weight determination.

G r o w t h trial. - - The effect of the 16 potting mixes on plant growth was examined in a separate 14-week trial.

Plants chosen for the growth trial were: (1) tomato 'Grosse Lisse'; (2) Boron ia h e t e r o p h y l l a F. Muell. -- as a representative of an Australian genus commonly used in the nursery trade: (3) P e p e r o m i a heder i fo l ia Hort. -- representing a slow-growing, popular indoor plant.

These plants were irrigated daily, but slow-release fertilizers at a rate of 1 kg m -3 3--4 month Osmocote R (15--5.2--12.5), 2 kg m -3 7--9 month Osmocote (18--4.8--8.3) and 1 kg m -3 Micromax R replaced the liquid feeding employed in the wilting trial. Plants were grown in a ventilated glass- house for 14 weeks prior to harvesting for top dry weight determination. There were 4 replications of single plants for each experimental pott ing mix.

RESULTS

P o t t i n g mixes . - - The term Coarseness Index (the cumulative percentage of particles > 1.0 mm diameter) proved to be a useful means of characterizing the particle-size distributions of the individual mixes into a single expression. In the following sections, CI is used frequently to aid in discussing the individual mixes and treatment effects. It should be noted that a term for "fineness" could equally have been defined and used in the discussion. Table II shows that CI varied between mixes from 36 to 79%. Since all mixes are dominated by pinebark, this ingredient had a major effect on the CI of the resulting mixes. For example, mixes with a coarse pinebark component (CMF and CM) had an average CI of 69%, whereas those wi thout this component (MF and M) had an average CI of 46%. The average effect of including river sand rather than sandy loam was to increase the CI by 12%. The incorporation of brown coal, which has a fairly uniform particle-size distribution (Fig. 1A), did not greatly affect the CI of any mix. For example, in those mixes with a coarse pinebark component , brown coal decreased the CI by an average of 3%, whereas in mixes wi thout coarse pinebark, brown coal increased the CI by 2%.

Water. re la t ions and aerat ion . - - The effect of the different pott ing mixes on some physical properties is shown in Tables III and IV. Only the main effects of the ingredients are shown, since factorial analysis of variance indicated that there were no significant interactions between ingredients. Mixes containing coarse pinebark components (CMF and CM) produced similar TWHC and AP. Similarly, there were no differences between mixes where the coarse pinebark component was excluded (MF and M). Between these two groups, however, the effect of the coarse pinebark was to substantially reduce TWHC and increase AP; both by about 30%. In contrast to the influence of the coarse pinebark, the inclusion of fine pinebark had no effect. For the other ingredients, sandy loam increased TWHC and decreased AP compared with coarse river sand, and the average effect of including brown coal in the mix was also to increase TWHC and decrease AP.

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TABLE III

Main effects of potting-mix components on water-holding capacity and air porosity after 2 h drainage from saturation

Treatment main effects Total water-holding capacity Air porosity

g/pot %v/v Arcsine %

A. Pinebark component CMF 311 44.4 0.67 38.3 CM 311 44.4 0.66 37.5 MF 409 58.4 0.51 24.1 M 385 55.0 0.56 27.8

LSD (P = 0.05) 33 0.04 (P = 0.01) 43 0.06

B. Sand component RS 329 47.0 0.63 34.8 SL 380 54.3 0.57 28.8

LSD (P = 0.05) 23 0.03 (P = 0.01) 31 0.04

C. Brown coal component --BC 328 46.9 0.63 34.7 +BC 380 54.3 0.57 29.0

LSD (P = 0.05) 23 0.03 (P = 0.01) 31 0.04

Table IV shows the inf luence o f the mixes on AWHC at Stage I and Stage II wilting. Data fo r Stage III wilt ing is n o t shown because Stage III was, in prac t ice , f o u n d t oo di f f icul t to accura te ly assess and in any case, plants are p roba b ly irreversibly damaged b e y o n d Stage III and of no fu r t h e r use to nu r se rymen . At b o t h Stage I and II, the e f fec t o f the coarse p inebark and of the sand on AWHC resembled t ha t fo r TWHC (see Table III), and there was a close re la t ionship be tween the n u m b e r of days to each wilting stage and AWHC. F o r addi t ion o f b r o w n coal, however , there was no increase in AWHC despi te the large increase in TWHC, and y e t t ime to wilt ing at Stage I decl ined.

Plant g rowth af te r 12 weeks was u n a f f e c t e d by p inebark and sand ingredients , bu t increased in b rown-coa l -amended mixes (Table IV).

R e g r e s s i o n a n a l y s i s . - - using the coarseness index (CI), it was possible to analyse results by regression analysis in addi t ion to fac tor ia l analysis o f variance. The regression da ta highl ighted some o f the general fea tures o f the mixes and ingredients (Figs. 2 and 3). Three o f the graphs (Figs. 2a, b and 3b) show tha t when b r o w n coal add i t ion was analysed as a pseudo-variable

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TABLE IV

Main effects of pott ing-mix components on available water, wilting and plant growth

Trea tment main effect Available water at wilting-stage: Days to wilt Shoot at stage: dry wt.

I II (g) I II

g /pot % v/v g/pot % v/v

A. Pinebark fractions CMF 172.4 24.6 201.7 28.8 4.1 5.8 5.64 CM 176.7 25.2 213.5 30.5 3.8 6.1 5.81 MF 276.3 39.5 303.0 43.3 5.9 7.8 6.14 M 268.2 38.3 295.0 42.1 6.3 8.0 6.06

LSD (P = 0.05) 25.4 25.4 0.9 0.9 0.65 (P = 0.01) 33.8 33.7 1.1 1.2 0.86

B. Sand fractions RS 206.3 29.5 234.7 33.5 4.6 6.4 5.84 SL 240.5 34.4 272.0 38.9 5.4 7.4 5.99

LSD (P = 0.05) 18.0 17.9 0.6 0.6 0.46 (P = 0.01) 23.9 23.9 0.8 0.8 0.61

C. Brown coal fractions --BC 222.0 31.7 248.5 35.5 5.3 7.1 5.53 +BC 224.8 32.1 258.2 36.9 4.7 6.8 6.29

LSD (P = 0.05) 18.0 17.9 0.6 0.6 0.46 (P = 0.01) 23.9 23.9 0.8 0.8 0.61

(+BC = +1; no BC = --1), it made a significant contribution to the regression equations.

TWHC declined linearly with increasing CI (Fig. 2a). There was no distinc- tion between alterations in either the pinebark or sand components, in that any change in either component that caused an increased CI resulted in a proportional decline in TWHC. The incorporation of brown coal into the media had a different effect. Brown coal substantially increased TWHC but had little effect on altering CI (see also Table II). The effect of brown coal, therefore, cannot be solely due to its influence on particle-size distribution. A further feature of Fig. 2a, which was not apparent in analysis of variance (Table III), was the interaction between brown coal and the other components. The increased TWHC due to brown coal was less at lower CI than at high CI. Figure 2b shows that AP increased with increasing CI, and that brown coal resulted in a decreased AP.

In the relationship between AWHC (at Stage I wilting) and CI, there was no significant regression component due to incorporation of brown coal

i-

u

70.

60.

so,

40-

o a .

i i ! i

)-

O

5 0 -

40-

30"

20 .

j o j

Y =2.50 + 0 .51X-0 .0SBC.X r = 0.920

& sb e'0 7b go COARSENESS INDEX

% part ic les > l . 0 m m diam.

Fig. 2. The relationship between water-holding and aeration properties of 16 experimental media and particle-size distr ibution expressed as a Coarseness Index. (a) Total water-holding capacity (% by volume); (b) percentage air porosity. Pinebark com- ponent: o, coarse--medium--fine; a, coarse--medium; 0, medium; A, medium--fine. Sand component : small symbols, sandy loam; larger symbols, river sand. Brown coal: open symbols, added brown coal; closed symbols, no brown coal. For regression equations, brown coal was analysed as a pseudo-variable: +BC = 1 ; --BC = --1.

(Fig. 3a). For all mixes, AWHC declined linearly and proportionally with increasing CI. This confirms that the additional water held in brown~oal- amended mixes after 2 h drainage was apparently not available for plant use at wilting Stage I (Table IV).

The relationship between days to reach wilting Stage I and CI also showed a significant influence due to addition of brown coal (Fig. 3b). This time the effect was constant such that, for a given CI, mixes with brown coal reduced wilting time by 0.7 days. As for the physical properties, the pinebark and sand components affected wilting time in proportion to their resultant changes to CI.

10

a .

• O •

~" 40 ~ •

< Y = 6 3 . 6 7 - 0 . 5 5 X r = 0 .914 •

10 V

u i ! I !

Z I.-

*" < I - (n

4

2 -

b .

~ " " , , , , , , ~ • , ~ a_

Y = 9 , 9 0 - 0 X 9 X - 0 . 3 5 ~ r = 0 . 8 9 0

£ s~ e'0 7b 8'0 COARSENESS INDEX

% pa r t i c l es ~ 1 . 0 r a m d i a m ,

Fig. 3. The relationship between tomato plant water-use in 16 experimental media and particle-size distribution expressed as a Coarseness Index. (a) Available water-holding capacity (% by volume) at wilting stage I (when shoot tips and leaves begin to wilt); (b) days to wilting Stage I. For details of symbols, see caption for Fig. 2.

G r o w t h trials. - - The main effects of the different ingredients on the growth of different plant species after 14 weeks is shown in Table V. Peperorn ia grew better in pinebark with a coarse component (CMF and CM) and best if fine fractions were also included (i.e. CMF). Boronia , on the other hand, favoured pinebark with a fine component (CMF and MF), irrespective of the inclusion of a coarse component . Tomato showed no preference for any pinebark component , as was also indicated by the wilting trial (Table IV). None of the species responded differently to the sand components , but all of the species grew better when brown coal was added to the mix; a result also established for tomato in the wilting trial.

II

TABLE V

Main effects of potting mix components on growth of Peperomia, Boronia and Lycopersicon

Treatment main effect Shoot dry wt. (g)

Peperomia Boronia Lycopersicon

A. Pinebark component CMF 3.98 4.92 5.48 CM 3.29 4.50 5.38 MF 2.78 4.95 6.04 M 2.59 4.30 5.32

LSD (P = 0.05) 0.36 0.49 0.75 (P = 0.01) 0.49 0.66 1.00

B. Sand component RS 3.13 4.84 5.65 SL 3.19 4.50 5.47

LSD (P = 0.05) 0.26 0.35 0.50 (P = 0.01) 0.34 0.47 0.70

C. Brown coal component - BC 2.98 4.45 5.30 +BC 3.34 4.88 5.80

LSD (P = 0.05) 0.26 0.35 0.50 (P = 0.01) 0.34 0.47 0.70

DISCUSSION

Physical properties. -- After 10 weeks of daily irrigation, none of the experimental mixes had developed aeration levels lower than 20% AP when measured 2 h after saturation. Air-filled porosities of between 15 and 25% are considered adequate for general nursery mixes (Handreck and Black, 1984). Thus, in the absence of limitations due to aeration, the most useful mixes are those that supply the most available water. The pinebark component had the dominating influence on water supply. Total water, available water and days to wilting were all substantially increased (Tables Ill and IV) by exclusion of the coarse pinebark component (defined here as all particles > 2 mm diameter). This improved water supply is essentially due to an increase in the proportion of small inter-particle pores, and a c o n c o m i t a n t decrease in large, rapidly free<lraining pores (Handreck, 1983) . There is evidence, however , t h a t wa te r supply is inf luenced by o the r proper t ies associated wi th p inebark particles. Airhar t et al. (1978) and P o k o r n y and Wetzstein (1984) have shown tha t p inebark particles can

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absorb substantial amounts of water internally. Moreover, hammer-milling not only reduces particle size, but is also likely, by random fracturing, to expose more of the internal pore space of particles for water adsorption. In the present experiments, 44% of TWHC was unavailable to plants at Stage I wilting in mixes containing coarse pinebark, whereas mixes wi thout coarse pinebark fractions retained only 31% as unavailable water. This difference probably reflects the internal porosi ty and surface adsorption properties of pinebark particles. For mixes with a high proport ion of large pinebark particles, internally adsorbed water may not be readily accessible by roots. However, for finer particles, water extraction may be improved, since the surface area: volume ratio is greater, and root penetration into cracks and pores in the particles need not occur over extended distances.

Coarse pinebark fractions (> 2 mm diameter) commonly represent 30--40% by volume of commercial potting mixes. The present results indicate that these fractions could be readily eliminated from a general pott ing mix so as to increase water availability wi thout any adverse effects on aeration.

A comparison of the two sand sources showed that the finer sandy loam increased TWHC and decreased AP compared with the coarse river sand. These effects showed no interactions with changes in the other potting-mix ingredients, bu t were lower in magnitude than those resulting from manipulation of the more substantial pinebark component . Thus, alteration of the sand source may be a simple means by which nurserymen can marginally manipulate their mixes to raise/lower TWHC or lower/raise AP according to the specific requirements of some plant species.

The effect of including brown coal on the physical properties of the mix was different to the other ingredients. Although addition of brown coal substantially increased the total water supply, the water available for plant use was not improved. This reduced water availability in brown coal mixes has been noted previously (Beardsell et al., 1979b). The other major feature of brown coal amendment is that its effects on the physical properties were largely independent of any influences on particle size distribution (Figs. 2 and 3). Taken together, these results indicate that brown coal is a highly porous medium and that adsorbed water is held under high tension upon, and/or within, the brown coal particles. Data shown in Table IV, although not statistically significant, further suggests that brown coal may yield some of this tightly-held water under the higher plant suctions developed at Stage II wilting.

Ingredients such as pinebark and sand appear to impart their influence on mixes in a non-interactive and additive way, thus allowing the prediction of physical properties of mixes formulated from different ingredients (Figs. 2 and 3; Brown and Pokorny, 1975; Beardsell et al., 1979a,b; Porkorny and Henny, 1984). It seems likely from the present work that the effect of ingredients may act largely through alterations made to the overall particle- size distribution of resulting mixes. As pointed out by Hoitink and Poole

13

(1980) and Handreck (1983), no formula is available to link physical properties such as TWHC, AWHC and AP to particle-size distribution. The use of the Coarseness Index (% of particles > 1.0 mm diameter} in the present paper provides this link, allowing the quantification of particle-size distribution and its correlation with physical properties. The use of this easily determined index requires further verification for different ingredients and mix formulations. However, it should be noted that porous ingredients, such as brown coal, may alter physical properties largely independently of their effect on the Coarseness Index.

Plant growth and performance. - - The influence of particle size and mix formulation on physical properties cannot be viewed independently of effects on plant growth. In each trial, and for each plant species, it was evident that the inclusion of brown coal increased shoot dry weight. For tomatoes grown in the wilting trial, the effect of brown coal on increasing plant growth was also reflected by a reduction in the number of days to wilting (Fig. 3b). It is likely that growth promotion by brown coal is due to its chemical, rather than physical, properties. Brown coal (lignite) has a very high cation exchange capacity (600 me 1-1}, which is approximately 3 times higher than that for aged pinebark (Handreck and Black, 1984}. Thus, as has also been suggested for coal cinders (Neal and Wagner, 1983}, addition of brown coal would be expected to improve the buffering capacity and reduce leaching of nutrients from potting mixes.

Pinebark fractions appeared to affect plant growth through influences on both water supply and aeration. Tomato and Boronia grew best in high water-holding mixes with fine pinebark fractions included. In both species, however, there was no benefit to plant growth from including a coarse pinebark component. Given that days to wilting is reduced by inclusion of coarse pinebark fractions (Table IV, Fig. 3b), the results again suggest that these fractions could successfully be eliminated from the mix when growing these, and other species with similar requirements. The slow-growing indoor plant Peperomia, on the other hand, appeared to respond best to mixes that included coarse pinebark fractions. Under the present experimental condi- tions of frequent irrigation, these plants may be subjected to water levels that are too high for optimum aeration. Therefore, only those mixes with inherently high air porosities near saturation were suitable for Peperomia.

REFERENCES

Airhart, D.L., Natarella, N.J. and Pokorny, F.A., 1978. The structure of processed pine bark. J. Am. Soc. Hortic. Sci., 103: 404--408.

Beardsell, D.V. and Nichols, D.G., 1982. Wetting properties o f dried-out nursery con- tainer media. Scientia Hortic., 17: 49--59.

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