Growth Media for Green Roofs: a trial for The Hills …€¦ · Final Report 12 August 2011 Growth...

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Final Report 12 August 2011

Growth Media for Green Roofs: a trial for The

Hills BARK BLOWER™ Company 2009 – 10

E. Charles Morris

School of Natural Science

Hawkesbury Campus

University of Western Sydney

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1. SUMMARY

There has been substantial overseas research into the environmental benefits of green roofs, such as

temperature reduction, control of stormwater runoff and increase in urban amenity. There is a need

for corresponding research in the Australian context: while the overall benefits are likely to be similar

to those found overseas, the local requirements for growing media and plant stock are more likely to

be different to those documented for the northern hemisphere or tropical locations where research has

been conducted to date.

The Hills BARK BLOWER™ quality team commissioned research by The University of Western

Sydney to test a number of growing media and soil depth combinations for their effects on plant

growth, water quality and temperature reduction, to determine what might work best for green roofs in

Sydney, Australia. Three soil mixes were used; a standard mineral soil mix, and two organic:mineral

mixes for comparison. One mix used 80% organic compost media to 20% mineral soil (80:20 mix),

and the second mix used a ratio of 50% organic to 50% mineral (50:50 mix). These mixes were

combined with three soil depths (100, 200 and 300 mm); each mix x depth combination was

established in two 3 x 3 m brick planting boxes, one of each which was randomised into two blocks

established at The Hills BARK BLOWER™ landscape yard at Kenthurst, Sydney, in late 2009. A

standard mix of 15 plant species was planted into each box in early December 2009, and a layer of

forest mulch applied as the surface layer in boxes. No fertiliser was supplied during construction. An

establishment period of six weeks was allowed, after which measurement of plant growth and water

quality was undertaken from late January to late March 2010 (trial period; Weeks 7 – 17). In the post-

trial period, fertiliser was added to one block of the experiment (August 2010). Plants were re-

measured for growth in October 2010 (week 45) and drainage water samples collected for analysis.

During the trial period there was little plant death, and plant growth differed little amongst the mix x

depth treatments. In the subsequent period up to 90 weeks, more plants died in the shallowest depth

treatment. Visible differences in plant growth and health became apparent, with best growth on the

50:50 mix, and then (in decreasing order) the 80:20 mix, and the standard mix.

Measurements of water quality showed high initial concentrations of inorganic chemicals in samples

collected from the first watering-in, with differences amongst mixes detected for some variables

(ammonia, phosphate and sodium) and amongst depths for some other variables (chloride, electrical

conductivity (EC) and pH). The concentrations for all variables declined steeply over time, and

differences amongst mixes and depths disappeared, so that values for most variables met the NHRMC

Australian Standard for Drinking Water by week 45.

Soil slumpage was minimal (<5mm) over all treatments for the period of observation. The daytime

temperature difference between a bare bitumen surface and the base of two 100 mm planter boxes was

15 – 25 oC over the course of two days in October 2010, when measurements were taken.

The results demonstrated that all the mixes were suitable for plant growth, and that 200 or 300 mm

soil depth was better for plant survival. Once past the ‘first flush’ of inorganic chemicals from the

mixes, the quality of drainage water was such that capture, storage and re-use for irrigation would be

feasible. Slumping of any of the mixes was very minor over the period of observation. The insulation

provided by the green roof set-ups in the trial was substantial, and comparable with that recorded for

overseas green roofs.

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2. INTRODUCTION

There has been a recent trend to locate gardens on top of urban buildings; such installations

are referred to as ‘green roofs’, to distinguish them from the traditional roof top of conventional

building material. There are a number of reasons driving this trend, ranging from environmental

benefits to the physical environment, to social benefits from such plantings.

One immediate benefit to the physical environment is the effect that green roofs can have on

ambient temperature. The phenomenon of ‘urban heat island effect’ refers to the elevated

temperatures experienced in urban environments, relative to non-urbanised ones. For example Susca

et al. (2011) documented an average difference of 2 0C difference between the most and the least

vegetated areas of New York. This effect arises because the high thermal mass properties of

traditional building materials such as concrete, steel etc results in more absorption and retention of

heat by buildings than occurs in non-urbanised environments. This absorbed heat is then re-radiated

by the buildings at night, giving the elevation in temperature referred to. Green roofs can reduce this

effect by intercepting the solar radiation before it reaches the underlying building materials, and

shedding it either by reflection, or using if to evaporate water from leaves (Takebayashi et al. 2007).

Wong et al. (2003) documented the heat gain and heat loss from different surfaces over a typical day

in Singapore; a bare hard surface gained 366 kJ/m2 and lost only 4.2 kJ/m

2; the same figures for a turf

lawn were a gain of 29 kJ/m2 and a loss of 62 kJ/m

2, and for a tree were gain of 15 kJ/m

2 and loss of

53 kJ/m2. The vegetated surfaces showed a net energy loss over a day, compared to the hard surface

which showed a strong net energy gain. Temperatures of the hard surfaces ranged up to 57 0C in the

middle of hot days in this study; by comparison, temperatures at the soil surface under vegetation in

green roofs was 20 – 30 0C less, depending on the species used (Wong et al. 2003). The decline in

temperature under vegetation compared to a bare surface was approximately 10 degrees in a Japanese

study (Takebayashi et al. 2007).

Stormwater issues such as volume and water quality can be improved by the installation of

green roofs. The growing substrate for the green roofs absorbs rainfall and stores it up to the capacity

of the medium/volume combination used; this acts to reduce immediate runoff and spread the

subsequent runoff over a longer period than a traditional hard surface roof. If surface runoff is trapped

and fed through a green roof set-up, concentrations of water pollutants such as nitrates and phosphates

can be reduced.

A number of social benefits can arise from use of green roofs, whose the introduction can

help alleviate the urban vista of large constructed surface areas of concrete, glass and steel. The

presence of the plants in the gardens brings the natural components of green foliage, colourful

flowers, shade, water features and other aesthetic and practical values to an otherwise bare cityscape.

Use of green roofs can enhance the environment for the occupants of the building on which it is

located if used for recreation, and enhance the view of that building by occupants of surrounding

buildings. Whilst intangible, these improvements can have direct effects on human health and well-

being. Improving the view from urban buildings with green roofs has also resulted in increased real

estate values in North America and the UK.

Current recommendations for green roofs are to use mineral-based growing media. An

argument against the use of organic media in roof top locations has been that shrinkage of the organic

media will impact adversely on plant growth. However, there are a number of reasons for considering

organic growth media for green roofs. Firstly, organic media can maintain a long-term supply of

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nutrients for plant growth in a way that mineral media cannot. Secondly, organic media have a lower

bulk density than mineral media, and so are considerably lighter, volume for volume, than mineral

media. This weight saving can be important in roof top locations. Saving weight on the growing

media used has another benefit: the current recommendation for depth of the (mineral) media is 100

mm. If a move to organic media is possible, the saving in weight would allow greater depths to be

used.

If an organic growth medium was to be used, current recommendations are for no post-

planting maintenance, including no further watering. A possible modification of this system would be

to store drainage water from the containers holding the media and plants, and re-use for watering. If

this was to be done, the quality of the drainage water from organic media requires examination, to see

whether it is suitable for the purpose.

An experimental trial of growth media, using a standard mineral-based mix and two different

organic formulations was established at a range of soil depths at Sydney, Australia, to answer the

above questions. The trial allowed comparison of plant growth on the two types of media, and at a

range of soil depths. A uniform mix of commercially available plant stock was used, to allow testing

of a wide range of potential species and cultivars. The specific questions being asked in the trial

were:

1. How would plant growth compare between the standard mineral mix and the two organic

mixes, and between the two organic mixes?

2. How would water quality compare amongst the growth media?

3. How would soil shrinkage compare amongst the media?

4. How did temperature compare between the base of planter boxes and the bare substrate?

3. METHODS

3.1 Design overview

The factors varied in the trial were growing mix and soil depth:

growing mix: three types,

o ‘standard’ (100% mineral)

and two experimental organic:mineral mixes

o 80% organic:20% mineral, termed hereafter as ‘80:20 mix’ (or ‘organic 1’)

o 50% organic:50% mineral, termed hereafter as ‘50:50 mix’ (or ‘organic 2’).

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Table 3.1 Recipe for three types of growing mixes.

80:20 Mix Recipe 15m³

Product Coir 35% 5.25 m³ Composted Pine Bark 45% 6.75 m³ Clinker Ash 20% 3 m³

50:50 Mix Recipe 15m³ Product Coir 25% 3.75m³ Composted Pine Bark Fines 25% 3.75m³ Scoria ≤ 6mm 40% 6m³ Clinker Ash 10% 1.5m³

Standard Mix Recipe 15m³ Product Scoria ≤ 6mm 50% 7.5m³ Bayswater Sand (ash) 40% 6m³ Coarse Sand 10% 1.5m³

soil depth: three levels:

o 100 mm

o 200 mm

o 300 mm

This gave nine combinations of mix x depth; each combination was replicated twice in 3 x 3 metre

bays constructed as two blocks at The Hills Bark Blower site at Kenthurst (Table. 3.2).

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Table 3.2: Lay-out of 3 x 3 metre cells in the two blocks of the experiment.

Block 1

1. Depth: 300 mm

Mix: 50% organic,

50% mineral

7. Depth: 300 mm

Mix: 100% mineral

13. Depth: 300 mm

Mix: 80% organic, 20 %

mineral

2. Depth: 200 mm

Mix: 100% mineral

8. Depth: 200 mm


mineral

14. Depth: 200 mm

Mix: 50% organic, 50%

mineral

3. Depth: 100 mm

Mix: 80% organic, 20

% mineral

9. Depth: 100 mm


mineral

15. Depth: 100 mm

Mix: 100% mineral

Block 2

4. Depth: 300 mm

Mix: 100% mineral

10. Depth: 300 mm


mineral

16. Depth: 300 mm


mineral

5. Depth: 200 mm

Mix: 80% organic, 20

% mineral

11. Depth: 200 mm

Mix: 100% mineral

17. Depth: 200 mm


mineral

6. Depth: 100 mm

Mix: 50% organic,

50% mineral

12. Depth: 100 mm


mineral

18. Depth: 100 mm

Mix: 100% mineral

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3.2 Construction of planter boxes

Brick-walled bays or planter boxes (3 x 3 metres) were constructed (at ground level) at The Hills Bark

Blower site at Kenthurst, NSW during October –November 2009. The height of the brick walls was

varied to suit the three planting depths used. North-south walkways allowed access to bays; the three

adjacent bays on the north-south axis shared a common wall (Fig. 3.1).

Fig. 3.1: View of the southern block (block 2) of planter boxes under construction. Walkway

between boxes visible.

The layers in the boxes (from the bottom up) were:

blue metal layer (Fig 3.2)

drainage cells (Fig. 3.3)

geofabric (Fig 3.4)

drainage sand (Fig 3.4)

growing medium (Fig 3.5)

forest mulch (Fig 3.5)

After installation of the blue metal, a length of PVC pipe was inserted at the lowest corner of the bay

to allow collection of drainage water (Fig. 3.4). The drainage cells, geofabric and a 25 mm layer of

drainage sand was laid over the base of each box before the growing medium was installed to the

specified depth. After planting out, a final layer of forest mulch was applied to each planter box as it

binds together and is suitable for windy environments (Fig. 3.5).

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Fig. 3.2: Close-up of a bay showing covering layer of blue metal and PVC pipe for collection of

drainage water.

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Fig. 3.3: Drainage cell layer in place and geofabric ready for roll-out.

Fig. 3.4: layer of drainage sand being applied over geofabric layer

Fig. 3.5: Completion of planting of standard array of plants into boxes: growing media in place,

covered by bark mulch layer. Water collection pipes visible in corner of each planter box.

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Table 3.3: Plant material used in the trial: scientific names, common names and growth form.

Scientific and Culitvar names Trade name Growth form

Westringia fruticosa ‘WES05’ PBR Mundi™ shrub

Myoporum parvifolium 'PARV01' PBR Yareena ™ woody ground cover

Anigozanthos hybrid var. 'Gold Velvet' Gold Velvet PBR graminoid

Dianella caerulea ‘DC150’ PBR Aranda ™ graminoid

Dianella caerulea ‘DCMPO1’ PBR Little Jess ™ graminoid

Liriope muscari ‘LIRF’PBR Isabella ® graminoid

Lomandra fluviatilis ‘ABU7’ PBR Shara ™ graminoid

Lomandra hystrix ‘LHBYF’ PBR Katie Bells ™ graminoid

Lomandra longifolia ‘LM300’ PBR Tanika ® graminoid

Phormium tenax ‘PHOS2’ PBR Sweet Mist ® graminoid

Pennisetum alopecuroides ‘PAV300’

PBR

Pennstripe ™ grass

Themeda australis var. ‘Mingo’ Mingo PBR grass

Gazania tomentosa ‘GT10’ PBR Tiny Tom ™ ground cover

Sedum pachyphyllum Jelly Bean Blue succulent

Succulent hybrid #53 succulent

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3.3 Plant measurements

Planting out was completed by the end of the first week in December 2009; the following six-week

period (from Monday 7 December; week 1 of the experiment) was allowed as an establishment period

for the plants. Water was supplied during this period (twice per day in week 1; once per day in week

2; every second day in weeks 3 – 6). No plant death occurred, so no replacement planting was

necessary.

The establishment period ended on 17 January 2010 (end of week 6); watering was withdrawn.

Sampling

Plants

The following data were recorded at each sampling occasion (Table 3.4):

survivorship: whether plants were alive or dead

growth: given the wide range of species and growth forms of the plants used, a number of

measurements were used to follow plant growth, with methods adapted to the plant in question

(Table 3.5 ). Plant height or stem length were measured with a ruler. Canopy height was measured

by standing a ruler in the plant canopy, inserting the ruler through a hole in a cardboard disc of 25

cm diameter which was then dropped onto the canopy; canopy height was taken as the distance

from the ground to where the disc was resting.

Table 3.4: Sampling times for plant survivorship and growth

Week of experiment Date Data

8 25 Jan survivorship; growth

12 22 Feb survivorship; growth

17 29 Mar survivorship; growth

45 8 Oct survivorship; growth

Table 3.5: Species used to follow plant growth, and measurement taken.

Scientific name Common name Measurement

Westringia fruticosa ‘WES05’ Mundi™ plant height (ruler)

Myoporum parvifolium Yareena™ stem length

Anigizanthos hybrid Golden Velvet PBR height of highest inflorescence (ruler)

Liriope muscari ‘LIRF’PBR Isabella PBR canopy height (disc method)

Lomandra hystrix

‘LHBYF’PBR

Katie Bells™ height (disk method)

Phormium tenax‘PHOS2’PBR Sweet Mist® height (disk method)

Gazania tomentosa ‘GT10’PBR Tiny Tom™ stem length

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3.4 Drainage water

Samples of drainage water (one larger sample of 1 L; two smaller samples of 250 mL) were taken

from the pre-inserted pipes shown in Fig. 3.5 at each sampling occasion, stored in plastic sample

collection bottles and sent immediately to ALS Laboratory Group for analysis. The range of

parameters measured is listed in Table 3.6. Water samples were taken from the initial watering-in, to

capture the ‘first flush’ of nutrients out of the growth media (Table 3.6). Further samples were taken

in weeks 10 and 14 (Table 3.7 ).

Table 3.6: Parameters measured for drainage water samples by ALS Laboratory Group.

Parameter ALS Package

code

Method Reference Limit of

Reporting

pH EA005 APHA 4500-H+ B 0.01 pH units

conductivity EA010 APHA 2510 B

solids - - total

dissolved (TDS)

EA015H APHA 2540 C 5

cations - Na EA093 APHA 3120 1

chloride ED045 APHA 4500-Cl- B 1

nitrate EK058 APHA 4500-NO3- I 0.01

reactive

phosphorus

EK071 APHA 4500 P - G 0.01

ammonia as N EK055 APHA 4500-NH3- H 0.01

Table 3.7: Sampling times for drainage water.

Week of experiment Date comments

1 7 Dec 2009 drainage water collected from

initial watering-in

10 8 Feb 2010 end of rainy week

14 8 Mar 2010 after one day of rain

45 8 Oct

46 15 Oct

3.5 Soil movement

A taut string line was laid across each of the planter boxes, stretched over the lip of the brick walls, to

give a stable reference position for horizontal movement of the soil. Distance from the horizontal

lines to the soil surface was measure for three randomly-selected points per line, to give a total of six

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readings per planter box; an average was calculated from these six readings. This measurement was

taken in weeks 8, 12, 17 and 45.

3.6 Temperature readings

Thermocouples were used to measure temperature at the base of planter boxes, and on the adjacent

bitumen substrate, for a 50-hour period from approximately 13:00 hours on 26 October to 15:00 hours

on 28 October 2010. Thermocouples were connected to Squirrel data loggers and set to record

temperature every 10 minutes. Thermocouples were set up at the base of one planter box for each of

the three mixes, in the 100 mm depth treatment. The data returned from one of the data loggers were

anomalous (80:20 mix), and so data are reported for only two planter boxes. Temperature data was

downloaded into Excel, and graphed vs. time. The difference between the bare substrate reading and

the planter box reading was calculated for each sampling time, and graphed vs. time.

3.7 Data analysis: trial period

Data for plant growth and water quality parameters were analysed to compare the effects of the soil

mixes, the soil depths, and time of sampling, by Repeated Measures Analysis of Variance (ANOVA).

This analysis is used to decide whether the differences between the treatments are too large to be

explained by chance sampling variation (differences said to be significant), or are within the range of

random sampling variation from a common population (non-significant differences). The analysis

also examines how the experimental factors combine, in a series of terms called interactions. If the

experimental factors combine independently of each other, the interaction terms are non-significant.

If the way in which one factor has an effect depends on which level of another factor it is combining

with, the interaction term is significant.

Repeated Measures ANOVA was used because the same experimental unit (plants, or drainage water

pipes) were sampled over time. In the terms of the analysis, mixes and depths were the between-

subject factors, and time was the within-subject factor. Data analysed were the block averages for the

measurements of plant growth, water quality, and soil shrinkage; analyses were conducted using the

Repeated Measures routine in SPSS 18. An important feature of any analysis is that the data fit the

assumptions of the method used (otherwise the conclusions drawn are not reliable). Mauchly’s W

was calculated to test for the fit of data to the assumptions, and if significant, data were transformed to

the log scale to better fit the assumptions of the test.

Post-trial period

With the addition of fertiliser to one of the two blocks in August 2010, the analysis outlined above

could no longer be applied.

However a comparison of the fertiliser effect could be carried out as a one-way ANOVA of the block

effect. This comparison was made for plant growth and water variables, using week 45 data.

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4. RESULTS

4.1 Plant growth: trial period

There was very little plant death over the first 17 weeks of the trial; only three plants died, and so data

are not presented on survivorship.

Plant growth was relatively unaffected by the growing media used, or the depth of the medium, during

this period. Neither mix, nor depth, nor any of their interactions, significantly affected plant growth

(Table 4.1). Time was the only significant term in the analysis of the measurements of plant growth

made from the end of the establishment period until week 17; this term was significant for five of the

six species (Table 4.1).

The significant time effect was due to changes in the plants as they either showed positive or negative

growth over this period. Growth was (largely) positive for Anigozanthus (inflorescence height) and

Phormium (canopy height) from week 8 to 17 (Fig. 4.1 a, d), but was strongly negative for Liriope

(canopy height) and L. hystrix (canopy height; Fig 4.1c, d); Westringia (plant height) showed growth

that was positive from weeks 8 – 12, but was subsequently negative until week 17 (Fig. 4.1 f). For

Gazania, (stem length) growth was fairly static (Fig. 4.1 b), and time was not significant for this

species (Table 4.1).

Table 4.1: Repeated Measures ANOVA of plant growth for weeks 8 – 17 (raw data). P values of F-

ratio for each term.

Source Anigozanthus Gazania Liriope L. hystrix Phormium Westringia

Between subjects

Depth .592 .148 .946 .211 .371 .485

Mix .840 .958 .557 .964 .811 .594

Depth * Mix .575 .977 .489 .940 .348 .111

Error

Within subjects

Time .017 .083 .0001 .00001 .050 .032

Time x depth .716 .896 .938 .494 .242 .963

Time x mix .855 .360 .659 .237 .500 .605

Time x depth x mix .609 .122 .564 .269 .949 .977

Error

Plant growth: post-trial period

Despite the application of fertiliser to one of the blocks in August 2010, measurements of plant

growth did not show any significant effect of this treatment in October 2010 (comparison of block

effect was not significant at P = 0.05 for all species). The treatment means for plant growth were thus

calculated from the two blocks and are shown in Fig. 4.1.

No pronounced or consistent differences amongst any of the growth mixes were evident over the post-

trial period of measurement. By week 45, nine of the sedum plants had died in addition to the

original 3 general plant deaths by week 17 (P Smith, pers. comm.). Growth of four of the species

showed not much change from week 17 (Anigozanthus, Fig. 4.1a; Gazania, Fig. 4.1 b; Phormium,

Fig. 4.1 e; Westringia, Fig. 4.1 f). The negative growth in canopy height observed earlier in L. hystrix

and Liriope was continued to week 45 (Fig. 4.1c, d).

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By week 90, there was extensive plant mortality in the 100 mm, non-fertilised beds. The best

performing media in these beds was the 50:50 mix, with 50% plant survival, and the worst performing

was the standard mix with only 1 plant surviving. By comparison, the fertilised plots at 100mm soil

depth had better plant survival (50% in the standard mix and more in the 80:20 and 50:59 mix; P

Smith, pers. comm.).

175

200

225

250

275

20 40 60

He

igh

t (c

m)

Time (weeks)

a. Anigozanthus

standard

80:20 mix

50:50 mix

150

175

200

225

250

275

300

20 40 60

He

igh

t (c

m)

Time (weeks)

b. Gazenia

standard

80:20 mix

50:50 mix

5075

100125150175200225250275300

20 40 60

He

igh

t (c

m)

Time (weeks)

c. Liriope

standard

80:20 mix

50:50 mix

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Fig. 4.1: Plant growth measurements for six species over the trial period (Weeks 8, 12 and 17) and the

post-trial period (week 45).

150

175

200

225

250

275

300

20 40 60

He

igh

t (c

m)

Time (weeks)

d. L. hystrix Katie Bells

standard

80:20 mix

50:50 mix

100

125

150

175

200

10 20 30 40 50

He

igh

t (c

m)

Time (weeks)

e. Phormium

standard

80:20 mix

50:50 mix

150

175

200

225

250

275

10 20 30 40 50

He

igh

t (c

m)

Time (weeks)

f. Westringia

standard

80:20 mix

50:50 mix

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4.2 Water Quality

Nitrate: trial period

Concentrations of nitrate in the drainage water were highest at week 1 (mean of 2.7 mg/L across all

treatments; Fig 4.2), with a fall-off in concentration at later sampling times (mean of 0.09 mg/L at

week 10; mean of 0.32 mg/L at week 14; Fig 4.2).

Differences between mixes and depths were not significant in the analysis; the only significant effect

was time, due to the fall in nitrate concentration at the two later sampling periods (Fig. 4.2). This

pattern of a decline in nitrate concentration was consistent across mixes and depths (interactions with

time not significant, Table 4.2).

Fig. 4.2: Mean nitrate concentration in the drainage against time. Means for pooled over the three

mixes and three depths; means shown are for raw data, bars = standard error.

Table 4.2: Repeated Measures ANOVA of nitrate concentration in water (log transformed data).

Source Sum of Squares df Mean Square F Sig.

Between subjects

Depth .252 2 .126 .496 .625

Mix .435 2 .217 .854 .458

Depth * Mix .491 4 .123 .482 .749

Error 2.291 9 .255

Within subjects

Time 22.251 2 11.125 61.214 .000

Time x depth 1.350 4 .338 1.857 .162

Time x mix .622 4 .155 .855 .509

Time x depth x mix .749 8 .094 .515 .830

Error 3.271 18 .182

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 2 4 6 8 10 12 14 16

Nit

rate

co

ncen

trati

on

(m

g/L

)

Time (weeks)

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Nitrate: post-trial period

The concentration of nitrate in drainage water at Week 45 remained largely within the range seen at

Weeks 9 and 14, with most values being in the range 0.01 – 0.27 mg/L (Table 4.3); two values

exceeded this range (0.37 mg/L for 50:50 mix at 100 mm depth; 0.55 mg/L for 80:20 mix at 200 mm

depth; Table 4.3).

Differences in the nitrate concentration in drainage water from fertilised and unfertilised blocks was

detected as a significant block effect (F1,16 = 6.598, P = 0.021; mean for fertilised block = 0.27 mg/L;

mean for unfertilised block = 0.06 mg/L).

Table 4.3: Concentrations of nitrate (mg/L) in drainage water from the three mixes at three depths at

week 45 from set-up.

potting mix soil depth

Mean Std. Error

standard (100%

mineral)

100 mm .04 .16

200 mm .16 .16

300 mm .27 .16

80:20

organic:mineral

100 mm .08 .16

200 mm .55 .16

300 mm .04 .16

50:50

organic:mineral

100 mm .37 .16

200 mm .01 .16

300 mm .01 .16

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Ammonia: trial period

Ammonia concentrations were highest at the first sampling time, ranging from low values (0.09

mg/L) in the standard mix, up to 5.1 mg/L for 50:50 mix and 7.2 mg/L for 80:20 mix (Fig 4.3).

Ammonia concentrations were substantially lower at subsequent sampling times and differed far less

amongst the treatments; values ranged from 0.01 – 0.035 mg/L at week 10 to 0.008 – 0.017 mg/L at

week 14 (Fig. 4.3).

Mix, time, and the interaction between mix and time were significant in the analysis (Table 4.4),

reflecting the overall decline in ammonia concentrations over time, with strong differences between

the mixes at the first week (standard < organics) disappearing subsequently (Fig. 4.3).

Fig. 4.3: Mean ammonia concentration (log scale) for the three mixes (standard , 80:20 mix, 50:50

mix) against time. Means for mixes pooled over the three depths; means shown are for raw data, bars

= standard error; logarithmic scale used on Y-axis as concentrations became very low at 10 and 14

weeks.

Table 4.4: Repeated Measures ANOVA of ammonia concentration in water (log (x+1) transformed

data).


Between subjects

Depth .016 2 .008 3.408 .079

Mix .882 2 .441 188.698 .000

Depth * Mix .020 4 .005 2.086 .165

Error .021 9 .002

Within subjects

Time 3.766 2 1.883 857.150 .000

Time x depth .030 4 .007 3.396 .031

Time x depth (G-G) .030 2.033 .015 3.396 .078

Time x mix 1.720 4 .430 195.734 .000

Time x depth x mix .033 8 .004 1.858 .131

Error .040 18 .002 .040

0.001

0.010

0.100

1.000

10.000

0 5 10 15

Am

mo

nia

co

nce

ntr

atio

n (

mg/

L)

Time (weeks)

standard

80:20 mix

50:50 mix

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Ammonia: post-trial period

The concentration of ammonia in drainage water from the post-trial sampling showed the trend

evident in week 17, remaining very low and beyond the limits of detection (<0.01 mg/L) in 16 of the

18 samples (data not shown). The two exceptions were both from the 80:20 organic mix, where

concentrations of ammonia were 0.04 mg/L in drainage water from one of the 200 mm depth bays,

and 0.02 mg/L from one of the 300 mm depth bays.

Phosphorus: trial period

Phosphorus concentration in week 1 varied from a low of 0.1 mg/L in drainage water from the

standard mix, to higher values with increasing organic content of the mix (2.3 mg/L from the 50:50

mix and 4.7 mg/L from the 80:20 mix (Fig. 4.4). Concentrations at later sampling times rose slightly

in drainage from the standard mix (0.7 – 0.8 mg/L), but declined in that from the organic mixes (2.2 –

2.4 mg/L in the 80:20 mix; 1.5 – 1.6 mg/L in the 50:50 mix; Fig 4.4).

These contrasting patterns of changes in phosphorus concentration between the mixes over time

resulted in significant differences for the mixes term in the analysis, both as a main effect (standard <

50:50 mix < 80:20 mix; Fig. 4.4), and in interaction with time (Table 4.5).

Time also interacted significantly with depth, and this pattern can be seen in Fig. 4.5. Phosphorus

concentration in drainage water from the 100 and 200 mm depths declined consistently over time to

be <1.3 mg/L at week 14, but did not do so in the 300 mm treatment, remaining in the range 2.3 – 2.5

mg/L (Fig. 4.5).

Fig. 4.4: Mean phosphorus concentration for the three mixes (standard, 80:20 mix, 50:50 mix) against

time. Means for mixes pooled over the three depths; means shown are for raw data, bars = standard

error

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 5 10 15Ph

osp

ho

rus

con

cen

trat

ion

(m

g/L)

Time (weeks)

standard

80:20 mix

50:50 mix

21


Fig. 4.5: Mean phosphorus concentration for the three depths against time. Means mixes pooled

within depths. Bars = SE.

Table 4.5: Repeated Measures ANOVA of phosphorus concentration in water (log transformed data).


Between subjects

Depth .775 2 .387 2.382 .148

Mix 8.507 2 4.253 26.153 .000

Depth * Mix .679 4 .170 1.043 .437

Error 1.464 9 .163

Within subjects

Time .066 2 .033 1.219 .319

Time x depth .471 4 .118 4.361 .012

Time x mix 3.605 4 .901 33.346 .000

Time x depth x mix .301 8 .038 1.391 .265

Error .486 18 .027 .486 18

Phosphorus: post-trial period

Phosphorus concentrations in the drainage water at week 45 remained within the range seen at the end

of the trial period, with higher values still associated with increasing organic content of the mix.

Concentrations in the 100% mineral mix ranged from 0.195 – 0.36 mg/L, increasing to 0.51 – 1.15

mg/L in the 50:50 mix, and to 0.74 – 2.67 mg/L in the 80:20 mix (Table 4.6).

Mean concentrations of phosphorus were 1.22 mg/L in water from the fertilised block, and 0.59 mg/L

from the unfertilised block; the differences were not statistically significant (F1,16 = 3.77, P = 0.070)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15

Ph

osp

hate

?

Co

ncen

trati

on

(m

g/L

)

Time (weeks)

100mm

200mm

300mm

22


Table 4.6: Concentrations of phosphorus (mg/L) in drainage water from the three mixes at three

depths at weeks 45 - 46 from set-up.


Mean Std. Error

standard (100% mineral)

100 mm .195 .530

200 mm .260 .530

300 mm .360 .530

80:20 organic:mineral

100 mm 1.345 .530

200 mm .740 .530

300 mm 2.670 .530


100 mm 1.150 .530

200 mm .510 .530

300 mm .925 .530

Sodium: trial period

Sodium concentrations in drainage water showed a complex pattern, so that mixes, depths, time and

all their interactions were significant in analysis (Table 4.7). The patterns of sodium concentration at

each depth in each mix are shown in Fig. 4.6. Sodium concentrations were highest initially in

drainage water from the standard mix, ranging from 154 to 275 mg/L (Fig. 4.6a); concentrations were

lower at this sampling time in the 80:20 mix (91 – 142 mg/L, Fig 4.6b) and in the 50:50 mix (167 –

175 mg/L, Fig 4.6c).

Concentration of sodium declined at the subsequent sampling times and differences between mixes

lessened; values were generally <50 mg/L at weeks 10 and 14 (Fig 4.6). A better view of this pattern

can be seen in Fig. 4.7, where the data from each depth were pooled within each mix. The difference

between the mixes at the first sampling time, and the subsequent decline in concentrations and

lessening of differences between mixes, are apparent.

23


Fig. 4.6:Mean concentration of sodium (mg/L) in drainage water from a. standard, b. 80:20, and c.

50:50 mixes over time. Bars = standard error. Legend shown in a.

0

50

100

150

200

250

300

0 5 10 15

So

diu

m c

on

cen

trati

on

(m

g/L

)

Time (weeks)

a. standard

100mm

200mm

300mm

0

50

100

150

200

250

300

0 5 10 15

so

diu

m c

on

cen

trati

on

(m

g/L

)

Time (weeks)

b. 80:20 mix

0

50

100

150

200

250

300

0 5 10 15

so

diu

m c

on

cen

trati

on

(m

g/L

)

Time (weeks)

c. 50:50 mix

24


Fig. 4.7: Mean concentration of sodium in drainage water from the three mixes over time (raw data);

standard, 80:20 mix, 50:50 mix. Data for the three depths pooled within each mix. Bars = standard

error.

Table 4.7: Repeated Measures ANOVA of sodium concentration in water (log (x+1) transformed

data).


Between subjects

Depth .652 2 .326 58.331 .000

Mix .282 2 .141 25.209 .000

Depth * Mix .116 4 .029 5.193 .019

Error .050 9 .006

Within subjects

Time 8.247 2 4.124 1064.613 .000

Time x depth .108 4 .027 6.948 .001

Time x mix .087 4 .022 5.621 .004

Time x depth x mix .101 8 .013 3.250 .018

Error .070 18 .004

Sodium: post-trial period

The concentration of sodium in drainage water in the post-trial period remained low, with values in

the range 14 – 21 mg/L (Table 4.8). The fertilisation of one block did not show up in the

concentrations of sodium in drainage water, mean concentrations in the two blocks being close in

value (block 1 mean = 17.7 mg/L; block 2 mean = 15.5 mg/L). The comparison of block means was

insignificant (F1,16 = 0.783, P = 0.386).

0

50

100

150

200

250

0 5 10 15

Sod

ium

co

nce

ntr

atio

n (m

g/L)

Time (weeks)

standard

80:20 mix

50:50 mix

25


Table 4.8: Concentrations of sodium (mg/L) in drainage water from the three mixes at three depths at

weeks 45 - 46 from set-up.


Mean Std. Error


100 mm 15.5 3.7

200 mm 15.5 3.7

300 mm 16.5 3.7


100 mm 16.0 3.7

200 mm 15.5 3.7

300 mm 17.5 3.7


100 mm 14.0 3.7

200 mm 18.0 3.7

300 mm 21.0 3.7

26


Chloride: trial period

Chloride concentration was176 mg/L in drainage water from the 100 mm depth at the initial sampling

time, but was higher in drainage water from the two greater depths (287 – 288 mg/L; Fig. 4.8).

Chloride concentrations fell at the following sampling times, ranging from 30 – 70 mg/L at 10 weeks

to 4 – 16 mg/L at 14 weeks (Fig.4.8).

This initial difference in chloride concentration in drainage water from the three depths, followed by

the decline in concentration and lessening of differences between depths appeared as significant

differences amongst depths, amongst times, and in the time x depth interaction in the analysis (Table

4.9). Differences amongst mixes were not significant, either as a main effect or interaction with time

(Table 4.9).

Fig. 4.8: Mean chloride concentration for the three depths against time. Means for the three mixes

pooled within depths. Bars = SE.

Table 4.9: Repeated Measures ANOVA of chloride concentration in water (raw data).


Between subjects

Depth 26560.0 2 13280.0 4.299 .049

Mix 7773.6 2 3886.8 1.258 .330

Depth * Mix 8699.9 4 2175.0 .704 .609

Error 27800.0 9 3088.9

Within subjects

Time 615368.0 2 307684.0 210.6 .000

Time x depth 29277.7 4 7319.4 5.010 .007

Time x mix 6398.2 4 1599.5 1.095 .389

Time x depth x mix 17613.4 8 2201.7 1.507 .223

Error 26298.0 18 1461.0

0

50

100

150

200

250

300

350

0 5 10 15

Ch

lori

de

(mg

/L)

Time (weeks)

100 mm

200mm

300mm

27


Chloride: post-trial period

The concentration of chloride in drainage water in the post-trial period continued the trend observed at

the end of the trial period and remained low, with values in the range 13.5 – 15.5 mg/L (Table 4.10).

The fertilisation of one block did not show up in the concentrations of chloride in drainage water,

mean concentrations in the two blocks being close in value (unfertilised block 1 mean = 13 mg/L;

fertilised block 2 mean = 15mg/L). The comparison of block means was not significant (F1,16 = 2.77,

P = 0.115).

Table 4.10: Concentrations of sodium (mg/L) in drainage water from the three mixes at three depths

at weeks 45 - 46 from set-up.


Mean Std. Error


100 mm 13.5 2.96

200 mm 11.5 2.96

300 mm 11.5 2.96


100 mm 14.5 2.96

200 mm 15.5 2.96

300 mm 15.5 2.96


100 mm 15.0 2.96

200 mm 14.0 2.96

300 mm 15.5 2.96

28


Total dissolved solutes (TDS): trial period

TDS concentration was c. 800 mg/L in drainage water averaged over all mixes and depths at the

initial sampling time, falling to 167 mg/L at week 10 and 101 mg/L at week 14 (Fig. 4.9 ).

Time was the only significant term in the analysis (Table ); differences amongst mixes, depths, and

their interactions with each other and with time were not significant (Table ).

Fig. 4.9: Mean TDS concentration in the drainage water against time. Means for pooled over the

three mixes and three depths; means shown are for raw data, bars = standard error.

Table 4.11: Repeated Measures ANOVA of TDS concentration in water (log (x+1) transformed data).


Between subjects

Depth 1.619 2 .810 3.427 .078

Mix .004 2 .002 .008 .992

Depth * Mix .863 4 .216 .913 .496

Error 2.126 9 .236

Within subjects

Time 12.796 2 6.398 29.884 .000

Time x depth 1.184 4 .296 1.383 .279

Time x mix .034 4 .008 .040 .997

Time x depth x mix 1.729 8 .216 1.009 .463

Error 3.854 18 .214

100

200

300

400

500

600

700

800

900

0 2 4 6 8 10 12 14 16

TD

S c

on

ce

ntr

ati

on

(m

g/L

)

Time (weeks)

29


TDS: Post-trial period

Concentrations of TDS in drainage water late in 2010 remained approximately at the values observed

towards the end of the experimental period. Concentration increased consistently with depth (71 – 95

mg/L at 100 mm depth; 93 – 107 mg/L at 200 mm depth; 117 – 140 mg/L at 300 mm depth) (Table ).

A significant block effect was detected (F1,15 = 6.49, P = 0.021) with the average concentration of

TDS in drainage water from the fertilised block (122 mg/L) being higher than that in the unfertilised

block (92 mg/L).

Table 4.12: Concentrations of TDS (mg/L) in drainage water from the three mixes at three depths at

weeks 45 - 46 from set-up.


Mean Std. Error


100 mm 93.2 18.2

200 mm 104.4 18.2

300 mm 134.2 18.2


100 mm 71.3 18.2

200 mm 106.6 25.7

300 mm 140.5 18.2


100 mm 95.2 18.2

200 mm 93.3 18.2

300 mm 116.7 18.2

30


Electrical conductivity (EC): trial period

The electrical conductivity (EC) ranged from 1,250 S/cm in drainage water from the 100 mm depth

up to c. 1,800 S/cm at the 200 mm and 300 mm depths (Fig. 4.10). EC values were substantially

lower at subsequent sampling times, ranging from 290 – 440 S/cm across the three depths at week

10 to 117 – 360 units at Week 14 (Fig. 4.10).

This decline in EC with time, and the initial differences amongst depths, led to significant differences

amongst depths, amongst times, and the interaction of depth and time in the analysis (Table 4.13).

Fig. 4.10: Mean EC (S/cm ) for the three depths against time. Means for the three mixes pooled

within depths. Bars = SE.

Table 4.13: Repeated Measures ANOVA of EC in water (raw data).


Between subjects

Depth 1008380.9 2 504190.5 25.808 .000

Mix 61808.1 2 30904.0 1.582 .258

Depth * Mix 130607.6 4 32651.9 1.671 .240

Error 175822.6 9 19535.8

Within subjects

Time 21744783.4 2 10872391.7 288.005 .000

Time x depth 553933.7 4 138483.4 3.668 .024

Time x mix 187877.3 4 46969.3 1.244 .328

Time x depth x mix 297273.4 8 37159.2 .984 .479

Error 679512.5 18 37750.7

0

250

500

750

1,000

1,250

1,500

1,750

2,000

0 5 10 15

EC

Time (weeks)

100 mm

200 mm

300 mm

31


EC: Post-trial period

EC values at weeks 45 – 46 were similar in range to the values at the end of the experimental period,

ranging from 157 – 303 units overall (Table 4.14). Values increased with depth within each mix

treatment, but did not vary consistently amongst the mixes (Table 4.14 ). A significant block effect

was detected (F1,15 = 5.897, P = 0.028), with EC in the fertilised block = 265 units, and in the

unfertilised block = 204 units.

Table 4.14: : EC in drainage water from the three mixes at three depths at weeks 45 - 46 from set-

up.


Mean Std. Error


100 mm 205.9 37.9

200 mm 229.6 37.9

300 mm 291.9 37.9


100 mm 157.7 37.9

200 mm 235.9 53.6

300 mm 303.3 37.9


100 mm 212.1 37.9

200 mm 206.9 37.9

300 mm 256.6 37.9

pH: trial period

The pH of the drainage water at the initial sampling ranged from values close to neutral in the 300

mm depth, to more alkaline values as soil depth became shallower (8.0 at 200 mm, 9.0 at 100 mm;

Fig. 4.11 ). These initial differences between the depths disappeared at the following sampling times,

when pH values fell in the range 7.0 – 7.5 (Fig. 4.11 ).

These early differences in pH amongst the soil depth treatments, and subsequent minimising of

differences with time, resulted in significant differences amongst depths, amongst times and the

interaction of depth and time being significant in analysis (Table 4.15 ).

32


Fig. 4.11: Mean pH for the three depths against time. Means for the three mixes pooled within

depths. Bars = SE.

Table 4.15: Repeated Measures ANOVA of pH concentration in water (raw data).


Between subjects

Depth 1.833 2 .917 4.382 .047

Mix 1.113 2 .556 2.660 .124

Depth * Mix .830 4 .207 .992 .460

Error 1.882 9 .209

Within subjects

Time 8.433 2 4.217 11.373 .001

Time x depth 7.036 4 1.759 4.744 .009

Time x mix .348 4 .087 .234 .915

Time x depth x mix .986 8 .123 .332 .942

Error 6.674 18 .371

pH: post-trial period

The range of pH values in drainage water samples at post-trial sampling was very similar to values

observed at week 14, ranging from 7.0 – 7.6 (Table 4.16 ). Differences between blocks were not

significant ((F1,16 = 0.39, P = 0.54); the fertilisation of one block did not affect the pH of the drainage

water.

There were slight differences in the range of pH values in drainage water from the three mixes; pH

values in the standard mix (7.4 – 7.6; Table ) were a little more alkaline than values in the 80:20 mix

(7.0 – 7.4) or the 50:50 mix (7.2 – 7.3; Table 4.16).

6.5

7.0

7.5

8.0

8.5

9.0

9.5

0 5 10 15

pH

Time (weeks)

100 mm

200 mm

300 mm

33


Table 4.16: pH in drainage water from the three mixes at three depths at weeks 45 - 46 from set-up.


Mean Std. Error


100 mm 7.4 .09

200 mm 7.6 .09

300 mm 7.6 .09


100 mm 7.2 .09

200 mm 7.4 .09

300 mm 7.0 .09


100 mm 7.3 .09

200 mm 7.3 .09

300 mm 7.2 .09

34


4.3 Soil movement: trial and post-trial period

The three mixes showed minimal soil movement during the period week 8 – 17; the distance from a

horizontal line to the substrate surface increased by about 2 mm over this period for the standard and

50:50 mixes (Fig. 4.12), indicating minor shrinkage (Fig. 4.13). This distance decreased by about 1.5

mm for the 80:20 mix (Fig. 4.12), indicating some minor swelling (Fig. 4.13).

This movement was so slight that no term was significant in the analysis (Table 4.17).

All three mixes showed very minor soil shrinkage (range -1 to -4 mm, Fig. 4.13) when re-measured in

week 45.

Fig. 4.12: Mean distance from horizontal line to substrate surface for the three mixes against time.

Means for the three depths pooled within mixes. Bars = SE

Fig. 4.13: mean net soil movement (initial – subsequent values) during the period of measurement.

35

40

45

50

55

0 10 20 30 40 50

Dis

tan

ce t

o s

oil

(mm

)

Time (weeks)

standard

80:20 mix

50:50 mix

-5

-4

-3

-2

-1

0

1

2

0 10 20 30 40 50

Soil

mo

vem

en

t (m

m)

Time (weeks)

standard

80:20 mix

50:50 mix

35


Table 4.17: Repeated Measures ANOVA of soil movement measurements (raw data).


Between subjects

Depth 193.669 2 96.834 .491 .628

Mix 1295.527 2 647.763 3.284 .085

Depth * Mix 762.242 4 190.561 .966 .471

Error 1775.115 9 197.235

Within subjects

Time 48.308 3 16.103 .740 .538

Time x depth 127.323 6 21.221 .975 .461

Time x mix 92.151 6 15.359 .706 .648

Time x depth x mix 800.118 12 66.676 3.064 .008

Error 587.605 27 21.763

36


4.4 Temperature measurements

The time course of temperature at the base of a planter box containing standard mineral mix (100 mm

depth) is shown in Fig. 4.14a. Temperature recordings encompassed three day-time periods and two

nights. Temperature at the base of the planter box ranged between 15.3 to 27 0C over this period,

while the temperature range for the adjacent bare bitumen substrate ranged from 15.5 to 47.7 0C (Fig

4.14a). The maximum temperature difference between the two thermocouples ranged up from -4.1 0C

at night up to 26.5 0C by day (Fig. 4.14b).

Fig. 4.14a. Time course of temperature at the base of a planter box with standard mineral mix (100

mm depth) or on adjacent bare bitumen substrate; b. temperature difference between bare substrate

and base of the planter box for the data in a. Measurements made over 26 – 28 October 2010.

10

15

20

25

30

35

40

45

50

Te

mp

era

ture

(0C

)

Time

a. standard mix

planter box ('C)

-10

-5

0

5

10

15

20

25

30

bar

e T

- b

ox

T (0

C)

Time

b. temperature difference

37


The temperature readings at the bottom of a planter box containing 50:50 mix and the adjacent bare

bitumen substrate are shown in Fig. 4.15a. Temperature at the base of the planter box ranged between

16.4 to 27.5 0C over this period, while the temperature range for the adjacent bare substrate ranged

from 14.5 to 47.8 0C (Fig 4.15a). The temperature difference between the two thermocouples ranged

up from -5.7 0C at night to 25.2

0C by day (Fig. Yb). By day-time, the temperature of the bare

substrate quickly rose to 15 – 25 0C above that of the base of the planter boxes for both mixes at 100

mm depth (Fig 4.14b, 4.15b).

10

15

20

25

30

35

40

45

50

Te

mp

era

ture

(Oc

)

Time

a. 50:50 mix planter box('C)

baresubstrate ('C)

-10

-5

0

5

10

15

20

25

30

bar

e T

- b

ox

T ((0

C)

Time

b. temperature difference

38


Fig. 4.15a. Time course of temperature at the base of a planter box with standard 50:50 mix (100 mm

depth) or on adjacent bare substrate; b. temperature difference between bare substrate and base of the

planter box for the data in a. Measurements made over 26 – 28 October 2010.

5. DISCUSSION

5.1 Plant growth

Plant growth was very similar on all the combinations of soil mix and soil depth used in the trial; none

of the mixtures and/or depths used emerged as clearly being better for plant growth than the others

during this period. Plant death was very limited at this stage. While species differed in the growth

they showed over the first 17 weeks of the experiment, with some showing positive growth, others

negative growth, and some a mix of the two, these differences were consistent over the all the

combinations of soil mix and depth used.

Over the longer period post-trial, some differences between depths and mixes have become apparent.

Plants died in greater numbers over the 2009 – 10 summer, mainly in the 100 mm depth treatment, the

shallowest of the depths used. Soil water storage depends on soil type and depth, and in hot

conditions, plants on shallower soil depths will exhaust the available water sooner than plants on

deeper substrates.

Addition of fertiliser to one block in August 2010 did not result in a detectable effect on plant growth

when plants were remeasured in October 2010. However dramatic changes came to be observed as

time passed, and at 90 weeks, a noticeable difference was observed between the media depths tested,

with more plant death observed in the shallowest boxes (100mm). Also at 90 weeks, healthier growth

was observed in the fertilised boxes compared to their unfertilised replicate (statistical comparison not

made). As the time since establishment of the trial elapses, plant growth appears to be strongest and

best on the 50:50 mix, followed by the 80:20 mix and then least in the standard.

5.2 Water quality

The strongest pattern apparent in almost all the different measures of water quality was that readings

from the initial flush were substantially different to subsequent readings. If differences between

mixes or depths were significant, it was in interaction with time, reflecting a trend for initial

differences to lessen at subsequent sampling times.

Initial differences between mixes were apparent for some variables (ammonia, phosphate, and

sodium), but these differences lessened and all but disappeared by the end of the sampling period. For

ammonia for example, the initial concentration in drainage water from the two organic mixes was

more than an order of magnitude higher than that from the mineral mix; but these large differences

were absent at the two subsequent sampling times. For phosphate, again the initial concentrations

were an order of magnitude higher in drainage water from the two organic mixes compared to the

standard, but the differences lessened over time. For sodium, initial concentrations were highest in

the standard mix rather than in the two organics, but these differences were not maintained at later

sampling times.

Differences between mixes were not significant for a further suite of variables: nitrate, chloride, EC

and pH. Differences between depths were significant for some of these variables, and with the same

pattern as for mixes; initial differences lessening with time. Chloride, EC, and pH (as well as

ammonia and phosphate) showed this pattern. For chloride and EC, initial values were highest in

39


drainage water from the 200 and 300 mm depth treatments compared to the 100 mm depth; the two

deeper treatments contained a larger volume of soil from which nutrients could be flushed. For pH,

the initial values were a little lower for the two deeper soil depths.

For two variables (nitrate, TDS), time was the only significant term, and differences between mixes

and depth were not significant. For these two variables, the trend of decreasing values over time was

the dominating pattern.

When the drainage water was tested again in October 2010, concentrations of all variables continued

the generally low values seen at week 17. So once the ‘first flush’ occurs, with high concentrations of

a number of water variables, values settle down to relatively low concentrations thereafter.

Interestingly, the addition of fertiliser was detectable as elevated concentrations of some variables in

drainage water from the fertilised block (significant for nitrate, TDS; approaching significance for

phosphorus).

Comparison of the concentration of the water quality variables measured showed that concentrations

in the drainage water had declined sufficiently to mostly meet the standard for drinking water by week

14, and certainly by week 45 (Table 5.1). This was the case for nitrate and ammonia. The only

variable which remained above drinking water standard at week 14 was sodium, but its concentration

had fallen to drinking water levels by week 45 (Table 5.1).

If irrigation water standards are used as the basis for comparison, sodium levels at week 45 fell in the

range for moderately tolerant crops (Table 5.1), exceeding the recommended standard for very

sensitive and sensitive crops. Concentrations of chloride were well below the standard for either root

or foliar injury of sensitive crops (Table 5.1). TDS concentrations in drainage water at weeks 17 and

45 met the standard for low-salinity water; EC values generally met the standard for low-salinity

water, with some higher values falling into the range for medium-salinity water (Table 5.1).

Table 5.1: Concentrations of water variables at the end of the trial period (Week 14) and at re-

measurement in Week 45. Water quality standards for drinking water shown for the World Health

Organisation (WHO), USA Environmental Protection Agency (EPA) and NHMRC Australian

Drinking Water Guidelines (2004); standard for irrigation water shown for ANZECC Australian

Water Quality Guidelines for Fresh and Marine Waters (1992). All units in mg/L

Variable Week 14 Week 45 drinking water irrigation water (ANZECC 1992)

nitrate (mg/L) 0.32 0.01 -0.27 50a,c

Ammonia

(mg/L)

0.008 – 0.017 <0.01 0.5d

phosphorus

(mg/L)

<3 <2.7 -

sodium

(mg/L)

<50 14-21 20b

180d

foliar injury: <115 for sensitive crops

chloride

(mg/L)

4 - 16 13.5 -15.5 20b

250d

root uptake: <110 for most sensitive crop

foliar injury: <175 for most sensitive crop

TDS (mg/L) 101 71 – 140 500d 0 – 175: low-salinity water

175 – 500: medium salinity water

EC (S/cm) 117 – 360 157 – 303 - 0 – 280: low-salinity water

280 - 800: medium-salinity water

a: WHO standard

b: USA EPA standard

c: NHRMC Australian Drinking Water Guidelines 2004: health guidelines

40


d: NHRMC Australian Drinking Water Guidelines 2004: aesthetic guidelines

*recommendation only

Two conclusions are apparent from the water quality measurements:

any differences between mixes in quality of drainage water disappeared relatively early, and

within the first 14 weeks.

once the ‘first flush’ phenomenon was past, concentrations of most inorganic chemicals that

measure water quality were sufficiently low that any re-use of drainage water would not pose

problems for plants in green roof set-ups.

Soil shrinkage

The measurements of distance to the soil surface showed that very little movement occurred over the

sampling period up to week 45. There was no evidence to support concerns about slumpage of either

of the organic mixes.

Temperature

The temperature difference between the base of the planter boxes and the bare substrate was expected;

the measurements conducted quantified this difference for the soil depth and time of year used. By

night, the bare substrate cooled to about 4 – 5 0C below the planter box temperature. By day, the

reverse occurred as the temperature of the (unshaded) bare bitumen substrate rose to 15 – 25 0C above

that of the base of the planter box. A slightly greater temperature difference would be expected for

the greater soil depths as soil depth confers insulating advantage. And the temperature difference may

become greater on days when the total heat load on a horizontal surface was greater than that

experienced on the days of measurement in October. The temperature differences due to green roofs

recorded in this trial are comparable to those measured in trials in Singapore (Wong et al. 2003) and

Japan (Takebayashi et al. 2007).

CONCLUSIONS

there was little difference between the standard mineral soil mix, and the two organic mixes

used, on plant survival or growth over the period of measurement (17 weeks).

in the post-trial period up to 90 weeks, there was more plant death in the 100 mm soil depth

treatment, especially if not fertilised. A greater difference in plant growth was observed

amongst the mixes with plant survival/ health being best in the 50:50 mix, followed by the

80:20 mix and then the standard mix.

there were initial differences between mixes and depths for some measurements of water

quality, but these lessened and disappeared as the ‘first flush’ effect (high initial values) was

replaced by longer-term, low values of the variables.

longer-term values for water quality parameters are under standards for irrigation water; reuse

of drainage water for irrigation could be feasible, given the low concentrations of the

inorganic chemical indicators of water quality

41


the two organic mixes did not show soil shrinkage or movement that differed greatly from the

standard organic mix

there was a substantial day-time temperature difference between bare substrate and the base

of the planter boxes (at 100 mm depth), with the bare substrate temperature exceeding that of

the planter box by 15 – 25 0C (October measurement period).

ACKNOWLEDGEMENTS

Renee Attard provided expert technical assistance for the plant measurements and water sampling,

and Paul Thomas for the temperature measurements. Ozbreed provided the plant stock used in the

trial.

REFERENCES

World Health Organisation (2006) Guidelines for Drinking Water Quality 3rd

Edition

http://www.who.int/water_sanitation_health/dwq/gdwq3rev/en/ Downloaded 1 August 2011

United States of America Environmental Protection Agency Current Drinking Water Regulations.

http://water.epa.gov/drink/standardsriskmanagement.cfm Downloaded 1 August 2011.

National Health and Medical Research Council (2004) Australian Drinking Water Guidelines 6.

National Water Quality Management Strategy, Australian Government.

Susca T, Gaffin SR & Dell’Osso GR (2011) Positive effects of vegetation: urban heat island and

green roofs. Environmental Pollution 159, 2119-2196.

Takebayashi H, Moriyama M (2007) Surface heat budget on green roof and high reflection roof for

mitigation of urban heat island. Building and Environment 42, 2971-2979.

Wong NH, Chen Y, Ong CL & Sia A (2003) Investigation of thermal benefits of rooftop garden in the

tropical environment. Building and Environment 38, 261-270.

http://www.who.int/water_sanitation_health/dwq/gdwq3rev/en/

http://water.epa.gov/drink/standardsriskmanagement.cfm

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