effects of artificial amendments in potting media on orthosiphon aristatus (cat’s whiskers) lants under water deficit stress

Thus, any factor which enables plants to better withstand water deficit stress via improvement in physical properties of soils and growing media, including WHC, will improve plant growth

SCHOOL OF LAND, CROP AND FOOD

EFFECTS OF ARTIFICIAL AMENDMENTS IN POTTING MEDIA ON ORTHOSIPHON ARISTATUS (CAT’S WHISKERS) PLANTS UNDER WATER

DEFICIT STRESS

Nguyen Trung Ta

A report submitted as a requirement for the degree of (Master of

Agricultural Studies) in The University of Queensland

School of Land, Crop and Food Sciences Faculty of Natural Resources, Agriculture and Veterinary Science

The University of Queensland, Gatton

November, 2008

DECLARATION OF ORIGINALITY

The work reported in this research project report has been carried out by the undersigned Where reference has been made to the results of other workers, appropriate acknowledgment of the source of information has been made

Author………

This is to certify that the academic style and manner of presentation of the research project report are appropriate to the discipline, that all requirements of The University

of Queensland in relation to the deposition of records of research have been met and that the information contained in the report is a true representation of the data collected

Project Supervisor………

ACKNOWLEGEMENTS

I would like to thank Professor Daryl Joyce who devoted his time to my work I am indebted to him for his supervision, support and patience throughout this project Thanks are also due to Mr Allan Lisle for his assistance with data analysis Many thanks also to Mr Quang-Son Dinh for his assistance with this report and the experiments Special thanks to technicians and general staff at the School of Land, Crop and Food Sciences They supported me with equipment and materials Finally, I gratefully acknowledge the support of my father, mother, sisters and brothers, who cared, worried and encouraged me while I was doing this work

ABSTRACT

Synthetic soil conditioners have long been used with a view to improving soils and other growing media Among these, polyacrylamide gel (PAG) and urea-formaldehyde resin foam (UFRF) are commonly utilised for enhancement of soil water holding capacity However, there is little or no information comparing and contrasting them

This project investigated the growth of Orthosiphon aristatus grown in either

composted pine bark or sand treated with UFRF and/or PAG, including under transient water deficit stress Effects of these soil amendments on media physical properties (viz bulk density, water content and air-filled porosity) were also assessed for composted pine bark and sand

UFRF and PAG [30% and 0.1% (v/v), respectively; manufacturers recommended rates]

were incorporated into the composted pine bark or sand for potted O aristatus Effects

on plant growth parameters were observed under water withholding intervals

Plant growth and shoot number were not increased by adding PAG UFRF also did not improve shoot length and number of plants, and the number of opened flowers PAG

incorporation actually reduced shoot number of O aristatus The combination of UFRF

plus PAG reduced plants shoot biomass in experiment 1 but not in experiment 2

No significant effects of UFRF and/or PAG on plant media water content were recorded

at field capacity (FC) PAG retained more moisture than other treatments at wilting time during plant water deficit stress cycles UFRF andespecially UFRFplusPAG prolonged

the time to plant wilting of O aristatus by 4-15 hours compared to control and PAG

alone in experiment 1 with composted pine bark However, UFRF and/or PAG did not increase time to wilting for plants in experiment 2 with composted pine bark or sand UFRF and/or PAG did not increase plant available water for plants in both composted pine bark and sand Stomatal conductances (Gs) were generally similar in all water deficit stress cycles

In terms of media physical properties, bulk densities were reduced and air-filled porosity increased by UFRF incorporation PAG incorporation also reduced bulk density of sand but not the composted pine bark PAG did not increase air-filled porosity of both composted pine bark and sand Volumetric soil water content of both

composted pine bark and sand was not increased by UFRF UFRF plus PAG incorporation did not have greater effects on bulk density and air-filled porosity than either UFRF or PAG incorporation alone

In conclusion, UFRF slightlydelayed the onset of water deficit stress in O aristatus, whereas PAG did not Bulk densities and air-filled porosity were reduced and increased,

respectively, by adding UFRF but not PAG These changes, as might be repeated, did not increase available water for plants In future work, optimum incorporation rates of

UFRF and PAG could be integrated for composted pine bark and sand

TABLE OF CONTENTS

DECLARATION OF ORGINALITY i

ACKNOWLEGEMENTS ii

ABSTRACT iii

TABLE OF CONTENTS v

LIST OF TABLES viii

LIST OF FIGURES ix

LIST OF APPENDICES xi

LIST OF ABBREVIATIONS xii

CHAPTER 1 1

INTRODUCTION 1

1.1 BACKGROUND 1

1.2 THE PROBLEM 2

1.3 OBJECTIVES 3

CHAPTER 2 5

LITERATURE REVIEW 5

2.1 SOIL-PLANT-AIR CONTINUUM (SPAC) 5

2.1.1 Soil water 5

2.1.1.1 Water holding capacity (WHC) 5

2.1.1.2 Water flow 6

2.1.2 Root and shoot growth 8

2.1.2.1 Root growth 8

2.1.2.2 Shoot growth 8

2.1.2.3 Root: shoot ratio 9

2.1.3 Air water 10

2.1.3.1 Vapour pressure deficit 10

2.1.3.2 Air movement 11

2.2 PLANT WATER USE 11

2.2.1 Water use efficiency 11

2.2.2 Water deficit stress 12

2.3 SOIL CONDITIONERS TO ENHANCE SOIL PHYSICAL PROPERTIES 12

2.3.1 General functions 12

2.3.1.1 Wet-ability and infiltration rate 13

2.3.1.2 Soil aeration and drainage 14

2.3.1.3 Soil water holding capacity 14

2.3.2 Major soil conditioner types 15

2.3.2.1 Natural inorganic materials 15

2.3.2.2 Natural organic materials 17

2.3.2.3 Synthetic materials 19

2.3.3 Utility in improving SPAC relations 24

2.3.4 Synthetic soil amendments and the environment 25

2.3.4.1 Polyacrylamide gel 26

2.3.4.2 Urea formaldehyde resin foam 26

2.4 CONCLUSION 27

CHAPTER 3 28

MATERIALS AND METHODS 28

3.1 INTRODUCTION 28

3.2 MATERIALS AND METHODS 28

3.2.1 Plant materials 28

3.2.2 Experiment preparation 28

3.2.3 Potting media treatments 29

3.2.4 Transient water deficit treatments 29

3.2.5 Data collection 30

3.2.6 Physical properties determination 31

3.2.7 pH and EC 31

3.2.8 Experimental design and analysis 32

CHAPTER 4 35

RESULTS 35

4.1 PHYSICAL PROPERTIES OF COMPOSTED PINE BARK AND SAND 35

4.1.1 Physical properties 35

4.1.2 pH and EC 35

4.2 EXPERIMENT 1 37

4.2.1 Growth phase 37

4.1.2 Stress phase 37

4.1.3 Shoot biomass 40

4.1.4 Stomatal conductance 40

4.3 EXPERIMENT 2 43

4.3.1 Growth phase 43

4.3.2 Stress phase 44

4.3.3 Shoot biomass 48

4.3.4 Stomatal conductance 48

CHAPTER 5 51

DISCUSSION 51

5.1 SUBSTRATE CHARACTERIZATION 51

5.2 PLANT GROWTH 52

5.2.1 Plant height 52

5.2.2 Shoot number 54

5.2.3 Flowering time and open flower number 55

5.2.4 Shoot biomass 55

5.3 WATER DEFICIT STRESS TOLERANCE 56

CHAPTER 6 58

CONCLUSION 58

BIBLIOGRAPHY 59

APPENDICES 75

LIST OF TABLES

Table 2.1 The amount of N fixed by some plants in the sub-tropics and tropics .18 Table 2.2 Soil aggregates with and without arbuscular mycorrhizal fungi 19 Table 2.3 Effects of various synthetic soil amendments on soil physical properties .22

Table 2.4 Effects of various synthetic soil amendments on soil water contents 22

Table 4.1 Estimated physical properties, pH and electrical conductivity of

composted pine park and sand 36

Table 4.2 O aristatus shoot height increments between beginning and end of water

stress cycles and time to flowering in composted pine bark (experiment 1) 37

Table 4.3 Time to wilting of O aristatus and relative pot weight loss during the

water deficit cycles in composted pine bark (experiment 1) .39

Table 4.4 Volumetric water content of composted pine bark at field capacity and

wilting time, and plant available water (experiment 1) .41

Table 4.5 Shoot biomass of O aristatus grown in composted pine bark (experiment

1) 43

Table 4.6 O aristatus shoot height increments between beginning and end of water

stress cycles and time to flowering in composted pine bark and sand (experiment 2) 45

Table 4.7 Time to wilting of O aristatus and relative pot weight loss during the

water deficit cycles in composted pine bark and sand (experiment 2) 47

Table 4.8 Volumetric water content of composted pine bark and sand at field

capacity and wilting time, and plant available water (experiment 2) 49

Table 4.9 Shoot biomass of O aristatus growing in composted pine and sand

(experiment 2) 50

LIST OF FIGURES

Fig 2.1 Representation of soil water content at saturation, field capacity and

permanent witling point 6

Fig 2.2 Schema of estimated plant - available soil water content for a range of soil

textural types 6

Fig 2.3 The origin and position of apical meristems in a bean seedling 9

Fig.2.4 Scanning electron micrographs of soil specimens: untreated soil (A), 8%

lime-stabilized soil (B) (original magnification of 500) 16

Fig 3.1 Experiment 1 in a shade house (22 days after potting) 1.5 L pots with 125

mm top diameter and 140 mm height 33

Fig 3.2 Experiment 2 in a plastic house (29 days after potting) 1.5 L pots with 125

mm top diameter and 140 mm height 33

Fig 3.3 Temperature (□) and RH (○) during experiment 1 (A) and experiment 2 (B)

Data are means ± standard error (SE) (n = 48) 34

Fig 4.1 Relative O aristatus shoot length (A) and shoot number (B), and O

aristatus open flower number (C) over time for control (○), UFRF (□), PAG

(∆) and UFRF+PAG(◊) treatments in composted pine bark Data on shoot length and number were recorded on days 9, 18, 27, 36, 45, 49, 58, and 62 divided by day 9 The number of open flowers was recorded on days 49, 52,

55, 58 and 62 Arrows indicate days of beginning and end of water deficit stress cycles Data are means ± SE (n = 15) 38

Fig 4.2 O aristatus treated with UFRF plus PAG (A), UFRF (B), PAG (C) and

control (D) after 60 hours from the last watering to FC in water stress cycle

2, the experiment 1 .42

Fig 4.3 Stomatal conductance of O aristatus under stress cycle 1 (A) and stress

cycle 2 (B) over time for control (○), UFRF (□), PAG (∆) and UFRF+PAG(◊) treatments in composted pine bark LSD = 31.11, 22.31 and 149.68 at

800 h and 1600 h day 1, and 1200 h day 4; respectively (A) LSD = 173.30, 17.28, 29.16 and 6.83 at 1200 h and 1600 h day 3, and 800 h and 1200 h day 4; respectively (B) ns is no significance Data are means ± SE (n = 4) 42

Fig 4.4 Relative O aristatus shoot length (A), shoot number (B), and open flower

number (C) over time for control (○, ●), UFRF (□, ■), PAG (▲, ∆) and UFRF+PAG (◊, ♦) treatments in composted pine bark (open symbols) and sand (solid symbols) Data on shoot length and number were recorded on days 1, 9, 18, 27, 36, 45, 54, 63, 69, 74, 83, and 86 divided by day 1 Open flower number was recorded on days 69, 74, 77, 80, 83 and 86 LSD = 0.071, 0.094, 0.20, 0.28, 0.42, 0.53 and 1.66 for days 9, 18, 27, 36, 45, 54, and 83; respectively (A) LSD = 0.70, 3.37, 10.82 and 13.06 for days 18, 36, 83 and 86; respectively (B) LSD = 21.4, 42.05, 62.42 and 85.81 for days 77, 80, 83, and 86; respectively (C) ns is no significance Arrows indicate days for beginning and end of stress cycles Data are means ± SE (n = 5) .46

Fig 4.5 Stomatal conductance of O aristatus in stress cycle 1 (A) and stress cycle 2

(B) for control (○, ●), UFRF (□, ■), PAG (▲, ∆) and UFRF plus PAG (◊, ♦) treatments in composted pine bark (open symbols) and sand (solid symbols) LSD = 46.50, 4.47, 35.32, 37.19 and 73.14 at 1200 h day 1, 1200 h day 2,

1600 h day 3, and 800 h and 1200 h day 4, respectively (A) LSD = 24.92, 37.95, 17.96 and 23.24 at 800 h, 1200 h and 1600 h day 2, and 800 h day 3, respectively (B) ns is no significance Data are means ± SE (n = 5) 50

LIST OF APPENDICES

Appendix 3.1 Stomatal conductance pattern of well - watered O aristatus in

preliminary study .77

Appendix 4.1 Analysis of variance (ANOVA) for the effect of UFRF and/or PAG

on time to wilting of O aristatus in transient water deficit periods,

evaluated using randomized complete block (experiment 1) .78

Appendix 4.2 Analysis of variance (ANOVA) for the effect of UFRF and/or PAG

on time to wilting of O aristatus in transient water deficit periods,

evaluated using randomized complete block (experiment 2) .79

Appendix 4.3 Analysis of variance (ANOVA) for the effect of UFRF and/or PAG

on relative pot weight loss in transient water deficit periods, evaluated using randomized complete block (experiment 2) 81

LIST OF ABBREVIATIONS

AMF Arbuscular mycorrhizal fungi

LSD Least significant difference

UFRF Urea-formaldehyde resin foam

CHAPTER 1

INTRODUCTION 1.1 BACKGROUND

Drought is a likely occurrence all over the world, especially in semi-arid regions It may occur at any time during a plants growth and development Reduced precipitation due to global warming, increased populations in some countries, and inadequate policies and a lack of concern over climate change are some principal reasons for water shortages (Handreck, 2008) These factors worsen drought How to get more benefit from available water for both crop and garden plants has received increasing attention from researchers in recent times

Water is essential for plant growth for a number of reasons For instance, it allows transport of minerals from soils into roots and shoots to sustain plant growth, and is also the primary medium for both chemical and biochemical processes of plant metabolism (Haman and Izuno, 2003) Water is a limited resource because just 2.8% of the water on

the earth is fresh (van der Leeden et al., 1990) Moreover, only about 1% is accessible

(Wallace and Bathchelor, 1997) Plants under low moisture supply have low leaf water and turgor potentials These restrictions impede leaf elongation and reduce stomatal conductance Most of the physiological functions of plants, such as protein synthesis, nitrogen metabolism and cell membrane function, are impaired under prolonged soil

moisture deficit stress (Barfield and Norman, 1983; Gomez-Cadenas et al., 1996; Bogoslavsky and Neumann, 1998; Shangguan et al., 2000; Saneoka et al., 2004) Water

deficit stress can also be a primary cause of impaired photosynthetic functioning Thus, low availability of soil water can be a major physical limit to plant growth and crop production

Various water saving measures have been investigated General strategies involve physical monitoring and the introduction of regulations to address users with low efficiency (Frederiksen, 1992), including legal water restrictions (Handreck, 2008) In developing countries, other methods may involve relocation of people to reduce population pressures in high risk areas (McWilliam, 1986) However, such social solutions have not brought about significant water savings for specific crops and gardens, especially in the current scenario of climatic change Soil and growing media quality and conservation are pivotal to any

sustainable agriculture and horticulture program Management of soil and other growing media to maintain or create substrate aggregates in support of water and nutrient retention for plants is vital in the face of scarce water A broad range of approaches are applied to maintain substrate productive capacity The success of substrate conservation and improvement is intimately linked to maintaining or creating aggregates That is, this physical characteristic is important not only for nutrient reserves, but also for water retention

Improvement of substrate physical properties, including water holding capacity (WHC)

of soils and growing media, is an important issue for many scientists Since mid last century, many studies to enhance soil WHC were conducted to improve soils and rehabilitate disturbed lands To mitigate against water deprivation and improve soil physical properties, various natural soil conditioners (e.g Bouyoucos, 1939; Carter,

1986; Davies et al., 1987; Hudson, 1994; Borselli et al., 1996; Cai et al., 2006; Ismail and Ozawa, 2007; Chamini et al., 2008) and artificial soil amendments (e.g Johnson, 1984; Henderson et al., 1991; Janczuk et al., 1991; Chan and Savapragasam, 1996; Huttermann et al., 1999; Panayiotis et al., 2003; Vacher et al., 2003; Nektarios et al., 2004; Nikolopoulou and Nektarios, 2004; Nikolopoulou et al., 2004; Arbona et al.,

2005; Ajwa and Trout, 2006; Chan and Joyce, 2007) can contribute to soil improvement and rehabilitation In this context, the conservation and judicious use of soil water can play an important role for sustainable plant production, especially under drying conditions Thus, any factor which enables plants to better withstand water deficit stress via improvement in physical properties of soils and growing media, including WHC, will improve plant growth and development

1.2 THE PROBLEM

Synthetic soil and growing media amendments have been used since the 1950s with a view to improving physical properties, including WHC of soils and growing media Among these, polyacrylamide gel (PAG) has been trialled widely, and urea-formaldehyde

resin foam (UFRF) has been tried recently for turfgrass (Nektarios et al., 2004; Nikolopoulou and Nektarios, 2004; Nikolopoulou et al., 2004), Lantana camara (Panayiotis et al., 2003) and ornamental plants (Chan and Joyce, 2007) It is considered

that PAG absorbs water and then acts as a water reservoir to release moisture slowly

during drought periods On the other hand, UFRF amendment may increase substrate WHC, but would probably release it relatively easily under drought conditions The success of UFRF and PAG synthetic soil amendments may be different in terms of mitigating transient water deficit stress UFRF and PAG have different characteristics, therefore they may, probably, amend soils and other growing media differently UFRF is made from three components; a resin solution, a hardening agent and a foaming agent (Veili, 1964) In contrast, PAG is defined as a homopolymer compound formed though polymerization of identical acrylamide and related monomers (Barvenik, 1994) Anionic PAG are manufactured by copolymerization of acrylamide and acrylic acid or the salt of this acid (Mortimer, 1991) PAG can be polymer bearing physical and chemical cross-links

within macromolecular chains, which has swelling ability or hydrophobic (Bouranis et al.,

1995) However, there is little or no information to compare these soil amendments

Therefore, comparing and contrasting evaluation is required to characterise UFRF versus PAG with regard to supporting plant growth and development under transient water deficit stress conditions PAG has been shown to decrease bulk density of silty loam and to increase hydraulic conductivity of sandy soil (Johnson, 1984; Terry and Nelson, 1986; Al-Darby, 1996) and a degraded hard-setting soil (Chan and Savapragasam, 1996), but there is little or no information about the effects of PAG on the physical properties of organic potting mix UFRF is claimed to increase air-filled porosity and water infiltration of heavy soils and water retention in light soils (Baader, 1999), but it has not been adequately characterised in sand or organic potting mix Therefore, questions remain as to which soil amendment is more beneficial to the physical properties of sand and organic potting mix media

1.3 OBJECTIVES

This study aims to fill the above-mentioned knowledge gap as to effects of UFRF and

PAG using Orthosiphon aristatus (Cats’ Whiskers) as the test species O aristatus is a

rainforest species of Australia and tropical Southeast Asia It is used in Australia as an

attractive ornamental plant O aristatus belongs to Lamiaceae family It prefers moist garden soils for best growth and it flowers in sun (Nan and Nicholson, 1996) O

aristatus is often seen on the tropical coast of Queensland It is claimed to have low

drought tolerance and is allegedly ‘water hungry’ However, very little is known about the response of this and many other ornamental plants to synthetic soil amendments

Understanding the importance of soil moisture to plant development as well as soil

ameliorants to prolong O aristatus turgor in drought conditions may have potential

implications for other species under transient water deficit stress In this general context, effects of UFRF and PAG on soil water supply for plant uptake under transient water deficit stress conditions were evaluated This study also measured effects of these specific amendments at manufacturers recommended rates on the physical properties of sand and organic potting mixes

Therefore, the specific objectives were:

1 To ascertain the comparative growth and physiological responses of O aristatus

under transient water deficit stress in composted pine bark and sand media treated with UFRF and/or PAG

2 To assess the effects of these soil amendments at manufacturers recommended rates

on the physical properties (i.e bulk density, air-filled porosity and water content) of composted pine bark and sand media

CHAPTER 2

LITERATURE REVIEW 2.1 SOIL-PLANT-AIR CONTINUUM (SPAC)

2.1.1 Soil water

2.1.1.1 Water holding capacity (WHC)

Soil moisture content is the amount of water stored in void spaces or pore spaces between soil particles and aggregates and on clay minerals above oven-dry (McLaren and Cameron, 1996) WHC or plant available water (PAW) is the amount of water for roots of plants to uptake The PAW or soil water storage for plant use in the soil is the difference between water content at field capacity (FC) and permanent wilting point

(PWP) (McLaren and Cameron, 1996; Schwab et al., 1996; White, 2006; Kirkham,

2007) This can be described by the formula:

PAW = FC - PWP (1)

FC is defined as the water content after the soil is completely wet and then allowed to drain for 1 to 2 days, depending on the soil type It is an upper limit of water available

to plants (McLaren and Cameron, 1996; Schwab et al., 1996; Or and Wraith, 2002;

Kirkham, 2007) PWP is water content at which plants cannot extract any more for

growth because water is held tightly in the pore spaces (Schwab et al., 1996) At PWP,

the amount of water in the soil is very low (i.e low water potential) and so plants growing therein become permanently wilted (Fig 2.1) because of the high degree of water deficit stress Or and Wraith (2002) suggested that the easiest method to measure soil water content (θ) is on a mass basis (i.e gravimetric):

mass of water (mass wet soil) - (mass oven dry soil)

mass of dry soil mass of oven dry soil

Soil texture is a main determinant of WHC Different soil types have dissimilar FC and PWP As can be seen from Fig 2.2, finer soil textures have greater PAW Silt loam has the greatest PAW because of low water content at PWP and also high water content at FC

(McLaren and Cameron, 1996) This is partly the result of improved aggregation, as silt loam includes a range of particles and pore sizes Water drains freely out through large pores between aggregates, but it is also stored in fine pores

Fig 2.1 Representation of soil water content at saturation, field capacity and permanent

witling point (From McLaren and Cameron, 1996)

Fig 2.2 Schema of estimated plant - available soil water content for a range of soil

textural types (From Or and Wraith, 2002)

2.1.1.2 Water flow

Water flow occurs in both saturated and unsaturated soils A soil is saturated when all soil pores are full with water (McLaren and Cameron, 1996; Radcliffe and Rasmussen, 2002) In this situation, all of the porosity is entirely filled with water and air in the soil

is virtually zero In contrast, there is both water and air in the pore spaces of unsaturated soils Water flow in soils may be constant or non-uniform when the soil is either saturated throughout the whole profile or in just some parts of the profile (McLaren and Cameron, 1996) Radcliffe and Rasmussen (2002) reported that water usually flows horizontally in saturated soil, whereas water movement in unsaturated soils is predominantly vertical In unsaturated soils, water flow is less steady than in saturated soils, and the hydraulic gradient is the driving force which pushes water flow through

the soil to the water table at depth (Marshall et al., 1996; McLaren and Cameron, 1996;

Radcliffe and Rasmussen, 2002) With decreasing water in pores, soil water develops increasingly negative potentials as it is held in the soil matrix by adhesion and cohesion forces (McLaren and Cameron, 1996) Water always flows from high potential (i.e approaching 0 kPa) to low potential (i.e below 0 kPa) zones because of water potential equilibrium as well as flow downwards and upwards or sidewards as consequence of capillarity (McLaren and Cameron, 1996) In contrast in saturated soils, hydraulic conductivity (Ks) is constant in stable structured soil because the pores are completely filled with water (McLaren and Cameron, 1996; Radcliffe and Rasmussen, 2002) Moreover, the soil water potential is similar everywhere because equilibrium is reached According to Darcy’s Law, the volume rate of water flow (Q) in soils can be measured per unit time through a column which is saturated for the length of the water-soil column (L) with area (A) and a hydrostatic pressure difference of ∆P = P2 - P1 and constant Ks (Jury and Horton, 2004)

2.1.2 Root and shoot growth

2.1.2.1 Root growth

The root system of plants is typically the underground portion and may be highly branched (Fosket, 1994) Roots originate from stems directly and from other roots A root has four distinct sections in the first dimension of growth; root cap, meristematic region, elongation region and maturate region (Fig 2.3) (Sussex, 1989; Glinski and Lipiec, 1990) Each root tip may create ~18,000 cells and extend up to ~5 cm About 14,000 new roots may arise in 1 day from a plant during a vegetative period (Glinski and Lipiec, 1990) Roots grow rapidly to reach to moist zones and grow in any direction When they encounter dry zones of the soil, they branch to seek the moist zones (McLaren and Cameron, 1996) In the second dimension, growth of roots involves forming secondary vascular tissues and periderm (Esau, 1977) The development of secondary vascular tissues involves expansion and division in radial directions At the end of secondary vascular tissue formation, xylem and phloem develop in width and the primary phloem is removed (Gregory, 2006) Xylem and phloem continue to develop to become the outer cover of the root All of the stages of root growth need water from soils, and healthy roots function in water and nutrient uptake to supply shoots

2.1.2.2 Shoot growth

Shoot organs consist of a variety of stems, leaves thorns, tendrils, flowers and fruits which arise from the embryos’ shoot pole (Fosket, 1994) Esau (1977) reported that the apex of the shoot originates initially from meristematic tissues which are the foundation

of the primary plant body Primary meristems are established during embryogenesis and consist of the root and shoot apical meristems (Fig 2.3) The shoot and a few early leaves are considered as a rule, formed in the embryonic period of ontogenesis (Fosket, 1994; Jesko, 1996) The main axis of shoot is plumule or stem, which usually has nodes and internodes, the region between nodes, from which a leaf primordium, bracts, buds and flowers are originated Branches are usually initialised as buds in the axils of leaves A shoot grows strongest at the beginning of seed germination (Kozlowski, 1971) A tree grows very fast until it reaches its final height, but the rate of growth varies among species The development of shoot in air and light depends entirely on water and nutrients from soils via roots (Jesko, 1996)

Fig 2.3 The origin and position of apical meristems in a bean seedling (From Sussex, 1989)

2.1.2.3 Root: shoot ratio

The relationship between roots and shoots can be expressed by the root and shoot dry weight ratio Water contributes up to 70 - 90% of total plant weight (Jesko, 1996) The

root: shoot ratio ranges from < 1 to > 1 (Vogt et al., 1996; Cairns et al., 1997; Werner

and Murphy, 2001) This ratio depends on various factors, including nutrient and water availability However, this ratio tends to decrease as the trees grow bigger (Werner and Murphy, 2001; Ritson and Sochaki, 2003) Water availability significantly influences the root: shoot ratio Barton and Montagu (2006) found that there is a significant effect

of irrigation between total root and shoot biomass in Eucalyptus camaldulensis

Irrigated trees showed an increase in root fresh weight from 8.6 to 12.3 kg and shoot fresh weight from 16.5 to 43.4 kg, but a decrease in root: shoot ratio from 0.54 to 0.28

Interestingly, the root tends to grow deeply and new roots can grow in drought stress conditions Drought stress encourages roots to penetrate more deeply to seek moisture from depth in the soil profile (Reid, 1990) For instance, in the drought stress condition

non-irrigated Vicia faba crop responded to water deficit by reducing its rate of height

increase and decreasing its rate of leaf-area expansion, but the rate of root growth still

increased from about day 78 after sowing (Husain et al., 1990) Although some roots

were dying from drought stress in surface horizons, new roots grew at depth throughout vegetative and reproductive phases of shoot growth

There is a continuous path of water through the soil to the plant, and plants then transpire water from their stomata into the air The roots grow in the darkness so they depends on energy made in the shoot by photosynthesis (Jesko, 1996) Conversely, the shoot growing in the air and light depends on water and mineral nutrients from soils via the root The soil - root system is a continuum in which water is generally in a one way

flow from the soil to the root (Molz, 1971) Kamboj et al (1999) reported that

rootstocks can affect hormonal balance influencing tree vegetative growth Rootstocks can also have influence on tree vegetative growth through mineral nutrition (Jones, 1971) and water relations (Olien and Lakso, 1986)

2.1.3 Air water

2.1.3.1 Vapour pressure deficit

Vapour pressure deficit (VPD) is the difference between saturation moisture pressure in the air or the maximum vapour content and the actual amount of water vapour in the air

at certain temperature and relative humidity (RH) (Kucera, 1954; Wang et al., 2004)

There is a typically continuous flow of water molecules from free water surfaces to the

drier air (Jensen et al., 1990) At equilibrium, the amount of water that goes to the air

and the vapour that returns from air to the water surface is similar and water loss from plant and soil environments decreases VPD occurs when water content in the air is lower than the saturated level Increased aridity of the air results in greater evaporation stress (Kucera, 1954), whereupon water evaporation depends directly on the RH and temperature Higher RH makes VPD lower and vice versa Low RH associated with a high capacity of the air to hold water drives plants to transpire and water to evaporate more High temperatures increase VPD which takes plants to a higher evapo-transpiratory state (Barfield and Norman, 1983; Ismail and Ozawa, 2007) Evapo-transpiration in a field crop situation can be calculated by using the following water balance equation (James, 1988):

2.1.3.2 Air movement

Air movement or wind velocity influences evaporation and transpiration There is a high correlation between evaporation and wind velocity because air movement transfers

water vapour from above surfaces (Kucera, 1954; Munoz-Perea et al., 2007) Kucera

(1954) showed a connection of evaporation and VPD and wind by velocity classes The evaporation rate increased with rising wind velocity Water loss rate at any different wind velocity class is much greater if there is an increase in moisture deficits Non-transpiring plant parts exposed to high solar radiation levels are particularly vulnerable

to high temperature damage when wind speeds are low (Barfield and Norman, 1983) Wind speed affects crop temperature because at very low wind speeds (i.e near convection) the crop temperature is above air temperature even in the well-watered canopy

2.2 PLANT WATER USE

2.2.1 Water use efficiency

Water use efficiency (WUE) or water productivity refers to the relationship between the biomass produced by plants and the water that the plants consume WUE is widely used for water management applications (Gleick, 2003) In grain crops like dry beans, WUE

can be defined as the ratio of grain yield to water consumption (Li et al., 2004; Perea et al., 2007) In ornamental plants, WUE can be considered as the ratio of shoot

Munoz-plus root mass to the amount of water used Higher WUE is achieved when lower amounts of water are used to produce a unit of dry matter (Ismail and Ozawa, 2007)

WUE tends to increase with increasing water supply (Li et al., 2004) Hence, the WUE

term is a tool to investigate means of achieving more productivity with less water In a

growing crop, WUE is usually calculated by using the following relationship (Gregory

et al., 2000; Al-Kaisi and Yin, 2003; Li et al., 2004):

and Yin, 2003), plant species and cultivars (Mahlooji et al., 2000; Ucan et al., 2007), climate (Yu et al., 2004), irrigation management (Hamdy et al., 2003), and soil types (Gregory et al., 2000) A deep, friable and fertile soil usually has high IR and WHC

which sets up plants to achieve high WUE provided that sufficient store water is available in the profile

2.2.2 Water deficit stress

Water deficit stress is the situation of plants subjected to low soil moisture regimes Water deficit stress is a main cause of reduction in photosynthesis, limiting growth rate which results in decreased final yield and production (Bogoslavsky and Neumann,

1998; Saneoka et al., 2004; Ismail and Ozawa, 2007) Plants under low soil moisture

potentials (i.e moisture contents near or at PWP) have low leaf water and turgor

potentials (Barfield and Norman, 1983; Gomez-Cadenas et al., 1996) This impedes leaf

elongation and reduces stomatal conductance Most of the cellular functions of plants; such as protein synthesis, nitrogen metabolism and cell membrane function can also be seriously impaired under prolonged soil moisture deficits (Bogoslavsky and Neumann,

1998; Saneoka et al., 2004) After 20 days of drying, stomatal conductance and photosynthesis rates of citrus plants reduced to about 50% and 90% lower than those of

well-watered plants (Arbona et al., 2005) Specific leaf area of three birch species (Betula ermanii, B maximowicziana and B platyphylla var japonica) reduced from

about 270 to 200 cm2 g-1dry weight with decreasing of soil moisture content from 60%

to 23%, respectively (Koike et al., 2003) Water stress was imposed after 10 days seedling

2.3 SOIL CONDITIONERS TO ENHANCE SOIL PHYSICAL PROPERTIES 2.3.1 General functions

Sub-optimal soil physical properties like low IR, poor drainage, runoff and erosion are main factors contributing to soil degradation An approach for soil conservation and rehabilitation

to improve soil physical properties is application of soil conditioners Soil conditioners consist of both natural and synthetic materials that can sustain important soil functions For example, soil conditioners provide energy, substrates and/or biological diversity to support biological activity Organic and inorganic amendments are considered to improve physicochemical properties of soils and other growing media They are widely utilised

enhance wet-ability and IR, improve aeration and drainage, and increase WHC

2.3.1.1 Wet-ability and infiltration rate

IR and water storage capacity are very important physical properties of the soil, especially in uneven rainfall conditions where they are major determinants for plant production IR is an important soil feature with respect to leaching, runoff and soil moisture availability Soil IR problems are commonly associated with fine-textured soils and water repellency Water repellency can reduce IR and increase runoff or overflow It prevents or at least hinders downward water movement Low IR on surface irrigated fields could lead to water standing on soil surface which results in high evaporation rates (Ajwa and Trout, 2006)

Wet-ability and IR can be improved and water repellency reduced by natural soil amendments, such as clay, gypsum and many organic matter types Soil wetting-ability

is reduced by surface sealing due to raindrop or irrigation drop impact Sealing usually results from the physical disintegration of surface soil aggregates caused by the impact energy of raindrops and physicochemical dispersion of clays which clogs pores beneath the surface (Agassi and Levy, 1991) This process generates low IR and wetting rate (WR), and leads to high runoff and soil loss However, soils with high clay content can

have strong aggregates provided the surface seal is not formed (Betzalel et al., 1995; Mamedov et al., 2001b) Aggregates increase IR (Mamedov et al., 2001b) because water repellency is reduced (McKissock et al., 2002) Mouldboard ploughing with gypsum added can also increase IR during irrigation (Warrington et al., 1989; Harrison

et al., 1992) IR is also improved by organic matter which contributes to soil quality via

aggregates Soil organic matter includes manures, peat and plant residues, can bind soil particles together Improved aggregates lead to better pore size distribution, increased

soil IR (Boyle et al., 1989) and reduced water repellency In addition, increased total

soil organic carbon content reduces soil bulk density and improves water infiltration of sandy loam soils (Franzluebbers, 2002) Hence, addition of soil organic matter is widely practiced to rehabilitate soils

Various types of synthetic soil and growing media amendments have been used to improve the IR and wetting-ability PAG has a large number of -NH2 and –COOH functional groups These groups affect soil aggregates due to their inherent water absorption characteristics

and by establishing bridges between soil particles and changing soil wet-ability (Janczuk et

al., 1991) This improves IR and reduces surface hardness of disturbed soils (Vacher et al.,

2003); e.g increased IR of loess (i.e calcic haploxeralf) and grumusol (i.e typic chromoxerert) Another soil conditioner, UFRF is claimed to reduce soil bulk density and increase aeration Such enhancements may contribute to higher IR and wetting ability

2.3.1.2 Soil aeration and drainage

Soil aeration and drainage are important determinants of soil productivity Aeration can

improve other soil physical properties, including WHC and drainage (Gebauer et al.,

1996) Soil aeration is reported to enhance water infiltration in relatively well-drained

soil (Franklin et al., 2006) Khan et al (2000) reported that plants are impaired as soon

as the oxygen supply in soils is limited due to poor aeration Clay can be used for improved soil aeration and drainage On one hand, high clay content can maintain

standing water during the growing season in rice fields (Tsubo et al., 2007), On the

other, high clay can also limit percolation losses and increase water retention of sand (Al-Darby, 1996) PAG absorbs water and can swell up to 500 times its own weight, and so they can markedly enhance water retention of sandy soils in particular (Johnson, 1984) Thus, PAG increased water at the FC in a coarse sand by 2 - 4 times compared with the absence of the polymer PAG also promoted water stability of macro-aggregates (> 0.25 mm) in degraded hard-setting soil (Chan and Savapragasam, 1996) Polymer improved water stability and reduced structural breakdown of the hard-setting soil to < 0.05 mm particles/fragments up on wetting PAG molecules on the soil surface

make more aerated (Janczuk et al., 1991) UFRF is alleged to improve the soil physical properties of water retention and root zone aeration (Nektarios et al., 2004) Such

synthetic soil amendments may improve air porosity and drainage of heavy soils

2.3.1.3 Soil water holding capacity

Many studies indicate that when soils are improved by stabilization through natural and artificial soil amendments, high soil moisture availability to plants is achieved Organic matter, like muck and peat, has significant impacts on structural improvement of soils, thereby resulting in increased available water (Bouyoucos, 1939; Jamison, 1953; Jamison and Kroth, 1958; Salter and Williams, 1967; Hudson, 1994) Deep rooted

legumes and mycorrhiza also have positive benefits for soil improvement and directly

or indirectly promote water availability for plants Soil WHC increased with clay amendment to a sandy soil (Emerman, 1996) Gypsum also can enhance soil WHC Various studies have shown that polymer amendments can also increase soil WHC (e.g

Johnson, 1984; Tue et al., 1985; Chan and Savapragasam, 1996; Huttermann et al., 1999; Arbona et al., 2005)

2.3.2 Major soil conditioner types

2.3.2.1 Natural inorganic materials

Natural inorganic or mineral soil conditioners are utilized to modify soil physiochemical properties Inorganic soil amendments include clay, lime, gypsum and more unusual substances such as vermiculite, fly ash and perlite These can be used to improve both sandy soils (e.g clay) and clay soils (e.g lime and gypsum) They result in improved aeration and drainage, and increased soil WHC

Clays are principally an option to increase soil moisture and improve physical properties in sandy soils In this capacity, they enhance water availability and crop productivity (Reuter, 1994; Ismail and Ozawa, 2007), including by limiting percolation losses while maintaining adequate IR and water retention (Al-Darby, 1996; Ismail and Ozawa, 2007) Also, addition of a small amount of clay minerals to the top of sandy soil

can decrease the water repellency of sandy soil (Harper and Gilkes, 1994; McKissock et

al., 2002) Soils with 60% clay have the ability to store water (Emerman, 1996;

Mamedov et al., 2001b) According to McKissock et al (2002), higher water contents

for sandy soils are found with the presence of clay containing either kaolinite or smectite fractions This benefit is a result of improved soil aggregation Aggregate stability in cultivated soils increases with clay content because the clay particles help to bind soil particles together (Kemper and Koch, 1966) Soils with limited clay content (e.g loamy sand) have high final IR and low runoff because pores are not clogged by

clay dispersion (Ben-Hur et al., 1985) However, high clay content acts as a cementing

material in soils stabilising soil aggregates against slaking under rain or sprinkling

irrigation drops (Emerson, 1977; Mamedov et al., 2001a) Overall, soils with increased

clay content can have increased aggregate stability, and high IR and low runoff

(Kemper and Koch, 1966; Lado et al., 2004)

While clay is used for sandy soils, gypsum and lime are typically utilised as soil amendments in agriculture to enhance soil properties, including soil moisture for clayey

and crusting soils Ben-Hur et al (1992) suggested that gypsum addition had very slight

effect in pure kaolinite clay to give a non-dispersing soil However, gypsum otherwise

improves soil structure due to increased cohesion between soil particles (Borselli et al., 1996), thereby IR of sodic crusting soils was improved by added gypsum (Shainberg et

al., 1989; Borselli et al., 1996) Gypsum was applied to maintain water infiltration

(Harrison et al., 1992) and generally improve soil physical properties (Mitchell et al.,

2000) Soil for cotton growing treated with gypsum at 5 t ha-1had improved soil

structure for water storages and increased water availability (Hall et al., 1994) Gypsum

treatment for a hard-setting Alfisol enhanced WHC at 87 - 176 days after sowing by from 14 mm to 46 mm higher than for the non-gypsum treatment for wheat Gypsum addition also results in improved aeration for hard-setting soils It took only 8.5 min to wet soil surface after ploughing with gypsum compared to 32.5 min without gypsum

(Hall et al., 1994) Similarly, when soils are mixed with lime, it reacts with soil particles and results in improvement of soil properties (Cai et al., 2006) Bell (1996) indicated

that clayey soils treated with lime showed a significant increase in soil moisture content Differences of soil aggregation and soil particles between lime-stabilized soils and untreated soil are clearly shown in Fig 2.4

Fig.2.4 Scanning electron micrographs of soil specimens: untreated soil (A), 8%

lime-stabilized soil (B) (original magnification of 500) (From Cai et al., 2006)

2.3.2.2 Natural organic materials

Many natural materials can contribute organic matter and improve soils Organic

conditioners are used to increase IR and water retention, promote soil aggregates,

reduce soil strength, improve aeration and provide substrate for microbial activity and

nutrient cycling Organic amendments are derived from living materials, such as roots

of plants, bacteria, fungi, insects, earthworms and soil pests They are typically

comprised of decomposed residues of dead plants and animals (Davies et al., 1972;

Beyer et al., 2002; Schroth et al., 2003) Although plants cannot utilise organic mater

directly for their growth, it is a vital component from a soil fertility perspective (Schroth

et al., 2003), as it improves soil aggregation and structural stability and water

infiltration According to Charman and Roper (2007), organic matter can provide

chemical, physical and biological components which are vital in enhancing soil fertility

because they help aeration and aggregation of soils Humus can make sands and clays

loamier and organic matter is often a source of nitrogen in the soil (Davies et al., 1972)

Many findings have indicated highly positive relationships between organic matter

content, and soil physical properties and soil WHC (e.g Bouyoucos, 1939; Jamison,

1953; Jamison and Kroth, 1958; Salter and Williams, 1967; Hudson, 1994) Muck, peat

and decayed horse-cow manure increased markedly the WHC of mineral sandy loam,

silt loam, clay loam and clay soils (Jamison, 1953) Beyer et al (2002) suggested that

organic matter increases water availability for crops, improves aggregation of soil

particles and may improve drainage Bouyoucos (1939) reported that an addition of

well-decomposed organic topsoil significantly increased PAW in many soil texture

studies, including for sandy loam, sand, silt and clay loam For example, the PAW

increased from 5.54% without manure to 29.63% upon incorporation 12% manure for a

sandy loam The volume of water held at PWP increased as organic matter increased,

but there was still an overall increase in PAW because the volume of water held by the

soil at FC always increased much more than that at PWP (Hudson, 1994)

Deep rooted legumes can be considered organic soil ameliorants They can maintain soil

productivity in low-input farming systems through nitrogen fixation, recovery of deep

nutrients and addition of organic materials for soils (Giller et al., 1994; Wortmann et

al., 2000) Legume crop phases in rotation contribute natural biological nitrogen

fixation to agriculture (Eaglesham et al., 1981; Peoples et al., 1991; Giller et al., 1994)

Many studies have shown that the amount of N fixed by plants is significant Legumes grown as fence trees in inter-cropping may add up to 20 t ha-1 year-1 of organic materials

(Schroth, 2003; Schroth et al., 2003) This practice can yield 23 to 250 kg of nitrogen fixation (Giller and Wilson, 1991; Swift et al., 1994), 28 kg of phosphorus, 232 kg of

potassium and 60 kg of magnesium ha-1 year-1 (Schroth, 2003) In a field study by

McDonagh et al (1993) on groundnut followed by maize on sandy soils in Thailand, the

contribution of biological nitrogen fixation to groundnut ranged from 100 to 130 kgha-1

A study in the northern Guinea savannas of Nigeria indicated that degraded lands can be rehabilitated by pasture legumes (Agbenin and Adeniyi, 2006) The use of legumes in pasture mixtures is seen as a valuable practice in improving nitrogen levels and leading to enhanced soil aggregation (Charman and Roper, 2007) However, for individual species, the proportion of plant N derived from nitrogen fixation varied, depending on water supply, inoculation crop rotation and cultivar (Table 2.1)

Table 2.1 The amount of N fixed by some plants in the sub-tropics and tropics

Legume food

crops Proportion of plant N kg N ha-1 crop-1

Vigna unguiculata 0.61- 0.76 47-105 (Eaglesham et al., 1982)

L leucocephala 0.34-0.39 76-133 (Sanginga et al., 1989)

L leucocephala 0.60-0.98 no data (Yoneyama et al., 1990)

Gliricidia sepium 0.26-0.75 no data (Peoples et al., 1991)

Among of the diverse communities of micro-organisms and invertebrate animals in soils, mycorrhizal fungi are important for nutrient cycling and conservation and soil

structural improvement (Swift et al., 1994; Brundrett et al., 1996; Bearden and Petersen,

2000; Allen, 2007) Arbuscular mycorrhizal fungi (AMF) can improve soil structures and alleviate stress of soil compaction because of their extended hypha network (Bethlenfalvay and Schuepp, 1994) and increased stability of soil aggregates (Bearden

and Petersen, 2000; van der Heijden et al., 2006) The hyphae of mycorrhizal fungi,

along with fibrous roots, can bind soil particles and micro-aggregates into larger aggregated units (Miller and Jastrow, 1992) Hence, the presence of AMF increased soil

aggregation ability grassland (Table 2.2) and can enhance plant resistance to drought and high salinity stress due to the increased absorption zone of mycorrhizal roots

(Hardie, 1985; Bethlenfalvay et al., 1988; Al-Karaki, 1998); including improving the

mineral nutritional status, especially phosphorus (Al-Karaki and Al-Raddad, 1997) Miller and Jastrow (1992) suggested that the effect of AMF on soil aggregation involves three related processes: Firstly, the arbuscular mycorrhiza fungi develop of hyphae into the soil matrix creating the main structure which has the important roles to keep primary soil particles together through physical entanglement Secondly, the micro-aggregates are formed in the complex of roots and hyphae Eventually, micro-aggregates and smaller macro-aggregates are enmeshed together by hyphae and roots to make the macro-aggregate structure of soils

Table 2.2 Soil aggregates with and without arbuscular mycorrhizal fungi (From van

der Heijden et al., 2006)

Soil aggregate class Mycorrhizal treatment

With AMF Without AMF

1996; Chan and Savapragasam, 1996; Arbona et al., 2005) For example, granulated

PAG treatment (650 kg ha-1) of fallow clay loam soil gave surface (0 - 5 cm) bulk densities substantially lower than no treatment and bulk densities ranged from 0.95 - 1.06 and 1.17 - 1.21 g cm-3, respectively (Terry and Nelson, 1986) Furthermore, soil

aggregates increased when soils were treated with PAG (e.g Callebaut et al., 1979; Terry and Nelson, 1986; Shainberg et al., 1990; Chan and Savapragasam, 1996) PAG

application at 1% boosted the aggregate stability of a silt loam soil from 0 to 48%

(Callebaut et al., 1979) The stability of soil aggregates upon PAG treatment was from

three to four times higher than that of non-treated soils (Terry and Nelson, 1986; Chan and Savapragasam, 1996) In addition, PAG can also reduce soil structural breakdown of degraded soils (Chan and Savapragasam, 1996) and improve the water hydraulic

properties of sandy soils (Al-Darby, 1996; Ozturk et al., 2005)

The use of the UFRF (HydrocellTM) as a soil amendment for turfgrass (Panayiotis et al., 2003; Nektarios et al., 2004; Nikolopoulou and Nektarios, 2004; Nikolopoulou et al.,

2004) and for an ornamental tree (Chan and Joyce, 2007) has been researched This product is claimed to hold up to 60% of its volume in water and result in improvement

of soil physical properties Nikolopoulou and Nektarios (2004) reported that bulk density of compacted sandy loam soil was reduced from 1840.6 kg m-3 to 1434.9 kg m-3 UFRF has been used in attempts to improve water retention and/or aeration in the plant

root zone (Nektarios et al., 2004; Chan and Joyce, 2007) These characteristics may

lead to improved air-filled porosity and water infiltration in heavy soils, and to enhance water storage in light soils or potting mixes This amendment did improve shoot growth,

but only gave minor improvements in root growth and plant establishment (Nektarios et

al., 2004; Chan and Joyce, 2007) UFRF as a soil ameliorant in composted pine bark

promoted leaflet numbers in Flindersia schottiana growing in pots under irrigation

(Chan and Joyce, 2007) Under transient water deficit, UFRF at 10% (v/v) was

beneficial in allowing F schottiana plants a longer time (1.5 fold) before they wilted

Positive effects of UFRF on plant growth were questioned

Soil properties are improved by synthetic soil amendments that contribute to soil water content enhancement (Table 2.4) If soil WHC is increased, then this generally leads to

higher PAW (Huttermann et al., 1999) For instance, a cross-linked PAG incorporated

at up to 1g kg-1 into a sandy soil increased PAW to 3 - 5 times higher than for the

untreated control (Johnson, 1984) Huttermann et al (1999) found the higher

concentration of super-absorbent PAG, the more water in a fossil desert soil after the last watering (e.g., 7.2 kg instead of 2.2 kg of water for the control soil) Moreover, less water was lost with PAG treatment in a drought stress period; viz 50% compared with 90% of water evaporated from the control soil Water is, therefore, effectively stored in expanded cross-linked PAG

Table 2.3 Effects of various synthetic soil amendments on soil physical properties

Granular PAG 650 kg ha-1 clay loam - decreased bulk densities

- increased water stable aggregates

- saturated conductivity decreased

- improved the water hydraulic properties

(Al-Darby, 1996)

Anionic polymer 0.01% (w/w) degraded

hard-setting

- lowered bulk densities

- increased water stable aggregates

- reduced structural breakdown

- increased IR

(Chan and Savapragasam,

1996)

Anionic PAGs ≤ 225 kg ha-1 disturbed - increased IR

- reduced surface hardness

- reduced sediment entrainment

(Vacher et al., 2003)

Urea-formaldehyde

resin foam

20% (v/v) sandy loam - increased in total porosity

- reduced bulk density and air-filled porosity

- increased water retention

(Panayiotis et al., 2003; Nektarios et al., 2004; Chan

and Joyce, 2007) Terralyt Plus (TP)

- increased soil aggregate stability

- increased penetration resistance

(Ozturk et al., 2005)

Table 2.4 Effects of various synthetic soil amendments on soil water contents

Water content (%)

Control Treatment

Cross-link PAG (Johnson,

UFRF (Panayiotis et al., 2003) 40% (v/v) Sandy loam Lantana camara L Outdoor 10.6 10.2

UFRF (Nektarios et al., 2004) 20% (v/v) Sandy loam Turfgrass Field 7.1 9.0

UFRF (Chan and Joyce, 2007) 30% (v/v) Clay soil F schottiana Nursery 47 37

UFRF (Chan and Joyce, 2007) 30% (v/v) Loamy soil F schottiana Nursery 32 27

2.3.3 Utility in improving SPAC relations

Soil amendments enhance soil water content by providing an additional water reservoir for the SPAC and thereby reduce water stress for plants Johnson (1984) suggested that extended intervals between irrigations, reduced transplanting shock and a buffer against wilting were brought by soil and growing media amendments All-in-all from the review

of results of experiments above, both natural and synthetic soil and growing media amendments that have ability tocombine high initial absorption with minimal retention tensions for binding-up water increase the water availability of most soils for most plants

The advantage is water conservation as irrigation water is gradually becoming a scarcer commodity and transient drought is an increasingly probable cause of crop and yield losses Soil amendments can extend the survival time for plants in drought For

example, cross-linked PAG helped survival of Pinus halepensis, with amendment at

0.4% hydrogel almost doubling the survival time compared to control plants (82 and 49

days survival, respectively) (Huttermann et al., 1999; Arbona et al., 2005) As with food crops, soil amendments can enhance the drought tolerance of ornamental trees (Arbona

et al., 2005) One effect is that more adventitious roots develop in the presence of soil

amendments under water deficit stress although growth of old root tips stop under water

stress (Huttermann et al., 1999)

WUE is higher in soils amended with soil conditioners When soils and plant are better supplied with water, they maintain higher photosynthetic rates because the stomata do not close due to water stress This effect depends on leaf age and environmental

conditions, such as temperature and leaf water potential (Arbona et al., 2005) When

soil aggregates are improved, conditions are better for soil storage of both water and fertilizers Eventually, both shoot and root matter increase significantly with higher WUE For example, the shoot and root dry matter and WUE of wheat increased in water-stressed condition when two wheat genotypes were colonized with biological AMF soil amendment (Al-Karaki, 1998) Both mycorrhizal-infected genotypes had shoot and root dry matter of 4.03 compared with 2.23 g/plant for shoots and 1.70 compared with 1.10 g/plant for roots Al-Karaki (1998) found that under both well-watered and water-stressed condition, WUE was higher in mycorrhizal plants That is, these plants used less water to produce one unit of shoot dry matter One of the reasons

is that the soils colonized by AMF can have significantly higher levels of soil aggregate

water stability, especially in the 1-2 mm size class (Schreiner et al., 1997)

Overall, each type of soil amendment, including synthetic soil amendments, has potential and specific characteristics to improve soil physical properties and WHC PAG and UFRF have very different chemical properties Therefore, their mechanisms

of action in amending soils are probably not the same Research to compare and contrast synthetic soil conditioners is required to inform effective utilization of these materials

2.3.4 Synthetic soil amendments and the environment

It is claimed that synthetic soil conditioners have no or very little impact on the environment Urea-formaldehyde resin foam (UFRF), like HydrocellTM is considered in the popular literature to be an immobile and biodegradable substrate It is broken down when exposed to ultraviolet (UV) rays from sunlight and is not considered harmful to humans and the environment Formaldehyde is claimed to be biodegradable in sewage treatment plants and to have weak toxic potential (DHA, 2006) It is also asserted that polyacrylamide (PAG) is degraded by common fungal and bacterial species in soils, UV radiation from sunlight, and by field conditions or practices such as tilling or other forces

Nonetheless, the use of these or other soil conditioners could possibly result in contamination of the environment, either directly or via breakdown products Some natural soil conditioners may contain non-essential or excess elements, like heavy metals (e.g., Cu, Ni, Hg, and Zn), and/or persistent organic compounds (Mullins and Mitchell, 1994) which can inhibit plant growth and pollute the soil and water environments The contamination of water, including surface and underground, from agricultural chemicals, such as pesticides and fertilizers (i.e nitrates), is a general public health concern Nitrogen fertilizers might contaminate drinking water resources For instance, nitrate concentrations in drinking water have increased in regions where agriculture and horticulture activities relying on nitrogen fertilizers (Muchovej and Rechcigl, 1995) The use of phosphorus fertilizer also constitutes a potential source of contamination for soils (Withers and Sharpley, 1995) Added chemicals and by-products can reach aquifers Therefore, the use of synthetic soil conditioners is potentially challenging PAG and UFRF could conceivably have adverse effects on the environment Thus, ‘are they really safe for the environment’ is still a question Sudies

to allay these concerns are probably warranted in order to ensure a cleaner, safer and more sustainable environment Questions concerning PAG and UFRF relate to residues

of acrylamide (ACM, CH2 = CHCONH2) and formaldehyde, respectively, in soil and water Potential hazards of these soil conditioners should be assessed via research

PAGs containing less than 0.05% ACM are considered safe for the environment

(Kirk-Othmer, 1979; NRCS, 2001; Sojka et al., 2007) Some controversy arose owing to reports by Smith et al (1996) and (1997) PAG in sunlight and glyphosate, photolytically degrades to ACM Smith et al (1997) indicated that PAG thickening

agents formulated with or without glyphosate-surfactant herbicide resulted in increased ACM concentrations, and PAG released ACM under environmental conditions They reported a slow release of ACM over 6 weeks study They conclude that PAG can degrade to ACM through a free radial process While there are some grounds for environmental and health hazards as soil and/or ground water contaminants, research of

MacWilliams (1978), Johnson (1985), Wallace et al (1986) and Ver Vers (1999)

indicates that PAG does not release ACM in the natural environment Moreover, PAG is

reported to be non-toxic (Sojka et al., 2007) and, as noted earlier to be degraded under

normal conditions such as sunlight and field tillage PAG is vulnerable to UV degradation, but does not release to ACM when degraded in presence of sunlight, glyphosate or both in normal conditions (MacWilliams, 1978; Ver Vers, 1999) Three types of water; tap, river and lake, and two outdoor waters from a forest were used to test ACM concentrations for 6 weeks by a tandem column high-performance liquid chromatography method (Ver Vers, 1999) The highest ACM concentration was 2.37

ppm at day 0 and 2.00 ppm at day 48 McCollister et al (1965) noted that PAG

thickening agents have no herbicidal properties because they do not have ability to pass biological membranes Moreover, ACM was not found in sandy desert conditions when cross-linked PAG was degraded for period of time (Johnson, 1985)

2.3.4.2 Urea formaldehyde resin foam

There is little information on UFRF impacts in the environment Any potentially adverse effects of UFRF soil conditioner on the environment is a question remaining for scientists because of a potential residue, formaldehyde Formaldehyde can release into