Dr Paul Truong, Leader, Erosion Control and Slope Stabilisation, Resource Sciences Centre, BRISBANE, AUSTRALIA
1. Introduction
Although Vetiver grass (Vetiveria zizanioides) has been used for land protection purposes for about 50 years, its real impact on soil and water conservation was only started in the late 1980's following the promotion by the World Bank through the Vetiver Network which was established by Dick Grimshaw.
The Vetiver Grass System (VGS) was first developed to protect farmlands from soil erosion and to conserve water. While this application still has a vital role in agricultural lands, its tolerance to highly adverse soil conditions will play a key role in the ever increasing field of environmental protection. In addition, the special physical and morphological attributes of vetiver grass provides unique opportunities for engineering applications.
2. Some special characteristics of vetiver grass
2.1 Morphological characteristics
Vetiver grass has neither stolons nor rhizomes and a massive finely structured root system. In some applications rooting depth has reached 3-4m in the first year. This massively thick root system binds the soil and at the same time makes it very difficult to be dislodged under high velocity flows. In addition this very deep root system has also made vetiver very drought tolerant, as an example vetiver did not only survive but continued to grow through the worst drought in Australia early in the 1990's.
Further advantages include :
2.2 Physiological Characteristics
2.3 Weed potential
It is very important any plants that are used for soil and water conservation do not become a weed in the local environment. It is therefore imperative that only sterile cultivars be used. From the three vetiver cultivars present in Australia, a sterile line was selected and rigorously tested for its sterility. From 1989 this cultivar has consistently produced no caryopses when grown under glasshouse and field conditions and in dryland, irrigated and wetland habitats. This cultivar also tolerates climatic extreme of the semi arid areas of central Australia, to the wet tropics of north Queensland, to the temperate regions of south eastern Australia. This cultivar was registered as Monto vetiver as the Monto district was the site of first field trial in Queensland.
Establishment of Monto vetiver is by vegetative means using root divisions or slips. Monto vetiver is readily grazed by cattle, dairy cows, sheep and horses as well as some native animals. All research, development and application works conducted in four states of Australia: Queensland, New South Wales, Victoria and Western Australia has used Monto vetiver (21).
Fiji has a history of more than 100 years of vetiver uses and for more than 50 years for soil and water conservation purposes and vetiver has not shown any weed potential (19).
2.4 Genetic Characteristics
There are three Vetiver species being used for soil conservation purposes: V.zizanioides, V. nigritana and V. nemoralis.
V.nigritana is native to Southern Africa and its application is mainly restricted to the sub continent. V. nemoralis is native to Thailand and being widely used for thatching for centuries and recently for soil conservation purposes. Both of these species are seeded varieties hence its application should be restricted to their home lands.
There are two V.zizanioides genotypes being used in Asia for soil and water conservation purposes:
The South Indian genotype is the main cultivar used for essential oil production and this is the genotype that being used around the world for soil and water conservation purposes because of its unique and desirable characteristics already mentioned. Recent results of the Vetiver Identification Program, by DNA typing, conducted by Adams and Dafforn (1) have shown that of the 60 samples submitted from 29 countries outside South Asia, 53 (88%) were a single clone of V.zizanioides.
The 53 samples tested came from North and South America, Asia, Oceania and Africa. Most interestingly among these cultivars are: Monto (Australia), Sunshine (USA), Vallonia (South Africa) and Guiyang (China). The implications are that, once the genotype is identified, all our R, D and A (Application) can be shared around the world. For example, as all vetiver research conducted in Australia have been based on Monto vetiver, all the Australian results presented at this Workshop can be applied with confidence in China when the Guiyang variety is used and similarly, Vallonia for Southern Africa.
3. Agricultural Applications
When planted in rows Vetiver plant will form dense hedges, a living porous barrier which slows and spreads runoff water and traps sediment. As water flow is slowed down, its erosive power is reduced and at the same time allows more time for water infiltrate to the soil, and any eroded material trapped by the hedges. An effective hedge will reduce soil erosion, conserve soil moisture and trap sediment in the runoff water on site.
Most hedges can achieve the same results as vetiver grass, but due to its extraordinary and unique morphological and physiological characteristics previously detailed vetiver has many advantages over other systems tested. (4).
3.1 Soil Erosion Control, Water Conservation and Crop Production.
India: Experiments were conducted on shallow gravelly Alfisol having 2.5% slope during 1990-95 to study the effect of vetiver hedges on soil and water conservation and yield of rainfed crops, sorghum + redgram and castor. Vetiver hedge on contour reduced the runoff and soil loss by 66% and 76% (1993-94) and 67% and 97% (1994-95), respectively in contour cultivation alone and increased mean soil moisture by 5% to 9% in the 0-45 cm depth. Increase crop yields due to vetiver hedges ranged from 7.0% to 22.4% (13).
Sri Lanka: The up and down planting system in tea plantations without erosion control has resulted in the loss of 1.15m of top soil during last century. Unplanned land clearing causes 250t/ha/year of soil loss. No research data is available but significant results have been observed in soil and water conservation on those lands protected by vetiver hedges (11).
Malaysia: Since 1992 through the effort of P.K. Yoon the VGS has been widely adopted by rubber plantation and orchard owners as their preferred method of erosion and sediment control and land stabilisation.
Indonesia: Although VGS is very effective in controlling soil erosion as compared with other grasses in the upland farming, adoption of VGS was slow. This was due to the low income of upland farmers which leads to the use of species that give direct short term benefit such as elephant grass, leucaena, King grass and setaria. Vetiver hedges have reduced the soil loss from 120t/ha/year (control) to 13.21t/ha/year and 0.56t/ha/year for second and third year respectively. (8)
Thailand: To conserve soil moisture in orchards, planting vetiver grass in semi circle of 2m radius downslope from the tree gave best results, this followed by semi circle of 4m radius and the planting of straight row 4m from the trees conserved the least moisture .
Study on row spacing at Chiang Mai showed that vetiver planted in single row had better growth than double row planting. Wider spacing also provided a higher tiller number and bigger hill size than close spacing. There were no significant differences in crop yield among treatments. Both single and double rows significantly reduced soil loss as compared to control plots and there was no differences in soil loss between the row treatments. Moreover, the vetiver plots tended to conserve soil moisture longer than the traditional farming practice (9).
Philippines: In term of nutritive value, vetiver has lower levels of macronutrients, particularly P (0.1%), but higher in micronutrients than the commonly used forage species such as para grass, Napier grass and some legumes (14). Therefore if used as the main feed for grazing animals, vetiver needs to be supplemented with some other concentrates.
Fiji: The concept of using vetiver grass for soil and water conservation purposes instead of conventional structures was first developed in this country for the sugarcane growers approximately 50 years ago. The vetiver system is now widely used as a standard practice for soil and water conservation, particularly on small farms. As in other countries, VGS has proven to be very effective in soil erosion control, and when properly implemented the system has improved sugarcane yield up to 55%. Despite this improvement some farmers are still reluctant to implement the VGS on new plots, and some have even removed the previously well established hedges. The results have been resulting in massive soil erosion and loss of cane production. The main reasons for low adoption and rejection of the VGS are complex and include socio economic factors, land use policy, agronomic practices and lack of extension effort (24).
Australia: Numerous applications have been developed for agricultural and forestry applications in Queensland from rainfed cropping in low rainfall regions to the wet tropics (22). VGS was used as a replacement of contour banks in steep sugarcane lands on the wet tropical coast where the traditional method of soil conservation using contour banks can present some problem for farm machinery operation as the channels and banks can be dangerous for large machinery and vetiver hedges offered a solution to that problem.
Under rain-fed farming conditions, it was found that mature Vetiver hedges can compete with the adjacent crops. In Queensland, two year old hedges reduced establishment, growth and yield of 2 rows of grain sorghum adjacent to vetiver hedge (1.5m) in drought conditions. Growth and yield were not affected when soil moisture is not limited as under irrigation (3). In addition the shading effect of tall hedges can severely affect establishment and early growth of seedlings in adjacent rows up to 1.5m from the hedges. Lower hedge height will reduce shading effects and water competition with crops at planting.
Vetiver hedges have been used very effectively in stabilising gully erosion in both cropping and grazing lands. When planted on contour line above the gully head, hedges will spread and slow down runoff water and stop the advancement of gully head. Following the control of active erosion at the gully heads, gully floors are quickly revegetated with native species. (21).
Other significant applications of the VGS in Australia include erosion control of dam walls against wave action, providing shade for sheep in treeless tropical grassland, road and waterway stabilisation in forestry plantations (16,21)
South Africa: While VGS has been used to protect crops such as bean, banana, chilli etc, the two crops that benefited most from the VGS are coffee and sugar cane.
At Sarabica Coffee Estate, vetiver hedges have replaced contour banks (terraces) as the standard soil conservation measure when planted between the rows of coffee plants. Hedges have reduced maintenance costs, facilitated traffic and provided mulch for weed control in addition to the soil and water conservation effects .
At Vallonia Sugar Estate near Durban, Maxime Robert has fully incorporated the VGS into the management of his farm in Natal. Vetiver has been used successfully for steep slope stabilisation, farm road and stream bank stabilisation, and contour terrace replacement. Recently he has extended his VGS into a macadamia orchard on steep lands.
Zimbabwe: VGS is being used extensively in large sugar estate in Zimbabwe, and similar to South Africa, VGS is instrumental in the stabilisation of farm roads, erosion control in sugarcane fields and particularly in drainage ditches and irrigation channels. Jano Labat has developed an unique method of stabilising drainage channels by planting a row at the top of the channel to trap sediment in runoff water from the road and another on the top of the concrete lined drain to protect it from erosion and at the same time trapping more sediment (Grimshaw pers.com.).
Malawi: VGS technologies are now a part of the government soil improvement package. After initial reluctance, the technology is now universally accepted by agricultural and soil conservation staff. The key to effective application has been the use of an A frame for establishing level contours.
3.2 Flood Erosion Control
Australia: VGS has been used successfully on floodplain of the Darling Downs, Queensland, as a supplement to or an alternative to strip cropping practice which relies on the stubble of previous crops for erosion control of fallow land and young crops. But during drought or when low stubble-producing crops such as sunflower and cotton are grown, very little protection is provided. Eight rows of vetiver totalling almost 6,000m, were planted at 90m intervals on a strip cropped site. Flume tests found that when fully established, the Vetiver hedges should provide adequate protection from floodwater over the 90m spacing on 0.2 - 0.35% land slope which is equivalent to five existing strips at this particular site (2).
Results over the last two years, including several major flood events, have been excellent, the hedges were successful in reducing flood velocity and limiting soil movement, resulting in very little erosion in fallow strips and a young sorghum crop was completely protected from flood damage. The incorporation of Vetiver hedges as an alternative to strip cropping on floodplains should result in more flexibility, more easily managed land and more effective spreading of flood flows in drought years and with low stubble producing crops. An added benefit is that the area cropped at any one time could be increased by up to 30%.
4. Environmental Applications
Both morphological and physiological characteristics of vetiver grass make it highly suitable for environmental protection in the areas of bio-remedial, filtering and rehabilitation.
4.1 Bio-remedial applications
Thailand: Research conducted in cabbage crops grown on steep slope (60%) indicated that Vetiver hedges had an important role in the process of captivity and decontamination of agrochemicals especially pesticides such as carbofuran, monocrotophos and anachlor preventing them from contaminating and accumulating in crops (12).
Other research found that vetiver grass can absorb substantial quantity of Pb, Hg, Cd in waste water (17). Methanol extracts of ground stem and root was found to be very effective in preventing the germination of a number of both monocot and dicot weed species. These results indicate the potential of vetiver extract as a natural pre-emergent weedicide. (18).
Australia: Landfill and industrial waste sites are usually contaminated with heavy metals such as As, Cd, Cr, Ni, Cu, Pb and Hg which are highly toxic to both plants and humans. The movement of these contaminated materials from these sites needs to be adequately controlled.
Research conducted in Queensland has shown that Vetiver is highly tolerant to very high levels of heavy metals such as As, Cd, Cr, Ni, Ca, Pb, Zn, Se and Hg. (23, 27). Results indicate vetiver grass is most suitable for use in the rehabilitation of such contaminated sites and work conducted in Queensland have conclusively shown that vetiver can be used to rehabilitate the highly erodible slopes and drainage lines and in reducing leachate from these contaminated sites (27).
Vetiver has also been found to be highly resistant to menatode. Young vetiver plants were found to be immune to all major species of Meloidogyne nematode, which makes vetiver highly suitable to be used as cover crop, companion or hedge row crop. (28).
4.1 Filtering
The barrier formed from the thick growth of vetiver hedges is also a very effective filter of both coarse and fine sediment in run-off water. These sediment need to be trapped on site or they will pollute and silt up streams, roads and other infrastructure. Chemical pollutants also often adsorbed by these sediments which when trapped by vetiver hedges will lessen off-site pollution.
Thailand: Vetiver hedges are used extensively in the stabilisation of earth banks and also to filter run-off water of farm dams and fish ponds.
Australia: Vetiver filter strips are being used extensively in Queensland to trap sediment in both agricultural and industrial lands. At a working quarry, vetiver hedges planted across waterways and drainage lines reduced erosion and trapped both coarse and fine sediment resulting in less sediment in the dam water.
4.3 Rehabilitation
Australia: Vetiver is highly successful in the rehabilitation of old quarries where very few species can be established due to the hostile environment. Vetiver is able to stabilise the lose surface first so other species can colonise the areas between hedges later. After two years the site was completely revegetated with vetiver and local species. (19). In Queensland Vetiver has been successfully used to stabilise mining overburden and highly saline, sodic and alkaline (pH 9.5) tailings of coal mines and highly acidic (pH 3.5) tailings of a gold mine. It has also been successfully used to stabilise and rehabilitate a highly erodible acid sulfate soil on the coastal plain where actual soil pH is around 3.5 and oxidised pH is as low as 2.8 (26). Vetiver has also been used to rehabilitate salt affected lands due to both dryland salinity and irrigation.(21)
South Africa: Tony Tantum in cooperation with several mining Companies, has proved that vetiver can be effectively used to stabilise/rehabilitate "slimes dams" (tailings). As a result of DNA typing by Adams and Dafforn (1), South African mining companies can apply vetiver research results conducted in Australia with confidence as Monto vetiver is identical to Vallonia vetiver in South Africa.
Works at de Beers diamond mine slimes dams at Premier (800 mm annual rainfall) and Koffiefonteine (300 mm) mines confirms that vetiver can survive in very harsh environments where surface temperatures of the slimes dam materials, that are black in colour, reach over 550C. At this temperature most seeds are unable to germinate. Vetiver hedges at 2 m VI (Vertical Interval) provides shade that cools the surface thus allowing germination of other grass seeds. These results show vetiver is truly a pioneer grass (10).
Tony Tantum has also been involved in successful stabilisation of slimes dams at platinum and gold mines. It is very important to stress that at all the successful sites (as compared to report of failure) strict technical standards have been set and applied. Tantum finds that chicken manure gives vetiver a great start.
Although vetiver is highly effective in mining rehabilitation and accepted by the major mining houses in South Africa, the mine rehabilitation section of the South African Chamber of Mines in Johannesburg still uses Hippo grass (Pennisetum marcorium). This species has simular morphological characteristics to vetiver but its tolerance levels to adverse conditions, particularly low soil pH, are not as good as vetiver.
5. Engineering Applications
Due to its special morphological characteristics such as stiff and erect stems, extensive and deep root systems vetiver has provided a very effective means of flood mitigation and step slope stabilisation.
5.1 Hydraulic properties
Australia :The pioneering work on the hydraulic characteristics of Vetiver hedges under deep flows were conducted in the above ground flume at the University of Southern Queensland. These hydraulic parameters were needed for the design and incorporation of Vetiver hedges into strip cropping layout to control flood erosion on the floodplain mentioned above (Fig. 1) (2).
(NOT INCLUDED)
In hydraulic terms, on slopes higher than 5%, apart from slowing the flow in the zone immediately upstream of each hedge, the hedges will not materially affect the volume or rate of flow. On the sediment trapping of Vetiver hedges, the most important factors are hedge spacing and the thickness (density) of the hedge. Closely spaced hedges will minimise the quantity of sediment entrained by the flow. Dense hedges will maximise the depth upstream, the length of the back water and the settling time for particles and hence proportion of the sediment trapped. As terraces form the length of the backwater will increase and the sediment trapping efficiency will also increase - provided the hedge grows out of the sediment layer, remains dense and upright and is not overtopped by the flow.(15)
5.2 Steep Slope Stabilisation
Batters of both cut and fill slopes can be effectively stabilised by establishing vetiver on contour lines. The deep root system stabilises the slope while the hedges spread run-off water, reduce rill erosion and trap sediment, providing a very favourable environment for the colonisation by local volunteer species.
Malaysia: The first batter stabilisation trials were conducted in 1992 by P.K. Yoon. The success of these trials and most importantly the results of innovative research and application conducted by Diti Hengchaovanich (7) have led to the large scale application of the VGS for steep batter stabilisation in major highways in Malaysia. The research established that root tensile strength and shear strength of vetiver root in soil blocks are equivalent to one third of mild steel reinforcement which are as strong as and in some cases stronger than many hardwood species which have been proven positive in the stabilisation of steep slopes.
Results from this study gave engineers the quantitative parameters needed for their design of civil construction projects. This has led to the wide application of the vetiver technology in Malaysia and the acceptance of Vetiver grass as an effective means of batter stabilisation by Main Roads and Railway engineers in Australia a well.
Malaysia is leading the world in the application of Vetiver for erosion and slope stabilisation in highway engineering, Hengchaovanich conceded that the design is still somewhat conservative, treating Vetiver as `a bonus' or added assurance. Once more design parameters, especially the evapotranspiration and hydraulic aspects, and track records come to light, bolder and more innovative designs maximising the full potential of vetiver grass should be adopted. (5,6).
Thailand: Although road batter stabilisation with vetiver is only limited to the northern regions of Thailand, its effectiveness has been varied as the quality of planting materials, planting methods, maintenance program and particularly the layout have not been of suitable designed. The results are excellent at some sites and bad failures occurred at others.
Philippines: Through the recommendation of the World Bank the Philippines Department of Public Works and Housing has recently started using Vetiver for highway batter stabilisation. The results so far have been disappointing because from the engineers, who specified vetiver application, to the labourers, who planted the vetiver had no previous experience or training in the use of vetiver for steep slopes stabilisation. These failures are due entirely to poor application. Vetiver is a tool, like any tools one has to learn how to use it properly to maximise results.
Australia: The first vetiver trial on batter stabilisation was conducted on a very steep (1:1) railway cutting in June 1992 on a highly erodible sodic soil in north Queensland. Monto vetiver planted on contour lines stabilised the batter first and the inter-row spaces were then completely colonised by local vegetation in six months . Fifteen months later this highly unstable slope was stabilised by a mixture of vetiver and local native vegetation (20).
Despite these outstanding results, the application of vetiver for batter stabilisation was not readily accepted by highway engineers until 1996 following the results presented by Mr Hengchaovanich at the first Australian Vetiver Workshop in November. From then Monto vetiver has been used to stabilise an embankment of 32 km of a very steep railway line on a mountain range in south east Queensland. Main Roads design engineers have now specified vetiver for both batter stabilisation and run-off water control.
A trial was started in 1995 to compare the effectiveness of a native Australian vetiver (Vetiveria filipes), Lomandra longifolia and Monto vetiver (V.zizanioides) in batter stabilisation. Following a prolonged rain period with 400 mm in total, the sections planted with the Lomandra and native vetiver collapsed while the Monto vetiver section remained intact. These results clearly show the unique characteristics of V. zizaniodies as compared with other vetiver species.
South Africa: Vetiver has not been used in major highways in South Africa. But its application in Kwa Zulu province in the stabilisation of both cut and fill batters, roads, drainage lines resulting from massive earth shaping in industrial parks, has gained wide acceptance.
6.0 CONCLUSION
From the results of research and the successes of numerous applications presented above, it is clear that we now have enough evidence that VGS is ready to move out of the farm gate, beyond the soil and water conservation applications in agricultural lands to the protection of the environment in general, with particular emphasis the rehabilitation of contaminated lands, mining wastes and bio-remedial applications including wetland and aquaculture.
For effective results, the emphasis should be on the all important APPROPRIATE DESIGN AND CORRECT APPLICATION TECHNIQUES. Unless these strict specifications are met, its effectiveness is lost
7. REFERENCES
1. Adams, R.P., Dafforn, M.R. (1997). DNA fingerprints (RAPDs) of the pantropical grass, Vetiviria zizanioides L, reveal a single clone, "Sunshine," is widely utilised for erosion control. Special Paper, The Vetiver Network, Leesburg Va, USA.
2. Dalton, P.A., Smith, R.J. and Truong, P.N. (1996). Vetiver grass hedges for erosion control on a cropped floodplain, hedge hydraulics. Agric. Water Management :31(1,2) pp 91-104.
3. Dalton, P. and Truong, P.N. (1996). Soil moisture competition between vetiver hedges and sorghum under irrigated and dryland conditions. Proc. First Int. Vetiver Conf., Thailand (in press).
4. Greenfield, J.C. (1989). Vetiver Grass: The ideal plant for vegetative soil and moisture conservation. ASTAG - The World Bank, Washington DC, USA.
5. Hengchaovanich, D. (1996). Use of vetiver grass for engineering purposes in Malaysia with particular reference to Slope Stabilisation and erosion Control. Proc. R&D and A of VGS in Queensland. Australia, p 27-35.
6. Hengchaovanich, D. and Nilaweera, N.S. (1996). An assessment of strength properties of vetiver grass roots in relation to slope stabilisation. Proc. First Int. Vetiver Conf. Thailand (in press).
7. Hengchaovanich, D. (1997). Slope stability enhancement and erosion mitigation by vetiver grass in engineering applications. (This workshop).
8. Hermavan, A. (1996). Status of vetiver grass in upland farming development in Indonesia. Proc. First. Int. Vetiver Conf. Thailand (in press).
9. Intaphan, P., Boonches, S. and Vathatum, S. (1996). Study of optimum rows and different plant spacings of vetiver grass for soil erosion control on sloping land. First Int. Vetiver. Conf. Thailand (in press).
10. Knoll, C. (1997). Rehabilitation with Vetiver. African Mining (Magazine). 2 (2):43.
11. Navaratnam, R. (1996). Vetiver grass hedgerows for soil and water conservation in tea land in Sri Lanka. A success Proc. First. Int. Vetiver Conf. Thailand (in press).
12. Pithong, J., Impithuksa, S. and Ramlee, A. (1996). Capability of vetiver hedgerows on the decontamination of agro chemicals residues. Proc. First. Int. Vetiver Conf. Thailand (in press).
13. Rao, C.N., Rao, M.S. and Raju, A.P. (1996). Effects of vetiver hedge on run-off, soils loss, soil moisture and yield of rainfed crops in Alfisol watershed of southern India. Proc. First. Int. Vetiver Conf. Thailand (in press).
14. Salamanca, E., Kanbo, N., Katogiri, S. and Exconde, A.(1997). Mineral concentrations and some physical properties of vetiver grass in La Union, Philippines. Proc First Nat Vetiver Workshop, Lyete, Philippines (in press).
15. Smith, R. (1996). The hydraulics and sediment trapping of vetiver hedges on steep slopes. Proc. R&D and A of VGS in Queensland. Australia, p 18-26.
16. Somes, T. (1994). Asian Grass to lift lambing percentages. Queensland Country Life Newspaper, Queensland, Australia (10 November 1994).
17. Suchada, K. (1996). Growth potential of vetiver grass in relation to nutrients in wastewater of Changwat Phetchubari. Proc. First. Int. Vetiver Conf. Thailand (in press).
18. Techapinyawat, S., Sripen, K., Komkriss, T. (1996). Allelopathic effects of vetiver grass on weeds. Proc. First Int. Vetiver Conf. Thailand (in press).
19. Truong, P.N. and Creighton, C.(1994). Report on the potential weed problem of vetiver grass its effectiveness in erosion control in Fiji. Div. Land Management. QDPI, Brisbane, Australia
20. Truong, P.N, McDowell, M., and Christiansen, I. (1995). Stiffgrass barrier with vetiver grass. A new approach to erosion and sediment control. Proc. Downstream Effects of Land Use Conf. Rockhampton, Australia. pp 301-304
21. Truong, P.N. (1996). An Overview of Research, Development and Application of the VGS overseas and in Queensland. Proc. R, D and A of VGS in Queensland. Australia p: 6-7.
22. Truong, P.N., Dalton, P.A., Knowles-Jackson, C., Evans, D.S. (1996). Vegetative Barrier with Vetiver grass: An alternative to conventional soil and water conservation systems. Proc. 8th Aust. Agronomy Conf. Queensland pp 500-553.
23. Truong, P.N. and Claridge, J. (1996). Effects of heavy metal toxicities on vetiver growth. Proc. First Int. Vetiver Conf. Thailand (in press).
24. Truong, P.N. and Gawander, J. (1996). Back from the future: Do's and don'ts after 50 years of vetiver utilisation in Fiji. Proc. First Int. Vetiver Conf., Thailand (in press).
25. Truong, P.N., Gordon, I. And Baker, D. (1996). Tolerance of vetiver grass to some adverse soil conditions. Proc. First Int. Vetiver Conf., Thailand (in press).
26. Truong, P.N. and Baker, D. (1996). Vetiver grass for the stabilisation and rehabilitation of acid sulfate soils. Proc. Second National Conf. Acid Sulfate Soils, Coffs Harbour, Australia pp 196-8.
27. Truong, P.N. and Baker, D. (1997 ). The role of vetiver grass in the rehabilitation of toxic and contaminated lands in Australia, ( This workshop).
28. West, L., Sterling, G. and Truong, P.N. (1996). Resistance of vetiver grass to
infection by root-knot nematodes (Meloidogyne spp). Proc. First Int. Vetiver Conf.
Thailand (in press).