Dr Paul Truong

Principal Soil Conservationist,ResourceSciencesCentre, Queensland Department of Natural Resources. Brisbane, Australia.

Introduction

Although vetiver grass (Vetiveria zizanioides L.) has been used for land protection purposes in tropical and subtropical countries for more than a century, its real impact as a low cost, effective and simple method of soil conservation in agricultural land only emerged in the late 1980s. This was a result of its development for soil and water conservation by the World Bank in India and subsequently its promotion by The Vetiver Network, which was founded by Dick Grimshaw (see Chapter 1).

While its role in protecting agricultural lands remains very important, scientific research conducted in the last 10 years has clearly demonstrated that due to its special morphological and physiological attributes, vetiver grass provides unique opportunities for other applications such as infrastructure and environmental protection (Truong, 2000).

It is very important to point out that to obtain maximum effectiveness vetiver grass has to be used correctly for each application. For instance, as a method of soil and water conservation in agricultural land, in addition to engineering principles, a sound knowledge of soil and plant sciences are needed. Therefore the application of vetiver grass is called Vetiver Grass Technology (VGT).

Special Characteristics of Vetiver Grass

Morphological characteristics

Vetiver grass has no stolons and very short rhizomes and a massive finely structured root system that can grow very fast and in some applications rooting depth can reach 3 — 4 m in the first year. This deep root system makes the vetiver plant extremely drought tolerant and very difficult to dislodge in strong currents (Hengchaovanich, 1998).

Stiff and erect stems, which can stand up to relatively deep water flow (Truong et al, 1995).

It is highly resistant to pests, diseases and fire (West et al., 1996; Chen, 1999). A dense hedge is formed when vetivers are planted close together acting as a very effective sediment filter and water spreader.

New shoots emerge from the base helping it to withstanding heavy traffic and heavy grazing pressure.

New roots are developed from nodes when the plant is buried by trapped sediment. Vetiver will continue to grow up despite the deposited silt, eventually forming terraces if the trapped sediment is not removed (Truong, 1999a).

Physiological characteristics

Valuable physiological characteristics include

• Tolerance to extreme climatic variation such as prolonged drought, flood, submergence and extreme temperature from —22 °C to 60 °C (Truong, 1999b; Xia et al., 1999; Xu and Zhang, 1999);

• Ability to regrow very quickly after the weather improves or soil ameliorants have been added after having been affected by drought, frost, salinity and other adverse soil conditions (Truong et al., 1995);

• Tolerance to a wide range of soil pH values (3.0 to 11.5) (Truong and Baker, 1998);

• High level of tolerance to herbicides and pesticides (Pithong et al., 1996; Cull et al, 2000);

• Highly efficient in absorbing dissolved nitrogen, phosphorus, mercury, cadmium and lead in polluted water (Pithong et al., 1996; Suchada, 1996);

• Highly tolerant to growing media high in acidity, alkalinity, salinity, sodicity and magnesium (Truong and Baker, 1996; Truong and Baker, 1998) and highly tolerant to aluminium, manganese and heavy metals such as arsenic, cadmium, chromium, nickel, lead, mercury, selenium and zinc in the soils (Truong, 1999c).

Ecological characteristics

Although vetiver is very tolerant to some extreme soil and climatic conditions mentioned above, as a C4 plant (see Chapter 2) it is intolerant to shade. Shade will reduce its growth and in extreme cases may even eliminate vetiver in the long term. Therefore vetiver grows best in the open and weed control may be needed during the establishment phase. On erodible or unstable ground vetiver first reduces erosion, stabilising the erodible ground (particularly steep slopes), before improving its micro environment so that other wild or sown plants can establish later. Because of these characteristics vetiver can be considered as a pioneer plant on disturbed lands.

Weed potential

It is imperative that any plants used for bioengineering or environmental protection do not become weeds. To comply with the very strict Australian rules on introduced plants, a sterile vetiver cultivar was selected (from a number of existing cultivars in Australia) and exhaustively and rigorously tested for eight years for its sterility under various growing conditions. This cultivar is registered in Australia as Monto vetiver.

In Fiji, where vetiver grass was introduced to the country more than 100 years ago, and it has been widely used for soil and water conservation purposes for more than 50 years, yet vetiver grass has not become a weed in this new environment (Truong and Creighton, 1994).

As with all sterile plants, vetiver can only be vegetatively propagated and planted. Planting materials are obtained by subdividing the crown of a mature plant and are supplied as slips or splits in various forms suitable for different applications.

Although vetiver grass is very resilient under the most adverse conditions it can be eliminated easily either by spraying with glyphosate herbicide or uprooting and drying out by hand or using farm machinery.

Genetic characteristics

There are two vetiver species being used for soil conservation purposes, Vetiveria zizanioides L. and V. nigritana. The latter is native to southern and western Africa and its application is mainly restricted to that sub continent. As V. nigritana is a seeded variety its application should be restricted to its homeland because it may become a weed when introduced to new environments.

There are two V. zizanioides genotypes being used for soil and water conservation and for land stabilisation purposes:-

• The wild and seeded northern Indian genotype;

• The sterile or very low fertility southern Indian genotype.

While the seeded genotype is only used in northern India, the southern, sterile genotype, the vetiver used for essential oil production, is the genotype that is being used around the world for soil and water conservation and land stabilisation purposes. Results of the Vetiver Identification Program, by DNA typing, conducted by Adams and Dafforn (1997) have shown that of the 60 samples submitted from 29 countries outside South Asia, 53 (88%) were a single clone of V. zizanioides. These 53 samples tested came from North and South America, Asia, Oceania and Africa and most interestingly among these 53 cultivars are Monto (Australia), Sunshine (USA), and Vallonia (South Africa).

The implication is that, once the genotype is identified, all the research, development and application can be shared around the world. For example, as all vetiver research conducted in Australia has been based on Monto vetiver, all the Australian results published can be applied with confidence anywhere in the world when this genotype is used. A summary of the Monto vetiver adaptability range is shown on Table 6.1.

Main Applications of Vetiver Grass Technology

The main applications of VGT are based principally on some of the unique characteristics of vetiver grass mentioned above, namely:-

• its thick growth, forming dense hedges when planted in rows;

• its massive, fine, thick and deep root system and

• its high level of tolerance to adverse climatic and edaphic conditions and heavy metal toxicities.

Vegetative barriers as water spreaders and sediment filter

Hydraulic properties of vetiver hedges

When planted in rows, vetiver plants will form a thick hedge and with their stiff stems these hedges can stand up to water flow of at least 0.6 m depth, forming a living barrier which impedes and spreads run-off water. Hydraulic characteristics of vetiver hedges under deep flow were determined by flume tests at the University of Southern Queensland, Australia for flood mitigation on the flood plain of Queensland (Dalton et al., 1996a) (Figure 6.1).

Table 6.1 Adaptability Range of Monto Vetiver in Australia and Other Countries.

Adverse Soil Conditions

Australia

Other Countries

Acidity

Aluminium level (Al Sat. %)

Manganese level

Alkalinity (highly sodic)

Salinity (50% yield reduction)

Salinity (survived)

Sodicity

Magnesicity

Heavy Metals Arsenic Cadmium Copper Chromium Nickel Mercury Lead

Selenium Zinc.

Location

Climate

Annual Rainfall (mm) Frost (ground temp.) Heat wave

Drought (without effective rain)

Fertiliser

Vetiver can be established on very infertile soil due to its strong association with mycorrhiza pH 3.3

Between 68%-87% >578 mgkg-1 pH 9.5 17.5 mScm-1 47.5 mScm-1 33% (exchange Na) 2,400 mgkg-1 (Mg)

200-600 mgkg-1

50-100 mgkg-1

N and P

pH 4.2 (with high level soluble aluminium)

pH 11.5

N and P, farm manure

Palatability

Nutritional Value

Dairy cows, cattle, horse, rabbits, sheep, kangaroo

Cows, cattle, goats, sheep, pigs, carp

Crude protein 3.3% Crude fat 0.4% Crude fibre 7.1%

Field trials using hydraulic characteristics determined by the above tests showed that vetiver 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.

Soil erosion and sediment control on sloping farmlands

As the vetiver hedge is a living porous barrier it slows and spreads run-off water and traps sediment. As water flow is slowed down, its erosive power is reduced and at the discharge depth equation q = a 5yby1

gradually varied backwater profile equation dy = So - Sf dx 1 - NF

Figure 6.1 Hydraulic model of flooding through vetiver hedges.

q = discharge per unit width y = depth of flow y1 = depth upstream

So = land slope Sf = energy slope NF = the Froude number of flow.

Figure 6.1 Hydraulic model of flooding through vetiver hedges.

same time allows more time for water to infiltrate to the soil, and the hedge traps any eroded material. Therefore an effective hedge will reduce soil erosion, conserve soil moisture and trap sediment on site. When appropriately laid out these hedges can also act as very effective diversion structures spreading and diverting run-off water to stable areas or proper drains for safe disposal.

This is in sharp contrast to the stratagem with the contour terrace/waterway system in which run-off water is collected by the terraces and diverted as quickly as possible from the field to reduce its erosive potential. All this run-off water is collected and concentrated in the waterways where most erosion occurs in agricultural lands, particularly on sloping lands and this water is lost from the field. With the VGT not only is this water conserved but also no land is wasted on troublesome artificial waterways.

Both research findings and field results in Australia, Asia, Africa and South America show that in comparison with conventional cultivation practices, surface run-off and soil loss from fields treated with vetiver were significantly lower and crop yield was much improved by as much as 20%. The yield increase was attributed mainly to uniform in situ soil and moisture conservation over the entire toposequence under the vetiver hedge system (Truong, 1993).

Erosion and sediment control on floodplain

VGT has been used as an alternative to strip cropping practice on the flood plain of Queensland, Australia. This practice relies on the stubble of previous crops for erosion control of fallow land and young crops. On an experimental site vetiver hedges that

Figure 6.2 This 2000 m long vetiver hedge was established to protect a sorghum crop from flood damage on the flood plain of the Darling Downs, Australia.

Figure 6.2 This 2000 m long vetiver hedge was established to protect a sorghum crop from flood damage on the flood plain of the Darling Downs, Australia.

were established at 90 m intervals provided a permanent protection against flooded water. Results over the last several years (including several major flood events) have shown that VGT is very successful in reducing flood velocity and limiting soil movement with very little erosion in the fallow strips (Figure 6.2) (Dalton et al., 1996a; Dalton et al., 1996b; Truong et al., 1996a).

The incorporation of vetiver hedges as an alternative to strip cropping on floodplains has resulted in more flexibility, more easily managed land and more effective spreading of flood flows in dry years particularly with low stubble producing crops such as cotton and sunflower. An added major benefit is that the area cropped at any one time could be increased by up to 30% (Dalton et al., 1996a).

Rehabilitation Saline and Acid Sulfate Soils

The spread of salinity in both dryland and irrigated lands is a major concern in low rainfall and semiarid regions of the world. Vetiver has been used very successfully in erosion control and rehabilitation of these salt-affected lands (Truong, 1994).

Acid Sulfate Soils (ASS) constitute a major component of arable lands in many tropical countries in Africa and Asia such as Thailand and Vietnam where rice is the main food crop. These soils are highly erodible and difficult to stabilise and rehabilitate. Eroded sediment and leachate from ASS are extremely acidic. The leachate from ASS has led to disease and death of fish in several coastal zones of eastern Australia. Vetiver has been successfully used to stabilise and rehabilitate a highly erodible acid sulfate soil on the coastal plain in tropical Australia, where actual soil pH is around 3.5 and oxidised pH is as low as 2.8 (Truong and Baker, 1996, 1998).

Infrastructure protection

The major impact of VGT in the late 1990s was in the area of steep slope stabilisation for infrastructure protection (Hengchaovanich, 1999; Xie, 1997; Xia et al., 1999).

Batter stabilisation

The main causes of slope instability are surface erosion and structural weakness of the slope. While surface erosion often leads to rill and gully erosion, structural weakness will cause mass movement or land slip.

Research conducted by Hengchaovanich and Nilaweera (1996) in Malaysia showed that the tensile strength of vetiver roots increases with the reduction in root diameter; this phenomenon implies that stronger fine roots provide higher resistance than larger roots. The tensile strength of vetiver roots varies between 40—180 Mpa for the range of root diameters between 0.2—2.2 mm. The mean design tensile strength is about 75 Mpa (equivalent to approximately one sixth of mild steel) at 0.7—0.8 mm diameter, which is the most common size for vetiver roots. This indicates that vetiver roots are as strong as, or even stronger than roots of many hardwood species, which have been proven positive for root reinforcement in steep slopes (Figure 6.3).

In a soil block shear test, Hengchaovanich and Nilaweera (1996) also found that root penetration of a two year old vetiver hedge with 15 cm plant spacing can increase the shear strength of soil in the adjacent 50 cm wide strip by 90% at 0.25 m depth. The increase was 39% at 0.50 m depth and gradually reduced to 12.5% at 1 m depth. Moreover, because of its dense and massive root system it offers better shear strength increase per unit fibre concentration (6—10 kPa/kg of root per cubic metre of soil compared to 3.2—3.7 kPa/kg for tree roots).

The major advantages of VGT over conventional engineering methods in infrastructure protection are:—

Figure 6.3 Vetiver Grass Technology is used as a bioengineering technique to stabilise this very steep cut batter of a road in the Philippines where landslides often occurred during the annual typhoon season. (Photo Credit: Noah Manarang).

• VGT provides a "soft" option using a green and natural method of stabilisation rather than the "hard" conventional engineering approach such as rocks and concrete structures;

• VGT is very cost effective when compared with the costs of conventional methods. VGT costs only 10-17% of the cost of rocks in country with low labour cost as in China (Xia et al., 1999) and between 27-40% in a country with high labour costs such as Australia (Bracken and Truong, 2000).

Landslide remediation

Landslides are often caused by the lack of structural strength of the ground on steep slopes and the event is triggered by over-saturation during heavy rainfall periods. The problem can be exacerbated by the presence of tall trees, which can be blown over by strong wind. Under natural conditions such as forests, deep rooted trees provide the structural reinforcement, but when deforestation was carried out for agriculture and forestry production or infrastructure construction this structural reinforcement was lost. When this occurred, landslide often resulted. Vetiver grass with its extensive root system provides the much-needed soil reinforcement in a much shorter time than trees. In addition, vetiver grass has no heavy canopy to blow over in strong wind (Miller, 1999).

Floods mitigation

The combination of the deep root system and thick growth of the vetiver hedges will protect the banks of rivers and streams under flood conditions. Its deep roots prevent it from being washed away while its thick top growth reduces flow velocity and its erosive power. In addition, when properly laid out hedges can be designed to direct water flow to appropriate areas (Truong, 1999b).

Very successful stream bank and riverbank stabilisation has been carried out in Australia, Malaysia and the Philippines.

Land rehabilitation

Mine rehabilitation

On site and offsite pollution control from mining wastes is a major application of VGT for environmental protection. Research conducted by this author has clearly established the extremely high levels of tolerance of vetiver grass aluminium, manganese and heavy metals such as arsenic, cadmium, chromium, nickel, copper, lead, mercury, selenium and zinc in the soils (Table 6.2) (Truong and Baker, 1998).

Table 6.3 shows that distribution of heavy metals in vetiver plant can be divided into three groups:-

• very little of the arsenic, cadmium, chromium and mercury absorbed was translocated;

• a moderate proportion of copper, lead, nickel and selenium was translocated (16% to 33%) and

• zinc was almost evenly distributed between shoot and root (40%).

Table 6.2 Threshold Levels of Heavy Metals to Vetiver Growth.

Thresholds to Plant Growth Thresholds to Vetiver Growth

(mgKg(mgKg

Table 6.2 Threshold Levels of Heavy Metals to Vetiver Growth.

Thresholds to Plant Growth Thresholds to Vetiver Growth

(mgKg(mgKg

+1 0

Post a comment