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In Western Australia, sand is a significant proportion of the residue (~ 50 %) produced from bauxite ore processing, and has historically been used as the growth media for vegetation establishment on residue disposal areas. This occurs because fresh residue materials are caustic from the processing of the ore and these materials require freshwater leaching. Because residue sands leach more readily than residue fines, vegetation has proven more successful on the sands. But, the long term sustainability of native vegetative communities grown in residue sand alone is in question.

Successful rehabilitation of residue sand embankments and the capping layer poses many challenges due to the inherently high pH (10 - 12), alkalinity, sodicity (ESP > 15), salinity (EC(1:5) > 4 dS m-1) and the low water holding capacity (< 20 %)

associated with freshly deposited residue sands. In addition to adverse conditions that exist prior to leaching, the sands are also low or deficient in many necessary plant nutrients including N, P, K, Ca, Mg, Mn, Cu and Zn (Meecham and Bell 1977a; Fuller and Richardson 1986; Bell et al. 1997; Gherardi and Rengel 2001; Gherardi and Rengel 2003a; Eastham and Morald 2006; Eastham et al. 2006). Many studies have developed partial solutions to these problems such as gypsum additions to correct pH and sodicity (Gupta and Singh 1988; Wong and Ho 1988; Wong and Ho 1991; 1993; Polcaro et al. 2000; Eastman and Morald 2004; Kopittke et al. 2004); and fertilizers which are applied at high rates alone or with organic amendments to increase plant available nutrients (Fuller et al. 1982; Williams and Hamdy 1982; Marschner 1983; Wong and Ho 1991; Bell et al. 1997; Jasper et al. 2000; Gherardi and Rengel 2003b; Courtney and Timpson 2004; Eastham and Morald 2006). Although these manipulations have been successful to varying degrees, low water holding capacity and the poor nutrient retention of sands still remain major constraints to long term self-sustaining vegetation.

A recent comprehensive study comparing inorganic and organic (poultry, composted poultry, compost and manure) fertilizer use on residue sands illustrates the lack of nutrient retention in residue sands and highlights the complexity of nutrient deficiencies that occur during rehabilitation (Eastham et al. 2006). All treatments received the best management practices of a gypsum addition (50 t ha-1) ripped in to 0.7 m, additions of KCl (100 kg ha-1), and a trace element mix (90 kg ha-1). Although pH (8.2 - 8.4) and salinity (EC (1:5) 0.66 - 0.82 dS m-1) had dropped significantly within

three months, major nutrient deficiencies were measurable, including N and P. Additionally low levels of organic matter (< 0.15 %) were recorded and plant foliar concentrations were low to deficient in P, K, Mg, Cu, Zn (Eastham et al. 2006). Vegetation and rehabilitation of bauxite residue sites are intensively managed, but due to the nutrient retention limitations of residue sands nutrient deficiencies are common.

This is further complicated by the reduced bioavailability of many trace nutrients due to the alkaline conditions (Gherardi and Rengel 2001; Thiyagarajan et al. 2009).

Soluble and exchangeable cations (Ca, Mg, Na and K) present a particularly interesting scenario in bauxite residue rehabilitation. Very high concentrations of Na exist in bauxite residues (as high as 99 cmol kg-1 in unaltered fines) in soluble and exchangeable forms and within the multiple forms of sodium aluminosilicates (mainly sodalite) termed the desilication product (DSP) (Wong and Ho 1995). It is estimated that up to 75 % of the Na in the residue may be slowly released from within the

aluminum-silicate micropore network of the DSP, representing a long term problem for pH and sodicity (Wong 1990). Exchangeable sodium percentage (ESP) can be as high as 60 - 85 in fresh residue sands and fines (Meecham and Bell 1977a; Wong and Ho 1993; Courtney and Timpson 2005). The soluble and exchangeable fractions are believed to be easily leached from the residue sands and large amounts of

phosphogypsum (up to 225 t ha-1) are applied to ameliorate the adverse growth

conditions for plants. Addition of phosphogypsum is used to remove Na from exchange sites by Ca displacement and a reduction of pH is believed to partially occur through the precipitation of calcium carbonate.

Phosphogypsum treated residue materials have a high concentration of Na and Ca in solution and this may result in competition amongst cations for plant uptake with Ca deficiencies possible (Kopittke and Menzies 2005a). Although Mg deficiencies have been the most prevalent among the cation deficiencies in the bauxite residue, seawater treatment of residues, which is a preferred method for many situations, introduces significant concentrations of Mg but may also induce Ca deficiencies in vegetation (Kopittke and Menzies 2005b). Concentrations of K are low in unfertilized fresh residue sand and current fertilizer practices do not appear to maintain K

concentrations at levels necessary for adequate plant uptake (Eastham and Morald 2006).

Revegetation is a key component for long-term rehabilitation of RDAs in terms of erosion control, site stability, water balance, pollution control and aesthetics. These issues will only be successfully managed if self-sustaining vegetation is established. Successful revegetation of RDAs in the Mediterranean type climatic region of south- western Western Australia is in turn dependent on a capping stratum which will satisfy water use and nutrient cycling requirements of the vegetation. Rehabilitation of residue sands alone has had limited success producing a growth media capable of supporting a self sustaining diverse native flora community.

Increasing the water and nutrient retention of the capping layer sands for the long term requires an alteration of the soil texture. It is hypothesized that only a small proportion of fines are required to improve the growth characteristics, while adverse effects may be produced from excessive amounts. To alter the bauxite sand (~ 92 % sand) to a loamy sand texture class requires an increase of 7 – 22 % of silt and clay (> 0.060 mm size particles) based on the United States Department of Agriculture (USDA) texture classification (Figure 1.1).

It is hypothesized that by increasing the residue fines content (< 0.060 mm size particles) by only 7 – 8 %, the texture change will increase meso- and micropore spaces, and in turn, increase the water holding capacity. Figure 1.2 shows a generalized

diagram of how available water content increases with increasing fines content. Increased water retention occurs because the additions of fines to sands decreases the macropores (> 0.060 mm dia.) which tend to be air filled and increases the meso- (0.002 - 0.060 mm dia.) and micropores (< 0.002 mm dia.) which retain water through

capillary tension. Water retained in mesopores is considered plant available water.

Figure 1.2. Generalized relationship between soil texture and pore water. (Ritter 2006)

Slight changes in this physical characteristic of the capping layer are expected to produce a beneficial increase in plant available water. Research has shown that

additions of silts and clays to sands increases plant available water capacity, and crop yield, even with small changes (4 – 20 %) in silt and clay content (Noble et al. 2001; Croker et al. 2004; Berthelsen et al. 2005; Soda et al. 2006; Suzuki et al. 2007). Additions of fines to sandy soils have also been shown to increase nutrient levels present and nutrient retention. Bentonite clay additions have been shown to increase cation exchange capacity and reduce the loss of cations during leaching in sandy agricultural soils (Noble et al. 2001; Croker et al. 2004; Berthelsen et al. 2005; Soda et al. 2006).

Organic matter additions such as compost, biosolids, and piggery or chicken manures have been shown to increase water retention and organic nutrient content, but these benefits only last for the short term (< 3 months) (Eastham and Morald 2006) if the growth media does not have the capacity to retain or recycle the organic matter. The lack of organic matter present is major concern as the additions of some types of organic amendments (e.g. chicken manure) in Western Australia have been restricted due to preceived health risks by the nearby communities. Silt and clay act as an important binding mechanism for accumulation of organic matter which does exist through binding into stable micro- and macro-aggregates (Edwards and Bremner 1967; Stevenson 1994; Wagner et al. 2007). These stable aggregates act as a storage

mechanism for nutrients which can be cycled within the soil ecosystem and made available to plants through microbial decomposition over the long term. Without these aggregates, nutrients such as NO3, P, S, K and exchangeable cations (Mg, Ca, Na, and

K) can be easily lost through leaching on sandy substrates such as residue sands. As fresh residue materials have very little organic matter (< 0.3 % (Wong and Ho 1993)), it is extremely important to ensure the growth media have the necessary texture

components to retain nutrients and build a base of organic matter through development of stable aggregates.

Due to the cost of acquiring and transporting a silt/clay mixture from off site, the treated residue fines offer a less expensive option for altering the particle size

distribution of the growth media. Once adverse characteristics of residue fines have been ameliorated through treatments (carbonation or seawater washing), the physical benefits of the fines additions should be similar to other silt/clay materials with similar particle size distribution.

Until recently the other by-product of bauxite refining, residue fines, was mostly ignored as a soil amendment for on-site capping of the RDAs due to its caustic nature

and slow leaching characteristics. Residue fines have been used successfully as an amendment on sandy, acidic agricultural soils and on mine spoils due to its acid neutralizing capacity, increased water holding capacity and nutrient retention capabilities (Barrow 1982; Koch and Bell 1983; Ward 1983; Vlahos et al. 1989; Summers et al. 1993; Browner 1995; Summers et al. 1996a; Summers and Pech 1997; Summers et al. 2001; Snars et al. 2003; Hanahan et al. 2004; Snars et al. 2004a; Snars et al. 2004b). The only previous attempt to use residue fines to amend bauxite residue sands showed very high rates of addition of unaltered residue fines only compounded the adverse caustic characteristics (Meecham and Bell 1977b). But after pretreatment (seawater washing or carbonation), residue fines have improved chemical

characteristics which limit the adverse characteristics of sodicity and alkalinity (Cooling et al. 2002; Menzies et al. 2004).

Residue fines have been treated to neutralize the alkalinity, using strong acids (Shannon and Verghese 1976; Shiao and Akashi 1977; Piga et al. 1993; Koumanova et al. 1997; Pradhan et al. 1998), or gypsum amendments (Gupta and Singh 1988; Ho et al. 1989; Wong and Ho 1991; 1993; Courtney et al. 2003; Courtney and Timpson 2004; Kopittke et al. 2004; Courtney and Timpson 2005; Ippolito et al. 2005; Xenidis et al. 2005) with varying degrees of success. Recent research has developed techniques of carbonation or seawater washing treatments to reduce the pH and remove excessive amounts of Na from residue fines, thus reducing the adverse caustic nature.

Carbonation of residue fines occurs by passing concentrated CO2 through a

slurry of residue fines (Cooling et al. 2002; Jones et al. 2006). Seawater washing occurs by mixing seawater and residues into a slurry to transport the residue fines or by in situ leaching using seawater irrigation. Many recent studies have been produced assessing the favorable chemical conditions after seawater washing of residues (Somes et al.

1998; McConchie et al. 2000; Hanahan et al. 2004; Kopittke et al. 2004; Menzies et al. 2004).

Carbonation instigates reductions in pH (from 12 to 9 - 10) due to CO2 reacting

with OH- to form HCO3- which then precipitates soluble and exchangeable Na to form

NaHCO3, which can then react to from more stable compounds (Cooling et al. 2002;

Jones et al. 2006). Not only does this affect the chemical composition, but carbonation has been proven to alter measurable physical characteristics such as shear strength and moisture retention (Nikraz et al. 2007). By carbonating the residue fines, the Na concentrations are lowered, but it is not known if plant nutrients will remain available when added to residue sand.

Seawater washing treatment of residue fines reduces the Na concentrations by precipitating Na as possibly either Na2CO3 or Na2SO4 or larger minerals such as

sodalite or nosean. Reductions in pH (from 12 to 8 - 9) arise due to removal of hydroxide ions into formations of Mg and Ca hydroxides and hydroxycarbonates (McConchie et al. 2000; Hanahan et al. 2004). Another significant benefit is the introduced cations from the seawater washing that displace the Na on the exchange sites. These nutrients include: Ca, Mg, K, and many others (Hanahan et al. 2004; Menzies et al. 2004; Wehr et al. 2006). Due to the relative decreases in Na and

increases in Ca, Mg, and K, seawater washed residue fines have lower ESP and greater plant available nutrients on exchange sites. In seawater washed fines, adsorption of phosphate is as much as 2.5 times greater than that of unaltered residue fines (Hanahan et al. 2004). Seawater washed residue fines also have significantly greater acid

neutralization capacity compared to unaltered fines due to additions of Ca and Mg compounds (Hanahan et al. 2004).

As an on-site amendment of residue capping sands, the addition of altered residue fines could be useful to increase water holding capacity and improve the media’s ability to retain nutrients and to introduce essential plant nutrients.