Advances in Soil Amendment: Vermicomposting, Alumina Refinery Residue and Cotton Production in Australia

The cotton industry is a major contributor to the economic wealth of some countries, including Australia (de Garis, 2013). In fact, more than 100 countries grow cotton commercially, with China (33 million bales/year) and India (27 million bales/year) among the biggest producers; Australia is the third largest exporter of cotton after the U.S. and India, supplying 3% of the world’s cotton. Australia also has the highest yields in the world (with highs of 10-12 bales/ha or about 2.1 t/ha) and exported an average of 4.3 million bales in 2012-2103 valued at $2.7 billion (CSIRO, 2015; Global Agricultural Information Network, 2013). 

However, in addition to problems associated with salinity and soil erosion, the over-abundant use of pesticides (including herbicides, insecticides and fungicides) and their downstream impacts on the environment (a global problem identified in earlier studies, such as those of Banuri [1999] in Pakistan), and the limited access to (and hence availability of) enough fresh water for viable production, one of the most pressing issues faced by the cotton industry is the capability of farmland to sustainability support long-term production. As a consequence, the need for a feasible and economically viable approach to soil amendment as been encouraged. 

The primary waste stream generated from cotton (Gossypium hirsutum) production is cotton gin trash (CGT), the rejected leaves, sticks, bolls and soil which remain after ginning. Disposing CGT as a solid waste (via either incineration or as a solid to landfill) is an option, but Knox et al. (2006) have also identified the ways CGT can be reused by the industry, which include spreading untreated CGT on soil to amend it, feeding it to livestock as a hay replacement, and composting it to produce fertiliser. Applying CGT as a soil additive at a rate of 6-15 t/ha has been shown to increase cotton yields, apparently due to increase moisture retention in soil (Knox et al., 2006), and this approach has been considered the most desirable. 

In some jurisdictions CGT has been classified as “hazardous” due to the presence of high concentrations of pesticides, particularly chlorpyrifos, chlorfurazuron and endosulfan which can potentially uptake into the cotton stalk (Crossan & Kennedy, 2006), thereby making either spreading CGT untreated on soil or using it as a feed source less attractive. In fact, downstream concerns associated with CGT feed being contaminated with residues of chlorfurazuron and endosulfan (although 56 chemicals associated with pesticide use in cotton production have been isolated) and their potential to find their way into the human food-chain, have been identified (Crossan et al., 2006).

For these reasons, feeding CGT to livestock has been ruled out entirely in Australia, and incineration is discouraged because of its atmospheric polluting and greenhouse gas potentials (i.e., as a result of direct, on-site burning). On- and off-site disposal of CGT as a solid waste is an option, but off-site disposal can be costly and is the least economically desirable approach. Therefore, amending soil by reusing CGT as a media for composting has been considered.

During the last 40 years composting CGT has been investigated, but with mixed results. For example, Gordon et al. (2006) found that while CGT produced a viable compost material, the composting process did not destroy either verticullium wilt (a disease caused by soil-borne fungi Verticillium albo-atrum and Verticillium dahliae) or miscellaneous weeds, including annual bluegrass (Poa annua), large crabgrass (Digitaria sanguinalis), purple nutsedge (Cyperus rotundas) and prickly sida (Sida spinose), all associated with “gin trash”. On the other hand, Griffis and Mote (1978a, 1978b) found composting CGT was lethal to all weeds present in the gin waste, including redroot pigweed (Amaranthus retroflexus) and Johnson grass (Sorghum halepense), and the process produced a viable compost. 

Similarly, using static aeration biopiles and standard turned windrows, Dı́az et al. (2002) found that when 55% CGT was mixed with 45% vinasse (obtained by fermenting molasses, sugarcane, maize or beetroot) by weight, peak temperatures of 45°C at 21 days in biopiles and 54°C at seven days in windrows were achieved, and both methods achieved stability and fertiliser of high value. Smith (2009) also found that when using an aerating composter temperatures rose to 55-68°C within 72 hours, thereby killing pathogens and the germination of any seeds; Smith showed he was able to produce 15m3 of highgrade compost every 24 hours using CGT as a carbon source via this method.

 Further areas of research relevant to soil amendment and cotton production are the related fields of vermiculture or “worm farming” (Edwards, 2004; Edwards et al., 2010) and vermicomposting (Dickerson, 2001; Monroe, 2014). Vermiculture is the process whereby any one of 1,800 species of earthworms (usually red earthworms Eisenia Fetida or tiger earthworms Eisenia Jetida) are cultivated or artificially reared for use in any number of applications, including as bait; vermicomposting is the process whereby the worms used in vermiculture produce “worm castings” or vermicasts (i.e., the waste by-product from digesting organic matter, such as household waste or manure).

 Vermicomposting can also be defined as the process whereby worms are actually fed compost in order to generate worm castings. In both cases, worm castings have nutritional value and can therefore be used as a “natural” organic fertiliser in agriculture. The following percentage components of worm castings provides a summary of the chemical properties of “average” worm castings: organic carbon (OC) = 20-30%; nitrogen (N) = 1.8-2.0%; phosphorus (P) = 1.3-1.9%; potassium (K) = 1.3-1.5%; calcium (Ca) = 3.0-4.5%; and <0.5% traces of magnesium (Mg), sodium (Na), sulphur (S), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B) and aluminium (Al) (Bourdon, 2011).

However, vermicomposting can concentrate heavy metals in worm castings if metals are present in the original composted material (e.g., zinc can increase by 70% from 14.5 mg/kg to 24.6 mg/kg as a result of vermicomposting), but more importantly can also increase the percentage of desirable macronutrients. For example, vermicomposting can increase N from 0.5% to 1.6% and P from 0.2% to 0.7% in worm castings when using animal manure as the feedstock, and N from 0.8% to 1.9%, P from 0.35% to 0.47%, K from 0.48% to 0.7%, and Ca from 2.3% to 4.4% when using garden waste as the feedstock. 

In the context of flowering pot plant production, Matta et al. (2008) have reported that the nutrient value of worm castings generated from the vermicomposting of sheep and cattle manure is greater than those derived from vermicomposting horse manure; Matta et al. have also reported that earthworms have a preference for animal manure generally over other types of organic wastes. Significant potential value for soil amendments, including amending soil for cotton production, thereby inheres to worm castings produced through vermicomposting (Dickerson, 2001, p. 1). 

Of interest to the present study are the trials of John Buckerfield and his team, whose early work using worm castings mixed with composted CGTas a soil amendment agent to improve cotton production is notable. Buckerfield (1999) found that various addition rates of composted CGT and manure, along with worm castings, increased cotton yields by 15% in the first year after application. Mysteriously, he also found that yields went down by 5% and 25% respectively in the two subsequent years, although he could not attribute these declines to the quality of soil amendment additives. This phenomenon, along with enhanced moisture retention was also investigated in vineyards (Buckerfield et al., 1999) with similar outcomes.

 Alumina refinery residue (ARR) or “red mud” has similarly been investigated over several decades as a possible means of amending agricultural soil. For example, in both its unmodified (i.e., high alkalinity >3,000 mg/kg and pH >12.5) and modified (i.e., alkalinity of <300 mg/kg and pH <10.0) forms, research conducted in many countries since the 1980s has been based on a supposition that ARR might help agricultural soil retain moisture, neutralize acidity, bind heavy metals, and enhance the presence, retention and availability of macronutrients (such as Fe) and micronutrients (such as Mn) in soil. 

ARR is typically composed of Fe (30-60%), Al (10-20%), Na (3-10%), titanium (4-18%), silica (3-50%) and Ca (2-8%) in oxide, hydroxide and/or oxy-hydroxide states. ARR is composed of a complex of metals and minerals, including hematite (Fe2O3), beohmite (ץ-AlOOH), gibbsite (Al[OH]3), sodalite (Na4Al3Si3O12Cl), anatase (TiO2), aragonite (CaCO3), brucite (Mg[OH]2), diaspore (ß-Al2O3.H2O), ferrihydrite (Fe5O7[OH].4H2O), gypsum (CaSO4.2H2O), hydrocalumite (Ca2Al[OH]7.3H2O), hydrotalcite (Mg6Al2CO3[OH]16.4H2O) and para-aluminohydrocalcite (CaAl2[CO3]2[OH]4.3H2O). As Caand silicon (Si) play a vital role in plant nutrition, disease prevention and nitrogen absorption efficiency, the existence of these elements may play a useful role in soil amendment. 

However, ARR may contain heavy metals and metalloids, including arsenic (As), chromium (Cr) , Cu, gallium (Ga), Mn, thorium (Th), uranium (U) and vanadium, although these are usually only present in trace concentrations of a few parts per million up to 100 mg/kg. While the potential presence of radio nuclides, such as lead (Pb), Th and U, have also raised ongoing health and environmental concerns, according to Gräffe et al. (2009) these elements are almost always found in nonradioactive states and in especially low concentrations.

 About 50% of ARR is amorphous, with crystalline constituents composed mainly of goethite and hematite, quartz, and rutile, anatase and/or ilmenite; many minor remnant phases from the original bauxite (e.g., mica and boehmite) and newly formed species (e.g., natroalunite and noselite) may be present. Upon contact with water, ARR imparts a pH of ±12.5, with elevated levels of electrical conductivity(EC) at 1.0-16 dS/m due to high concentrations of Na; >80% of ARR particles are <10 micron and thus the media has a high charge-to-mass ratio and a high surface area-to-mass ratio (about 300 mm2 /g), a bulk density of 1.53 g/cm3 , anda specific gravity of 3.2g/cm3 (all data on ARR from Deelwal et al., 2014; Fergusson, 2009, 2015; Gräffe et al., 2009). 

According to Snars et al. (2003), the global research effort into ARR has concentrated on its potential effects in a number agricultural contexts, including P-retention of sandy soils to prevent phosphate (PO4) run-off to rivers and estuaries (Summers et al. 1996a; Summers and Pech, 1997), the so-called “liming” of acidic soils (Simons, 1984), and importantly for this study, increasing yields of horticultural crops and pastures with and without the addition of N, P and K fertilisers (Summer et al., 1996b; Summers et al., 2001).

Research has also been conducted on the use of ARR to beneficiate compost and the composting degradation process. Waddell et al. (2002) investigated the impact of ARR on retaining soluble P in compost added to sandy soils and its impact on the growth of long scarlet radishes (Raphanus sativas). Starting concentrations of 0.83% total P in compost were detected, achieved as a result of ARR addition rates between 1-5% (on a w/w basis) to sandy soils. Results indicated that radish yields from crops grown in sandy soils amended with ARR-compost were significantly higher than a control soil with and without unamended compost, but yields were not as high as crops grown in sandy soils amended with composts containing other clay-like additives, including zeolite. However, Waddell et al. reported a decreased loss of P in soil and a greater retention of P in radish roots when ARR addition rates increased, with P concentrations of 0.38% in roots and 0.24% in leaves 11 weeks after application; Waddell et al. also found that ARR significantly aided the retention of N in sandy soils. 

Thiyagarajan et al. (2011) similarly studied to role of ARR and compost and their relation to Mn-retention in sandy soils, and Anderson et al.(2011) investigated whether amending ARR sand (i.e., the courses and fraction of ARR, called the desilication product or DSP present at concentrations of between 3% and 50%) with ARR fines (i.e., the fine sand fraction which is present at concentrations of between 1% and 5%) would improve its suitability as a growth medium. Anderson et al. found that the addition of ARR “fines” more than doubled plant-available water and increased extractable nutrient concentrations relative to unaltered sand, and increased extractable K, P, S, Mg and B in the growth medium. However, ARR treatments also increased both the electrical conductivity and exchangeable Na percentage of the growth medium. 

This author has reported on the use of a modified form of ARR in both composting and agriculture. Fergusson (2009) documented the application of modified alumina refinery residue (MARR), i.e., ARR which had been modified using seawater and/or the addition of Mg and Ca salts to reduce total alkalinity caused by high concentrations of Na and chloride (Cl) in the original ARR, in a biosolids composting trial at a municipal council site in Queensland. In that study, composting time to stability of biosolids and green waste was reduced from a standard 11-14 week composting period to seven weeks, the amount of carbonaceous green waste required for compost to reach maturity was reduced by 50%, and average biopile temperatures averaged 65⁰C over seven weeks. 

The composting process generally follows three basic phases: first, a mesophilic or “moderate temperature” phase, which lasts between a few days and a week as the temperature rises above 40⁰C; second, a thermophilic, or “high temperature” phase, which can last for a few days up to a few months at temperatures above 50⁰C; and third, a “cooling” or maturation phase when temperatures return to 40-45⁰C. Each phase is dominated by mesophilic, then thermophilic, and then again mesophilic microorganisms, including actinomycetes and saprophytes.

The finding that MARR increases average biopile temperatures to an average of 65⁰C over seven weeks may not be entirely desirable, as target average temperatures for compost are usually 50⁰C, with a minimum of only three consecutive days required at temperatures greater than 55⁰C (although some jurisdictions require temperatures above 65⁰C for at least three days). Thus, the higher temperatures observed when using MARR in compost may potentially destroy the very beneficial and desirable mesophilic bacteria present during the cooling phase. 

However, this author has found no evidence in the literature pertaining to the relationship between vermicomposting and MARR or to the relationship between vermicomposting, MARR and cotton production. This shortfall in data begs the question of whether the combination of vermicomposting and MARR might play a role in cotton production, leading to the present study which asks the following research questions: 1) What effect does the addition of MARR have on the chemical and biological properties of worm castings; and 2) Does the application of a combination of worm castings generated from the composting of municipal household waste and MARR improve cotton yields?