Published online 27 October 2006
Published in J Environ Qual 35:2293-2301 (2006)
DOI: 10.2134/jeq2005.0455
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
Co-composting of Acid Waste Bentonites and their Effects on Soil Properties and Crop Biomass
Wannipa Sodaa,*,
Andrew D. Noblea,
Shinji Suzukia,
Robert Simmonsa,
La-ait Sindhusenb and
Suwannee Bhuthorndharajb
a International Water Management Institute (IWMI), Southeast Asia Regional Office, c/o WorldFish P.O Box 500 GPO, 10670 Penang, Malaysia. S. Suzuki, present address, National Agricultural Research Center for Hokkaido Region, Hitsujigaoka Toyohira-ku Sapporo 062-8555, Japan
b Land Development Department (LDD), Chattuchak, Bangkok 10900, Thailand
* Corresponding author (w.soda{at}cgiar.org)
Received for publication December 13, 2005.
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ABSTRACT
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Acid waste bentonite is a byproduct from vegetable oil bleaching that is acidic (pH < 3.0) and hydrophobic. These materials are currently disposed of in landfills and could potentially have a negative impact on the effective function of microbes that are intolerant of acidic conditions. A study was undertaken using three different sources of acid waste bentonites, namely soybean oil bentonite (SB), palm oil bentonite (PB), and rice bran oil bentonite (RB). These materials were co-composted with rice husk, rice husk ash, and chicken litter to eliminate their acid reactivity and hydrophobic nature. The organic carbon (OC) content, pH, exchangeable cations, and cation exchange capacity (CEC) of the acid-activated bentonites increased significantly after the co-composting phase. In addition, the hydrophobic nature of these materials as measured using the water drop penetration time (WDPT) decreased from >10 800 s to 16 to 80 s after composting. Furthermore, these composted materials showed positive impacts on soil physical attributes including specific surface area, bulk density, and available water content for crop growth. Highly significant increases in maize biomass (Zea mays L.) production over two consecutive cropping cycles was observed in treatments receiving co-composted bentonite. The study clearly demonstrates the potential for converting an environmentally hazardous material into a high-quality soil conditioner using readily available agricultural byproducts. It is envisaged that the application of these composted acid waste bentonites to degraded soils will increase productivity and on-farm income, thus contributing toward food security and poverty alleviation.
Abbreviations: SB, soybean oil acid waste bentonite • PB, palm oil acid waste bentonite • RB, rice bran oil acid waste bentonite • RH, rice husk • RHa, rice husk ash • CL, chicken litter • OC, organic carbon • CEC, cation exchange capacity • WDPT, water drop penetration time • pHw, pH measured in water • pHCa, pH measured in calcium chloride
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INTRODUCTION
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SANDY LIGHT-TEXTURED SOILS are common throughout the globe and it is estimated that they cover approximately 900 million ha (Driessen et al., 2001). While these soil types (Arenosols, Acrisols) predominate the arid and desert regions of the world, these soils occur extensively within the semi- and humid-tropics and support the livelihoods of large populations. Within Thailand, the upland regions of the northeast occupy approximately one-third of the entire land area of Thailand. Soils in this region are light-textured with clay contents of <15%. These soils are aeolian in origin and have formed under a rainfall regime that exceeds 1200 mm per annum and are predominantly acidic in reaction with low soil organic matter (Kheoruenromme et al., 1998). The combination of inappropriate land management and overexploitation of these limited natural resources has resulted in significant soil degradation thereby making them marginal for crop production.
An important step in addressing the low productivity of these degraded soils is to tackle the fundamental problem of diminished nutrient-holding capacity (as indicated by CEC) associated with the decline in soil organic matter (OM). A possible approach to correcting the aforementioned issues is through the application of natural materials or industrial waste that are readily available to be a resource to poor farmers.
Bentonite is a 2:1 layer silicate containing smectite minerals, usually montmorillonite. It is used in a large number of applications ranging from foundry molds to stockfeed supplementation. The annual production is estimated to be 8 million tonnes (Virta, 2001). Bentonite is also used to remove a variety of impurities in the vegetable oil industry including phosphotides, fatty acids, gums, and trace metals followed by decolorization. This results in the production of light-colored and stable oils acceptable to consumers (Foletto et al., 2002). Acid-activated calcium bentonites are the preferred form of clay for use as the absorbing agent during the decolorizing, clarification, and refining process in the manufacture of vegetable oils. Acid-activated clays are produced by treating calcium bentonites with mineral acids (i.e., hydrochloric or sulfuric acids). This alters the structure, chemical composition, and physical properties of the clay and significantly increases the adsorption capacity (Foletto et al., 2002).
The reuse of acid waste bentonites is an area of potential opportunity for cost savings in the oil processing industry and has the added benefit of protecting the environment. In addition, these waste oil bentonites can be used in rejuvenating degraded soils through an enhancement of the cation exchange capacity (CEC) and increased water absorption capacities. The application of either modified or naturally occurring bentonites have been shown to increase the soils cation exchange properties and increase crop productivity (Noble et al., 2001, 2004). However, a significant problem in the reuse of acid waste bentonites from vegetable oil processing is their hydrophobic nature due to the presence of fats and oils on their surfaces as well as their acidic nature. This reduces the ability of these materials to absorb nutrients and water. Croker et al., (2004) applied acid waste bentonite to a degraded light-textured soil at rates of 40 t ha–1 and observed increases in CEC from 0.6 to 1.9 cmolc kg–1. However, due to nutrient insufficiencies (N and K) and the acidic nature of these materials, overall productivity of the maize test crop was limited. Hence the objectives of the study were (i) to determine whether composting of waste oil bentonite with rice husk, rice husk ashes, and chicken litter could remediate the hydrophobic and acidic nature of the acid waste bentonite, and (ii) to assess the potential of these materials as soil amendment.
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MATERIALS AND METHODS
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Chemical Characterization of Materials
Samples of acid waste bentonite were obtained from three oil processing companies (Morakot, King, and Tip) in Bangkok, Thailand. These companies specialize in the processing of soybean, rice bran, and palm oil, respectively. Samples were air-dried and passed through a 2-mm mesh sieve. Electrical conductivity (EC) and pH was measured in water and 0.01 M CaCl2 at a clay/solution ratio of 1:5. Exchangeable cations calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+), and CEC were determined using 1 M NH4–acetate buffered at pH 7.0 (Rayment and Higginson, 1992). Organic carbon (OC) was determined by wet oxidation using the Walkley and Black method as modified by Rayment and Higginson (1992) and total N by Kjeldahl steam distillation. In addition, the co-composting materials, namely chicken litter, rice husk, and rice husk ash were similarly assessed for EC, pH, CEC, exchangeable cations, OC, and total N using the previously described methods.
Co-composting
A total of nine composting treatments were studied (Table 1). Ratios of the different components making up the composting material were mixed air dry. In addition, to each treatment (1 through 9), 4, 5, or 6 kg of dolomitic lime was applied to neutralize residual acidity associated with the acid-activated bentonite. The amount of dolomite applied was determined based on the chicken litter added, as the latter had significant amounts of alkalinity. The mixed treatments were then placed in adequately drained, covered outdoor concrete composting bins (1 x 1 x 1m). The amount of water added to each bin varied slightly depending on the treatment. However, the composting materials in all bins that received water had the same consistency, namely, no excess water was released when samples were squeezed with the ball of the hand, this being the standard compost moisture content assessment practice employed in Thailand. A further set of control treatments (10 through 12) were included where the acid waste bentonites were placed in composting bins with no additions of composting materials and subjected to the same mixing routines. Temperature within the composting bins was monitored on a daily basis at 3 random points within the pile (20-cm depth). The compost materials were turned at 20, 45, and 75 d and water reapplied to maintain the desired moist condition without waterlogging. Samples from each of the bins were collected on three occasions during the composting period, air-dried and chemically analyzed. Changes in hydrophobicity were assessed on all samples on each sampling date using the WDPT methodology of Bisdom et al. (1993).
Biomass Production Experiment
A pot experiment was established in an evaporatively cooled greenhouse and the materials (Table 1) generated from the first phase of the study evaluated for biomass production. To undertake the experiment, over 400 kg of degraded light-textured sandy soil (0 to 15 cm) was collected from an experimental site at the Animal Nutrition Development Station, Department of Livestock, Chiang Yuen, Mahasarakham province, Northeast Thailand. The soil was classified as a Satuk series (Imsamut and Boonsompoppan, 1999) or isohyperthermic Oxic Paleustult (Soil Survey Staff, 1990). The soil was air-dried, sieved through a 2-mm sieve and thoroughly mixed to ensure homogeneity. General attributes of the soil indicate it is acidic (pHCa 4.0) and has a low CEC (1.8 cmolc kg–1) and OC content (0.5%). The gravimetric soil moisture content of the air-dried soil used in the current study was 0.01 kg kg–1 (water/oven dry soil). With respect to the soil texture, sand, silt, and clay content were 834, 55, 111 g kg–1, respectively.
The design of the biomass experiment was a 4 x 12 + 1 incomplete factorial design consisting of 4 application rates of 0, 15, 30, 60, and 120 t ha–1 (18.0, 37.5, 75.0, and 150 g) composted material on an oven-dry basis and a control (no compost added). Composted materials were thoroughly mixed with 3 kg of soil and placed in PVC pots, with a height of 19 cm and a 21-cm i.d. All treatments were replicated 3 times. The resultant 147 pots were placed in a complete randomized block design on six benches within the greenhouse. Before the transfer of the treatments to their respective pots, for each treatment replicate, five samples of 0.5 kg each were randomly collected for the subsequent determination of soil chemical properties and WDPT. Chemical properties were determined on all samples using the methodology previously described. Two consecutive maize crops were grown to assess the persistence of the response to applied treatments. After the final crop had been harvested, the soils in containers were sampled for physical and chemical analysis.
To prevent initial nutrient and soil losses through drainage and soil slumping during watering, a glass fiber matting was placed at the bottom of each pot. This facilitated capillary rise while preventing waterlogging and potential loss of soil and nutrients through surface leaching. During the first 2 wk following seedling emergence, 200 mL of water was applied every third day to the base of the pots to avoid soil slumping and the development of a compacted soil. Subsequent to the establishment of an adequate root system, 500 mL of water was carefully applied every other day to the soil surface and the drainage tray removed to facilitate free drainage.
Following the initial irrigation to field capacity, 8 pregerminated seeds of Metalaczil (35%) DF mixed maize were planted 2 cm below the soil surface and a plastic bag was placed over the pot to reduce evaporation and prevent the formation of a resistant soil crust to which the study soils are susceptible. Three days after planting, three seedlings were removed from each pot and the plastic bag covers removed. Five plants were left in each pot for the remainder of the study.
To reduce the influence of spatial variation in solar radiation on crop growth, the pots were rotated within each bench on a daily basis and between benches on a weekly basis. Eleven days after transplanting, 5 mL of standard Ruakura micronutrient solution was applied to each pot to ensure adequate micronutrient nutrition (Smart et al., 1998). Weeds were removed manually as required.
The 5 plants from each pot were combined to form a composite sample at each harvest (45 d after germination). The composite plant samples were subsequently oven-dried at 65°C for 48 h and their weight determined. For each pot all subsurface root biomass was carefully removed.
Bulk Density and Water Retention
To determine changes in the bulk density and water retention of soils associated with the application of co-composted (PB, SB, and RB) and noncomposted (P-PB, P-SB, and P-RB) acid waste bentonites, these materials were mixed at rates 0, 15, 30, 60, and 120 t ha–1 with 3 kg of the air-dried soil and placed in free-draining PVC pots. Annually, the northeast of Thailand experiences several rainfall events >50 mm (Noble et al., 2004). Therefore, to simulate such an event, 50 mm of deionized water was applied to the bentonite soil mixture in the PVC pot and allowed to drain freely for 3 d at room temperature. After equilibration, three undisturbed soil samples 47 mm in diameter and 30 mm in height were collected using a stainless steel cylindrical core. The soil cores were capillary-saturated using deionized water and then allowed to equilibrate at a matric potential of –10 kPa (approximately equivalent to the field capacity 
fc) on a suction table. The volumetric moisture content and bulk density of the samples were determined after oven-drying the sample at 110°C for 24 h. Using disturbed samples, the matric potential of the soil was measured from –0.2 to –6.1 MPa using the freezing point depression method (Suzuki, 2004). Volumetric moisture content at the permanent wilting point (i.e., –1.5 MPa in matric potential, wp) was estimated from these determinations. Crop available water content for crop growth (av) was calculated as the difference between fc and wp.
Statistical Analysis
An analysis of variance (ANOVA) was used to estimate the significance of treatment effects using the GENSTAT 5 (Release 3.22) statistics program (Genstat Committee, 1993).
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RESULTS AND DISCUSSION
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Chemical Characterization of Treatments
Selected chemical attributes of the acid-activated bentonites (acid waste bentonite) after the bleaching process of the three contrasting vegetable oil bentonites and three co-composting materials are presented in Table 2. The absorption of fats and oils associated with the bleaching of vegetable oils results in elevated OC content of the acid waste bentonites (Table 2) when compared to natural bentonites. Total N content in the acid waste bentonites ranged from 0.5 to 1.3 g kg–1 suggesting higher levels of N in the palm oil when compared to the two other acid waste bentonite sources (Table 2).
The other components used in the composting process, namely chicken litter, rice husk, and rice husk ash had variable nutrient compositions (Table 2). As expected with ashing, rice husk pH increased this being associated with the formation of oxides. The chicken litter had a neutral to slightly acidic reactivity (Table 2). In general, exchangeable cations in the rice-based materials were low when compared to the chicken litter. Of importance with respect to the chicken litter is the elevated K+ level (77.3 cmolc kg–1). As evidenced by the elevated EC and difference between CEC and sum of bases, most of the cations extracted from the chicken litter, rice husk, and rice husk ash were in a soluble form (Table 2). This would imply that these nutrients are readily available for plant uptake as well as potentially available for leaching. Chicken litter had the highest total N content (25.5 g kg–1 N) compared to other materials and hence would be an effective source of N during the composting phase (Table 2).
Composting Process: Changes in Pile Temperature
For brevity changes in pile temperature are produced for those treatments receiving the ratio of acid waste bentonite/rice husk/rice husk ash/chicken litter 1:1:1:2 and for the noncomposted acid waste bentonite (Fig. 1). In general the temperature in the piles increased rapidly within 10 d of the start of the composting process and declined dramatically at Day 20, which coincided with the piles being turned (Fig. 1a). With each successive turning of the piles (i.e., Days 20, 45, and 75) the maximum temperature reached was less than the previous period. The gradual decline in temperature of the piles from Day 20 to Day 75 is indicative of the progressive decline in microbial activity and of the composting process coming to completion.
In the noncomposted acid waste bentonite bins (i.e., those treatments with the only acid waste bentonites) the temperature of the piles remained relatively constant throughout the composting phase except for the rice bran oil acid waste bentonite, where after 20 d the temperature of the pile increased to a maximum of 30°C above ambient (Fig. 1b).
Changes in Chemical Attributes with Composting
Changes in chemical characteristics after 84 d of the composting phase are presented in Table 3. The pHw of the co-composted acid waste bentonites increased with composting when compared to the original acid waste bentonites (Table 2), these increases being associated with the addition of the rice husk ash, chicken litter, dolomitic lime, and alkalinity generated through the composting process. The mean pH of the co-composted acid waste bentonites increased to 7.2 as compared to an initial mean pH of 3.7. Significant increases in pH within individual acid waste bentonite treatments were observed with increasing additions of chicken litter (Table 3). In general, the quality of the compost material with respect to available nutrients increased in those treatments receiving chicken litter, rice husk, and rice husk ash. Calcium and Mg2+ concentrations increased along with the CEC (Table 3). A cursory assessment of CEC and the sum of basic cations indicates that a significant proportion of the exchangeable cations are not associated with the exchange complex and are therefore in a soluble and mobile form.
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Table 3. Selective chemical characters of co-composted treatments 84 d after the initiation of the composting process.
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Treatments that did not receive additions of rice husk, rice husk ash, and chicken litter did not undergo significant changes in chemical properties (Table 2 and 3).
One of the main reasons for undertaking co-composting with readily available agri-waste products was to reduce the hydrophobic nature of the acid waste bentonites associated with the deposition of oils and fats on the surface of these materials. The WDPT for the acid waste bentonites before co-composting ranged from 25 to >10 800 s for the soybean and palm oil acid waste bentonites respectively (Table 2). After the co-composting the WDPT generally declined from the initial original values (Table 2 and 3). With increasing additions of chicken litter to the compost mix there was a significant increase in the WDPT (P < 0.001) (Table 3), although these values were substantially lower than the original rare acid waste bentonites (Table 2). In those treatments that did not undergo co-composting, the WDPT either declined from their original values in the case of the RB, or remained the same, in the case of the SB and PB (Table 3). The decline in the hydrophobicity of the RB after 84 d may in part be attributed to the rise in pile temperature due to specific properties of this waste (Fig. 1b).
Changes in Soil Chemical Attributes
Selected chemical properties of soils collected at the conclusion of the pot study in the greenhouse are presented in Table 4. It is clearly evident that increasing rates of application of the co-composted acid waste bentonites resulted in a significant increase in soil pH (P < 0.001) (Table 4). The greatest increase in pHCa associated with the application of co-composted acid waste bentonites was to pH 6.6, observed in the RB03 treatment applied at 120 t ha–1 (Table 4). Contrasting this, the application of the noncomposted acid waste bentonite treatments (P-SB, P-PB, and P-RB) had a mixed effect on the soils reactivity (Table 4).
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Table 4. Selective chemical properties of soils treated with co-composted and non-co-composted acid waste bentonites at the conclusion of the study.
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The EC of the 1:5 extract ranged from 0.0 dS m–1 in the unamended soil to 0.9 dS m–1 in the case of the non-co-composted soybean oil acid waste bentonite (P-SB) at a rate of 120 t ha–1 (Table 4). The range in EC values do not exceed the threshold value associated with sensitivity to salinity in maize of 1.5 to 3.0 dS m–1 as published by Mass (1990). However, these relatively high values indicate a high level of soluble salts that are not associated with the soils exchange complex. A plot of ECEC (
Ca2+ + Mg2+ + Na+ + K+) versus CEC measured in an ammonium acetate-buffered system clearly demonstrates the presence of a significant proportion of the measured cations being in a ‘soluble’ form (Fig. 2). If all of the measured cations were held on the exchange complex, then a plot of ECEC versus CEC would fall along the 1:1 line (Fig. 2). Clearly this is not the case for all treatments, suggesting that a significant proportion of the extracted cations are not associated with the exchange complex and are potentially leachable.
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Fig. 2. Relationship between measured CEC and the sum of exchange cations for all treatments at the conclusion of two cropping cycles with maize.
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Increasing rates of application resulted in significant increases in the amount of CEC generated, with the greatest increase in CEC being observed with the application of non-co-composted palm oil acid waste bentonite (P-PB) at a rate of 120 t ha–1 (Table 4). A clear benefit associated with the application of these materials to degraded soils is an enhancement of the CEC resulting in enhanced capacity to retain and supply nutrients to a growing crop.
As expected, increasing additions of co-composted acid waste bentonites resulted in a significant increase in the OC content of the soils (Table 4). The greatest increases were observed in the non-co-composted acid waste bentonite treatments (P-SB, P-PB, and P-RB). Increases in OC would have contributed to the observed increase in CEC. This may reflect a greater resistance to mineralization of the OC in these treatments.
In general, water penetration into those soils treated with the co-composted acid waste bentonites was rapid (Table 4). Clearly the co-composting process and time significantly reduced the hydrophobicity of these materials.
Changes in Soil Physical Attributes
For brevity and clarity changes in the bulk density, the moisture content at the permanent wilting point (wp) and the available water content (av) are presented in Fig. 3 for those treatments receiving the ratio of acid waste bentonite/rice husk/rice husk ash/chicken litter 1:1:1:3 (i.e., SB03, PB03, and RB03) and for the noncomposted acid waste bentonite (i.e., P-SB, P-PB, and P-RB). Increasing rates of application of co-composted acid waste bentonite treatments resulted in decreases in the bulk density with a significant decrease observed at application rates of 120 t ha–1 (Fig. 3a). Contrasting this, the bulk density of soils with the application of noncomposted P-PB, P-SB, and P-RB acid waste bentonites was higher than that of the control soil over all application rates (Fig. 3b). Croker et al. (2004) demonstrated that application of waste oil bentonites to a light-textured sandy soil resulted in increases in the bulk density of soils due to slumping and collapse of the soil structure. The consequence of decreased bulk density observed for the co-composted acid waste bentonite treatments is increased total porosity of the soils (Gupta et al., 1977). Therefore the result indicates that the co-composted acid waste bentonites reduce slumping and collapse of soil structure through the composting process, resulting in decreases in the bulk density (i.e., increases in the total porosity) of soils.
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Fig. 3. Selective physical properties [(a) and (b), bulk density; (c) and (d), moisture content; (e) and (f), available water content] of soils treated with co-composted and non-co-composted acid waste bentonites. Vertical bars are the LSD0.05 between treatment means.
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For all co-composted and non-co-composted acid waste bentonites, the soil moisture content at the permanent wilting point (wp) increased significantly with increasing application rates from 15 to 120 t ha–1 (Fig. 3c and 3d). Co-composted RB03 acid waste bentonite treatment resulted in a significant increase in the available water content for crop growth (av) at any given application rate when compared with the soil receiving no treatment (Fig. 3e). In addition, significant increases in av were observed for non-co-composted P-PB with increasing rates of application from 30 to 60 t ha–1 (Fig. 3f). Significant increases in av with the application of co-composted acid waste bentonite treatments were also observed for PB01 and PB02 (data not shown). However, av remained relatively unchanged for the soils receiving other co-composted acid waste bentonite treatments. This is due to a parallel increases in the field capacity (fc) and wp since av is the difference between fc and wp. It worth noting that there was a consistent decline in av with increasing rates of application for P-RB. Similarly, a significant decrease in av was detected for P-SB at application rates of 60 t ha–1 (Fig. 3f). This is due to increases in wp with the application of P-SB and P-RB being greater than increases in fc (Fig. 3d).
Soil water retention at lower matric potentials (approximately <–100 kPa) predominantly depends on adsorptive forces between the soil solid surface components (including soil particles and organic matter) and the soil solution, hence it is significantly affected by the specific surface area of soils. The wp is fairly well correlated with the surface area of a soil and would represent, roughly, about 10 molecular layers of water if it were distributed uniformly over the solid surface of the soil (Hillel, 1998). This indicates that an increase in wp is associated with an increase in the specific surface area of the soil. As previously indicated, the observed wp of the degraded light-textured sandy soil evaluated was significantly correlated with the co-composted and non-co-composted acid waste bentonite treatments (Fig. 3c and 3d). It is suggested that the inferred increases in specific surface area (as indicated by increased wp) is due to the applied bentonite and organic matter. In addition, the application of bentonite and organic matter in the co- and non-co-composted treatments resulted in significant increases in CEC. Figure 4 illustrates the highly significant relationship between wp and CEC (R2 = 0.779, P < 0.01). Since negative surface charge is directly related to the specific surface area of clay minerals and organic matter (Jury et al., 1991), the results of this study indicate that increases in CEC of the test soil through the incorporation of co- and non-co-composted acid waste bentonites were due primarily to an increase in the specific surface area (and associated negative charges) as derived from the bentonite and organic matter.
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Fig. 4. The relationship between gravimetric soil moisture content at the permanent wilting point (x) and cation exchangeable capacity (y) of the light-textured sandy soils applied at different ratios of co-composted and non-co-composted acid waste bentonites.
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Crop Response
The cumulative dry matter production of maize over the two cropping cycles for the co-composted treatments are presented in Fig. 5. In the case of PB and RB treatments, increasing rates of application resulted in a corresponding increase in maize dry matter production (Fig. 5a and 5b). Co-composted RB treatments gave the greatest overall response, this being achieved at the highest rate of application. Dry matter production doubled with the application of 15 t ha–1 and steadily increased with increasing rates of application thereafter (Fig. 5b and 5c). At the highest rate of application dry matter production had increased by approximately 3 times (Fig. 5b and 5c).
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Fig. 5. Cumulative dry matter yields for the co-composted acid waste bentonite sources [(a) soybean oil bentonite, (b) palm oil bentonite, (c) rice bran oil bentonite]. Vertical bar is the LSD0.05 between treatment means.
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Contrasting this, the application of co-composted SB treatments resulted in significant increases in productivity to 30 t ha–1, thereafter productivity tended to plateau with no significant gains (Fig. 5a). This suggests that certain limiting factors may have influenced dry matter production at the two highest rates of application. This may in part be related to the N content of the co-composted material. The co-composted SB treatments had lower total N contents when compared to the PB and RB co-composted materials (Table 3). As no additions of N were made over the two cropping cycles, it is plausible that N may have contributed to the observed yield plateau.
The dry matter responses of maize to additions of non-co-composted acid waste bentonites over the two cropping cycles are presented in Fig. 6a and 6b. During the first cropping cycle, the addition of non-co-composted acid waste bentonites resulted in a significant decline in dry matter production when compared to the control soil, clearly demonstrating the negative impact of these materials on productivity (Fig. 6a). However, during the second cropping cycle the overall productivity was higher than the previous cycle and significant responses to the first two rates of application were generally observed (Fig. 6b). This would suggest that some of the limiting factors that were operative in the first cropping cycle had been eliminated or reduced before and/or during the second cycle. However, it should be stressed that these treatments, although showing an improvement in the second cropping cycle, did not achieve productivity levels commensurate with those observed in the co-composted treatments. Hence the direct disposal of these non-co-composted acid waste bentonites to land is not to be recommended.
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Fig. 6. Dry matter production of maize associated with the application of non-co-composted acid waste bentonites at (a) first (time 1) and (b) second (time 2) harvests. Vertical bars are the LSD0.05 between treatment means.
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CONCLUSION
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Composting waste oil bentonites with readily available agricultural byproducts improved the chemical and physical characteristics of these materials. In addition, the incorporation of these composted materials in soil also enhances the chemical and physical properties of a degraded light-textured sandy soil. The application of composted materials significantly increased the soil pH, soil organic matter, clay content, and nutrient supplying capacity. Further, they have had a positive impact on soil physical attributes resulting in a decrease in bulk density and an increase in available water content. However, these composted materials are not fertilizers and hence there will be a need to add nutrients commensurate with crop requirements. The process of co-composting these materials reduces their potential negative impact on the environment through the neutralization of their acid reactivity and a decline in their hydrophobicity. Such processing, where there are locally available farm waste products (i.e., rice husk, chicken litter) can be utilized to produce an excellent soil amendment may be suitable for the establishment of small businesses in close proximity to processing plants. Such a development can turn an environmentally hazardous waste into a high quality soil amendment that would have a retail value.
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ACKNOWLEDGMENTS
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The research team, both IWMI and the Land Development Department (LDD) of the Royal Thai Government, would like to express our gratitude to the Division of Microbiology of the Thai Department of Agriculture for their facility during the composting period, and especially, Mrs. Bhavana Likhananont, who provided technical guidance. The Thai Ruam Jai Vegetable Oil Company Ltd. (rice bran oil), the Industrial Enterprise Company Ltd. (palm oil), and the Morakot Industries Public Company Ltd. (soybean oil) are also acknowledged for their contribution of the raw materials of acid waste bentonite and acid-activated bentonite.
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REFERENCES
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ABSTRACT
INTRODUCTION
), palm oil (
), and rice bran oil (
), respectively.