Catena 122 (2014) 54–60

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Effects of rice-husk ash on soil consistency and compactibility

Jili Qu ⁎, Beibei Li, Tianle Wei, Chencai Li, Baoshi LiuUniversity of Shanghai for Science and Technology, Environmental and Architectural College, 200093, China

⁎ Corresponding author. Tel.: +86 21 55153383.E-mail addresses: qujili@hotmail.com, qujili@icloud.co

http://dx.doi.org/10.1016/j.catena.2014.05.0160341-8162/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 June 2013Received in revised form 21 March 2014Accepted 20 May 2014Available online 27 June 2014

Keywords:Soil workabilitySoil trafficabilityLiquid limitPlastic limitProctor compaction test

The use of rice-husk ash (RHA) as a soil amendment is new and its effects on soil mechanical properties has notbeen well studied. This laboratory study aimed to assess the effects of rice-husk ash with different rates of 0%,10%, 20%, and 30% (v/v) on soil consistency limits and soil compactibility parameters in soils with differenttextures. Rice-husk ash applications in all experimental soils significantly (p b 0.05) increased liquid limit (LL)and plastic limit (PL) values. The effectiveness of rice-husk ash on LL and PL was more pronounced in soilswith low clay content. As compared with the control, the highest application dose of rice-husk ash (30%) in-creased LL with the rates of 29.1%, and 25.9%, in HA (Halaquept) and PL (Plagganthrept), respectively. But, thehighest LL values were obtained from 20% rice-husk ash application in UD (Udifluvent). On the average, rice-husk ash application increased PL by3.4%, 10.3%, and 14.1%with 10%, 20%, and 30% application rates, respectively,as compared to the control. Rice-husk ash application decreased maximum dry bulk density (MBD), butincreased optimumwater content (OWC). In all the soils studied, the lowest MBD and the highest OWCwere ob-tained from thehighest application dose of rice-husk ash. As comparedwith the control, thehighest rice-husk ashapplication dose (30%) decreased theMBDwith the rates of 7.2%, 8.8%, and 9.0%, inHA, PL andUD, but it increasedtheOWCvalueswith the rates of 21.6%, 31.9%, and 25.5%. The findings presented in this study clearly showed thatthe application of rice-husk ash increases the soil resistance to mechanical forces, since an increase in OWCmayimply that soil is more easily tilled in higher moisture contents without any deformation which also provideshigher workable range.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Soil amendments vary greatly in their origin, composition, applica-tion rate and expected or claimed mode of action (Wallace and Terry,1998). These actions include: improvements in soil structure, aerationand drainage, increasing soil water holding capacity, reducing soilcompaction, tillage and hardpan conditions, higher workability range,encouraging root development and increasing yield. Many organic andinorganic soil amendments have been extensively used for improvingsoil characteristics. As a soil conditioner, the use of rice-husk ash (RHA),a horticultural grade medium for all application, is not so common.Rice-husk ash is a powder obtained by burning of rice-husk, an agricul-tural by-product, at a temperature less than 1000 °C. China is one of thelargest countries planting rice, with an annual rice-husk productivity ofabout 39 million tons, being plenty of rice-husk resource (Chen, 2012).Although there aremany practical or potential industrial uses, the naturalstacking or burning of rice-husk is the general processing method formost of the rice-processing companies, which not only occupies theland resource, but also leads to environmental pollution and a fire poten-tial. It has been one of the main concerns for environmental agents andrice processors. The rice-husk ash obtained by burning rice husk under

m (J. Qu).

1000 °C is loosely consisted of nano-scale SiO2 gel particles (about50 nm) in diameter. Therefore, the introduction of rice-husk ash intofarming loam can not only give play to the physical filling effect, improvethe particle size distribution of soil, but also promote the second hydra-tion reaction by its high chemical activity, increasing the workabilityrange and reducing the compactability of loam (Chen, 2012).

In this study, the Atterberg limits and the Proctor compaction ofdifferent types of local soils were focused on to evaluate their effectson soil behaviors. The Atterberg limits and the Proctor compaction testare used by agricultural engineers for classifying soils. Various authorshave proposed to derive soil workability estimates using existing datafrom standard soil testing methods, water retention data (Dexter andBird, 2001), consistency limits (Mueller et al., 1990), and Proctor com-paction test data (Wagner et al., 1992). Soil consistence is described interms of the soil conditions at differentwater states, fromdry to viscous.Soil consistency has important implications to agricultural, engineering,and industrial uses of the soil (Hemmat et al., 2010; Lal and Shukla,2004). Few agronomists have used consistency limits in their compac-tion research (Mapfumo and Chanasyk, 1998; Mosaddeghi et al.,2009). With the help of consistency limits, the optimum and workablewater content range for tillage operations without undue effort andwith minimum risk of structural damage could be determined (Dexterand Bird, 2001). Many studies showed that a significant and positivecorrelation exists between optimum soil water content for tillage and

55J. Qu et al. / Catena 122 (2014) 54–60

PL (plastic limit) and/or LL (liquid limit) (Barzegar et al., 2004; Dexterand Bird, 2001; Mueller and Schindler, 1998a; Mueller et al., 1990,2003; Reeve and Earl, 1989; Terzaghi et al., 1988). Plastic (PL) and liquidlimits (LL) are useful because they are direct measures of soil mechanicalbehavior and represent an integration of soil properties (Soane et al.,1972), which can be used to estimate properties such as compressibility(Ball et al., 2000).

Another parameter that canbeused todepict themechanical behavioris the compactibility of soil. Compactibility can be defined as the increasein density with increasing applied stress. Several factors such as soil tex-ture, inherent bulk density, structural stability, organic matter, solublesalts andmost importantlywater content and compactive effort influencesoil compactibility (Thacker et al., 1994). These factors also influence soilworkability (Larson et al., 1994). The soil workability status is clearlyrelated to the moisture content at the plastic limit (Dexter and Bird,2001; Mueller and Schindler, 1998b; Smedema, 1993). The workablesoil water content is a little lower than PL (Godwin and Spoor, 1977;Spoor and Godwin, 1977). Dexter and Bird (2001) reported that the opti-mum soil water content for tillage occurs near the 0.9PL. Keller et al.(2007) found values of optimum soil water content for tillage in therange 0.7–0.9PL on four different Swedish soils.Maximumwater contentsfor optimum soil workability are nearly 0.6–0.9PL, or thewater content atmaximumProctor density (Mosaddeghi et al., 2009;Mueller et al., 2003).The optimum water content varies with the property of the soil, but ingeneral, it lies in the vicinity of plastic limit of the soil (Huang et al.,2009). Some researchers also related soil workability and trafficability tothe Proctor compaction test (Wagner et al., 1992). The Proctor test hasbeen employed to characterize resistance of agricultural soils to compac-tion and for evaluating the compaction status of soils (De Kimpe et al.,1982; Ekwue and Stone, 1995, 1997; Felton and Ali, 1992; Hakanssonand Lipiec, 2000; Thomas et al., 1996; Wagner et al., 1994; Zhang et al.,1997). Parameters to compare compactibility of soils are the maximumsoil bulk density (MBD) under the Proctor test and the optimum watercontent (OWC) at which the maximum soil bulk density is reached. Theagronomic importance of these parameters is elucidated by Wagneret al. (1994). Wagner et al. (1992) also found that the best soil fragmen-tation in tillage is obtained at the Proctor optimum water content.

As tillage plays a significant role in agricultural crop production, itshould be scheduled carefully in order to obtain the optimum soil struc-ture. The response of soil structure to tillage crucially depends on thesoil water content. When tilled outside of these limits, not only largeclods can be produced but also soil structural damage can occur.Wagner et al. (1992) appointed the Proctor critical water content asthe optimum water content for tillage. Mueller et al. (2003) also founda strong relationship between the soil workability and the Proctor criticalwater content. Increase in liquid limit (LL), plastic limit (PL), and theProctor optimum water content (OWC) will not only cause lesscompactable and more easily tilled soils, but also higher workable rangeand more soil resistance to mechanical forces, which are mainly depen-dent upon soil water suction. Therefore, the objective of this study wasto determine the effects of rice-husk ash application on consistency limits(LL, PL, and PI) and the Proctor compaction test parameters and indirectlyto probe the effect of rice husk ash on soil mechanical forces.

Table 1Chemical component of rice-husk ash (%).

Constituent SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 P2O3 Cl−1

quantity 87.89 0.66 0.55 2.41 0.56 2.5 0.14 0.3 0.82 0.25

2. Materials and methods

This paper evaluates the effects of rice-husk ash (RHA) on somemechanical properties of Shanghai soil. This study was conductedunder laboratory conditions with a relative humidity of 65 ± 5% andan average temperature of 21 ± 2 °C. The experimental soil sampleswere collected from the 0 to 20 cm depth of commonly distributedsoil great groups in the agricultural fields of Shanghai, China (31°14′N,121°29′E). Soils were classified as Halaquept (HA), Plagganthrept (PL),and Udifluvent (UD) according to Keys to Soil Taxonomy (2003, NinthEdition) issued by the United States Department of Agriculture (USDA).

The soil samples were air-dried and crumbled to pass a 4 mm sieve.Rice-husk ash passed through a 2 mm sieve was applied with the ratesof 10%, 20%, and 30% on volume/volume (v/v) basis, corresponding toweight/weight (w/w) basis of 2.0%, 3.9%, and 5.9% for HA, 2.5%, 4.9%,and 7.4% for PL and 2.4%, 4.8%, and 7.1% for UD. Soil and rice-straw ashwith defined amounts were uniformly mixed and conveyed to the ex-perimental pots. The control soil without rice-straw ash applicationwas alsomixed itself in order to reduce experimental errors on structuralparameters because of mixing. The mixtures were then filled into thirtysix plastic containers (40 cm in length and 25 cm in wide) to a depth of15 cm. Soils were incubated for three months at near field capacity byaddingwater with 3 days intervals under constant laboratory conditions.

The rice-husk ash (RHA) was an industrial waste residue from KaidiGreen Energy Power Plant. It is a powder obtained by burning the ricehusk, an agricultural by-product, at about 1000 °C. The rice-husk ashwas black–gray powder, which had a 76% residue on 45 μm sieve, 42%residue on 63 μm sieve, and with a loss on ignition of 5%. Table 1 showsthe chemical composition of rice-husk ash. It can be seen fromTable 1that the main component of rice-husk ash is SiO2. While the SiO2 inrice-husk represents an amorphous state, it demonstrates certain poten-tial activity (Chen et al., 2011). Table 2 shows the general characteristicsof the soils prior to the experiment.

Particle size distribution was determined using the sieving method;pH and electrical conductivity were measured according to McLean(1982) and Rhoades (1982a). Soil organic matter was determinedusing the Smith–Weldon method (Nelson and Sommers, 1982). Limecontent of the soils was determined with “Scheibler Calcimeter” asdescribed in Nelson (1982). Cation exchange capacity was determinedwith flame photometer using sodium acetate–ammonium acetate buff-ered at pH7 (Rhoades, 1982b). Bulk densitywas determined as describedby Blake andHartge (1986). The Casagrande devicewas used tomeasurethe liquid limit (LL), by the three-point Casagrande method. The plasticlimit (PL) was determined using “the 3-mm rod formation method”(McBride, 1993). The difference between LL (liquid limit) and PL (plasticlimit) is defined as plasticity index (PI). The standard Proctor method(ASTM, 1992) was applied. Subsamples of about 2.5 kg were spraymois-turized in order to reach at least eight different water contents. Followingthe method, amounts of soil from these homogenized wet subsampleswere compacted in three layers in a compaction chamber, volume of0.911 × 10−3m3. Each layer received 25 blows of a 2.5 kg falling hammerfrom 0.305m height. The weight of the wet compacted soil in the cham-ber was determined. Then the samples were dried in an oven at 105 °Cfor 24 h, and weighed again to estimate the moisture content and drybulk density.

Size distribution of soil aggregates was determined using the dryand wet-sieving methods developed by Kemper and Rosenau (1986).Air-dried aggregates were separated by placing 100 g of air-dried soilson the top of a stack of five sieves (5, 2, 1, 0.5 and 0.25mm in diameter).The soils were sieved for 10 min on a ro-tap sieve. Dry aggregates re-maining on each sievewere collected andweighed.Water-stable aggre-gates were estimated following the standard wet-sieving method.Briefly, 50 g composite soil samples representing each dry aggregatesize class were placed on the topmost of a nest of sieves with diametersequaling to 2, 1, 0.5, and 0.25 mm, respectively. The sieves were placedin a sieve holder of the Yoder type aggregate analysismachine (DM200-II) and sieved inwater for 30min at a rate of 30 cycle/min. The resultantaggregates on each sieve were dried at 105 °C for 24 h and weighed.According to the size range of 5–2, 2–1, 1–0.5, and 0.5–0.25mm, respec-tively, the percentage of water-stable aggregate was determined. Themass of b0.25 mm aggregate was calculated by the difference between

Table 2General physical and chemical properties of the soils.

Materials

HA(Halaquept) PL(Plagganthrept) UD(Udifluvent

Clay(%) 7.71 15.37 31.40Silt(%) 60.33 68.51 50.1Sand(%) 31.96 16.12 18.50Textural class SILT LOAM SILT LOAM SILTY CLAY LOAMGreat group Halaquept Plagganthrept UdifluventpHa 8.0 8.1 8.1ECa(mS cm−1) 0.21 0.16 0.12CEC(cmol(+)kg−1) 12.94 12.30 15.00CaCO3(%) 4.32 5.30 7.00Organic matter (%) 0.97 1.56 1.59Bulk density (gcm−3) 1.53 1.22 1.26

a Determined in 1:2.5 (soil:water) extract. EC:electric conductivity. CEC:cationexchange capacity.

56 J. Qu et al. / Catena 122 (2014) 54–60

the initial sample weight and the sum of sample weights collected onthe 2, 1, 0.5, and 0.25 mm sieve nest. The water stable indices, i.e., theaggregate stability or themeanweight diameter (MWD)were calculatedaccording to the method of Kemper and Rosenau (1986).

MWD ¼Xnþ1

1

ri−1 þ ri2

�mi

Where ri is the sieve aperture of the ith sieve; mi is the weightpercentage of the ith sieve.

Analysis of variance (ANOVA) was performed by SPSS StatisticalPackage (SPSS 19.0, SPSS Inc., 2011) using GLM. The Duncan's MultipleRange Test was used for testing mean differences.

3. Results and discussion

3.1. Effects of rice-husk ash on consistency limits (LL, PL, and PI)

3.1.1. Effects of rice-husk ash on liquid limits (LL)Rice-husk ash (RHA) applications in all three soils significantly

(p b 0.05) increased LL values (Table 3), because of its potential activity(Chen et al., 2011). Increases in the application doses of rice-husk ashincreased LL values in HA and PL. In HA and PL soils, the highest LLvalues were obtained from the maximum dose (30%) of rice-husk ashapplication. However, the application of rice-husk ash had a less signif-icant effect on LL values of UD soil, due to its high clay content and

Table 3Effects of rice-husk ash application on liquid limit (LL), plastic limit (PL), and plasticityindex (PI) of soils (Mean ± SD).

Soil Application rate (v/v) LL (%) PL (%) PI (%)

HA Control 38.2 ± 0.6d 23.0 ± 0.3d 15.2 ± 0.3b10% 47.2 ± 0.3c 28.4 ± 0.4c 18.8 ± 0.5b20% 48.1 ± 0.3b 29.2 ± 0.4b 18.9 ± 0.2a30% 49.3 ± 0.4a 30.0 ± 0.5a 19.3 ± 0.4a

PL Control 40.9 ± 0.3c 28.5 ± 0.6b 12.4 ± 0.3d10% 49.3 ± 0.4b 29.0 ± 0.4b 20.4 ± 0.1c20% 49.9 ± 0.2b 33.0 ± 0.3a 16.9 ± 0.2b30% 51.5 ± 0.4a 33.7 ± 0.3a 17.8 ± 0.2a

UD Control 54.7 ± 0.3d 36.7 ± 0.2b 18.0 ± 0.2d10% 59.4 ± 0.2c 34.1 ± 0.1d 25.3 ± 0.1c20% 60.1 ± 0.3a 35.0 ± 0.2c 25.1 ± 0.1a30% 58.8 ± 0.3b 37.0 ± 0.1a 21.8 ± 0.2b

General Control 44.6 ± 4.5d 29.1 ± 2.4d 15.5 ± 2.8d10% 52.0 ± 2.5c 30.1 ± 2.3c 21.9 ± 2.3c20% 52.7 ± 6.1b 32.1 ± 3.5b 20.6 ± 8.9a30% 53.2 ± 5.2a 33.2 ± 2.9a 20.0 ± 7.3b

Letters followed in each row (capital letters) show differences between soils, while lettersin columns (small letters) show differences between application rates (Mean ± SD).Mean differences were tested at the level of p b 0.05.

content of micro-aggregated clay. In HA, the increasing rates in LLvalues were 23.6%, 25.9%, and 29.1% for 10%, 20%, and 30% rice-huskash doses, respectively. These rates were 20.5%, 22.0%, and 25.9% forPL soil. On the other hand, the highest LL values were obtained from20% rice-husk ash application dose in UD. The effectiveness of rice-husk ash on LL increases in soil with lower clay content. While increasein the rate of LL values was the highest in HA, it was lowest in UD.Correlation coefficients between LL and rice-husk ash applicationwere found as 0.893**, 0.875**, and 0.715** for HA, PL and UD, respec-tively (Table 5). These results indicate that the effectiveness of rice-husk ash application on LL decreased with increasing clay content ofsoil.

3.1.2. Effects of rice-husk ash on plastic limits (PL)Effects of rice-husk ash (RHA) on plastic limit were also significant

(p b 0.05) (Table 3). As in liquid limit, increases in the applicationdoses of rice-husk ash increased plastic limit values of HA and PL soils,and the highest increases were obtained from the 30% rice-husk ash ap-plication. Compared with the control sample, the highest increasingrates in plastic limit of HA and PL soils were 30.4%, and 18.2%, respec-tively. Additionally, significant correlations were obtained betweenrice-husk ash application doses and PL (Table 4 and 5). The highest PLvalues were obtained also with the application of 30% rice-husk ash inUD. These rates of rice-husk ash increased PL by only 0.82% as comparedwith the control. In the case of doses of 10% and 20% rice-husk ash appli-cation in UD, the PL values decrease instead. The results indicate that asclay content increases, the effect of rice-husk ash on PL of soil becomesless significant. Hemmat et al. (2010) and Smith et al. (1985) have re-ported that an increase in soil specific surface area and activity resultsin an increase in the LL and PL values.

3.1.3. Effects of rice-husk ash on plastic index (PI)The plasticity index (PI) reflects the range of moisture content over

which the soil is susceptible to compaction by external forces. Thehigher the PI value, the greater the range of moisture over which thesoil is susceptible to compaction. Effect of rice-husk ash (RHA) applicationon soil PI was not clear.While LL (liquid limit) has a significant role in theincrease of PI, increase in PL may minimize this effect. Because of the in-creases in LL and PL values, the highest PI values vary with applicationdoses (Table 3). In HA, as compared with the control, PI increased withan increase in application doses, but the same situationwas not obtainedfor in PL andUD soils. In general, PI increasedwith increase in applicationdoses, but no significant differences were obtained.

In this study the higher plasticity index of UD soil than that for HAand PL soil means that the UD soil is prone to substantial compactionover a wider moisture content range than either the HA and PL soils.According to Jumikis (1984) and Mapfumo and Chanasyk (1998) aPI b 7 indicates that a soil is of lowplasticity, 7 b PI b 17 indicatesmediumplasticity while PI N 17 indicates high plasticity. Using these guidelinesthe UD soil used in this study is highly plastic while the HA and PL soilshave medium plasticity and therefore are less prone to severe compac-tion because of the narrow moisture range within which deformationwould occur.

3.2. Effects of rice-husk ash on Proctor test parameters

It was found that effects of rice-husk ash (RHA) on themaximumdrybulk density (MBD) and the optimum water content (OWC) were sta-tistically significant (p b 0.05) (Table 6), and the relationship betweenwater content and dry bulk density of the soils at different levels ofrice-husk ash is illustrated in Fig. 1 using the standard Proctor test.The result clearly indicates that addition of rice-husk ash to soil extendsthe range of trafficability without causing compaction. It is importantespecially for fine textured soils. In all of the same application ratesof the experimental soils, the highest MBD and the lowest OWCvalues were obtained from HA, which had the lowest clay content.

Table 4General correlation relationships among indices of soil tested.

Aggregate stability (AS)

RHA Clay Silt Sand b0.25 0.25–0.5 0.5–1. 1–2 2–5 N5 LL PL PI OWC MBD

AS RHA −Clay .000 −Silt .000 −.710 −Sand .000 −.649 −.076 −b0.25 −.537 −.164 .811 −.644 −0.5–0.25 −.513 −.986 .818 .512 .077 −1–0.5 −.126 .905 −.942 −.264 −.266 −.040 −2–1 −.441 −.731 .038 .994 −.107 .352 .032 −5–2 .604b .258 −.723 −.634 −.300 −.834a −.152 −.385 −N5 .831a .847 −.227 −.954 −.457 −.729a −.067 −.633b .727a −LL .447 .984 −.573 −.774 −.021 −.735a .396 −.567 .614b .556 −PL .517 .131 .606 −.839 −.023 −.010 −.565 −.506 .054 .489 −.111 −PI .178 .972 −.855 −.452 −.009 −.634b .574 −.287 .512 .284 .914a −.504 −OMC .797a .230 .523 −.890 −.159 −.266 −.514 −.460 .422 .580b .247 .742a −.087 −MBD −.927a −.569 −.175 .995 .296 .481 .336 .510 −.581b −.758a −.448 −.642b −.128 −.949a −

RHA: rice husk ash; AS: aggregate stability; LL: liquid limit; PL: plasticity limit; PI: plasticity index; OWC: optimumwater content; MBD: maximum dry bulk density.a Correlation is significant at the 0.01 level.b Correlation is significant at 0.05 level.

57J. Qu et al. / Catena 122 (2014) 54–60

On the control samples, the MBD values were found as 1.66, 1.60 and1.55 g cm−3 for HA, PL and UD soils, respectively. For these soils theOWC values were found as 17.6%, 18.2%, and 20.8%. The optimumwater content for the maximum dry bulk density increased as the clay

Table 5General correlation relationships among indices of three types of soils tested.

Aggregate stability (AS)

RHA b0.25 0.25–0.5 0.5–1 1–2

HA RHA _b0.25 −.864a _0.5–0.25 −.525 .485 _1–0.5 .547 −.169 −.512 _2–1 −.650b .494 .761a −.484 _5–2 .192 −.313 −.814a .033 −.666b

N5 .846a −.835a −.790a .389 −.874a

LL .893a −.774a −.794a −.513 −.910a

PL .884a −.799a −.763a .535 −.879a

PI .784a −.694b −.800a .451 −.912a

OMC .946a −.829a −.705b .538 −.814a

MBD −.973a .857a .584b −.535 .733a

PL RHA _b0.25 .191 _0.5–0.25 −.958a −.286 _1–0.5 −.512 −.246 .369 _2–1 −.460 −.369 .452 .385 _5–2 .861a .029 −.763a −.684b −.547N5 .911a .122 −.885a −.611b −.600b

LL .875a −.186 −.819a −.359 −.239PL .934a .425 −.923a −.514 −.367PI .490 −.613b −.418 −.093 −.043OMC .990a .142 −.951a −.506 −.403MBD −.973a −.134 .912a .553 .410

UD RHA _b0.25 −.905a _0.5–0.25 .107 −.297 _1–0.5 .100 −.146 .225 _2–1 −.267 .154 −.038 .623b _5–2 .540 −.511 −.403 −.591b −.589b

N5 .920a −.894a .134 −.172 −.396LL .715a −.577b −.232 −.028 −.414PL .172 −.268 .450 −.012 .060PI .445 −.306 −.352 −.015 −.324OMC .976a −.912a .290 .147 −.241MBD −.976a .869a −.155 .028 .378

RHA: rice husk ash; AS: aggregate stability; LL: liquid limit; PL: plasticity limit; PI: plasticity inda Correlation is significant at the 0.01 level.b Correlation is significant at 0.05 level.

content increased. Larson et al. (1980), Carig (1987), and Barzegaret al. (2000) also reported similar results.

TheMBDdecreasedwith an increase in rice-husk ash (RHA) applica-tion, while the OWC values increased significantly (p b 0.05) (Fig. 1). In

2–5 N5 LL PL PI OWC MBD

_.610b _.784b .978a _.498 .978a .987a _.690b .927a .968a .915a _.402 .954a .958a .973a .901a _−.304 −.889a −.899a −.919a −.893a −.951a _

_.822a _.789a .800a _.736a .808a .735a _.530 .486 .832a .236 _.874a .892a .920a .917a .568 _−.878a −.882a −.922a −.892a −.593b −.987a _

_.650b _.624b .629b _−.116 .281 −.522 _.498 .338 .936a −.788a _.424 .894a .661b .205 .393 _−.559 −.940a −.712a −.180 −.440 −.974a _

ex; OWC: optimumwater content; MBD: maximum dry bulk density.

Table 6Effect of rice husk ash on maximum dry bulk density (MBD) and optimumwater content(OWC) (Mean ± SD).

Soil Application rate (v/v) MBD (g·cm−3) OMC (%)

HA Control 1.66 ± 0.01a 17.6 ± 0.1d10% 1.61 ± 0.01b 18.6 ± 0.1c20% 1.58 ± 0.15c 20.0 ± 0.1b30% 1.54 ± 0.01d 21.4 ± 0.3aMean 1.60 ± 0.05A 19.4 ± 1.0B

PL Control 1.60 ± 0.02a 18.2 ± 0.5d10% 1.57 ± 0.03b 20.6 ± 0.3c20% 1.52 ± 0.02c 21.6 ± 0.2b30% 1.46 ± 0.01d 24.0 ± 0.2aMean 1.54 ± 0.08C 21.1 ± 2.8A

UD Control 1.55 ± 0.01a 20.8 ± 0.3d10% 1.50 ± 0.01b 22.8 ± 0.3c20% 1.47 ± 0.01c 23.5 ± 0.3b30% 1.41 ± 0.02d 26.1 ± 0.3aMean 1.48 ± 0.05B 23.3 ± 2.0B

General Control 1.60 ± 0.02a 18.9 ± 0.5d10% 1.56 ± 0.02b 20.7 ± 1.0c20% 1.52 ± 0.03c 21.7 ± 1.7b30% 1.47 ± 0.06d 23.8 ± 2.0a

Letters followed in each row (capital letters) show differences between soils, while lettersin columns (small letters) show differences between application rates (Mean ± SD).Mean differences were tested at the level of p b 0.05.

Water content (%)

1.48

1.50

1.52

1.54

1.56

1.58

1.60

1.62

1.64

1.66

10 12 14 16 18 20 22 241.46

HA

0(control)102030

Application rate(%)

12 14 16 18 20 22 24 26 28 30

1.42

1.44

1.46

1.48

1.50

1.52

1.54

1.56

1.58

1.60

Water content (%)

PL

0(control)102030

Application rate(%)

18 20 22 24 26 28 30 321.30

1.32

1.34

1.36

1.38

1.40

1.42

1.44

1.46

1.48

1.50

1.52

1.54

1.56

Water content (%)

0(control)102030

Application rate(%)

UD

Dry

bul

k de

nsity

(g.

cm-3

)D

ry b

ulk

dens

ity (

g.cm

-3)

Dry

bul

k de

nsity

(g.

cm-3

)

Fig. 1. Proctor compaction test curves of the soils studied.

58 J. Qu et al. / Catena 122 (2014) 54–60

all of the soils studied, the lowest MBD and the highest OWC valueswere obtained from the highest application dose (30%) of rice-huskash (Table 6). According to a previous study (Aksakal et al., 2013), itwas clearly seen that AS (aggregate stability) is a significant factor inthe decrease of the MBD and increase of the OWC values. In thisstudy, the correlation coefficient between AS and MBD was foundas −0.581* (particle size group 2–5 mm) and −0.758** (particlesize group N5 mm), respectively (Table 4). The correlation coefficientbetween AS and OWC was found as 0.422 (particle size group 2–5 mm)and 0.580* (particle size group N5 mm), respectively. It was inversein the case of other particle size groups with only a loose correlation.Several researches have demonstrated that an increase in soil structuralstability decreases soil compactibility (Aksakal et al., 2004; Barzegaret al., 1996; Baumgart and Horn, 1991). Application of rice-husk ashincreasedwater content at liquid limit (LL), plastic limit (PL), and valuesof OWC in which the maximum compaction occurred. Increase in rice-husk ash application rate increased the OWC values of the controlgroups which were 0.42LL and 0.65PL. These values were found as0.40, 0.41, and 0.45LL and 0.69, 0.68, and 0.62PL for 10%, 20%, and 30%rice-husk ash application rates, respectively. Many studies showedthat significant and positive correlation exists between the OWC forPL and LL (Barzegar et al., 2004; Dexter and Bird, 2001; Mosaddeghiet al., 2009; Mueller and Schindler, 1998a; Mueller et al., 1990, 2003;Reeve and Earl, 1989; Terzaghi et al., 1988). The results presented inthis study showed that AS is a significant factor in the decrease of theMBD and increase of the OWC values, hence, this situation shows thatsoils amended with rice-husk ash are resistant to mechanical forces.In agricultural aspect, increases in the water content of LL, and PL inOWC values may increase the field capacity, which make soils moreeasily tilled in higher moisture contents without any deformation.

3.3. Agronomical implications of the findings

The application of rice-husk ash (RHA) increases the OWC anddecreases the MBD. In this study, OWC is positively correlated withthe aggregate stability (0.422 for particle size group 2–5 mm and0.580* for particle size group N5mm),whileMBD is negatively correlatedwith AS (−0.581* for particle size group 2–5 mm and −0.758** forparticle size group N5 mm). That means that the application of rice-husk ash can also improve significantly the formation and stability ofthe soil aggregate, especially for larger size group (N2 mm). The forma-tion and stability of the soil aggregates play an important role in the

crop production and soil degradation prevention. An increase in the for-mation of macroaggregates by the addition of rice-husk ash (RHA) indi-cates that the RHA is able to increase the soil aggregation. The organicmaterials introduced by the RHA may act like a glue to cementmicroaggregates into macroaggregates in which larger pore spaces arepresent between micro-aggregates. The aggregate stability not only af-fects the movement of water and air in the soil but also influences the

59J. Qu et al. / Catena 122 (2014) 54–60

water holding capacity, root penetration, seedling emergence, runoff anderosion.

Many studies showed that the workable soil water content is a littlelower than water content at plastic limit (Dexter and Bird, 2001;Godwin and Spoor, 1977; Mueller and Schindler, 1998b; Smedema,1993; Spoor and Godwin, 1977). Maximumwater contents for optimumsoil workability are nearly 0.6–0.9PL, or the water content at maximumProctor density (Mosaddeghi et al., 2009; Mueller et al., 2003). In thisstudy, the application of RHA increases the water content at plasticlimit, which means that the range of water content in which the soilcan be easily tilled extends. Therefore the soil can be tilled at higherwater content, improving the soil workability. While the decrease ofMBD (maximum bulk density) caused by the application of RHA to soilincreases the resistance of soil to compaction caused by mechanicalforce, namely, the soil can be tilled at highermechanical forcewithout de-formation, improving the soil trafficability.

4. Conclusion

The results of this study clearly indicated that rice-husk ash (RHA)application enhances soil physical and mechanical properties. Theconclusions derived from our study can be summarized as follows:

1. Rice-husk ash applications in all three soils significantly (p b 0.05)increased liquid limit and plastic limit values. The effectiveness ofrice-husk ash on LL and PL increased with the decreases in clay con-tent and vice versa.

2. Increase in rice-husk ash application not only decreased the MBDvalues, but also increased the OWC values significantly (p b 0.05)in all of the soils studied. The higher the rice-husk ash applicationrate, the lower the MBD and the higher the OWC values.

3. Increase in rice-husk ash application rate increasedOWC values of thecontrol groups that were 0.42LL and 0.65PL. Application of rice-huskash increased LL and PL as well as the OWC at nearly the same rate.Therefore there may be significant and positive correlation betweenLL, PL and OWC in which the maximum compaction occurred.

In conclusion, the application of rice-husk ash (RHA) decreased theMBD, but increased the OWC values of soils, and improved the stabilityof soil aggregate, thus, this situation leads soils more resistant to me-chanical forces. Also, increase in the water content at LL (liquid limit),PL (plastic limit), and OWC (optimum water content) values maymake soils more easily tilled in higher moisture contents without anydeformation. It could be concluded that it might be possible to extendthe range of workability by amending soil with rice-husk ash.

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