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http://journals.tubitak.gov.tr/agriculture/

Turkish Journal of Agriculture and Forestry Turk J Agric For(2017) 41: 357-371© TÜBİTAKdoi:10.3906/tar-1705-25

Influence of vermicompost application on soil consistency limits and soil compactibility

Serdar SARI1, Ekrem Lütfi AKSAKAL1,*,**, İlker ANGIN2,*1Department of Soil Science and Plant Nutrition, Faculty of Agriculture, Atatürk University, Erzurum, Turkey

2Department of Agricultural Structures and Irrigation, Faculty of Agriculture, Atatürk University, Erzurum, Turkey

1. IntroductionSoil is a critical part of successful agriculture and is the original source of the nutrients that we use to grow crops. Healthy soils are essential for healthy plant growth, human nutrition, and ecosystem services (Sahin et al., 2002; Kalisz et al., 2015; Cucci et al. 2016; Sönmez et al., 2016).

Sustainable management practices to improve soil quality by increasing soil organic matter (SOM) content is important to support economic crop production with enhanced ecosystem services (Razzaghi et al., 2016). Balanced recycling of organic and inorganic amendments such as wastes, composts, manures, mulches, cover crops, biosolids, and chemical fertilization is an integral component of sustainable management practices (Giménez-Morera et al., 2010; Sadeghi et al., 2015; Hazbavi and Sadeghi, 2016; Ni et al., 2016; Yagüe et al., 2016). While recycling is a socioeconomically viable way of disposing waste materials, it also alleviates environmental problems caused by waste accumulation (Ferreras et al., 2006; Angin et al., 2012). However, these materials may

contain soluble salts, which could accumulate in the soil following consecutive application to agricultural land and reach levels toxic to the cultivated crops. Therefore, usage of these materials requires careful and controlled use.

Vermicomposting, which is defined as the process of stabilized organic materials being produced by the synergistic activities of earthworms and diverse microorganisms, has been reported as one of the environmentally compatible and cost-effective ways for municipal solid waste management (Logsdson, 1994; Garg et al., 2006). Several studies have reported the positive effects of vermicompost on crop productivity and yield parameters (Atiyeh et al., 2000; Arancon et al., 2004; Gutierrez-Miceli et al., 2007; Joshi et al., 2015; Oo et al., 2015; Datta et al., 2016). Likewise, the beneficial effects of vermicompost on selected physical properties of soil were reported by Aksakal et al. (2016). However, available information related to vermicompost effects on soil consistency limits and compactibility associated with moisture content is limited.

Abstract: Soil organic matter (SOM) is a composite indicator of soil quality. Recycling of organic wastes is one of the environmentally compatible and economically viable options to increase and/or maintain SOM content. Our study was conducted to evaluate the effects of vermicompost (0%, 0.5%, 1%, 2%, and 4% w/w) on the consistency limits and the Proctor compaction test parameters of three texturally different soils (sandy loam, loam, and clay) from similar climatic conditions and cropping management practices. Results showed that vermicompost increased the optimum moisture content (OMC) with an associated decrease in maximum dry soil bulk density (MBD). The increase in OMC at 4% vermicompost was 64.2%, 42.4%, and 33.9% for sandy loam, loam, and clay, respectively, when compared to the control. Similarly, the rate of decrease in MBD at 4% vermicompost was 8.9%, 10.9%, and 10.4% for sandy loam, loam, and clay, respectively, than that of the control. Vermicompost increased the moisture contents of both liquid (LL) and plastic (PL) limits at OMC, in which the MBD occurred. While the OMC at control was 53% of LL and 75% of PL, these values were found as 55%, 56%, 56%, and 57% of LL and as 75%, 80%, 80%, and 76% of PL for 0.5%, 1%, 2%, and 4% vermicompost rates, respectively. Our results indicate that vermicompost could be applied to minimize physical degradation and compaction caused by the adverse effects of frequent tillage at higher soil moisture contents. The electrical conductivity of vermicompost used in this study was high and it requires careful and controlled use in terms of soil salinity when applied in high doses.

Key words: Vermicompost, Atterberg limits, Proctor compaction test, soil mechanical properties, soil friability, workability

Received: 05.05.2017 Accepted/Published Online: 02.08.2017 Final Version: 10.11.2017

Research Article

* These authors contributed equally to this work. ** Correspondence: elaksakal@atauni.edu.tr

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One of the mechanical properties is the Atterberg limit, often used as a sensitive indicator of a soil’s susceptibility to compaction or maximum dry soil bulk density (MBD) (Dexter and Bird, 2001; Hemmat et al., 2010; Aksakal et al., 2013). Field soil workability, the ability of the soil to undergo necessary tillage operations without causing smearing or compaction, is closely related to the Atterberg limits and Proctor compaction tests (Mueller et al., 1990, 2003; Dexter and Bird, 2001; Keller et al., 2007; Mosaddeghi et al., 2009; Aksakal et al., 2013). The Proctor compaction test permits the determination of MBD across a range of moisture contents, which is important for determining soil response to compaction (Mapfumo and Chanasyk, 1998; Blanco-Canqui et al., 2009). The optimum moisture content (OMC) range for tillage, which causes minimum soil structural degradation, can be estimated with the help of the Atterberg limits (Dexter and Bird, 2001). The OMC for soil workability is little lower than the plastic limit (PL), because the PL is generally regarded as the most suitable moisture content at which the field soil can be trafficked and tilled without considerable structural degradation (Dexter and Bird, 2001; Dexter and Birkas, 2004; Blanco-Canqui et al., 2009). Maximum water contents for workable soil conditions are reported to be nearly 60% to 90% of the PL, or the water content at maximum Proctor density (Wagner et al., 1992; Mueller et al., 2003; Keller et al., 2007; Mosaddeghi et al., 2009). The close relationship between OMC for tillage operation and the PL and/or liquid limit (LL) has been reported by several researchers (Dexter and Bird, 2001; Mueller et al., 2003; Arvidsson and Bölenius, 2006; Dexter and Czyż, 2007; Keller et al., 2007; Dexter and Richard, 2009; Aksakal et al., 2013).

Conventional tillage, which affects soil structure, is closely related to the soil moisture content. If tillage is not timed properly (wetter or lower than the OMC), it is expected to degrade the soil structure and require excessive energy to operate (Dexter and Bird, 2001; Keller et al., 2007). It is expected that an increase in the Atterberg limits and the Proctor OMC will not only minimize compactibility but will also cause a higher workable range and hence resistance to external mechanical forces (Aksakal et al., 2013).

As vermicompost is reported to improve soil physical properties by improving aggregate stability (AS), total porosity, and air and water permeability with an associated decrease in bulk density, and penetration resistance (Aksakal et al., 2016), its effect on consistency limits and the Proctor compaction test parameters needs to be evaluated. Our hypothesis is that vermicompost will improve soil consistency limits and the Proctor compaction test parameters by influencing OMC. The study was conducted to evaluate the effects of vermicompost (control, 0.5%, 1%, 2%, and 4% weight/weight) on the consistency

limits and the Proctor compaction test parameters of 3 different textured soils (sandy loam, loam, and clay) under laboratory conditions.

2. Materials and methods2.1. Soil and experimental designA pot study was performed under laboratory conditions, where mean air temperature and relative humidity were set to 25 ± 2 °C and 60 ± 5%, respectively, with the widely distributed great soil groups Ustorthent (sandy loam), Fluvaquent (loam), and Pellustert (clay) in East Turkey (Soil Survey Staff, 2006). Composite soils were sampled to a depth of 0–20 cm from agricultural fields under similar tillage and crop management practices in Erzurum, Turkey.

The field-moist soil samples were crumbled by hand, air-dried for 2 weeks under shade at laboratory conditions (~25 °C), and sieved through an 8-mm mesh. Each plastic pot [(40 cm in length and 25 cm in width) to a depth of 15 cm] contained 17, 16, and 14 kg of soil for sandy loam, loam, and clay, respectively. Commercial animal waste vermicompost processed by earthworms, Eisenia fetida (Lumbricidae, Oligochaeta), for a duration of 6 months in windrows was used in this study. Vermicompost was sieved (4-mm mesh) and added to the potted soil at weight/weight (w/w) of 0%, 0.5%, 1%, 2%, and 4%, respectively. The pots were arranged in a factorial combination of 3 soils × 5 vermicompost rates with 3 replications for each treatment combination using a completely randomized design.

To simulate the length of a vegetation period, soil and vermicompost mixtures were incubated for 90 days. In order to sustain mineralization of organic matter and microbial activity during this incubation time, the soils were kept at field moisture capacity (~330 cbar) by adding water at 3-day intervals (243 mL for sandy loam, 349 mL for loam, and 468 mL for clay). In order to minimize moisture loss, the containers were covered with stretched fabric and placed into an incubator (4 °C) after the end of the incubation period. 2.2. Vermicompost properties and soil analysisInitial characteristics of soils and vermicompost are presented in Table 1. The chemical components of soils and vermicompost were determined using a WDXRF Spectrometer (Rigaku ZSX-100e) equipped with a 4-kW rhodium tube. The standard hydrometer method was used for soil mechanical analysis (Gee and Or, 2002). While soil pH and electrical conductivity were measured in 1:2.5 soil:water mixtures, the pH and electrical conductivity of vermicompost were measured in saturation extracts (Rhoades, 1996; Thomas, 1996). Organic matter contents of soils and vermicompost were determined using the Smith–Weldon and loss-on-ignition methods, respectively (Nelson and Sommers, 1996). A Schreiber calcimeter was

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used to determine lime contents of soils (Loeppert and Suarez, 1996). Cation exchange capacity (CEC) of soil was determined by using the 1-M neutral ammonium acetate method followed by micro-Kjeldahl distillation (Sumner and Miller, 1996). The bulk density was determined by following the standard core method (Grossman and Reinsch, 2002). The pycnometer method was used to determine soil particle density (Flint and Flint, 2002).

A standard drop-cone penetrometer was used to determine the LL (McBride, 2002). The PL was determined using the rod formation method (McBride, 2002). The shrinkage limit (SL) was measured using the ASTM D427-04 standard method (ASTM, 1992). The plasticity index (PI) was calculated as the difference in moisture content between the LL and PL. The friability index (FI) was calculated by subtracting the SL from the PL.

The standard Proctor method (ASTM, 1992) was used to determine the compaction test curves, OMC, and MBD. For each pot, 2.5 kg of subsample was moisturized to achieve different water contents. Following this procedure, the homogenized wet subsamples were compacted in a compaction chamber (volume of 0.911 × 10–3 m3) in three layers. Each layer received 25 blows with the help of 2.5 kg rammer dropped from a height of 30.5 cm. After this procedure, the wet and compacted soil was weighed and dried at 105 °C for 24 h until a constant weight was obtained. The samples were reweighed to calculate the moisture content and dry bulk density.2.3. Data analysisSPSS 20.0 (IBM Corp., Armonk, NY, USA) was used to perform multivariate statistical analysis. Data normality was checked with the Kolmogorov–Smirnov test. Soil type and vermicompost application rates and their possible interactive effects on dependent variables were analyzed using two-way ANOVA. Tukey’s multiple comparison test was used for mean separation at P ≤ 0.05, unless otherwise mentioned. Significance of correlations among tested variables was tested using the Pearson correlation method.

3. Results and discussionThe nutrient contents of organic materials such as sewage sludge, composts, and manures are relatively high and may increase soil electrical conductivity due to the release of nutrients in their content. However, increase in electrical conductivity is not responsible for any salinity problem when applied at moderate levels (Kirchmann and Bergström, 2001; Lazcano et al., 2008). The electrical conductivity (EC) of the vermicompost used in this study was high (5.69 mS cm–1) (Table 1) in accordance with the permissible limits (<2 mS cm–1) of composts (Wiemer and Kern, 1992; Hoornweg et al., 2000). The declared limit values and the salt sensitivity of plants are different, but electrical conductivity values above 4 mS cm–1 are at

risk. For this reason, the vermicompost used in this study requires careful and controlled use in terms of soil salinity when applied in high doses.3.1. Effects of vermicompost on soil consistency limits The effect of vermicompost application rates on the LL, PL, and SL was similar for all soils, because the soil × vermicompost interaction was nonsignificant (Table 2). However, the effect of vermicompost on PI and FI was different among the soils with a significant soil × vermicompost interaction. Among the soils, the highest values of LL, PL, PI, SL, and FI were obtained for clay soil. While the observed parameters were positively and significantly correlated with clay content, they were negatively correlated with silt and sand contents (Table 3). Clay, sand, and organic matter contents, which influence the soil diffused double layer characteristics, have significant effects on the Atterberg limits (Lal and Shukla, 2004; Hemmat et al., 2010; Aksakal et al., 2013). De Jong et al. (1990) stated that clay content is the most important determinant of the Atterberg limits. Keller and Dexter (2012) pointed out that soils with negligible amounts of organic matter should contain at least 10% of clay content to be plastic. However, if organic matter is present, this value can be lower.

Vermicompost applications significantly (P ≤ 0.05) increased the LL, PL, SL, and FI values (Table 2). The effectiveness of vermicompost on these parameters increased with higher application rates. Among the vermicompost application rates, the highest LL values were observed with 4% vermicompost across soils. The increase in LL values at 4% vermicompost was 52.5%, 32.5%, and 16.9% for sandy loam, loam, and clay soil, respectively, as compared with the control. Likewise, the highest PL values were found at 4% vermicompost, which were 31.27%, 41.25%, and 53.34%, respectively. When compared to the control, the increase in PL values at 4% vermicompost were 62.6%, 52.1%, and 30.1% for sandy loam, loam, and clay, respectively. An increase in both LL and PL values was expected based on the fact that organic matter amendments as vermicompost increase soil moisture content due to their water-holding capacity (Rixon et al., 1991; Bhushan and Sharma, 2002). While organic matter content strongly influences the PL, the impact of organic matter on the LL was limited. Both the PL and LL are strongly influenced by clay type and contents in soil. While the increase in water adsorption capacity induced by the organic matter increases the LL and PL, the organic matter-induced aggregation is expected to decrease them by affecting the total surface area of the particles, particularly in kaolinite and illite, but to a lesser extent with montmorillonite (Malkawi et al., 1999). Our previous study (Aksakal et al., 2016) showed that vermicompost application in all three different textured soils significantly increased the organic matter

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content of the mixtures. Correlation coefficients between vermicompost application and organic matter content were 0.908**, 0.885**, and 0.954** for sandy loam, loam, and clay, respectively (Table 4). The effect of vermicompost on the LL and PL increased with lower amounts of clay contents. While the increases in LL and PL values were highest in sandy loam soil, both LL and PL were lowest in clay soil. Correlation coefficients between vermicompost and LL

were 0.974**, 0.958**, and 0.936** for sandy loam, loam, and clay, respectively. Similarly, the correlation coefficients between vermicompost and PL were 0.925**, 0.940**, and 0.962**, respectively for different textured soils (Table 4). In coarse-textured soils, the soil organic matter content has a greater influence on the increase of the specific surface area and consequently on water retentions (Smith et al., 1985; Hemmat et al., 2010).

Table 1. Initial characteristics of soils (0–20 cm) and vermicompost (mean ± SD) (Aksakal et al., 2016).

PropertiesSoils and Material

Soil I Soil II Soil III Vermicompost (VC)

Clay (%) 16.59 ± 1.21 25.85 ± 1.09 64.24 ± 0.04 - Particle size distribution (%)

Silt (%) 24.54 ± 0.16 40.67 ± 2.52 19.14 ± 0.07 - 4000–3000 µm 1.12

Sand (%) 58.87 ± 1.20 33.48 ± 1.44 16.62 ± 0.07 - 3000–2000 µm 1.38

Textural class Sandy loam Loam Clay - 2000–1000 µm 5.54

Great soil groupδ Ustorthent Fluvaquent Pellustert - 1000–500 µm 10.79

CEC (cmol kg–1) 22.54 ± 1.28 40.67 ± 1.34 47.01 ± 1.48 - 500–420 µm 2.72

CaCO3 (%) 0.46 ± 0.02 0.51 ± 0.03 0.85 ± 0.04 - 420–297 µm 10.59

Organic matter (%) 1.93 ± 0.07 1.24 ± 0.09 1.12 ± 0.03 34.91 ± 1.12 297–250 µm 0.70

pH 6.57 ± 0.08§ 7.75 ± 0.02§ 7.26 ± 0.04§ 8.17 ± 0.04Ψ 250–100 µm 4.93

EC (mS cm–1) 0.54 ± 0.09§ 0.85 ± 0.05§ 1.06 ± 0.08§ 5.69 ± 0.11Ψ 100–74 µm 13.32

Particle density (g cm–3) 2.66 ± 0.02 2.63 ± 0.02 2.67 ± 0.02 2.23 ± 0.02 74–53 µm 1.63

Bulk density (g cm–3) 1.32 ± 0.02 1.21 ± 0.02 1.07 ± 0.03 0.58 ± 0.01 <53 µm 47.28

Field capacity (%Pv) 19.12 ± 0.93 25.83 ± 1.15 44.12 ± 1.21

XRF

ana

lysis

(con

cent

ratio

n, %

)

O 47.29 46.96 47.71 47.55

Ca 3.85 4.88 2.10 17.19

Si 31.67 30.25 32.51 14.22

Mg 1.44 1.51 1.77 5.13

K 2.09 1.75 1.93 3.20

Al 8.43 8.32 8.90 2.67

P 0.17 0.16 0.05 1.49

S 0.04 0.05 0.07 0.74

Fe 1.82 2.42 2.18 0.56

Na 1.75 1.42 0.48 0.54

Mn 0.04 0.06 0.05 0.04

Sr 0.01 0.01 0.01 0.03

δ Soil Survey Staff (2006).§ Determined in 1:2.5 (soil:water) extract.Ψ Determined in saturation extract.

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While AS plays an important role in the swelling behavior of soil, information on the relationship between AS and consistency limits is limited. The AS is often conducted in fractions of 1–2 mm in size, whereas the LL and PL were done in fractions of less than 0.42 mm. Significant positive correlations between these parameters were observed (Table 4). The correlation coefficients between AS0.42 – LL and AS0.42 – PL were 0.953** and 0.925** for sandy loam, 0.775** and 0.755** for loam, and 0.720** and 0.677** for clay, respectively (Table 4). The results

suggest that measuring AS in fractions of <0.42 mm can be a good indicator of the relationship of AS with LL and PL.

The relationships of vermicompost with LL, PL, and PI are presented in Table 2 and Figure 1. Averaged across soils, irregular fluctuations in PI were observed. Results show that an increase in LL might not truly reflect an increase in PI because of the higher variations in PL. No consistent relationship was observed between the vermicompost application and the extent of the response on the PI. Among the vermicompost application rates, the highest PI

Table 2. Effects of vermicompost on LL (liquid limit), PL (plastic limit), PI (plasticity index), SL (shrinkage limit), and FI (friability index) of soils (mean ± SD)§.

Soils usedApplicationrate (%, w/w)

LL (%) PL (%) PI (%) SL (%) FI (%)

Sandy loam (Ustorthent)

0% 27.09 ± 1.11 d 19.23 ± 1.06 c 7.86 ± 0.34 b 6.74 ± 0.32 d 12.49 ± 0.74 c

0.5% 29.15 ± 0.69 d 21.47 ± 1.10 c 7.68 ± 0.55 b 7.30 ± 0.27 cd 14.17 ± 0.83 c

1% 32.62 ± 1.47 c 22.50 ± 1.64 bc 10.12 ± 0.22 a 7.96 ± 0.47 bc 14.54 ± 1.17 bc

2% 36.90 ± 0.44 b 25.74 ± 1.53 b 11.16 ± 1.10 a 8.73 ± 0.30 b 17.01 ± 1.23 b

4% 41.31 ± 1.80 a 31.27 ± 1.63 a 10.04 ± 0.18 a 11.04 ± 0.53 a 20.23 ± 1.10 a

Mean 33.41 ± 5.44 C 24.04 ± 4.49 C 9.37 ± 1.50 C 8.35 ± 1.59 C 15.69 ± 2.92 C

Loam(Fluvaquent)

0% 38.14 ± 1.21 d 27.12 ± 0.94 d 11.02 ± 0.33 ab 9.50 ± 0.32 d 17.62 ± 0.62 d

0.5% 40.43 ± 0.58 cd 28.46 ± 1.08 cd 11.97 ± 0.68 a 9.90 ± 0.25 cd 18.56 ± 0.84 cd

1% 42.38 ± 1.15 c 31.87 ± 1.07 bc 10.51 ± 0.45 bc 10.80 ± 0.32 bc 21.07 ± 0.76 bc

2% 45.79 ± 1.29 b 34.43 ± 1.62 b 11.36 ± 0.33 ab 11.23 ± 0.41 b 23.20 ± 1.19 b

4% 50.55 ± 1.57 a 41.25 ± 2.12 a 9.30 ± 0.58 c 14.05 ± 0.58 a 27.20 ± 1.54 a

Mean 43.46 ± 4.61 B 32.63 ± 5.33 B 10.83 ± 1.02 B 11.09 ± 1.68 B 21.54 ± 3.67 B

Clay (Pellustert)

0% 69.76 ± 1.39 c 40.99 ± 1.57 c 28.77 ± 0.65 a 15.78 ± 0.45 d 25.21 ± 1.14 c

0.5% 71.38 ± 1.21 c 42.82 ± 0.57 bc 28.56 ± 0.73 a 16.19 ± 0.24 cd 26.63 ± 0.34 bc

1% 73.66 ± 1.06 bc 45.28 ± 1.36 b 28.38 ± 0.35 a 17.01 ± 0.37 bc 28.27 ± 0.99 b

2% 76.87 ± 1.73 b 50.75 ± 1.25 a 26.12 ± 0.75 b 17.74 ± 0.41 b 33.01 ± 0.86 a

4% 81.55 ± 2.21 a 53.34 ± 1.03 a 28.21 ± 1.39 a 20.31 ± 0.47 a 33.03 ± 0.60 a

Mean 74.64 ± 4.54 A 46.64 ± 4.96 A 28.00 ± 1.23 A 17.41 ± 1.69 A 29.23 ± 3.43 A

Vermicompost effects

0% 44.99 ± 19.21 e 29.11 ± 9.60 d 15.88 ± 9.77 10.67 ± 4.03 d 18.44 ± 5.59 e

0.5% 46.99 ± 18.95 d 30.92 ± 9.46 d 16.07 ± 9.57 11.13 ± 3.96 d 19.79 ± 5.51 d

1% 49.55 ± 18.60 c 33.22 ± 9.98 c 16.33 ± 9.04 11.92 ± 4.03 c 21.30 ± 6.01 c

2% 53.19 ± 18.21 b 36.97 ± 11.07 b 16.22 ± 7.46 12.57 ± 4.04 b 24.40 ± 7.05 b

4% 57.80 ± 18.33 a 41.95 ± 9.68 a 15.85 ± 9.30 15.13 ± 4.12 a 26.82 ± 5.64 a

Soil × VC (p) 0.654 0.168 0.001 0.980 0.019

§ Different capital letters in each column show differences between soils, whereas lowercase letters in columns show differences between application rates. Mean differences were tested at the level of P ≤ 0.05.

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Tabl

e 3.

Pea

rson

corr

elat

ion

coeffi

cien

ts b

etw

een

para

met

ers (

for a

ll so

ils).

VC

Cla

ySi

ltSa

ndO

MA

ggre

gate

stab

ility

(fra

ctio

ns, m

m)

LLPL

PISL

FIO

MC

<0.4

20.

42–0

.84

0.84

–2.0

02.

00–6

.46.

4–12

.7>1

2.7

Mea

n

Cla

y0

Silt

0–.

548*

*

Sand

0–.

896*

*0.

120

OM

0.69

3**

–.53

3**

0.02

50.

619*

*

Aggregate stability (fractions, mm)

<0.4

20.

665*

*–.

528*

*0.

073

0.58

9**

0.90

9**

0.42

–0.8

40.

616*

*–.

452*

*0.

080

0.49

4**

0.75

6**

0.80

0**

0.84

–2.0

00.

650*

*–.

646*

*0.

351*

0.58

0**

0.82

4**

0.81

1**

0.66

8**

2.00

–6.4

0.60

6**

–.29

0–0

.314

*0.

511

0.79

2**

0.77

2**

0.73

4**

0.66

6**

6.4–

12.7

0.74

9**

–.43

4**

0.11

80.

453*

*0.

863*

*0.

855*

*0.

776*

*0.

880*

*0.

823*

*

>12.

70.

625*

*–.

587*

*0.

057

0.66

6*0.

901*

*0.

895*

*0.

857*

*0.

809*

*0.

816*

*0.

868*

*

Mea

n0.

714*

*–.

537*

*0.

071

0.60

0**

0.91

8**

0.92

5**

0.87

2**

0.89

1**

0.88

0**

0.95

5**

0.95

5**

LL0.

247

.964

**–0

.483

**–0

.887

**–0

.351

*–0

.349

*–0

.287

–0.4

50**

–0.1

45–0

.225

–0.4

18**

–0.3

42*

PL0.

429*

*.8

71**

–0.3

26*

–0.8

61**

–0.2

08–0

.226

–0.1

84–0

.266

–0.0

96–0

.066

–0.3

03*

–0.2

060.

964*

*

PI0.

001

.983

**–0

.629

**–0

.833

**–0

.492

**–0

.466

**–0

.384

**–0

.632

**–0

.190

–0.3

98**

–0.5

17**

–0.4

75**

0.94

6**

0.82

6**

SL0.

356*

.914

**–0

.410

**–0

.868

**–0

.247

–0.2

66–0

.220

–0.3

49*

–0.0

89–0

.123

–0.3

37*

–0.2

520.

985*

*0.

989*

*0.

884*

*

FI0.

471*

*.8

33**

–0.2

68–0

.847

**–0

.180

–0.1

98–0

.159

–0.2

10–0

.100

–0.0

29–0

.278

–0.1

750.

940*

*0.

996*

*0.

779*

*0.

971*

*

OM

C0.

446*

*.8

65**

–0.3

42*

–0.8

46**

–0.1

96–0

.200

–0.1

57–0

.282

–0.0

66–0

.066

–0.2

73–0

.191

0.95

5**

0.97

5**

0.83

7**

0.97

2**

0.96

5**

MBD

–0.2

92–.

936*

*0.

376*

0912

**0.

341*

0.33

3*0.

271

0.41

3**

0.18

90.

210

0.42

0**

0.33

5*–0

.982

**–0

.968

**–0

.903

**–0

.974

**–0

.953

**–0

.968

**

VC

, Ver

mic

ompo

st; O

M, o

rgan

ic m

atte

r; LL

, liq

uid

limit;

PL,

pla

stic

lim

it; P

I, pl

astic

ity in

dex;

SL,

shrin

kage

lim

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363

SARI et al. / Turk J Agric ForTa

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.84

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00–6

.46.

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M0.

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*

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939*

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*

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0.89

7**

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957*

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953*

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951*

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928*

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968*

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875*

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940*

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870*

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854*

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929*

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Mea

n0.

988*

*0.

930*

*0.

941*

*0.

972*

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969*

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964*

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990*

*0.

948*

*

LL0.

974*

*0.

888*

*0.

953*

*0.

956*

*0.

931*

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923*

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959*

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917*

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972*

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925*

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900*

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925*

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932*

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531*

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0**

0.73

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0.69

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0.55

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720*

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538*

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926*

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952*

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704*

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911*

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MBD

–0.9

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43**

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79**

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74**

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44**

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50**

–0.9

01**

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49**

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33**

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83**

–0.8

45**

–0.6

77**

–0.8

36**

–0.8

44**

–0.9

17**

Loam (Fluvaquent)

OM

0.88

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<00.

420.

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874*

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316

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0.93

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.70.

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844*

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793*

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0.77

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0.81

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Mea

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0.93

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–0.9

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Clay (Pellustert)

OM

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Aggregate stability (fractions, mm)

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513

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0.41

4

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*

>120

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823*

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858*

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763*

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Mea

n0.

781*

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772*

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720*

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906*

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8**

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0.19

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SL0.

919*

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572*

0.69

5**

0.65

0**

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8*0.

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796*

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632*

0.57

6*0.

815*

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348

0.55

9*0.

713*

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708*

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985*

*–0

.571

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864*

*

OM

C0.

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877*

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607*

0.45

60.

501

0.47

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485

0.66

8**

0.61

0*0.

827*

*0.

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0.84

9**

0.77

7**

MBD

–0.9

47**

–0.8

89**

–0.7

57**

–0.6

81**

–0.6

90**

–0.4

61–0

.560

*–0

.797

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.751

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.850

**–0

.875

**0.

392

–0.8

10**

–0.8

67**

–0.8

67**

364

SARI et al. / Turk J Agric For

Figure 1. Relationship between vermicompost application rate and LL, PL, and PI.

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SARI et al. / Turk J Agric For

values for sandy loam, loam, and clay were obtained with 2% (11.16%), 0.5% (11.97%), and control (28.77%) doses, respectively. Stanchi et al. (2009) reported a strong positive correlation between Alpine soil organic matter content and LL and PL, but a nonsignificant effect of organic matter on PI for surface and subsurface soils. In contrast, Blanco-Canqui et al. (2006) found a strong positive correlation between soil organic carbon and PI. After examining 44 soils, McBride and Bober (1989) stated that PI decreases with soil organic matter content increases. We observed an inverse relationship (r = –0.492**) between soil organic matter content and PI (Table 3). However, when the correlation of each soil was examined, the correlation coefficients between organic matter content and PI were 0.531*, –0.625*, and –0.266 for sandy loam, loam, and clay, respectively (Table 4). Differences in soil inherent characteristics associated with parent material, clay type and content, and mineralogy might be among the factors causing the inconsistency (De Jong et al., 1990; Hemmat et al., 2010). The increase in the organic matter content of mineral soil (<5% organic matter) often causes an increase in both the LL and PL (Lal and Shukla, 2004). Therefore, an increase in organic matter content is expected to exert consistent effects on the PI. The higher PI of clay than that of sandy loam and loam indicated that the clayey soils are more prone to compaction over a wider range of moisture contents than that of coarse textured soils.

Vermicompost application significantly increased the SL and FI in all soils (Table 2). The highest SL and FI values were observed with the 4% vermicompost application. The increasing rates in SL values at the highest vermicompost dose (4%) as compared with those of the control were 63.8%, 47.9%, and 28.7% for sandy loam, loam, and clay, respectively. When compared with the control, the SL of sandy loam increased by 8.3%, 18.1%, 29.5%, and 63.8% at 0.5%, 1%, 2%, and 4% vermicompost applications, respectively. Similar results were observed for loam and clay. The highest values of SL were obtained for clayey soil due to the higher clay contents than that of other soils. Our results suggest that the magnitude of shrinkage increases with increasing clay contents due to the micropore volume, the pore volume of the clay matrix organized in clay aggregates, and the interaggregate pore volume (Boivin et al., 2004). The increase in SL values with vermicompost application can be related to the increase of soil organic matter, which increases the moisture content because of its high absorptive capacity for water (Bhushan and Sharma, 2002; Hemmat et al., 2010). This justification is in close agreement with the close relation between soil organic matter content and the SL. The correlation coefficients were as 0.920**, 0.904**, and 0.951** for sandy loam, loam, and clay, respectively (Table 4).

The feasibility of cultivation and the optimal tillage period for exerting minimum disturbance on the soil structure can be extended by an increase in FI (Tivy, 1990). In accordance with the PL and SL values, the highest FI values were found when 4% vermicompost was applied to the soil (Table 2). In general, the FI increased with an increase in vermicompost application rates. These results indicated that vermicompost-applied soils became friable at relatively higher moisture contents as compared to the control, which allows the soil to be fairly workable under high moisture contents. Correlation coefficients between soil organic matter content and the FI were 0.883**, 0.877**, and 0.846** for sandy loam, loam, and clay, respectively (Table 4). Similar results were also reported by McBride and Bober (1989), Watts and Dexter (1998), Dexter and Bird (2001), and Hemmat et al. (2010).3.2. Effects of vermicompost on Proctor test parametersEffects of vermicompost application on OMC and MBD are presented in Table 5. Moreover, the relationship between moisture content and dry bulk density of soil at different vermicompost levels using the standard Proctor compaction test is shown in Figure 2. The effect of vermicompost application on OMC and MBD was similar across soils, as the soil × vermicompost application was nonsignificant. While vermicompost applications significantly increased OMC, they decreased the MBD of soils (Table 5; Figure 3). Averaged across soils, the highest OMC and the lowest MBD values were reached with 4% vermicompost application. The higher the vermicompost application, the higher the OMC but lower the MBD values. For all vermicompost rates, the lowest OMC and the highest MBD values were obtained from sandy loam, which has the lowest clay contents. Results showed that the OMC increased and the MBD decreased with an associated increase in clay content. Similar results were reported by other researchers (Larson et al., 1980; Carig, 1987; Barzegar et al., 2000; Aksakal et al., 2013).

Higher OMC values by 23.60%, 24.59%, 27.23%, and 30.20% at 0.5%, 1%, 2%, and 4% vermicompost application rates, respectively, were observed as compared with the control (21.04%). For the MBD, these values were determined as 1.70, 1.63, 1.60, 1.56, and 1.53 g cm–3 for the control, 0.5%, 1%, 2%, and 4% application rates, respectively (Table 5). These results clearly indicate that vermicompost application to soil extends the range of field workability. An extension in the range of field workability is expected to allow tillage of the soil more easily without any substantial compactions and/or deformations under mechanical forces. While vermicompost application was positively correlated with the OMC, it was inversely related with the MBD (Tables 3 and 4). The correlation coefficients between vermicompost and OMC were 0.972**, 0.972**, and 0.885** for sandy loam, loam, and clay, respectively. In

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contrast, the correlation coefficients between vermicompost and MBD were –0.944**, –0.966**, and –0.947** for sandy loam, loam, and clay, respectively. Averaged across soils, the correlation coefficient between vermicompost – OMC was significant (r = 0.446**) as compared to a nonsignificant correlation coefficient of –0.292 between vermicompost – MBD.

Several studies showed that an increase in soil structural stability significantly decreases soil compatibility (Baumgart and Horn, 1991; Barzegar et al., 1996; Aksakal

et al., 2013). Agricultural management practices that favor organic matter addition and/or accumulation are known to increase AS (Batjes, 2014; Yazdanpanah et al., 2016). It was reported that AS can be improved by organic amendments as organic matter increases the cohesive forces between mineral particles and organic components (Chenu et al., 2000; Yazdanpanah et al., 2016). Positive effects of vermicompost on AS were reported in our previous study (Aksakal et al., 2016). It is obvious that AS significantly increased OMC with as associated decrease in MBD, as

Table 5. Effects of vermicompost on OMC and MBD of soils (mean ± SD)§.

Soils used Application rate(%, w/w) OMC (%) MBD (g cm–3)

Sandy loam(Ustorthent)

0% 14.43 ± 0.81 d 1.91 ± 0.02 a

0.5% 16.01 ± 0.21 cd 1.83 ± 0.02 b

1% 18.11 ± 1.11 c 1.80 ± 0.02 bc

2% 20.72 ± 1.09 b 1.77 ± 0.02 cd

4% 23.69 ± 0.71 a 1.74 ± 0.01 d

Mean 18.59 ± 3.50 C 1.81 ± 0.06 A

Loam(Fluvaquent)

0% 19.80 ± 0.18 c 1.74 ± 0.01 a

0.5% 22.07 ± 0.30 b 1.69 ± 0.01 ab

1% 23.40 ± 1.39 b 1.66 ± 0.01 b

2% 26.47 ± 0.84 a 1.60 ± 0.02 c

4% 28.19 ± 0.57 a 1.55 ± 0.04 c

Mean 23.99 ± 3.19 B 1.65 ± 0.07 B

Clay(Pellustert)

0% 28.90 ± 1.77 c 1.44 ± 0.03 a

0.5% 32.71 ± 2.05 bc 1.37 ± 0.01 b

1% 32.28 ± 0.93 bc 1.35 ± 0.01 bc

2% 34.51 ± 0.65 b 1.31 ± 0.02 cd

4% 38.71 ± 1.48 a 1.29 ± 0.01 d

Mean 33.42 ± 3.54 A 1.35 ± 0.06 C

Vermicompost effects

0% 21.04 ± 6.41 d 1.70 ± 0.20 a

0.5% 23.60 ± 7.39 c 1.63 ± 0.20 b

1% 24.59 ± 6.28 c 1.60 ± 0.19 c

2% 27.23 ± 6.05 b 1.56 ± 0.20 d

4% 30.20 ± 6.73 a 1.53 ± 0.19 e

Soil × VC (p) 0.315 0.201

§ Different capital letters in each column show differences between soils, whereas lowercase letters in columns show differences between application rates. Mean differences were tested at the level of P ≤ 0.05.OMC, Optimum moisture content; MBD, maximum dry bulk density.

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shown by the significant correlations among the variables. The correlation coefficients between ASMean – OMC were 0.971**, 0.933**, and 0.610* as compared to correlations of –0.933**, –0.948**, and –0.751** between ASMean – MBD for sandy loam, loam, and clay, respectively (Table 5).

Soil LL and PL are considered as useful indexes of soil physical properties, such as strength and compressibility, which provide indicators for management operations. Vermicompost application increased water content at the LL and PL of OMC, at which the maximum soil compaction

Figure 2. Proctor compaction test curves of the soils studied.

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occurred. For sandy loam, the OMC values were found to be 55%, 56%, 56%, and 57% LL and 75%, 80%, 80%, and 76% PL for 0.5%, 1%, 2%, and 4% vermicompost application rates, respectively, as compared to 53% LL and 75% PL for the control. Similar rates of increases were obtained for

loam and clay. A number of studies showed a significant and positive correlation of OMC with LL and PL (Terzaghi et al., 1988; Mueller et al., 1990; Dexter and Bird, 2001; Mueller et al., 2003; Barzegar et al., 2004; Mosaddeghi et al., 2009; Aksakal et al., 2013). Overall, the correlation

Figure 3. Relationship between vermicompost application rate and OMC and MBD.

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coefficient between OMC – LL and OMC – PL was found as 0.955** and 0.975**, respectively (Table 3).3.3. ConclusionsResults showed that vermicompost amendments significantly influenced soil mechanical properties, and the effects were more pronounced at 4% vermicompost application. Averaged across soils, vermicompost significantly increased the optimum moisture content with an associated decrease in the maximum dry bulk density of soil. As a result, the soil’s strength in response to external mechanical forces (such as compaction) increased.

Increase in optimum moisture contents of the LL, PL, and FI indicated that vermicompost-applied soils become more friable at relatively higher moisture contents as compared to the control. This allows the soil to be more favorably workable under higher moisture contents. Our results clearly show that vermicompost applications will extend the range of field workability with tillage equipment.

AcknowledgmentThis study was supported by a grant from Atatürk University (BAP - 2013/374).

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