Evaluating the long-term effects of tillage systems on soil structural quality using visual assessment and classical methods

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INTRODUCTION

Soil structure and associated properties (e.g., compaction, aggregate stability, porosity) are complex dynamic character-istics affected by inherent (e.g., particle size distribution) and anthropogenic factors (Guimarães, Lamandé, Munkholm, Ball, & Keller, 2017), therefore, considered as important soil quality indicators in comparing different land uses and tillage practices (Moncada, Penning, Timm, Gabriels, & Cornelis,

2017). Several direct or indirect methods have been proposed to evaluate the soil structural quality of surface soils. Visual methods have been increasingly preferred to directly examine the impacts of land use and management practices on soil structure due to the simplicity, reliability and low cost of the assessment procedure (Guimarães et al., 2017; Leopizzi, Gondret, & Boivin, 2018). Field level visual evaluation of soil structural quality provides reliable information on the current state of soil and guides farmers in evaluating the

R E S E A R C H PA P E R

Evaluating the long- term effects of tillage systems on soil

structural quality using visual assessment and classical methods

İsmail Çelik

1

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_{Hikmet Günal}

2

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_Mert Acar

1

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_{Nurullah Acir}

3

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Zeliha Bereket Barut

4

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_Mesut Budak

5

1_{Department of Soil Science and Plant}

Nutrition, Faculty of Agriculture, Çukurova University, Adana, Turkey

Nutrition, Faculty of Agriculture, Gaziosmanpaşa University, Tokat, Turkey

Nutrition, Faculty of Agriculture, Kırşehir Ahi Evran University, Kırşehir, Turkey

4_{Agricultural Machinery and Technologies}

Engineering, Faculty of Agriculture, Çukurova University, Adana, Turkey

Nutrition, Faculty of Agriculture, Siirt University, Siirt, Turkey

Correspondence

İsmail Çelik, Department of Soil Science and Plant Nutrition, Faculty of Agriculture, Çukurova University, Adana, Turkey. Email: icelik@cu.edu.tr

Funding information

TUBITAK (Scientific and Technological Council of Turkey), Grant/Award Number: 115 O 353

Abstract

Current agricultural practices and their impacts on the sustainability of crop produc-tion can be evaluated by simple and reliable soil structure assessment tools. The study was conducted to determine the effects of long- term (2006–2017) tillage sys-tems on structural quality of a clayey soil using the visual evaluation of soil structure (VESS) and classical field and laboratory measurements. A field experiment with seven tillage systems, representing both traditional and conservation tillage meth-ods, was conducted on a clayey soil in the Cukurova region, Turkey. Soil samples from 0–10, 10–20 and 20–25 cm depths were analysed for mean weight diameter (MWD), porosity and organic carbon. Penetration resistance (PR) was determined in each treatment plot. The VESS scores (<2) of upper 0–5 cm indicated a good structural quality for all tillage systems. The VESS scores were positively related to PR and MWD and negatively to macroporosity (MaP) and total porosity. In reduced and no- till systems, poorer soil structures were observed in subsurface layers where firm platy and angular blocky structures were defined. Mean VESS score (3.29) in 20–25 cm depth where PR was 3.01 MPa under no- till indicated a deterioration of soil structural quality; thus, immediate physical interventions would be needed. Lower VESS scores and PR values under strategic tillage which was created by ploughing half of no- till plots in November 2015 indicated successful correction of compaction caused by long- term no- till. The results suggest that the VESS approach is sensitive and useful in distinguishing compacted layers within the topsoil.

K E Y W O R D S

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effects of management practices on the sustainability of their productions (Ball et al., 2017).

The effects of tillage practices on soil structure and structure- related properties like pore size distribution and crop productivity are being discussed for a long time (Munkholm, Heck, & Deen, 2013; Nouri, Lee, Yin, Tyler, & Saxton, 2019). The impacts of tillage systems on soil quality have generally been assessed through measurements of bulk density (Sharma et al., 2005), porosity (Pires et al., 2017), penetration resistance (Moraes, Debiasi, Carlesso, Franchini, & Silva, 2014), hydraulic properties (Strudley, Green, & Ascough, 2008) and aggregate stability (Çelik et al., 2019) because of their high sensitivity to disturbance. However, considerable specialization, tedious sampling and sophisticated laboratory analyses are required to obtain re-liable results by classical methods (Guimarães et al., 2017). Therefore, the visual evaluation of soil structure (VESS) method which does not require long- term training and ex-pensive equipment (Johannes, Weisskopf, Schulin, & Boivin, 2017) has been preferred to determine structural quality of soils under different management and cultivation practices (Guimarães et al., 2017; Leopizzi et al., 2018; Moncada et al., 2017). The VESS is a reliable tool for identifying the compaction in surface horizons and helps in the management of compacted soils which may cause yield loss (Hargreaves et al., 2019). Despite the aforementioned merits of the VESS method, the major drawback is the lack of information on features of soil layers below the 25 cm depth of soil surface where severe compaction might occur in conventionally tilled soils. Therefore, Emmet- Booth, Forristal, Fenton, Bondi, and Holden (2019) pointed out that VESS may not always pro-vide reliable information on soil structural quality related to the effects of agricultural practices and suggested to examine at least 40 cm depth.

Porosity, size, shape and strength of aggregates are the most common attributes determined in soil structural qual-ity assessments (Guimarães et al., 2017). The VESS methods focus on size, strength and shape of soil aggregates and root-ing density, which have significant effects on water storage and transport, root development and nutrient uptake (Ball et al., 2017). The VESS results have been compared with penetration resistance (Guimarães et al., 2017), bulk den-sity (Newell- Price, Whittingham, Chambers, & Peel, 2013), aggregate stability and soil water (Moncada et al., 2017) to determine their reliability. Structural quality of soils signifi-cantly influences root distribution, water and nutrient uptake, which ultimately determines plant productivity (Cherubin et al., 2016). Therefore, soil structural quality assessment under different tillage systems could provide useful insights to evaluate the effectiveness of methods on productivity function of soils (Munkholm et al., 2013). In this study, the VESS, mean weight diameter, penetration resistance, poros-ity, organic carbon and organic carbon/clay ratio were used

TABLE 1

Monthly and annual average temperature, evaporation and rainfall data of the study area during experimental period (2006–2017)

January February March April May June July August September October November December Annual

Mean air tempera

-ture (°C) 9.4 11.3 14.5 18.2 22.1 26.2 28.9 29.7 26.8 22.2 16.0 11.1 19.7 Total evaporation (mm) 42.1 54.9 90.5 121.2 162.5 206.9 235.6 225.0 169.4 117.0 70.4 46.6 1,542.2 Total rainfall (mm) 90.0 85.4 69.4 47.2 53.1 21.7 7.0 4.1 33.5 41.1 61.7 111.9 626.0

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in soil structural quality assessment. The ultimate objectives of this study were to assess the effects of different long- term (2006–2017) tillage systems on soil structural quality and compare soil structural quality scores obtained through the VESS method with established field and laboratory- based measurements of soil physical properties.

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MATERIALS AND METHODS

2.1 _|

Study area and details of tillage

experiment

The study was conducted on Agricultural Experimental Station of the Cukurova University, Adana, Turkey. The

soil of the experimental field is classified as a fine, smec-titic, active, mesic Typic Haploxerert (Soil Survey Staff, 1999), with a clayey texture (50% clay, 32% silt and 18% sand) in 0–30 cm depth and the soils formed over old ter-races of Seyhan River. The mean pH, electrical conduc-tivity and calcium carbonate were 7.82, 0.15 ds m−1 and 244.0 g kg−1_{in 0–30 cm soil depth, respectively (Celik et al.,} 2011). The climate is Mediterranean and mean annual tem-perature between 2006 and 2017 was 19.7°C, precipitation was 626.0 mm, about 75% of which falls during the winter and spring, and the annual potential evapotranspiration was 1,542.2 mm (Table 1; Appendix 1). High temperatures and low rainfall in summer months (June–August) cause high evapotranspiration (667.5 mm) in the region. Therefore, crop

TABLE 2 Summary of tillage methods and equipment used in the experiment

Tillage methods Soil tillage for winter wheat Total weight of equipment (kg) Soil tillage for second crop maize and soybean Total weight of equipment (kg)

Conventional tillage with

stubbles (CT- 1) • 2nd crop stover chopping• Moldboard plough

(30–33 cm)a

• Disc harrow (13–15 cm, 2 passes)

• Floating (2 passes) • Drill (4 cm)

30,225 • Wheat stubble chopping

• Heavy tandem disc harrow (18-20 cm)

• Disc harrow (13–15 cm, 2 passes) • Floating (2 passes)

• Planter (8 cm)

29,865

Conventional tillage with

stubbles burned (CT- 2) • 2nd crop stover burning• Moldboard plough

(30–33 cm)

• Disc harrow (13–15 cm, 2 passes)

25,585 • Wheat stubble burning

• Chisel plough (35–38 cm) • Disc harrow (13–15 cm, 2 passes) • Floating (2 passes)

• Planter (8 cm)

24,888

Heavy disc harrow

re-duced tillage (RT- 1) • 2nd crop stover chopping• Heavy tandem disc harrow

(18–20 cm, 2 passes) • Floating (2 passes) • Drill (4 cm)

• Rotary tiller (13–15 cm) • Floating (2 passes) • Planter (8 cm)

21,485

Rototiller reduced tillage

(RT- 2) • 2nd crop stover chopping• Rotary tiller (13–15 cm)

• Rotary tiller (13–15 cm) • Floating (2 passes) • Planter (8 cm)

21,485

Heavy disc harrow zero

soil tillage (RT- 3) • 2nd crop stover chopping• Heavy tandem disc harrow

(18–20 cm) • Floating (2 passes) • Drill (4 cm)

• Herbicide treatment • No-till planter (8 cm)

12,955

No- till or zero tillage (NT) • 2nd crop stover chopping

• Herbicide treatment • No-till drill (4 cm)

12,955

Strategic tillage (ST)b _{• 2nd crop stover chopping}

• Herbicide treatment • No-till drill (4 cm)

12,955

a_{Figures in parenthesis are average working depth of the equipments.}

b_{This treatment continued as NT from 2006 until November 2015. Afterwards, it was tilled with moldboard plough only once in November 2015 and then, the same}

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production without irrigation in summer is not possible in the Mediterranean region. Similar to Cukurova Plain where the experimental field is located, Rana and Katerji (2000) stated that 75% of the available water in the Mediterranean region is used in crop production.

The experiment was laid out according to a randomized complete block design with three replications, and the ex-periment was initiated in 2006 with six tillage systems. The land had been used for wheat production about 50 years prior to the experiment. During wheat production, field had been deeply ploughed by a moldboard plough and stubbles were burnt in most of the years. In the experiment, the tillage sys-tems were comprised of two conventional (CT- 1 and CT- 2), three reduced (RT- 1, RT- 2 and RT- 3) and two no- till (NT and ST) systems. The half of NT plots (240 m2_{) were tilled with} moldboard plough only once in November, 2015 to a create strategic tillage (ST) system. Reduced and no- till systems are considered as conservation tillage systems. The details of till-age systems are presented in Table 2. The tilltill-age plots were 12- m wide and 40- m long (480 m2_{) with a 4 m buffer zone} between the plots.

Winter wheat (Triticum aestivum L.)–corn (Zea Mays L.) and wheat–soybean (Glycine max. L.) rotations were established in all treatments on alternate years. A non- selective herbicide (500 g ha−1_{glyphosate) was used to} con-trol weeds in conservation tillage systems 2 weeks prior to sowing. Fertilizers were applied in the seedbed at rates of 172 kg N ha−1 and 55 kg P2O5 ha−1 for wheat, 250 kg N ha−1 and 60 kg P2O5 ha−1 for corn and 120 kg N ha−1 and 40 kg P2O5 ha−1 for soybean. The wheat was sown in the first week of November and harvested in the first week of June. Wheat in all treatment systems was harvested using a small plot harvester (HEGE 125) with a working width of 1.35 m. Average precipitation and evaporation in June were 21.7 and 206.9 mm, respectively. Soils were mostly very dry at the time of wheat harvest, and irrigation (90 mm) was needed to help emergence of the second crops. The corn and soy-bean were sown in the third week of June and harvested in the second week of October. Average precipitation and evap-oration in November were 61.7 and 70.4 mm, respectively. Precipitation was always enough for the germination of wheat seeds. Corn and soybean were sprinkler- irrigated once in every 13- day (six times) during the growing period, and no irrigation water was applied to the wheat.

2.2 _|

Soil sampling and analyses

Disturbed and undisturbed soil samples were collected at three locations in each plot from 0–10, 10–20 and 20–25 cm depths after the harvest of wheat in June 2017. All analyses were performed in three soil samples. The organic carbon (OC) content in disturbed samples was determined using the Walkley–Black Method (Nelson & Sommers, 1996).

Bouyoucos hydrometer method was used to determine the particle size distribution (Bouyoucos, 1962). Undisturbed soil samples were taken by using a steel core sampler of 100 cm3_{volume. Total porosity (TP) was determined in} undisturbed water- saturated samples assuming no air trapped in pores. Microporosity (MiP) (radius < 4.5 μm) was determined from the volumetric water content, using a pressure membrane apparatus at field capacity (−33 kPa). Macroporosity (MaP) (radius > 4.5 μm) was calculated as the difference between TP and MiP (Danielson & Sutherland, 1986).

Mean weight diameter (MWD) of water stable aggregates was measured in disturbed and air- dried samples by using an instrument similar in principle to the Yoder wet sieving ap-paratus containing 4.0, 2.0, 1.0 and 0.5 mm meshes. After sieving through an 8- mm sieve, 50 g soil sample was placed on the first sieve (4 mm) and slowly moistened to avoid a sud-den rupture of aggregates. Moistened soil was sieved in dis-tilled water, and soil above each sieve was dried after 10 min of 30 oscillations per minute (along a 4 cm amplitude). The resistant aggregates on each sieve were dried at 105°C for 24 hr, weighed and corrected for the sand fraction to obtain the proportion of the true aggregates. The method modified by Kemper and Rosenau (1986) was used to calculate the MWD as follows:

where, Xi is the mean diameter of each size fraction (mm) and

Wi is the percentage of total sample mass in the

correspond-ing size fraction after the deduction of sand mass.

Penetration resistance (PR) (0–25 cm) was measured at the same time of soil sampling by a hand- pushing electronic cone penetrometer (Eijkelkamp Penetrologger 06.15.SA) fol-lowing ASAE standard procedures (ASAE, 1994), using a cone with 1 cm2_{base area and 80 cm driving shaft; readings} were recorded at 10 mm intervals. Twelve PR measurements in each plot were performed at around the small pits (four for each pit) used for VESS assessments. Volumetric soil mois-ture content was determined on the field using a time- domain reflectometry (TDR) (Soil Moisture 6050X3K1B- MiniTrase Kit) along with PR measurements. High clay and water con-tents limit the use of TDR; therefore, TDR readings must be calibrated to the studied soil to obtain more realistic volu-metric water content (Acar, Çelik, & Günal, 2017; Stangl, Buchan, & Loiskandl, 2009). The calculated coefficient of determination (R2 = 0.81) for water content measurements ob-tained with TDR was comparable to that (R2_{= 0.84) reported} by Dasberg and Dalton (1985) and lower than those (ranging from R2_{= 0.91–0.99) determined by Zanetti, Cecílio, Silva,} and Alves (2015). The TDR readings have been converted to

MWD =

n

∑

i=1

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FIGURE 1 Soil structures defined during VESS assessment for different tillage systems [Colour figure can be viewed at wileyonlinelibrary. com]

CT-1

RT-1 RT-2 RT-3

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real moisture contents using the following equation created for the experimental soil.

x = Volumetric moisture determined by the TDR instrument; y = Calibrated volumetric moisture content

2.3 _|

Visual evaluation of soil structure

The VESS method is a modification of the Peerlkamp spade test (Peerlkamp, 1959), and the sizes, shapes and visible pores of broken soil fragments and aggregates are the main features considered in evaluation. The VESS scoring was carried out according to the guideline provided by Guimarães, Ball, and Tormena (2011) on three locations in each treatment plot. The VESS assessments were conducted after wheat har-vest in June 2017 at the same day of soil samplings. A soil block of 20 × 10 cm to 25 cm depth was sampled for VESS (Figure 1). The depth of each layer with different structural features was recorded. Structure and characteristics of each layer were compared to the VESS scoring chart to give one of five soil structural quality scores. Summary of descrip-tions for the VESS scoring categories from 1.0 (good soil structural quality) to 5.0 (the worst soil structural quality) has been presented in Ball et al. (2017). The scores were as-signed for the layers identified according to the VESS chart, and then weighted average of scores were calculated for the soil layers.

Each layer was identified and scored based on the criteria presented in VESS description charts. The weighted scores for 0–10, 10–20 and 20–25 cm depth intervals and 0–25 cm depth were calculated using the following equation (Cherubin et al., 2017).

where, VESS is the weighted average VESS score, Sqi and Ti are the score and thickness of each identified soil layer, respec-tively and TT is the total thickness of soil sample.

Pedological description of soil structures (Soil Survey Division Staff, 2017) was visually compared with the VESS scores using the VESS chart. Granular structure cor-responds to the best quality score (1.0) and composed of fine aggregates which are needed in the seedbed to ensure good seed- soil contact for moisture uptake. Sub- angular blocky structure is compatible with the VESS score of 2.0 and 3.0 depending on the size of aggregates. All very fine (<5 mm) and most of fine (5 to <10 mm) sub- angular blocky structures were defined with 2.0 while all other

sub- angular blocky structures were scored as 3.0. Large (>10 cm) angular blocky or platy structures with few mac-ropores and cracks were scored as 4.0. Very compacted massive structure was scored as >4.0.

2.4 _|

Statistical evaluations

A two- way analysis of variance (ANOVA) was conducted to test the significance in the data. Tillage was the main factor, whereas depth was regarded as subfactor. Duncan's multiple range test at 95% probability was used as post hoc where ANOVA indicated significant differences. Correlations between VESS scores and PR, MWD, MiP, MaP, TP, OC, OC:clay ratio and crop yields were also tested using Spearman's correlation coefficients at p < 0.05 level of significance. Principal component analysis (PCA) was con-ducted to reveal multiple relationships among soil structural quality indicators and crop yields. All statistical analyses were performed using the SPSS statistical package (version 21.0, SPSS Inc.).

3 _|

RESULTS AND DISCUSSION

Four types of soil structures namely granular, sub- angular blocky, angular blocky and platy were identified under seven tillage systems. Surface layers in all systems except NT have been harrowed at 13–15 cm before seeding in order to make the structure very fine for a good seed- to- soil contact. Higher organic matter content, root density and biological activity in the surface horizons (Celik et al., 2011) likely caused the for-mation of granular aggregates which correspond to a ‘good’ soil structural quality.

3.1 _|

Visual evaluation in soil structural

quality assessment

Tillage systems had a significant (p < 0.01) effect on VESS, PR, MWD, OC:clay ratio and MaP (except for 0–10 cm, p < 0.05; Figures 2–4; Table 3). Although clay soils par-ticularly low in OC:clay ratio are considered highly sus-ceptible to compaction, Johannes, Weisskopf, et al. (2017) indicated that clay soils also have a higher potential to form a good and crumbly structure. The lower VESS scores under CT treatments compared to RT and NT systems in-dicated a reasonably good soil structural quality despite the increase in tillage density (Figure 2). In contrast to our findings on good soil structural quality under CT systems, Tuchtenhagen, Lima, Bamberg, Guimarães, and Mansonia (2018) reported better soil structural quality with the ab-sence of tillage under native grass compared to CT system. Better soil quality under native grassland was attributed to

y = 1.049x − 3.0839 R2= 0.81 VESS = n ∑ i=1 SqiTi TT

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the lack of tillage, high plant residue and higher organic matter content in surface layers. The pressure exerted dur-ing plantdur-ing, fertilizer and herbicide application and har-vest caused higher VESS scores in NT compared to CT (Table 2 and Figure 2). Although better soil structural qual-ity was defined under CT system compared to NT by the VESS method, this should not mean that CT system has no detrimental impact on soil structural quality. Because, the VESS method assesses soil structural quality only up to 25 cm depth; hereby, the VESS cannot capture degradation in structural features below 25 cm depth (Ball et al., 2015) and may not suggest reliable management requirement

(Emmet- Booth et al., 2019). Therefore, methods such as SubVESS (Ball et al., 2015, 2017) or DS (Emmet- Booth et al., 2019) considering below 25 cm depth are needed to assess the structural quality of conventionally tilled soils. Moreover, the beneficial effect of tillage as indicated by low VESS scores will prevail only for the short term due to the adverse impact of breaking aggregates on the loss of organic carbon (Cherubin et al., 2017).

The OC:clay ratio was proposed as a pertinent criterion when considering structural quality of soils (Johannes, Matter, et al., 2017), due to the contribution of organic carbon (OC) to resistance and resilience of soil structure. Our results are con-tradicting with the criterion of Johannes, Matter, et al. (2017) who recommended that a ratio of 1:8 (OC:clay) indicates an optimum soil structural quality, 1:10 a reasonable soil struc-tural quality, and <1:13 represents most likely unacceptable soil structural quality, which needs improvement. Despite the good structural quality indicated by VESS scores, the OC:clay ratio under all tillage systems within 0–25 cm depth was lower than 1:13 (Table 3) which corresponds to a bad soil structural quality for all tillage systems. The discrepancy of OC:clay ratio criteria with our results is related to the differ-ences in clay and OC contents of studied soils. Clay content in our experimental site was 50%, whereas clay content of soils in Johannes, Matter, et al. (2017) ranged from 9.9% to 34.3% (mean value of 20.5%). In order to obtain a reasonably good structural quality, OC content in experimental site should be at least 5 g/100 g which may not be reached at all in the stud-ied region. However, low OC:clay ratio after the long- term RT and NT systems indicated that structure is still vulnerable and longer period is needed to obtain a stable soil structural quality.

Overall VESS scores for 0–25 cm depth under CT sys-tem changed from 1.0 (first layer) to 3.0 (second or third layer) which did not indicate any concern for soil structural

FIGURE 2 Visual evaluation of soil structure under different

soil tillage treatments. Lower case letters indicate significant differences (p < 0.01). Error bars indicate standard error. CT- 1, conventional tillage with stubbles; CT- 2, conventional tillage with stubbles burned; NT, no- till or zero soil tillage; RT- 1, heavy disc harrow reduced tillage; RT- 2, rototiller reduced tillage; RT- 3, heavy disc harrow zero soil tillage; ST, strategic tillage [Colour figure can be viewed at wileyonlinelibrary.com]

4 Tillage: p < 0.01

Depth: p < 0.01 Tillage*Depth: p < 0.01

Visual evaluation of soil structure

3 2 1 0 CT-1 CT-2 RT-1 RT-2 0–10 cm 10–20 cm 20–25 cm RT-3 NT ST

FIGURE 3 Penetration resistance (MPa) under different soil

tillage treatments. Lower case letters indicate significant differences (p < 0.01). Error bars indicate standard error. CT- 1, conventional tillage with stubbles; CT- 2, conventional tillage with stubbles burned; ns, not significant; NT, no- till or zero soil tillage; RT- 1, heavy disc harrow reduced tillage; RT- 2, rototiller reduced tillage; RT- 3, heavy disc harrow zero soil tillage; ST, strategic tillage [Colour figure can be viewed at wileyonlinelibrary.com] 4 _{Tillage: p < 0.01} Depth: p < 0.01 Tillage*Depth: ns 3 2

Penetration resistance (MPa)

1 0

CT-1 CT-2 RT-1 RT-2

0–10 cm 10–20 cm 20–25 cm

RT-3 NT ST

FIGURE 4 Mean weight diameter (mm) of water stable

aggregates under different soil tillage treatments. Lower case letters indicate significant differences (p < 0.01). Error bars indicate standard error. CT- 1, conventional tillage with stubbles; CT- 2, conventional tillage with stubbles burned; NT, no- till or zero soil tillage; RT- 1, heavy disc harrow reduced tillage; RT- 2, rototiller reduced tillage; RT- 3, heavy disc harrow zero soil tillage; ST, strategic tillage [Colour figure can be viewed at wileyonlinelibrary.com]

1.0

Mean weight diameter (mm) of water stable aggregates

0.8 0.6 0.4 0.2 0.0 CT-1 CT-2 RT-1 RT-2 0–10 cm 10–20 cm 20–25 cm RT-3 NT ST Tillage: p < 0.01 Depth: p < 0.01 Tillage*Depth: p < 0.01

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quality (Appendix 2). Similar VESS scores confirming good soil structural quality in upper 25 cm depth have been re-ported by Emmet- Booth et al. (2019) who also stated that the VESS scores between 25–40 cm depth indicated degradation in soil structure at the majority of sites. In contrast to the VESS scores in CT, VESS scores under RT and NT reached to 4.0 (second or third layer) and showed a decrease of total porosity indicating some changes needed to prevent further degradation of soil structural quality (Figure 2, Appendix 2

and Table 3). Ball et al. (2017) pointed that soil layer with soil structural quality between 1.0 and 2.9 does not limit plant growth. Despite the non- significant differences be-tween the VESS scores, all soils under different tillage sys-tems had a reasonably good soil structural quality (<2) in the upper 5–7 cm depth. Harrowing in surface horizons prior to each seeding operation led to the lower VESS values in the CT systems. The weighted mean VESS scores of surface soils (0–10 cm) for the CT systems were 1.08 (CT- 1) and

TABLE 3 Mean porosity, MaP/MiP, OC and OC:clay values for different tillage treatments

Soil Tillage MiP (cm3_cm−3₎ _{MaP (cm}3_cm−3₎ _{TP (cm}3_cm−3₎ _MaP/MiP _{OC (%)} _OC:Clay

0–10 cm

CT- 1 0.392#_±0.01ab&_ns _{0.132 ± 0.02ab ns} _{0.523 ± 0.01ab ns} _{0.34 ± 0.04ab ns} _{0.83 ± 0.01d**} _{1:35 ± 0.00d**}

CT- 2 0.386 ± 0.01b ns 0.135 ± 0.01ab * 0.521 ± 0.01ab ns 0.35 ± 0.03ab* 0.73 ± 0.02e** 1:40 ± 0.00e**

RT- 1 0.396 ± 0.01ab ns 0.131 ± 0.01ab** 0.528 ± 0.00ab** 0.34 ± 0.04ab** 1.16 ± 0.02c** 1:25 ± 0.00c**

RT- 2 0.410 ± 0.00ab* 0.104 ± 0.01b ns 0.513 ± 0.01b** 0.25 ± 0.02b ns 1.28 ± 0.01b** 1:22 ± 0.00ab**

RT- 3 0.406 ± 0.01ab ns 0.118 ± 0.01b ** 0.523 ± 0.00ab** 0.29 ± 0.02ab* 1.38 ± 0.02a** 1:20 ± 0.00a**

NT 0.418 ± 0.01a ns 0.109 ± 0.01b ns 0.527 ± 0.01ab** 0.26 ± 0.02b ns 1.41 ± 0.02a** 1:20 ± 0.00a**

ST 0.393 ± 0.01ab ns 0.153 ± 0.01a* 0.546 ± 0.01a** 0.39 ± 0.04a* 1.25 ± 0.05b** 1:23 ± 0.00bc**

ANOVA ns p < 0.05 ns p < 0.05 p < 0.01 p < 0.01

10–20 cm

CT- 1 0.395 ± 0.01c 0.118 ± 0.01a 0.513 ± 0.01b 0.30 ± 0.02a 0.75 ± 0.02c 1:38 ± 0.00c

CT- 2 0.397 ± 0.00bc 0.110 ± 0.01ab 0.507 ± 0.01b 0.28 ± 0.03ab 0.64 ± 0.02e 1:45 ± 0.00d

RT- 1 0.415 ± 0.01a 0.080 ± 0.00c 0.495 ± 0.01b 0.19 ± 0.01c 0.71 ± 0.01d 1:41 ± 0.00c

RT- 2 0.399 ± 0.00abc 0.095 ± 0.01bc 0.494 ± 0.01b 0.24 ± 0.02bc 0.85 ± 0.02b 1:34 ± 0.00b

RT- 3 0.401 ± 0.00abc 0.093 ± 0.01bc 0.494 ± 0.01b 0.23 ± 0.01bc 0.76 ± 0.01c 1:38 ± 0.00c

NT 0.403 ± 0.00abc 0.089 ± 0.00bc 0.493 ± 0.00b 0.22 ± 0.01c 0.79 ± 0.01c 1:37 ± 0.00c

ST 0.414 ± 0.01ab 0.124 ± 0.00a 0.538 ± 0.01a 0.30 ± 0.01a 1.09 ± 0.01a 1:27 ± 0.00a

ANOVA ns p < 0.01 p < 0.01 p < 0.01 p < 0.01 p < 0.01 20–25 cm CT- 1 0.407 ± 0.01a 0.093 ± 0.00b 0.500 ± 0.01ab 0.23 ± 0.02b 0.70 ± 0.01bc 1:43 ± 0.00bc CT- 2 0.395 ± 0.01a 0.092 ± 0.00b 0.487 ± 0.01bc 0.23 ± 0.01b 0.61 ± 0.01d 1:47 ± 0.00bc RT- 1 0.403 ± 0.00a 0.090 ± 0.00b 0.492 ± 0.01abc 0.22 ± 0.00b 0.60 ± 0.01d 1:48 ± 0.00c RT- 2 0.389 ± 0.00a 0.084 ± 0.01b 0.473 ± 0.01c 0.22 ± 0.02b 0.67 ± 0.01c 1:43 ± 0.00bc RT- 3 0.396 ± 0.01a 0.092 ± 0.00b 0.488 ± 0.01abc 0.23 ± 0.01b 0.69 ± 0.02bc 1:42 ± 0.00bc

NT 0.398 ± 0.01a 0.096 ± 0.01b 0.494 ± 0.01abc 0.24 ± 0.02ab 0.73 ± 0.02b 1:41 ± 0.00b

ST 0.398 ± 0.00a 0.113 ± 0.00a 0.510 ± 0.00a 0.28 ± 0.01a 0.93 ± 0.03a 1:32 ± 0.00a

ANOVA ns p < 0.01 p < 0.05 p < 0.05 p < 0.01 p < 0.01

Tillage ns p < 0.01 p < 0.01 p < 0.01 p < 0.01 p < 0.01

Depth ns p < 0.01 p < 0.01 p < 0.01 p < 0.01 p < 0.01

Tillage x

Depth ns ns ns ns p < 0.01 p < 0.01

Note: Changes with depth obtained in Duncan test. #, mean value for three replicates; ±, standard error of the mean; &, differences in the same column are presented with the same letters (Duncan, p ≤ 0.05).

Abbreviations: CT- 1, conventional tillage with stubbles; CT- 2, conventional tillage with stubbles burned; MaP, macroporosity; MiP, microporosity; ns, not significant; NT, no- till or zero soil tillage; OC, organic carbon; RT- 1, heavy disc harrow reduced tillage; RT- 2, rototiller reduced tillage; RT- 3, heavy disc harrow zero soil tillage; ST, strategic tillage; TP, total porosity.

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1.13 (CT- 2) which were significantly lower than those re-corded under NT system. The seeder used in NT system was equipped with a disc and shanks reaching to 7–8 cm, to open a long narrow trench and lay fertilizer and seeds. This su-perficial disturbance along with plant residues eliminates the compaction in surface layers. However, the highest weighted mean VESS scores in surface horizons were obtained for NT as 2.63 (Figure 2 and Appendix 2).

The agricultural practices from tillage to harvest in RT and NT systems had detrimental impacts on soil structural quality of subsurface horizons which were clearly detected in field survey and reflected in VESS scores (Figures 1 and 2). Weak soil structures were defined in subsurface layers of RT and NT and relatively friable structure for CT systems. Despite the slight differences between NT and RT systems in soil struc-tural quality values at 10–20 cm depth, the VESS scores for the RT and NT systems were statistically similar indicating a comparable soil quality after 11 years of crop production (Figure 2). Continuous pressure of machineries during seed-ing and harvestseed-ing and absence of tillage under NT system resulted in significantly higher VESS scores in subsurface lay-ers compared to CT systems. Similarly, Johannes, Weisskopf, et al. (2017) pointed out that the scores between 3.0 and 4.0 in-dicate loss of structural quality due to the mechanical stresses. The mean VESS scores ranged from 1.78 (CT- 2) to 2.94 (NT) in 10–20 cm and from 2.13 (CT- 2) to 3.29 (NT) in 20–25 cm. Ball et al. (2017) reported that mean VESS score of ≥3.6 in-dicating the need for some management changes to prevent the further deterioration of soil structural quality. Although weighted mean VESS scores were < 3.6, individual higher VESS scores (≥3.6) were recorded in most of the subsurface horizons in RT and NT systems. The compacted subsurface layers in RT and NT systems were also characterized by dense large angular and hard to break clods (Figure 5). In compatible

with our findings, Hargreaves et al. (2019) stated that large, angular aggregates in the top 0–10 cm of the trampled soil are the signs of compaction. Non- porous angular structures in subsurface layers were scored as 3.0 by Castioni et al. (2018). Similar to our findings, Garbout, Munkholm, and Hansen (2013) have also reported poorer soil structure with a firm structure below topsoil for direct drilling (2.9) and a relatively friable structure (2.2) for ploughing.

Both the compacted layer and compaction depth had sig-nificant spatial dependence even within a single experimental plot. Considering the individual layer scores, the compacted layers (≥3.5) were observed at 13 and 18 cm depths in some locations of RT- 1, 10, 14, 17, 18, 19 and 20 cm depths in RT- 2, 16, 19 and 21 cm depths in RT- 3 and 8, 13 and 19 cm depths in NT treatment (Appendix 2). High VESS scores at soil surface indicate a possible surface compaction which may retard plant growth at early stages. Ball et al. (2017) also identified highly compacted layers with ≥4.0 at different depths, and compaction of surface layers was attributed to the pressure applied by grazing, and subsurface compaction (below 15 cm) was accredited to the stress exerted by tractor wheels. However, due to variation of compaction depth even within a single treatment plot, the differences in compaction depths in this study cannot be solely linked to the weights and pressure of equipment used during crop production (Table 2; Appendix 2). Despite the effectiveness of VESS in locating the compacted layer, spatial dependence in occurrence and position of compaction even under the same tillage treatment complicates to provide specific information for the manage-ment of soil compaction (Appendix 2).

Local loosening due to the superficial working depth in NT during planting was not enough to mitigate the compaction of subsurface layers (Çelik et al., 2019). Therefore, half of NT plots were deep ploughed to remove the compaction and promoted the desired physical conditions. The weighted mean VESS score was significantly reduced from 1.38 to 1.11 in 0–10 cm, from 2.94 to 2.32 in 10–20 cm and from 3.29 to 2.31 in 20–25 cm depth by introducing the ST (Figure 2). The ST treatment has also led to a marked decrease in the PR as in VESS values (Figure 3) and increase in MaP and TP (Table 3). The results confirmed that the VESS method is sensitive to the changes in soil structure due to the differences in tillage intensity.

3.2 _|

Penetration resistance in soil structural

quality assessment

Reduced or absence of tillage significantly increased PR of soils. Despite the excess stress of equipment used in CT sys-tems (Table 2), the lowest PR values were observed under CT- 1 and CT- 2. The PR measurements were carried out after the wheat harvest in June which is characterized by low pre-cipitation and high evaporation (Table 1; Appendix 1). The PR values changed between 0.93 (CT- 1) and 3.01 MPa (NT)

FIGURE 5 Large blocky structure with cracks, earthworm holes

under reduced tillage (RT- 2, 17–25 cm and Sq = 4.0) [Colour figure can be viewed at wileyonlinelibrary.com]

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within 0–10, 10–20 and 20–25 cm layers (Figure 3). The de-gree of compaction under CT systems seemed not critical for plant root development. Moraes et al. (2014) suggested to in-crease the commonly used 2 MPa critical limit for penetration of roots in the absence of cracks and root channels to 3 MPa for chisel ploughing of high clayey soil in RT and to 3.5 MPa in NT system. Hampering of root development through subsoil compaction indicated by high VESS and PR values (Figure 2 and 3) resulted in significantly lower wheat and soybean yields under NT system (Table 5) which do not support the sugges-tions of Moraes et al. (2014). Therefore, critical limit for root penetration as well as sustainable crop growth should be con-sidered as <2 MPa regardless of tillage systems.

The validity of VESS method has been supported by other quantitative physical attributes. Significant positive correla-tions (p < 0.01) of PR measurements with VESS scores at three depths show the effectiveness of VESS method in detect-ing compacted layers (Table 6). Similar to our results, signif-icant relationships have been reported between VESS scores and physical soil quality attributes for different types of soils (Castioni et al., 2018; Cherubin et al., 2017; Tuchtenhagen et al., 2018). The PCA analysis using the varimax rotation and components extraction with Eigenvalues ≥1.0 yielded four principal components (PCs) that collectively accounted for the 80.64% variance in data (Table 4). The first two PCs accounted for 57.01% variance in the data. VESS scores and corn yield were negatively correlated with PC1, whereas MaP, TP, OC and OC:clay ratio were positively correlated with the PC1. The second PC was positively influenced by PR and MWD (Table 4). There was no variable positively or negatively as-sociated with PC3, whereas PC4 was positively correlated

with MiP. These results indicated that VESS, MiP, MaP, TP, OC, corn, PR, OC:clay and MWD probably influenced the variation in the data. Regarding the contribution of observed variables towards variance in the data, OC and OC:clay ratio had the highest contribution followed by TP and MaP in PC1. Similarly, PR and MWD had the highest contribution towards observed variability in PC2. It can be concluded that variables with the highest contribution towards PC1 and PC2 are the major contributors towards VESS scores.

Higher VESS values have been ascribed by higher com-paction and greater resistance of aggregates to break into small pieces (Tuchtenhagen et al., 2018). The compatibility of high and low VESS scores with PR values is an import-ant indicator for the reliability of VESS method in assessing the structural soil quality. Similar to the high VESS scores in RT and NT systems in our experiment, clayey surface soils under long- term NT system in Brasil were identified with 4.0 scores (Tuchtenhagen et al., 2018). Contrary to the reports of Johannes, Matter, et al. (2017) on positive correlation between OC:clay ratio and soil structural quality, we have observed that OC:clay ratio decreased with increasing soil structural quality.

3.3 _|

Mean weight diameter in soil

structural quality assessment

Intensive and repeated tillage operations under CT systems reduced the aggregate size represented by low MWD, which lead to lower the VESS scores. In contrast, decreasing or avoiding the disturbance significantly increased the MWD (Figure 4), and larger aggregates were found under RT and NT systems (Figure 1). The MWD ranged from 0.26 (CT- 2)

TABLE 4 Factor loadings of principal component analysis and contribution of the observed variables towards cumulative variability

Variables

Factor loading Contribution of variables (%)

PC1 PC2 PC3 PC4 PC1 PC2 PC3 PC4

Penetration resistance 0.10 0.88 −0.01 −0.23 0.27 28.30 0.01 4.58

VESS −0.61 0.58 −0.19 −0.31 10.43 12.25 2.46 8.23

Mean weight diameter 0.26 0.91 0.05 −0.06 1.93 30.29 0.15 0.32

Microporosity 0.20 0.32 −0.45 0.78 1.11 3.73 14.35 51.81 Macroporosity 0.66 −0.49 0.07 −0.46 12.14 8.89 0.31 18.16 Total porosity 0.81 −0.27 −0.26 0.09 18.66 2.73 4.67 0.71 Wheat −0.39 −0.26 0.55 0.09 4.19 2.50 21.34 0.70 Corn −0.60 −0.19 0.42 0.38 10.11 1.27 12.60 12.61 Soybean 0.15 −0.43 −0.55 −0.08 0.66 6.70 21.07 0.49 Organic carbon 0.87 0.22 0.36 0.13 21.25 1.81 9.18 1.41 Organic carbon/clay 0.83 0.21 0.44 0.11 19.24 1.54 13.86 0.99 Eigenvalue 3.55 2.73 1.42 1.17 Variability (%) 32.23 24.78 12.95 10.68 Cumulative variability % 32.23 57.01 69.96 80.64

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to 0.86 mm (NT) within 0–10, 10–20 and 20–25 cm layers (Figure 4). Higher aggregate sizes under RT and NT were in accordance with those reported by Hajabbasi and Hemmat (2000) who found that finer aggregates were significantly higher under CT compared to the NT and RT systems.

The MWD had significantly high positive correlations with VESS scores and PR values, while had significant nega-tive correlations with crop yields (Table 6). Contrary to what was expected, the significant positive correlations between MWD and VESS and PR may be related to the high clay and low OC content of soils studied. Since clay is an important binding agent of aggregates, MWD can be still higher at high VESS scores and PR values. Another example for the effect of OC on stabilizing the soil structure has been reported by Schjønning, Elmholt, Munkholm, and Debosz (2002) who found a dense topsoil on a low carbon soil under conventional tillage, whereas porous structure for a neighbouring soil with significantly higher OC content. The discrepancy of larger MWD and larger VESS scores can also be attributed to the shrink- swell process which is important in structure formation of clay rich soils (Pal et al., 2006), if a low OM level does not stabilize the bulk volume. Soil sampling and VESS assessment have been carried out at small water content (Table 5), mean-ing that the soil was shrunk and eventually split into pieces by cracks (Figure 1). This is also in accordance with the relative changes in MaP/MiP ratio between the treatments (Table 3).

3.4 _|

Porosity in soil structural

quality assessment

The increase in tillage intensity decreased the volume of MiP and increased the MaP in 0–20 cm depth. The lowest

(0.386 cm3_cm−3_{) and the highest (0.418 cm}3_cm−3_{) MiP values} in 0–10 cm depth were obtained in CT- 2 and NT systems, re-spectively (Table 3). The pore volume was significantly higher in the topsoil (0–10 cm) than the subsurface layers. The vol-ume of pores in most of the tillage systems decreased markedly with the depth. The MaP decreased with increasing soil depth, though decrease was only significant under CT- 2, RT- 1, RT- 3 and ST systems (Table 3). The decrease in TP with increasing VESS scores has been considered as a remarkable degradation feature (Johannes, Weisskopf, et al., 2017). Although the dif-ference in TP between tillage systems was not significant in 0–10 cm depth, significant differences were obtained for 10–20 and 20–25 cm depths. The TP in these layers slightly decreased with the decrease in tillage intensity. The highest pore volume was observed after ploughing the NT plots (ST).

The VESS may be useful indicator of compaction but it is surprising that compaction (as measured by PR) was not also reflected in the differences of MaP or TP. The VESS scores had only a relatively weak negative correlations with MaP and TP in 10–20 cm (p < 0.05) and TP in 20–25 cm (p < 0.05) depths (Table 6). This is probably related to the shrink- swell nature of experimental soil which had high clay and low organic carbon content. Lower MaP and TP val-ues are expected in compacted layers; however, both valval-ues were not low as expected in the experimental plots (Table 3). Despite the higher VESS scores and PR values in compacted layers, the formation of cracks in the shrinking process cre-ated larger pores. In contrast to our findings, Emmet- Booth et al. (2019) reported significant correlation between VESS scores and TP in 20 cm depth.

Development of a platy structure was detected below 15 cm of surface due to the use of shallow tillage equipment

Tillage methods

Moisture Wheat Corn Soybean

Volumetric, % kg ha−1 CT- 1 20.47#_{±0.27 ab}& _{5,101 ± 127 ab} _{10,186 ± 129 b} _{4,193 ± 22 ab} CT- 2 18.90 ± 0.61 b 4,857 ± 99 bc 10,813 ± 356 a 4,123 ± 49 b RT- 1 20.40 ± 0.44 ab 5,093 ± 12 ab 9,933 ± 245 b 4,310 ± 105 a RT- 2 20.25 ± 0.30 ab 4,663 ± 70 c 10,078 ± 166 b 4,071 ± 28 b RT- 3 20.43 ± 0.42 ab 5,210 ± 89 a 9,728 ± 171 b 3,893 ± 73 c NT 21.34 ± 0.51 a 4,838 ± 77 c 10,064 ± 148 b 3,878 ± 13 c ST 21.20 ± 0.88 a 4,665 ± 61 c 10,039 ± 157 b 4,294 ± 38 a ANOVA ns p < 0.01 p < 0.01 p < 0.01

Note: #, mean value for three replicates; ±, standard error of the mean; &, differences in the same column are presented with the same letters (Duncan, p ≤ 0.05).

Abbreviations: CT- 1, conventional tillage with stubbles; CT- 2, conventional tillage with stubbles burned; ns, not significant; NT, no- till or zero soil tillage; RT- 1, heavy disc harrow reduced tillage; RT- 2, rototiller reduced tillage; RT- 3, heavy disc harrow zero soil tillage; ST: strategic tillage;

*Crop yield is the mean values except for ST since it has been established in 2015. Wheat is the mean value of 11 wheat harvests, corn is the mean of six harvests, and soybean is the mean of the five harvests. The crop yields in ST are the mean values of 2 wheat, 1 corn and 1 soybean.

TABLE 5 Moisture contents of

0–30 cm depth during VESS assessments and PR measurements and mean long- term (2006–2017)* crop yields for different tillage treatments

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(heavy tandem disc and rotary tiller) under RT treatments (Figure 6). Formation of dense platy structure under RT sys-tems is in correspondence with the results of Munkholm et al. (2013) who reported compacted platy structure below tillage depth under NT and RT systems and attributed to the use of a rotary harrow in secondary tillage. Cavalieri et al. (2009) re-ported that platy structure may not always point to a degraded

structure, in contrast, these soils may be more productive than the VESS method indicated. The decrease in tillage intensity under RT systems led to the formation of compacted large angular blocky clods that were very hard, difficult to break down, non- porous and correspond to a ‘poor’ soil structural quality. Mueller et al. (2013) also stated that clayey soils may extremely be compacted and hard to break- up under RT

TABLE 6 Correlation among indicators of soil structural quality and wheat, corn and soybean yields

PR VESS MWD MiP MaP TP OC OC:Clay Wheat Corn Soybean

0–10 cm PR 1.00 VESS 0.46** 1.00 MWD 0.86** 0.53** 1.00 MiP 0.24 0.11 0.33* 1.00 MaP −0.20 −0.27 −0.32* −0.73** 1.00 TP 0.16 −0.05 0.10 0.10 0.52** 1.00 OC 0.79** 0.43** 0.85** 0.27 −0.23 0.12 1.00 OC:clay 0.75** 0.47** 0.83** 0.20 −0.21 0.05 0.96** 1.00 Wheat −0.34* −0.14 −0.31* −0.17 0.05 −0.35* −0.03 0.02 1.00 Corn −0.34* −0.14 −0.27 −0.13 −0.03 −0.34* −0.24 −0.26 0.35* 1.00 Soybean −0.27 −0.46** −0.44** −0.19 0.28 0.18 −0.50** −0.48** 0.01 −0.11 1.00 10–20 cm PR 1.00 VESS 0.67** 1.00 MWD 0.74** 0.58** 1.00 MiP 0.19 0.07 0.22 1.00 MaP −0.25 −0.37* −0.15 −0.29 1.00 TP −0.19 −0.38* −0.13 0.26 0.81** 1.00 OC 0.43** 0.36* 0.70** 0.05 0.26 0.22 1.00 OC:clay 0.41** 0.34* 0.70** 0.03 0.23 0.14 0.93** 1.00 Wheat −0.20 0.03 −0.41** −0.10 −0.33* −0.38* −0.47** −0.32* 1.00 Corn −0.15 0.07 −0.32* −0.22 −0.20 −0.33* −0.40** −0.34* 0.35* 1.00 Soybean −0.45** −0.38* −0.23 0.29 0.27 0.45** 0.01 0.01 0.01 −0.11 1.00 20–25 cm PR 1.00 VESS 0.49** 1.00 MWD 0.70** 0.39* 1.00 MiP −0.01 −0.23 −0.05 1.00 MaP 0.12 −0.13 0.15 −0.18 1.00 TP 0.02 −0.38* −0.03 0.72** 0.47** 1.00 OC 0.34* 0.07 0.43** −0.11 0.45** 0.25 1.00 OC:clay 0.27 0.03 0.37* −0.39* 0.50** 0.00 0.85** 1.00 Wheat −0.26 0.25 −0.47** −0.24 −0.10 −0.33* −0.19 0.00 1.00 Corn −0.30 0.17 −0.34* −0.15 −0.31* −0.42** −0.39* −0.24 0.35* 1.00 Soybean −0.33* −0.46** −0.13 0.15 0.18 0.23 −0.04 0.02 0.01 −0.11 1.00

Abbreviations: MaP, macroporosity; MiP, microporosity; MWD, mean weight diameter; OC, organic carbon; PR, penetration resistance; TP, total porosity; VESS, visual evaluation of soil structure.

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system. Large distinct macropores (mostly through cracks) often containing hairy roots and rare earthworms were also observed in these layers (Figure 5).

3.5 _|

Crop yield and soil structural quality

Structural quality of soils has direct or indirect impacts on crop growth and yield (Munkholm et al., 2013); thus, the in-dicators proving information on soil structural quality help to identify the problems causing variation in crop yield. The differences in wheat, corn and soybean yields between tillage practices were significant (p < 0.01) due to the obvious dif-ferences in soil structural quality. However, the difdif-ferences in the amount of yield were not very high (Table 5). Application of fertilizers slightly compensated the problems arising from the physical restraints to shallow rooted wheat growth.

The increase in tillage density and depth significantly in-creased the crop yields. Compacted subsurface layers in RT and NT systems had a negative impact on crop yield which was mostly lower under RT and NT systems compared to the CT (Table 5). Lack of tillage decreased the wheat, corn and soybean yields under NT at a rate of 5.2%, 1.2% and 7.5% compared to CT- 1 and 0.4%, 6.9% and 5.9% for CT- 2. Similar results were obtained for wheat and soybean yields comparing the CT and RT systems. The findings for higher yields under CT systems are in accordance with the results of Sharma et al. (2005) who found higher sorghum (Sorghum vulgare L.) and castor (Ricinus communis L.) yields under CT compared to RT systems. Despite the improvement in soil structure and accumulation of crop residue on surface, Hajabbasi and Hemmat (2000) reported lower grain yields under NT compared to CT and RT due to heavy texture and initial low organic matter content of soils in an arid region of Iran.

Crop yields and PR, VESS, MWD and TP values mostly had negative correlations which explain the de-creases in yields under NT system. The decrease or ab-sence of tillage in NT system caused higher VESS scores and PR values which both considered as the indicators

of compaction. However, the correlations between crop yields and soil structural properties were highly variable (Table 6). For surface layer, the highest negative correla-tion was obtained between soybean yield and VESS scores (0.46), while the correlations between soybean yield and PR and TP were not significant. In contrast, the correla-tions between wheat yield and PR, MWD and TP were significant, while the correlation between VESS scores and wheat yield was not significant. Therefore, it is very hard to make a certain conclusion to say that a particular structural property better explains the variability in crop production than others.

The results revealed that soil structural quality is not the only driver of crop production; hence, significant relation-ship may or may not be present between crop yield and soil structural quality indicators. Significantly important negative correlation between soybean yield and VESS values and lack of correlation for wheat and corn yields support the above discussion.

4 _|

CONCLUSIONS

This study was conducted to investigate the long- term ef-fects of conventional and conservation tillage systems on soil structural quality of a clayey soil using visual evaluation, field and laboratory measurements. The study also assessed for the first time the feasibility of VESS method in Turkey under various tillage practices. Eleven years of RT and NT systems resulted in the formation of compacted layers (VESS score ≥ 3.5) below seeding/tillage depths with platy and large dense angular blocky structures. Lack of tillage, pressure ap-plied by planter, fertilizer spreader, herbicide sprayer and harvest machine along with low organic carbon:clay ratio of the soil are the major causes of poor structural stability in NT plots.

The overall VESS scores and PR values for each of the tillage systems indicated that the soil structural quality is bet-ter under conventional tillage systems compared to RT and NT systems, despite the lower MWD values. However, since the VESS method assesses the structural stability of surface soils (25 cm depth), further assessments are needed to evalu-ate the structural quality below the 25 cm depth where might have potentially restrictive layers. Soils under NT system had a significantly lower soil structural quality than all other till-age systems investigated.

One- time moldboard tillage in NT systems improved soil structural quality indicated by significantly lower VESS scores and PR values; however, yield responses of such im-provement were not significant. Significant correlations be-tween VESS scores and PR values indicated the ability of VESS to detect compaction damage caused by the tillage and monitor changes in soil quality over time. Nevertheless, high

FIGURE 6 Platy structure in reduced tillage (RT- 1, 18–25 cm

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spatial variability in VESS scores and position of compacted layer hindered to provide specific information for the man-agement of compaction.

ACKNOWLEDGEMENTS

We acknowledge the financial contribution of Scientific and Technological Council of Turkey (TUBITAK), Grant Number 115 O 353.

ORCID

İsmail Çelik https://orcid.org/0000-0002-8650-2639 REFERENCES

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How to cite this article: Çelik İ, Günal H, Acar M, Acir

N, Bereket Barut Z, Budak M. Evaluating the long- term effects of tillage systems on soil structural quality using visual assessment and classical methods. Soil Use Manage. 2020;36:223–239. https ://doi.org/10.1111/ sum.12554

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APPENDIX 1

MONTHLY AVERAGE RAINFALL, TEMPERATURE AND EVAPORATION DATA OF THE STUDY AREA DURING EXPERIMENTAL PERIOD (2006–2017)

Year/

Month January February March April May June July August September October November December

Total rainfall (mm) 2006 36.3 131.6 46.2 9.3 19.8 4.5 41.3 0.0 37.4 156.3 91.5 0.0 2007 34.1 127.0 75.7 115.4 32.0 24.8 1.2 0.0 0.5 18.2 83.0 125.4 2008 32.2 55.1 25.6 18.5 19.2 4.8 0.0 9.6 16.8 31.5 45.3 58.2 2009 145.7 130.1 135.5 34.3 28.7 0.0 22.7 0.0 33.5 15.2 130.1 131.8 2010 113.9 67.6 14.8 89.3 56.6 1.4 0.7 0.0 1.7 30.8 0.0 194.5 2011 76.5 92.4 107.0 78.3 105.6 49.4 0.0 0.0 4.1 5.8 44.1 156.4 2012 262.0 122.9 57.9 21.4 79.9 17.1 14.0 0.1 0.0 63.4 128.3 298.4 2013 46.2 55.7 54.3 54.0 61.5 0.9 0.0 19.8 31.9 40.1 6.1 21.5 2014 35.7 36.5 47.7 22.1 34.9 89.8 3.5 0.2 95.4 54.9 66.5 106.4 2015 107.5 122.0 135.1 21.5 65.7 4.8 0.4 10.9 130.0 32.1 10.5 0.6 2016 138.4 83.1 67.1 36.6 87.9 45.6 0.2 8.2 39.8 0.0 11.9 216.3 2017 52.0 0.8 65.4 65.9 45.9 17.3 0.0 0.0 11.2 44.3 122.7 33.0 Mean temperature (°C) 2006 9.1 10.7 14.1 18.4 22.1 25.7 27.9 29.1 26.1 21.6 13.8 9.6 2007 9.0 11.2 14.1 16.5 23.4 26.5 29.6 29.5 26.5 23.3 15.6 10.3 2008 7.4 9.6 16.3 19.2 21.0 26.6 29.0 30.0 26.7 22.3 16.8 9.8 2009 9.3 10.7 12.6 17.1 21.9 26.9 28.3 29.1 25.3 23.5 15.1 12.7 2010 11.2 12.3 15.6 18.4 22.2 25.9 28.5 30.8 28.0 22.4 18.4 13.0 2011 10.4 11.3 13.5 16.6 21.0 25.4 28.5 29.3 27.0 21.4 12.9 10.1 2012 8.4 9.1 11.5 18.9 21.6 26.9 29.1 29.8 27.6 22.5 17.0 11.1 2013 9.9 12.5 14.8 18.6 23.6 26.1 28.6 29.2 25.8 19.9 17.7 10.3 2014 11.5 12.3 15.6 19.0 22.0 25.6 28.4 29.3 26.3 21.3 15.1 13.4 2015 9.4 11.2 14.6 16.9 22.5 25.0 28.4 30.0 28.4 23.4 17.5 11.8 2016 8.7 13.9 15.7 20.5 21.6 27.1 29.5 29.9 26.3 23.1 15.6 9.0 2017 8.7 10.7 15.2 18.5 21.8 26.2 30.4 29.9 27.8 22.2 15.9 12.6 Total evaporation (mm) 2006 44.3 37.6 67.1 94.9 179.3 212.9 212.4 225.9 178.6 101.4 56.0 55.7 2007 48.4 48.1 90.9 131.6 172.6 227.2 258.5 223.4 167.7 131.3 63.6 50.8 2008 55.9 71.5 107.2 140.8 182.0 273.9 289.0 258.5 187.4 132.8 80.4 54.2 2009 57.3 44.0 88.1 131.0 199.4 238.4 264.5 257.5 189.8 161.5 83.4 47.3 2010 42.6 77.6 108.4 146.2 179.4 217.0 211.4 244.8 200.7 107.3 101.2 64.7 2011 54.8 58.9 78.4 92.9 148.0 179.3 207.5 219.6 147.5 126.5 68.8 37.3 2012 26.6 54.2 99.3 109.0 141.5 210.5 242.8 248.0 176.6 97.9 46.6 25.1 2013 29.6 49.2 92.7 92.5 156.8 202.8 237.0 207.2 152.7 118.1 64.1 64.8 2014 41.0 57.4 100.1 137.3 157.0 178.8 219.7 199.4 145.1 104.3 58.0 32.3 2015 37.9 35.5 80.0 106.5 148.9 171.4 206.7 204.9 167.1 99.0 77.4 55.0 2016 34.1 62.8 91.9 142.7 141.9 195.4 231.8 205.3 155.3 112.7 77.9 32.6 2017 32.7 61.5 82.3 128.5 143.7 175.2 246.3 205.6 164.6 111.0 67.6 39.5

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APPENDIX 2 VESS scor

es in sam

pling points under tillag

e tr eatment Point 1 Point 2 Point 3 Point 4 Point 5 Point 6 Point 7 Point 8 Point 9 T (cm) Sq T (cm) Sq T (cm) Sq T (cm) Sq T (cm) Sq T (cm) Sq T (cm) Sq T (cm) Sq T (cm) Sq 1 _0–11 1 0–9 1 0–14 1 0–8 1 0–11 1 0–8 1 0–12 1 0–8 1 0–8 1 12–25 3 10–25 3 15–25 3 9–25 1.5 12–22 1.5 9–23 1.5 13–25 2 9–16 1.5 9–25 2 23–25 2 24–25 2 17–25 2 2 _0–4 1 0–9 1 0–5 1 0–12 1 0–12 1 0–10 1 0–7 1 0–9 1 0–9 1 5–17 2 10–18 2 20 2 13–25 1.5 13–25 1.5 11–25 2 8–16 1.5 10–16 1.5 10–25 2 18–25 2.5 19–25 3 21–25 3 17–25 2 17–25 2 1 _0–7 1 0–5 1 0–6 1 0–9 1 0–11 1 0–9 1 0–12 1 0–9 1 0–13 1 8–12 2 6–25 2 7–15 1.5 10–17 3 12–18 2 10–21 2.5 13–25 4 10–19 2.5 14–25 3 13–25 3 16–25 3 18–25 3.5 19–25 2.5 22–25 3 20–25 3 2 _0–10 1 0–9 1 0–8 1 0–8 1 0–11 1 0–7 1 0–7 1 0–6 1 0–5 1 11–19 3 10–25 3.5 9–17 2 9–16 3 12–18 2.5 8–18 2.5 8–13 2 7–25 2 6–20 2 20–25 3.5 18–25 3.5 17–25 4 19–25 3.5 19–25 3 14–25 3.5 21–25 3 3 _0–7 1 0–9 1 0–7 1 0–9 1 0–10 1 0–11 1 0–9 1 0–5 1 0–9 1 8–14 2 10–15 2 8–15 2 10–18 2 11–15 2 12–20 3 10–19 2 6–14 2 10–18 2 15–25 2.5 16–25 3 16–25 3 19–25 4 16–25 4 21–25 3.5 20–25 3 15–25 3 19–25 2.5 NT 0–8 1 0–8 1 0–9 1 0–7 1 0–8 1 0–5 1 0–9 1 0–8 1 0–7 1 9–17 2 9–18 3 10–18 2 8–25 3.5 9–19 2.5 6–12 3 10–22 3 9–22 3 8–25 3 18–25 3 19–25 4 19–25 3 20–25 3 13–25 4 23–25 3 23–25 3 ST 0–8 1 0–9 1 0–7 1 0–9 1 0–13 1 0–15 1 0–9 1 0–10 1 0–10 1 9–15 2 10–15 3 8–16 2.5 10–21 3 14–25 2 16–25 2 10–20 3 11–19 2 11–20 2.5 16–25 3 16–25 2.5 17–25 2 22–25 2 21–25 2.5 20–25 2.5 21–25 2 Abbreviations:

1, conventional tillage with stubbles;

2, conventional tillage with stubbles burned; NT,

till or zero soil tillage;

1, heavy disc harrow reduced

tillage;

2, rototiller reduced tillage;