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Article

Unearthing Soil Structure Dynamics under Long-Term No-Tillage System in Clayey Soils

by
Kopano Conferance Phefadu
1,* and
Lawrence Munjonji
1,2
1
Department of Plant Production, Soil Science and Agricultural Engineering, University of Limpopo, P/Bag X1106, Polokwane 0727, South Africa
2
Risk and Vulnerability Science Centre, University of Limpopo, P/Bag X1106, Polokwane 0727, South Africa
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(18), 13478; https://doi.org/10.3390/su151813478
Submission received: 25 July 2023 / Revised: 29 August 2023 / Accepted: 1 September 2023 / Published: 8 September 2023
(This article belongs to the Section Soil Conservation and Sustainability)
Browse Figures
Versions Notes

Abstract

:
Soil structure is a sensitive and dynamic soil physical property that responds rapidly to different tillage systems, and thus it requires constant monitoring and evaluation. The visual evaluation of soil structure (VESS) and subsoil visual evaluation of soil structure (SubVESS) methods were used to assess the soil structure quality of clayey soils subjected to different tillage systems. The tillage systems were no-tillage (NT) and conventional tillage (CT), with virgin fields (VGs) used as controls. The study was conducted at Tshivhilwi and Dzingahe in Thohoyandou, Vhembe District, Limpopo Province, South Africa. The soil structure quality at Tshivhilwi, as determined by VESS and SubVESS, was found to be poor. However, at Dzingahe, both the VESS and SubVESS scores responded to the impact of tillage. VESS showed a fair (Sq = 2.25) soil structural quality in the NT system, poor quality (Sq = 3.57) in the CT system and moderately poor quality (Sq = 3.05) in the VG. Similarly, at the same location, the SubVESS scores were moderately good in the NT system, moderately poor for the CT system and fair in the VG. The differences in the responses of VESS and SubVESS at the two locations were attributed to differences in the duration of the NT system. The VESS and SubVESS results were supported by selected measured soil physico-chemical properties such as bulk density and porosity. In conclusion, the findings of this study showed that VESS and SubVESS were able to effectively differentiate between the impacts of tillage systems on soil structural quality. The soil structure quality was better under NT than CT at Tshivhilwi and Dzingahe.

1. Introduction

Soil structure is a sensitive and dynamic soil physical property. It rapidly responds to management practices, land use changes, moisture and temperature regimes [1]. As a result, it requires frequent assessment and monitoring. It is most regularly assessed when evaluating soil quality under various tillage systems and land uses [2] and is regarded as a general soil quality indicator [3]. Soil tillage systems are the major contributors to soil structural modifications [4,5,6,7]. The resultant soil structure can influence other soil properties such as aeration, water retention, availability and movement. Therefore, assessing soil structural quality is a key component of soil quality monitoring and assessment [8].
Traditional methods used for quantifying soil structural parameters are generally expensive as they need complicated equipment. They are also time consuming and require an in-depth knowledge of soil science. Furthermore, soil structure is commonly characterised qualitatively on the basis of class, grade and type [9], which lacks detail on its quality. Considering these challenges, semi-quantitative visual soil structure evaluation methods can provide a more detailed assessment.
The primary visual methods of assessing soil structure focus on describing rooting, soil aggregates and porosity [3]. One of them is the visual evaluation of soil structure (VESS). The VESS method was developed to assess soil structural quality using a description chart to compare aggregate and root features to assign a soil quality score [10]. VESS scores reflect the effect of agricultural management practices such as tillage on soil quality [11]. Numerous methods developed for topsoil visual assessment, like VESS, put more emphasis on compaction status [12]. Where necessary, SubVESS can be used to assess the subsoil structural quality. VESS has been validated in its application together with some soil physical, chemical and biological properties and has proven to be effective in assessing soil structure quality and therefore soil quality [2,13,14,15,16,17]. VESS can enable farmers and land users to frequently assess and monitor soil quality as it is cheap, easy to execute and rapid. Despite their reported effectiveness, neither VESS nor SubVESS are commonly used in South Africa and have not been tested enough, especially on subtropical clayey soils.
Conservation (i.e., no-tillage) and conventional tillage systems may alter soil structure regardless of the texture. Tillage systems gradually modify soil physical properties, which can lead to increased soil compaction [18]. Conventional tillage temporarily encourages larger soil pores than a no-tillage system, especially in the topsoil layer [19]. Soil structural changes that result from conventional soil preparation affect bulk density, porosity, water retention and storage, aeration and aggregate stability [20]. The no-tillage system, over time, can also have a negative or positive impact on some of these parameters. The adoption of no-tillage has challenges such as soil compaction and the stratification of organic matter [21]. There is variability in the execution of these tillage systems, more especially for no-tillage; hence their impacts are not always the same.
The main purpose of this study was to assess the impact of long-term no-tillage system on soil structure quality in clayey soils. The study hypothesises that (i) the structural quality and parameters of clayey soils vary significantly across different tillage systems; and (ii) a tillage system has the same impact on soil structural quality and parameters at different locations.

2. Materials and Methods

2.1. Site Description

The study was carried out at two locations in Thohoyandou, Vhembe district, Limpopo province, South Africa. Location 1 was at Tshivhilwi (22°50′54″ S, 30°38′38″ E, 512 m above sea level), where the no-tillage field was 6 ha, with maize planted throughout the year in rotation with legumes and vegetables, and it was under irrigation. Maize was the only crop cultivated in the conventional tillage field. Location 2 was at Dzingahe (22°55′32″ S, 30°31′00″ E, 662 m above sea level); the no-tillage field was 2 ha, with the main crops being maize and ground nuts, which were intercropped under dryland conditions. Maize was the only crop cultivated in the conventional tillage field. The virgin fields at both locations were not cultivated; however, livestock belonging to the local community were allowed to graze. The frequency and intensity of grazing were unknown as the livestock were not managed or controlled. Furthermore, the virgin fields were open and the livestock were not enclosed, which allowed them to move freely. The no-tillage fields in Tshivhilwi and Dzingahe had been untilled for 8 and 40+ years, respectively, while the number of years of tillage of the conventional tillage fields was estimated to be about 50 years. Both study sites had an average annual rainfall of 762 mm, a minimum temperature of 15 °C and a maximum temperature of 28 °C.

2.2. Soil Sampling

Soil samples were collected from no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Tshivhilwi and Dzingahe. The virgin field (VG) was used as a control treatment at each location. Five sampling points were randomly selected in a portion (area = 1000 m2) of each field per location considering the homogeneity of the soil. Soil samples were dug up at the selected sampling points. The sampling depths were 0–30 cm and 30–60 cm. A total of 60 soil samples (30 topsoil and 30 subsoil) were collected from both locations. Visual (i.e., VESS and SubVESS) methods were used to assess soil structure quality in the field, and other selected soil parameters were also analysed in the laboratory to validate the outcome of the visual observations.

2.3. VESS and SubVESS

Visual assessment of soil structure quality in the field was carried out with the VESS [22,23] and SubVESS [12] methods. First, the VESS was carried out. Then, a soil pit (1 m × 1 m × 0.7 m) was dug for the SubVESS assessment. The VESS method was used to assess soil structure in the topsoil (0–30 cm) based on the key parameters, namely, aggregates, porosity and roots. Then, a score rating from Sq1 to Sq5 (Sq1–2 = good, Sq2–3 = fair, Sq3–Sq5 = poor) was assigned. The SubVESS method was used to assess soil structure in the subsoil (30–60 cm) based on key parameters, namely, mottling, strength, porosity, roots and aggregates. Then, a score rating from Ssq1 to Ssq5 (Ssq1–3 = good, Ssq4 = fair, Ssq5 = poor) was assigned.

2.4. Data Collection

Soil bulk density (BD) was determined by collecting samples with stainless steel cylindrical core samplers with an internal diameter of 5 cm and 5 cm height from each field at the 0–30 cm and 30–60 cm depths. The cylindrical cores were used to measure the bulk density as the mass in grams of the oven-dried soil per volume of core in cubic centimetres. The bulk density was then calculated using the obtained oven-dried mass of each sample and the volume of the core [24]. After calculating the BD, the pore percentage was then calculated using the bulk density values with the following formula: %porosity = 1 − B u l k   d e n s i t y P a r t i c l e   d e n s i t y × 100 ; a particle density of 2.65 g/cm3 was used. Particle size distribution was determined by the Bouyoucos method [25]. Soil organic carbon was analysed using the Walkley and Black method [26]. Soil pH was measured with a pH meter model (Lab 845 Set/BL19 pH) in a 1:2.5 (v/v) soil: water and soil: KCl solution suspensions. Soil electrical conductivity (EC) was measured (Lab 945 Set/LF435T) in a 1:2.5 (v/v) soil: water suspension [27].

2.5. Statistical Analysis

The collected data were subjected to analysis of variance (ANOVA) and multivariate analysis of variance (MANOVA) at a 95% confidence interval (p ≤ 0.05) to compare the parameters measured between the tillage systems at each location using IBM SPSS statistics 29.0 statistical software. The Pearson correlation coefficient was used to check the relationship between the parameters at each location. The means of the measured parameters in the same tillage systems at the locations were compared only using MANOVA.

3. Results

3.1. VESS and SubVESS as Influenced by Tillage System

At Tshivhilwi, the VESS (Sq) and SubVESS (Ssq) scores did not show any significant differences between the tillage systems. The soil structure quality was poor for all the tillage systems (NT: Sq = 3.53; CT: Sq = 4.12; VG: Sq = 3.67). Even though no significant differences were observed, NT had the lowest Sq score and CT had the highest. The SubVESS (Ssq) scores were also poor, with an equal score of 5 for all the tillage systems (Figure 1a). At Dzingahe, the VESS (Sq) scores varied significantly (p = 0.009) between NT and CT but there was no significant difference between the NT and VG or between the CT and VG tillage systems. The topsoil structure quality was fair (Sq = 2.25) for NT and poor for CT (Sq = 3.57) and VG (Sq = 3.05). The SubVESS (Ssq) scores did not show a significant difference between the tillage systems. Subsoil structure quality was moderately good for NT (Ssq = 3.80) and VG (Ssq = 3.60) and moderately poor for CT (Ssq = 4.20) (Figure 1b). Topsoil structure quality was better in NT than CT at both locations. Overall, the tillage systems did not have a significant effect on the soil structure quality except VESS (p = 0.03) at Dzingahe.

3.2. Soil Physico-Chemical Properties

At Tshivhilwi, the bulk density (BD) and porosity showed no significant differences between the tillage systems in the 0–30 cm soil depth. No-tillage had the lowest (1.32 g/cm3) and CT the highest (1.38 g/cm3) value. On the other hand, BD varied significantly between NT and CT (p ≤ 0.001), between NT and VG (p = 0.004) and between CT and VG (p = 0.002) in the 30–60 cm soil depth. Conventional tillage had the highest BD (1.57 g/cm3) value, followed by VG (1.39 g/cm3), and the lowest value was seen in NT (1.23 g/cm3); therefore, NT was less compacted than CT but both bulk densities were low (Figure 2a). The pore percentage ranged from 46.56 to 48.43% in the 0–30 cm soil depth. In the 30–60 cm soil depth, porosity indicated a significant difference (p < 0.05) between NT and CT and between NT and VG. No-tillage had the highest porosity (52.38%), followed by VG (45.35%), and the lowest porosity was seen in CT (40.81%) (Figure 2b). Organic carbon (OC) was non-significant in all the tillage systems in both soil depths. However, VG had the highest OC in both depths, followed by NT, and CT had the lowest score. The values ranged from 1.52 to 1.82% in the 0–30 cm soil depth and from 1.01 to 1.34% in the 30–60 cm soil depth (Figure 2c). The pH (water and KCl) was acidic (6.52–6.67 and 5.10–5.42, respectively) in all the tillage systems in the 0–30 cm soil depth, whereas in the 30–60 cm soil depth it ranged from acidic to slightly alkaline (5.45–5.67 and 6.71–7.22, respectively). The electrical conductivity ranged from 0.24 to 0.34 mS/cm for the 0–30 cm and from 0.20 to 0.32 mS/m for the 30–60 cm soil depth, and the soils were non-saline (Table 1).
At Dzingahe, BD differed significantly between NT and VG and between CT and VG in the 0–30 cm and 30–60 cm soil depths. In the 0–30 cm soil depth, the virgin field had the highest BD (1.32 g/cm3), followed by CT (1.22 g/cm3), while NT (1.19 g/cm3) had the lowest. The same trend was observed in the 30–60 cm soil depth, with VG the highest (1.44 g/cm3), followed by CT (1.26 g/cm3) and NT (1.20 g/cm3) (Figure 3a). So, the soils in all the tillage systems and both depths were not compacted. However, NT was less compacted than CT. Porosity varied significantly between NT and VG (p = 0.02) in the 0–30 cm soil depth and between NT and VG (p = 0.04) and CT and VG (p = 0.03) in the 30–60 cm soil depth. No-tillage had the highest porosity (55.10%) and VG (50.35%) had the lowest in the 0–30 cm soil depth. NT (53.31%) and CT (53.62%) varied slightly but had a greater porosity compared to VG (47.65%) in the 30–60 cm soil depth (Figure 3b). Organic carbon differed significantly between CT and VG (p = 0.03) in the 30–60 cm soil depth only. Conventional tillage (2.42% and 1.51%) had the highest OC, followed by NT (2.32% and 1.42%), and VG (1.92% and 1.00%) had the lowest OC in both depths (Figure 3c). OC decreased with depth as expected, because topsoils usually contains more OC than subsoils. The pH (water and KCl) was acidic (6.18–6.53 and 4.64–5.33) in all the fields and depths, while the electrical conductivity showed that the soils were non-saline (0.20–0.24 and 0.16–0.22 mS/cm) (Table 2). Generally, the tillage systems did not have a significant effect on the measured physico-chemical properties at Tshivhilwi. However, at Dzingahe, they were significantly (p = 0.02) affected by the tillage systems. There were notable effects on individual soil properties that could have resulted from the specific type of tillage system.

3.3. Pearson Correlations among the Soil Physico-Chemical Properties at Tshivhilwi and Dzingahe

At Tshivhilwi, the VESS score correlation with BD (r = 0.13), OC (r = 0.14), silt (r = 0.08) and sand (r = 0.06) in the 0–30 cm soil depth was positive. A very weak negative correlation of the VESS score with porosity (r = −0.09) and clay (r = −0.10) was found in the same soil depth. The negative correlation of BD with clay (r = −0.12) and sand (r = −0.12) in the 0–30 cm soil depth was very weak; however, there was a highly significant and strong negative correlation between porosity and BD (r = −0.72) in the same soil depth. Bulk density in the 30–60 cm soil depth also showed a significantly strong negative and moderate positive correlation with porosity (r = 0.87) and silt (r = 0.53), respectively. Sand showed a weak positive correlation with porosity (r = 0.16) in the 0–30 cm soil depth. A very weak negative correlation of porosity with OC (r = −0.09), clay (r = −0.07) and silt (r = −0.02) was observed in the 0–30 cm soil depth. Porosity showed a significant moderate positive and negative correlation with clay (r = 0.63) and silt (r = −0.59) in the 30–60 cm soil depth, respectively. However, sand in this depth showed a very weak negative correlation with porosity (r = −0.12).
At Dzingahe, there was a highly significant moderate positive correlation between the VESS and SubVESS scores (r = 0.62). VESS had a weak positive correlation with BD (r = 0.33), OC (r = 0.23), clay (r = 0.12) and sand (r = 0.07), whereas a weak negative correlation was observed with porosity (r = −0.34) and silt (r = −0.30) in the same depth. A very weak positive correlation of the SubVESS score with clay (r = 0.004) and sand (r = 0.05) was found, while BD (r = −0.02), porosity (r = −0.14), OC (r = −0.01) and silt (r = −0.07) showed a very weak negative correlation with the SubVESS scores in the same soil depth. The correlation between BD and porosity (r = −1.00) in the 0–30 cm soil depth was very strong and highly significant. There was a very weak positive correlation between BD and sand (r = 0.04) in the 0–30 cm soil depth, but OC (r = −0.03), clay (r = −0.02) and silt (r = −0.03) showed a very weak correlation with BD in the same soil depth. Bulk density in the 30–60 cm soil depth correlated negatively and significantly with porosity (r = −0.68) and OC (r = −0.54). The negative correlation of BD with clay (r = −0.10) and silt (r = −0.14) was weak. Only sand showed a very weak positive correlation with BD (r = 0.26) in the same depth. A very weak positive correlation of porosity with OC (r = 0.04), clay (r = 0.02) and silt (r = 0.02) was observed, along with a very weak negative correlation with sand (r = −0.03), in the 0–30 cm soil depth. Porosity in the 30–60 cm soil depth indicated a weak positive correlation with OC (r = 0.21) and silt (r = 0.45), although the correlation with clay (r = −0.10) and sand (r = −0.18) was weakly negative.

3.4. Comparison of Soil Physico-Chemical Properties under the Same Tillage Systems between Tshivhilwi and Dzingahe

The physico-chemical soil properties under the respective tillage systems were generally not affected by the study site. However, significant differences were identified in some soil properties under similar tillage systems between Tshivhilwi and Dzingahe. Bulk density (p = 0.004), porosity (p = 0.04), organic carbon (p = 0.01) and clay content (p = 0.03) in the 0–30 cm soil depth and structure quality (VESS and SubVESS) (p ≤ 0.001 and p = 0.01, respectively) showed significant difference between the NT fields. The topsoil structure quality was poor (Sq = 3.53) at Tshivhilwi and fair (Sq = 2.52) at Dzingahe. The subsoil structure quality was poor (Ssq = 5) at Tshivhilwi and moderately fair (Ssq = 3.80) at Dzingahe (Figure 4). Dzingahe also showed a lower bulk density (1.19 g/cm3) than Tshivhilwi (1.32 g/cm3) (Figure 5), but the clay content (37.60%) was greater at Tshivhilwi than at Dzingahe (26.53%). Porosity and organic carbon were relatively higher at Dzingahe (55.10% and 2.32%, respectively) than at Tshivhilwi (48.28% and 1.74%, respectively).
It was also found that bulk density (p = 0.013 and p ≤ 0.001), porosity (p = 0.013 and p ≤ 0.001) and organic carbon (p = 0.007 and p = 0.048) in both soil depths (0–30 cm and 30–60 cm) and subsoil structure quality showed significant difference between the conventional tillage fields. The SubVESS scores indicated a poor structure quality at Tshivhilwi (Ssq = 5.00) and Dzingahe (Ssq = 4.20) (Figure 4). Bulk density was highest at Tshivhilwi (1.37 g/cm3 and 1.57 g/cm3) than at Dzingahe (1.22 g/cm3 and 1.26 g/cm3) (Figure 5). Dzingahe (54.09% and 53.62%) had higher porosity than Tshivhilwi (48.43% and 40.81%). Organic carbon was also higher at Dzingahe (2.42% and 1.51%) than at Tshivhilwi (1.52% and 1.01%). Subsoil structure quality was the only parameter that varied significantly (p ≤ 0.001) between the virgin fields: it was poor (Ssq = 5.00) at Tshivhilwi but good (Ssq = 3.60) at Dzingahe.

4. Discussion

The soil structure quality at Dzingahe was found to be better than that at Tshivhilwi when compared across the tillage systems. The results suggested that the tillage systems did not exclusively alter the soil structure but other practices such as cropping systems and residue management could have contributed to the changes [11,28,29]. It was observed that the tillage systems were not practised in the same way in these two locations and that the duration of NT was also different, with a gap of more than 30 years. No-tillage at Tshivhilwi had been active for 8 consecutive years, while at Dzingahe it had been practised for more than 40 years. The visual assessment with VESS and SubVESS indicated that the soil structure quality was good at Dzingahe and moderate to poor at Tshivhilwi. This could be attributed to the duration of NT and also to the intensity of the activities at Tshivhilwi, as the field is utilized throughout the year, while at Dzingahe the field is planted once a year during the rainy season. It was clear that the degree of the impact of these tillage systems on the soil structure was different. However, both NT and CT can result in soil structural damage or improvement depending on their management [30,31]. The VESS and SubVESS scores indicated a better soil structure quality under NT than under CT at both locations. This could be due to the operations carried out in the respective tillage systems, especially the lower soil disturbance in NT [11,32]. Bulk density (BD) values were also shown to be lower in NT than in CT, which was similar to the discovery of [33].
The specific effects of no-tillage and conventional tillage systems on the soil structure could depend on the soil texture, mainly the amount and type of clay present, which might be the case at the study sites of this research. This was also identified by [34]. The authors showed that fine soils scored higher than coarse soils. The results suggest that, over time, both NT and CT can lead to deterioration or improvement of the soil structure quality at different soil depths, depending on how they are executed [4,7]. The common problem in NT is the topsoil compaction that occurs over time [35], which can cause damage to the soil structure and affect permeability. However, the structural damage can be severe under CT because the soil is mechanically turned and aggregates are destroyed during seedbed preparation. Soil compaction under CT generally happens below the plough layer (±25 cm). Hence, the bulk density in the subsoil was higher than in the topsoil, although this did not indicate that the soil was compacted. The clay content of the soil, together with the tillage systems in the cultivated fields, could have contributed extensively to the nature of the soil structure in the top 30 cm. The virgin fields also exhibited poor soil structure at both locations, which may be attributed to inherent properties like texture and/or to some extent the impact of the grazing animals. Animals can damage soil structure through compaction when they move around and graze the field. In addition, they can also make the soil surface bare in some parts if they overgraze, thus exposing the soil to further structural damage.
The structural variation between the 0–30 cm and 30–60 cm soil depths was logical and could have been caused by higher clay content and lower organic matter in the subsoil, which is in agreement with the findings of [36]. Obour and others found that clay content had a strong effect on mottling in the 20–45 cm soil depth and on aggregates and rooting in the 45–65 cm soil depth. Mottling was mostly identified in the 30–60 cm soil depth, although in some pits at Tshivhilwi it was evident in the 0–30 cm soil depth. Mottling, which refers to patches of colour mixed with the dominant soil colour, is generally caused by poor soil aeration and drainage. The poor air and water permeability of the soil is a result of reduced macroporosity, which is common in soils with a high clay content and/or that are compacted. These clay and/or compacted soils tend to have poor soil structure. It was found that clay content had more effect than compaction on the soil structure. As such, where the clay content was high, the VESS and SubVESS scores were also high. Alternatively, where the clay content was low, the VESS and SubVESS scores were also low. However, soil bulk density was shown to have minor divergence with the VESS and SubVESS scores. It was inconsistent between the fields in both soil depths and locations.
The VESS scores showed a weak positive and negative relationship with bulk density and porosity, respectively, at both locations. Ref. [13] discovered almost similar results, where bulk density correlated positively with VESS scores. The authors further indicated VESS scores were related to an increase in bulk density, which may cause a reduction in macroporosity, increased water retention and reduced permeability. Ref. [15] found a strong relationship between VESS scores and bulk density, porosity and organic carbon. It was also observed in this study that the poor soil structure as assessed by VESS cannot be due to compaction, as the BD values were within the normal range [37]. Given the clay and OC content of the soils, the low BD could be a result of the dominance of micropores and few macropores. Macroporosity is naturally limited in heavy clay soils and affects the soil’s ability to transmit water and air [38]; hence, there were visible mottles on the assessed soils.
Generally, soils with poor structure have low carbon, but, in this study, OC was relatively higher in all the tillage systems at both locations in the topsoil (0–30 cm). Ref. [39] discovered that visually assessed good structure quality soils have higher OC to clay ratios than those with poor structure quality. On average, the structure quality in the topsoil was moderately poor but the bulk density was optimally low. The negative relationship between bulk density and SubVESS scores in the same soil depth could be attributed to the increased clay content compared to that in the topsoil [36]. Ref. [40] found that bulk density decreased with an increase in clay content, whereas the SubVESS scores indicated a poor structure even though the bulk density was low. The increased clay content in the subsoil reduces mostly the macroporosity, while the micropores are not severely affected. The significant positive relationship between VESS and SubVESS that was identified at Tshivhilwi supports the use of these two methods together, especially where VESS indicates poor soil structure. Although the VESS and/or SubVESS scores can be similar for the respective tillage systems, it is important to note that during the assessment, some key parameters such as mottling and strength varied when scoring at different sampling points. This means that even though the scores e were similar (e.g., both Sq5 = poor) the degree of quality may differ. This was revealed by the low bulk density, which showed a significant difference between the tillage systems in the 30–60 cm soil depth where the SubVESS scores were all poor (i.e., Ssq = 5). It is acknowledged that laboratory analysis cannot be abandoned completely, but VESS can be used as a detector tool for early soil structural changes that will give a guide on the remediation.

5. Conclusions

VESS and SubVESS were able to effectively differentiate between the impacts of tillage systems on soil structural quality where NT had been practised for a long period (40+ years), while it could not do so where NT had been practised for a few years (8 years). The contrasting tillage intensity caused the differences in soil structure quality between the tillage systems and study sites. The soil structure quality was better under NT than CT at Tshivhilwi and Dzingahe. Opposing impacts of NT and CT on soil structure quality were identified between the study sites. The visual assessment outcome has shown to be site specific considering the combination of management practices and clay content. The VESS and SubVESS scores were related to quantitative parameters such as BD and we have corroborated their effectiveness for assessing soil structural quality. If VESS indicates moderate to poor soil structure, further assessment in the soil depth below 30 cm with SubVESS is recommended. Since both tillage systems have shown temporal effects relating to the changes in soil structural quality, more research is suggested on the use of VESS for monitoring spatio-temporal changes of soil structural quality under different soil–crop management practices and soil textures.

Author Contributions

Conceptualization, K.C.P. and L.M.; methodology, K.C.P.; software, K.C.P.; validation, K.C.P. and L.M.; formal analysis, K.C.P. and L.M.; investigation, K.C.P.; resources, K.C.P.; data curation, K.C.P.; writing—original draft preparation, K.C.P.; writing—review and editing, K.C.P. and L.M.; visualization, K.C.P. and L.M.; supervision, L.M.; project administration, K.C.P.; funding acquisition, K.C.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Research Foundation—Thuthuka, grant number 129567. And The APC was funded by Department of Research Administration and Development at the University of Limpopo.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available upon request.

Acknowledgments

The authors acknowledge the National Research Foundation—Thuthuka; Department of Research Administration and Development at the University of Limpopo; Department of Plant Production; Soil Science and Agricultural Engineering at the University of Limpopo; and Risk and Vulnerability Science Centre at the University of Limpopo.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a,b) VESS and SubVESS assessment under different tillage systems at Tshivhilwi and Dzingahe. Sq = VESS score, Ssq = SubVESS score, NT = no-tillage, CT = conventional tillage, VG = virgin field. The letters a and b indicate significant difference.
Figure 1. (a,b) VESS and SubVESS assessment under different tillage systems at Tshivhilwi and Dzingahe. Sq = VESS score, Ssq = SubVESS score, NT = no-tillage, CT = conventional tillage, VG = virgin field. The letters a and b indicate significant difference.
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Figure 2. (a–c) Soil bulk density (BD), porosity (%P) and organic carbon (OC) measurements under different tillage systems at Tshivhilwi. Sq = VESS score, Ssq = SubVESS score, NT = no-tillage, CT = conventional tillage, VG = virgin field. The letters a, b and c indicate significant difference.
Figure 2. (a–c) Soil bulk density (BD), porosity (%P) and organic carbon (OC) measurements under different tillage systems at Tshivhilwi. Sq = VESS score, Ssq = SubVESS score, NT = no-tillage, CT = conventional tillage, VG = virgin field. The letters a, b and c indicate significant difference.
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Figure 3. (a–c) Soil bulk density (BD), porosity (%P) and organic carbon (OC) measurements under no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Dzingahe. The letters a and b indicate significant difference.
Figure 3. (a–c) Soil bulk density (BD), porosity (%P) and organic carbon (OC) measurements under no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Dzingahe. The letters a and b indicate significant difference.
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Figure 4. VESS (Sq) and SubVESS (Ssq) scores under no-tillage (NT) and conventional tillage (CT) systems at Tshivhilwi (L1) and Dzingahe (L2).
Figure 4. VESS (Sq) and SubVESS (Ssq) scores under no-tillage (NT) and conventional tillage (CT) systems at Tshivhilwi (L1) and Dzingahe (L2).
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Figure 5. Bulk density (BD) under no-tillage (NT) and conventional tillage (CT) systems at Tshivhilwi (L1) and Dzingahe (L2). BD30 = bulk density in the 0–30 cm soil depth, BD60 = bulk density in the 30–60 cm soil depth.
Figure 5. Bulk density (BD) under no-tillage (NT) and conventional tillage (CT) systems at Tshivhilwi (L1) and Dzingahe (L2). BD30 = bulk density in the 0–30 cm soil depth, BD60 = bulk density in the 30–60 cm soil depth.
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Table 1. Soil pH (KCl), pH (W) and electrical conductivity (EC) under no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Tshivhilwi.
Table 1. Soil pH (KCl), pH (W) and electrical conductivity (EC) under no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Tshivhilwi.
Tillage SystempH (KCl)pH (W)EC (mS/cm)
Soil depth (0–30 cm)
NT5.42 (0.53)6.67 (0.55)0.34 (0.13)
CT5.10 (0.28) 6.52 (0.35)0.16 (0.05)
VG5.18 (0.43)6.55 (0.50)0.24 (0.09)
Soil depth (30–60 cm)
NT5.67 (0.55)7.07 (0.50)0.25 (0.15)
CT5.45 (0.40)6.71 (0.36)0.20 (0.12)
VG5.46 (0.36)7.22 (0.42)0.32 (0.13)
pH (KCl) = pH in potassium chloride solution, pH (W) = pH in water. The values in brackets are standard deviations (SD).
Table 2. Soil pH (KCl), pH (W) and electrical conductivity (EC) under no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Dzingahe.
Table 2. Soil pH (KCl), pH (W) and electrical conductivity (EC) under no-tillage (NT), conventional tillage (CT) and virgin field (VG) at Dzingahe.
Tillage SystempH (KCl)pH (W)EC (mS/cm)
Soil depth (0–30 cm)
NT4.89 (0.08)6.23 (0.11)0.20 (0.07)
CT5.10 (0.31)6.37 (0.24)0.24 0.05)
VG4.77 (0.31)6.18 (0.23)0.20 (0.10)
Soil depth (30–60 cm)
NT5.06 (0.24)6.40 (0.29)0.16 (0.09)
CT5.33 (0.40)6.53(0.29)0.22 (0.04)
VG4.64 (0.37)6.41(0.36)0.20 (0.07)
pH (KCl) = pH in potassium chloride solution, pH (W) = pH in water. The values in brackets are standard deviations.
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Phefadu, K.C.; Munjonji, L. Unearthing Soil Structure Dynamics under Long-Term No-Tillage System in Clayey Soils. Sustainability 2023, 15, 13478. https://doi.org/10.3390/su151813478

AMA Style

Phefadu KC, Munjonji L. Unearthing Soil Structure Dynamics under Long-Term No-Tillage System in Clayey Soils. Sustainability. 2023; 15(18):13478. https://doi.org/10.3390/su151813478

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Phefadu, Kopano Conferance, and Lawrence Munjonji. 2023. "Unearthing Soil Structure Dynamics under Long-Term No-Tillage System in Clayey Soils" Sustainability 15, no. 18: 13478. https://doi.org/10.3390/su151813478

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