Edaphic Response and Behavior of Agricultural Soils to Mechanical Perturbation in Tillage

Abstract

Mechanical perturbation constrains edaphic functionality of arable soils in tillage. Seasonal soil tool interactions disrupt the pristine bio-physio-mechanical characteristics of agricultural soils and crop-oriented ecological functions. They interfere with the natural balancing of nutrient cycles, soil carbon, and diverse organic matter that supports soil ecosystem interactions with crop rooting. We review soil working in tillage, associated mechanistic perturbations, and the edaphic response of affected soil properties towards cropping characteristics and behavior as soil working tools evolve. This is to further credit or discredit the global transition to minimum and no-till systems with a more specific characterization to soil properties and edaphic crop-oriented goals of soil tooling. Research has shown that improvement in adoption of conservation tillage is trying to characterize tilled soils with edaphic states of native soil agroecosystems rendering promising strategies to revive overworked soils under the changing climate. Soil can proliferate without disturbance whilst generation of new ecologically rich soil structures develops under more natural conditions. Researchers have argued that crops adapted to the altered physio-mechanical properties of cultivated soils can be developed and domesticated, especially under already impedance induced, mechanically risked, degraded soils. Interestingly edaphic response of soils under no-till soil working appeared less favorable in humid climates and more significant under arid regions. We recommend further studies to elucidate the association between soil health state, soil disturbance, cropping performance, and yield under evolving soil working tools, a perspective that will be useful in guiding the establishment of future soils for future crops.

1. Introduction

As population pressure on arable land increases, soil tillage has been intensified, leading to a decline in the per capita availability of arable land. Thus, soil resources are under tremendous pressure and concerns about ensuring sustained soil productivity are being voiced more vociferously in the wake of episodic climatic change. Soil tillage endeavors of cropland expansion to feed the ever-growing population have been associated with a quality deterioration of soil health. This is because current soil processing in tillage focuses more on exploitation of soil and water with application of conventional nutrients and less on reclamation, restoration, improvement, and sustainability [1,2].

The loss of edaphic function of soil is accompanied by abundant obliteration of microbial ecology due to seasonal mechanical disturbance and anthropogenic disruptions in arable lands [3]. While mechanical manipulation of soil contributes to management techniques selected to attain stable crop yields [4] across space and time, the current challenge is prevention of mechanically induced quality deterioration of soil properties and health. As such, it is imperative to review the mechanistic characteristics of soil disturbance in tillage and its response behavior due to heterogeneous properties of arable soils. In this paper, we review soil tool disturbances and their implications on edaphic behavior of soil brought about by tool induced disruptions on arable lands.

2. Soil Disturbance in Tillage

Arable soil tillage is the largest soil perturbation activity on Earth [5], and, astonishingly, the direct and indirect benefits of such global soil disturbances have a limited framework for its estimation [6]. Increased intensity of this global soil activity has dramatically impaired soil microbial community support and ecosystem functions [7]. Experience shows that albeit tillage offers favorable soil tilth, inhibits weed competition, fastens deeper plant residue incorporation, and controls pathogens in tilled soils, certain important and critical ecological crop requirements are not achieved [8].

2.1. Perturbable Properties of Soil Tillage

Tillage practices significantly affect soil characteristics. Numerous soil properties measure the quality characteristics of a healthy soil viz. microorganisms, organic matter levels, bulk density (ρ), infiltration rates, water-holding capacity among other soil, and physical, biological, microbial, and mechanical states under a range of tillage practices. [9,10]. Mechanistic soil disturbances in tillage thought to fracture the soil disrupt the structure and have been found to negatively impact virtually all soil quality characteristics [11]. This is more so because tillage implements, wheeling devices, and soil processing tools induce variable forces, weights, geometric widths, and tillage depths into the soil [12,13]. They also have variations in soil overturn intensity as meted out by soil-tool and implement design (for instance moldboard vis-à-vis disc plows). Thus, interactions of soil with tooling implements and processes, such as tractive and pneumatic soil-wheeling interactions and traversing from trailed tandem traffic, lead to complex stochastic characteristic responses from soils under tillage [14,15].

According to Keen et al. [16] agricultural farmland environment demonstrated complex and a significant characteristic change in soil properties with depth. This was particular to soil structure, mechanical impedance (penetrometer resistance), soil density, soil pore interconnectivity, moisture content, and ρ amongst other soil physical and mechanical states [17]. Thus, the dynamic behavior of soils and their interactions with tillage tools and residues complicates the characteristics of the soil environment for precise tillage predictions. As such, accuracies in cutting forces, displacement, soil loosening within variable furrow profiles, straw, and residue cover displacement and incorporation during tillage are often ill defined. However, measurable characteristics of structural status and functionality of arable soil may be evaluated using hydro-mechanical traits that include penetration resistance, water storage, saturated hydraulic conductivity (Ksat), infiltration capacity, aeration, and diffusivity metrics [18]. The potential usefulness of tillage and dynamic properties of soil as a characteristic metric of soil ecological functioning [19] is underpinned by changes in arable functionality following land-use conversions from a natural to tilled state. This has further been evidenced by a rapid change in soil carbon and mechanical resistance amongst other properties in response to land use conversion from a natural to tilled state compared to a slow and lengthy period of soil ecosystem equilibration following tillage abandonment [20].

2.2. Intrinsic Characteristics of Edaphic Soil Disturbance

Unlike in natural soil states, managed (tilled) soil structures in arable lands undergo seasonal breakup, loosening, manipulation, and fragmentation of soil massif [21]. This disrupts and complicates seasonal structural state of soil and affects agroecosystem dynamics of arable soils. Soil tillage tool interactions disrupt important “ecological legacy” and introduces plant roots to homogenized artificial environments with limited ecological cues for soil-root engagement. Critically, the crop-oriented structural goals of soil and its pedogenetic state and benefits are substantially impinged by mechanically induced perturbations, compared to undisturbed natural structural states. A long history of massive and continuous conventional tillage with a limited empirical base has demonstrated tillage as an ordinarily “practiced art, but a neglected science”, as paraphrased by Or et al. [5]. The appraisal of conservation tillage and associated practices of conservation agriculture [22] raises doubts regarding perceived benefits of conventional tillage. Natural soil structure in native lands have been associated with richer ecological functions, naturally balanced nutrient cycles, soil carbon, and diverse vegetative matter and humus that support higher ecosystem interactions with crop roots compared to tilled lands.

Anecdotal evidence demonstrates that apart from promoting accumulation of soil, organic carbon [23] avoidance of mechanical perturbation allowed the accumulation of bio-pores and organic matter in agricultural soils [24]. Avoidance of soil perturbation improves edaphic functionality and the persistent habit of macrofauna to increase soil biodiversity. Moreover, empirical evidence suggests that mature and stable ecosystems in natural untilled soil states supported ecological and edaphic functions by encouraging soil-root interaction, root morphology, and efficient penetration and exploration of root volumes into the soil. Conversely, tilled soils have a significant reduction in bio-linked characteristics connected to biological activities (microbial biomass, earthworm biomass and abundance), while mechanical impedance is commonly pronounced due to tractor wheeling, especially under conventional tillage [25].

3. Tilled and Untilled Soil States

The natural state, untilled lands attain steady physio-structural and bio-mechanical stability and functioning due to complex biological aggregation, accumulation of bio pores, particle stabilization, and organic residue binding and enmeshing. This is promoted by ecological activity, soil bound organic carbon, and embedded plant roots that reuse and preferentially exploit existing biological hotspots, such as bio pores [26]. Moreover, the untilled natural state biologically formed soil conglomerations and aggregates are pedogenetically embedded within the structure of the soil matrix and become ecologically stable, biochemically active, and edaphically rich. However, the natural untilled structural state of soil is highly fragile and can easily be disrupted by an instance passage of a heavy tractive device or tillage implement.

In the converse, a different ill-defined structure results from machine-induced perturbations in tillage of arable soils due to soil tooling, tractive wheeling, and trailed traversing. The hazy structure undergoes seasonal soil throws, removal, movement and displacement, vertical and lateral layer mixing, forward and backward ruptures, cut edge soil flows, upheaval, and topsoil burial. As such, tilled soil structure is characterized by mechanical fragmentation and subsequent coalescence, consolidation, and integration of loose soil fragments that deter the edaphic functionality and crop-oriented goals of soil [5].

3.1. Dynamics of Soil Properties in Tillage

Due to the spatial temporal, heterogeneous, and multivariate characteristic of soils, researchers have reported variable and multifaceted response characteristics of soil properties in tillage. For instance, Kahlon [27] investigated infiltration characteristics, ρ, porosity (ε), Ksat, and penetration resistance and reported effects of numerous effects of varying degrees as a consequence of soil tooling. However, there has been an overwhelming consensus to some extent on the influence of tillage and its principal effects on some constitutive characteristics of a healthy soil in agricultural farmlands, as exhibited by the resultant cropping characteristics.

According to Ahmad et al. [28], there was an inverse relationship between ρ and concentration and uptake of NPK, dry matter accumulation, grain yield, and press wheel penetration resistances. Unlike the inherent characteristics the dynamic soil physical and mechanical properties were among the most sensitive to tillage and soil-tool-interactions. However, due to limited empirical data and due to soil and climate variabilities, the temporal changes of physio-mechanical properties of soil that result from specific arable management practices under soil tillage are poorly understood. For instance, there was increased [29], decreased [30], and no effect [31] on Ksat under NT compared to intensive till system. Moreover, several studies have reported improved [32], reduced [33], and no effect [34] of soil water holding capacity under NT systems. In a separate work, Hu et al. [35] conducted a short-term study on dynamic behavior of soil characteristics as affected by tillage and penetration impedance under silt loams. Due to temporal changes, there were significant (p < 0.01) effects on interactions between tillage with time for all soil properties [35] indicating that the effects were changing with time. According to Hu et al. [35], there was a significant deterioration of soil physical quality by decreased macro porosity, available soil water content (θ), and Ksat due to soil compaction in tillage. Further, there are reports that the topsoil (0–10 cm) was quite susceptible compared with subsurface soils (10–20 cm) in connection to characteristic changes and physical soil degradation irrespective of tillage practice [36]. More studies on the dynamics of soil properties by Hu et al. [35] showed that although penetration resistance effects were short lived, other soil physical characteristics varied significantly for every two adjacent measurement periods.

Most of the soil tillage studies have focused on the dynamic properties of soils and their changes in relation to the inherent characteristics and with land use management practices and tillage methods. Researchers have reported the need to consider temporal effects on, for instance, hydraulic properties when representing soil processes and modeling agricultural systems. This is because tillage effects, say for instance, compaction, gradually disappeared with time due to natural recovery of physical properties over time to their original undisturbed structural states.

3.2. Tillage Methods and Physio Mechanical Properties of Soil

Tillage tools, practices, and methods have produced mixed results and have received dynamic responses from heterogeneous soils due to spatial-temporal variabilities across the soil matrix in arable fields. As such, researchers have reported variable interactive effects of tillage methods coupled with soil-tool interactions on soil characteristics and resultant cropping properties and patterns.

Long-term studies by Benjamin [37] showed that corn cropping alone had more pronounced effects (30–180% greater Ksat) on tillage than either corn–soybean rotation or corn–oats–alfalfa rotations under NT than under both moldboard and chiseling. However, there was a greater (23–91% greater) volume of large soil pores (radii > 150 mm) in chiseling than in NT, but not consistent across all soil types [37]. This further complicated the quantitative prediction of tillage outcomes as affected by cropping regimes under arable soil-tool interactions. Soil infiltration and sorptivity capacity has been experimented and reported by numerous investigators across the world. For duplex soils, NT resulted in higher sorptivities in southeastern Australia [38]; while during a 10-year study in New Zealand, Horne et al. [39] reported that NT resulted in the lowest infiltration rates compared with MT and CT on a silt loam.

However, in northwestern Canadian prairies, Azooz & Arshad [32] reported lower rates of soil infiltration in CT than in NT under silt and sandy loam gray luvisols. This contrasted with multiple Latin American studies summarized and documented by Alegre et al. [40] showing reduced infiltration rates in NT compared to conventional disking. Others, such as Agus & Cassel [41], have a prominent history of tillage research and its effects on soil processes including NT effects on infiltration, hydraulic characteristics, and behavior. For instance, the effects of disking and subsoiling on soil leaching depths were tested for two years. Subsoiling increased the depth of leaching for both the 1st and 2nd year compared with disk tillage, however, there was no significant correlation of leaching depths with mechanical impedance [41]. Czyż [42] established negative effects of tractive and tandem traffic on aeration and ρ under barley cropping. Additionally, Osunbitan et al. [43] reported increased ρ with time on loamy sand in Southwest Nigeria, because of rainfall particle resettlement after tillage. This was associated with increased bulk soil density by 48% under NT, disc-disc (DD) tillage by 55%, plow-harrow (PH) tillage by 57%, and manual hoe (MH) by 61% [43]. On the other hand, Hammel [44] reported that tillage had the most significant effect on ρ, but not on mechanical impedance after a 10-year study on the effects of tillage on ρ and soil impedance in northern Idaho under continuous MP, minimum (chiseling), and NT practices. Chen et al. [45] studied short-term tillage effects for three methods (CT 5.0–7.5 cm, NT, and subsoiling) on penetration impedance under poorly drained clays in Canada for 2 years. The study showed that traffic wheeling created hardpans at a depth of 175 mm during each fall, but was naturally removed in winter. Values of CI in the 2nd year of sampling in spring were 34% lower than during the previous fall, however, the values continuously increased with depth to the hardpan, though hardpans were absent at 175 mm depth under each spring of measurement campaign [45]. The researchers observed that the effect of tillage on CI was less significant compared to weather because CI in the fall of the first year was not statistically different between CT and NT, but equal in spring of the second year regardless of tillage practice [45]. These results contrasted findings by Osunbitan et al. [43], who reported increasing CI with time under all tillage practices, while Castrignano et al. [46] reported consistently rising CI values in NT after studying soil strengths at various depths of a fine silty clay vertisols. These researchers [46] reported high temporal cross-correlation for spatial variance of soil strengths under two tillage practices measured at different tillage depths (MP-40–45 cm and disking-20–25cm). However, they reported a small range of horizontal variance and nonexistent poor temporal correlation of spatially variated soil strengths resulting from randomly variated surface soil impedance, as controlled by θ [46]. This was because random rainfall patterns-controlled water content of the soil, rendering tillage management practice ineffective at producing noticeable statistical differences for distinguishing soil strength. However, studies performed by Chen et al. [45] showed that θ responded positively to seasonal rainfall variations and tillage methods. These researchers [45] established that NT generated an 8% increase, while subsoiling generated an 11% decrease in θ compared to CT.

We further explored specific experimental findings that reported sensitivities to structural and pore geometry of soils under different tillage methods. The NT system showed a lower ρ near the surface of sandy clay loams and no significant changes at 2cm intervals for any incremental depths throughout the top 30 cm during transition to NT under fine loamy soils [47]. On a similar study, Chang & Lindwall [33] reported a lack of any significant differences in ρ among crop rotations for 8 consecutive years under tillage. However, they reported a higher ρ in NT than CT after 10 years of continuous soil working under wheat cropping [33]. Similarly, other interesting long-term studies by previous researchers reported contrasting results [32]. In their investigation, Drees et al. [48] sought to elucidate the reported deviations and differences of unsaturated hydraulic conductivities between CT and NT under silt loams. Average pore sizes of soils under NT methods were greater compared to CT. This was further corroborated by numerous earthworm channels under NT and quite absent in CT [48].

A likeminded study was conducted to establish the dynamic effect of tillage on soil properties, by comparing NT, MP, and CP in clay soils under barley and it was reported that tillage had no effect on total ε [49]. Contrastingly, Alegre et al. [40] compared NT and MT with conventional disk tillage and concluded that the latter had lower ρ in the surface soil, along with increased macro porosity. However, the effect of plowing led to short-term increases in macro and total ε compared to NT [50]. Similarly, a long-term study by Moret & Arrúe [51] reported lower (p < 0.05) Ksat of soils under NT compared to CT and RT. This was despite showing significantly (p < 0.05) high and lowest mean micropores, but lower per unit area respective water conducting macropores under CT, RT, and NT long-term soil tillage systems, respectively. A study of temporal variable θ in a clayey calcic soil by Josa & Hereter [52] near Barcelona, showed that natural compaction processes eliminated macropores that contained plant-available water and reduced crop yield under NT. Findings from this study contradicted other studies that reported increased macropore under NT. This highlights the necessity for considering site-specific conditions, such as hydraulic responses to local climates as a function of soil properties.

Tillage methods have a significant influence on soil hydro-physical properties. For instance, after 18 months of studying hydro physical properties of soils under tillage, Peña-Sancho et al. [53] reported the highest bulk density and pore size distribution and lowest under NT compared to CT and RT. However, primary tillage soil loosening practices of soil improved α, S, and K even though pre-tillage values were recovered with associated surface crusting alluded to post-tillage rainfalls. Additionally, values of bulk density and pore size distribution increased, while scaling factor and SDexter index declined due to post-secondary tillage falls [53]. Researchers have reported multifaceted behavior of θ dynamism in response to tillage. For instance, laboratory investigations by Azooz & Arshad [32] showed that CT had lower water retention characteristics than NT, while the recharge coefficient (signifying rate of wetting) was significantly high in NT. These findings contrasted earlier results by Cresswell et al. [34], whose work reported insensitivities of tillage to θ under silt loams in New Zealand. Thus, amassed investigative evidence on the changes in θ has not been empirically generalized. Previous researchers reported that arable soils exhibited variations in response to K. Some have reported lower hydraulic conductivities in NT compared to CT after 10 years of tillage in the 30–60 mm depth interval on a loam soil [33]. Despite reports on the declining total ε and infiltration rates associated with increased bulk densities and aggregate sizes under NT compared to MT and CT, Horne et al. [39] showed that Ksat did not significantly differ under a 10 year study. Mahboubi et al. [54] performed a long-term (28 year) study for comparison of tillage tools viz. NT, chisel, and moldboard plowing, and comparatively reported great significant mean hydraulic conductivities in NT. Long-term studies by Logsdon et al. [55] and Azooz & Arshad [32] on CT and NT under sandy loam gray luvisols and silt loams showed that NT kept soil structural pore geometry and continuity undisturbed. This significantly contributed to higher Ksat in NT than in CT for both sandy loam gray luvisols and silt loams [32].

In summary, dynamism of soil properties under managed agricultural soils in tillage produced mixed results and inconsistent responses to macropore connectivity, total ε, soil bulk densities, infiltration rates, Ksat, and moisture contents across soils, climates, tillage practices, implements, and spatial-temporal variabilities. Thus, it is not possible to generalize the results of previous researchers due to such dynamic factors and variabilities and leave the hypothesized findings to the fate of soil types, climate and implements geometry, and tillage practice.

4. Edaphic Functionality of Arable Soils

The edaphic property of soil determines the growth and development of plants. Edaphic factors of soil agroecosystem integrate an intricate network of biological, physio-chemical, and mechanistic properties of soil with microbial community. Researchers, such as Buhk et al. [56], Honnay et al. [57], and Knudsen et al. [58], assert that soil edaphic functions must always be considered for sustainable land management. However, few studies have looked at the effect of tillage on edaphic properties of soil at a field level. This is because edaphic quantification of the impacts of soil quality and biodiversity losses requires long-term investigations in replicated experimental systems due to complex, slow, and large spatial heterogeneity of arable soil processes [59,60,61,62,63,64,65,66].

Edaphic characteristics viz. soil moisture content, air, temperature, organic matter, and microorganisms affect plant growth and development. Changes in soil characteristics resulting from tillage activities have significant adverse impacts on the edaphic functionality of soils. It is estimated that about two-thirds of poor crop yield is caused by soil edaphic function disruption [2]. Triantafyllidis et al. [67] studied the effect of anthropogenic interventions, such as tillage and land-use types, on edaphic properties of soil. Compared to abandoned lands, there was decreased species diversity in olive and maize cropped fields [67].

Long lasting land-use intensification has a negative impact on agroecosystem dynamics, such as soil properties and plant species. Over the years the underlying natural patterns have been overlapped by tillage intensification and the contribution of parent material has been masked while the effect of floristic diversity has reduced. According to Balzan et al. [68], agricultural intensification in arable lands has led to a reduction of plant species and functional diversity of agricultural soil ecosystems. Kosmas et al. [69] reported significant differences in soil properties and plant species diversity between abandoned and tilled lands. It was estimated that the greatest improvement factor was increased soil organic matter (SOM) in the topsoils of abandoned lands [69]. Bhattacharyya et al. [70] and Ramachandran et al. [71] reported that, apart from continuity, pore size and arrangement, plant root growth and biological activities created better pore connectivity and improved hydraulic conductivity of soils in untilled soils. Thus, alleviation of multiple adverse effects on edaphic factors requires a holistic approach [2] that includes the use of new tools and strategies that have emerged in conservation tillage.

4.1. Tillage Method and Edaphic Response of Soils

Soil tool disturbances promoted plant rooting to some extent, as opposed to enhancing biological activities in the soil [72,73,74]. Evidently NT stimulated ecological networks, increased SOC, and improved soil water storage and retention characteristics, among other things [75]. However, this did not always result in improved yield under all conditions since crop performance and yield under NT was also dependent on soil type, cropping characteristics, and climatic conditions. Reports from comparative emerging trends from numerous studies showed that there was no generalized suitability of NT practice under all farming conditions [76,77]. This suggests that there are certain advantages offered by carefully managed conventional soil disturbances where inveterate and substantial NT yield losses are economically unacceptable. According to Pittelkow et al. [77], the greatest benefit of NT occurred under rainfed arid agriculture. It was interesting to note that NT appeared less favorable under humid climates [21] and more significant in more arid regions. This is because where soil aeration was necessary, higher soil density under humid climates limited soil gas transport under NT [78,79]. On a side note, however, crops that were sensitive to penetration impedance, including those investing in underground tubers, performed worse under NT due to reduced mechanical perturbation and higher mechanical resistances [80,81].

The advantageous effect of NT on properties emanating from climate mitigation have been investigated. Numerous researchers have reported an increased accumulation of soil carbon at the top and much less in the subsoil soil layers after decades of residue incorporation, although at slow rates [81,82]. However, in some instances inter-annual soil management and variabilities made it difficult to detect the response of slow SOC accumulation [66,83]. It, thus, becomes arguable that decades of persistent variable responses of SOC to land management changes, especially conversion from natural state to managed (tilled) state, is significant evidence of the degraded structural state of soil. Moreover, this is arguably the adverse impact of mechanical perturbation on edaphic soil functioning. Moreover, the differences brought about by soil management need to be evaluated not only by the effects captured on integrated trends of soil properties functionality, but also on cropping characteristics.

A consistent picture of the effects of land use change by introduction or abandonment of tillage has been shown clearly by Ledo et al. [84]. They reported increased soil carbon in perennial than annual cropping due to absence of mechanical perturbation trends under perennial cropping. A study by Reicosky et al. [85] showed that soil tillage resulted in organic carbon homogenization within the tilled layers and eventual accumulation near the surface under NT. However, fairly-minded expectations of carbon sequestration under NT by Powlson et al. [86] and Paustian et al. [19] was found contrary to metadata analysis by Luo et al. [87]. This was because accumulation of soil carbon did not show any significant difference between conventional and NT soil management regimes [86]. Nonetheless, numerous studies have reported gradual increase in an accumulation of soil carbon under NT, even though significant differences became more detectable after several decades of conversion [23,88,89].

4.2. Soil Edaphic Properties and Mechanical Impedance

Numerous researchers have conducted studies aimed at characterizing cropping characteristics of agricultural soils with mechanical perturbation. Cropping methods and associated soil disturbances have reported significant results. For instance, compared to NT, chiseling was found to be a viable strategy for mitigating soil physical constraints of rooting and yield [90].

Use of pre-crops enhanced root growth and yield by providing bio pores and root channels in the soil profile through reduction of mechanical and water stresses resulting from compaction and soil desegregation [90]. In separate studies, amassed evidence showed that plant heights shoot emergence levels, leaf area, biomass dry matter (DM), stem diameter, wet and dry root masses, and grain mass yield had significant incongruities with soil cone index (CI), ρ, soil aeration, ε, and mechanical impedance of soils [24,91,92,93,94,95,96,97]. Olubanjo & Yessoufou [95] explored the effects of penetration resistance on crop nutrient absorption levels. They reported higher nutrient concentration in treatment with low penetrometer resistances. These researchers have assured a reduction in the plant’s nitrogen, potassium, magnesium, and sodium absorption ability by 13.5%, 51.4%, 50.4%, and 51.5%, respectively, when penetration resistance was increased from 1.5 to 5.2 MPa. However, calcium and phosphorus concentrations were higher when penetration resistance was varied between 3 Mpa and 4 Mpa. In their study, Olubanjo & Yessoufou [95] concluded that unfavorable soil conditions created by increasing mechanical impendence and associated penetration resistance was responsible for reduced mineral uptake.

Researchers have proven reduction of concentration and uptake of nutrients due to structural compaction of soils [95,98,99,100,101]. Grath & Arvidsson [102] reported a lower concentration of macronutrients in pea and barley under highest compaction wheel loads compared to the non-compacted control (that showed higher grain nitrogen) in sandy loamy soils. Researchers concluded that compacted soil structures significantly restricted aeration and reduced nodulation and nitrogen fixation [102,103,104,105]. Similar studies by Guan et al. [106] showed that nutrients uptake decreased under increasing mechanical impedance of managed soils. Further, extensive research by Jourgholami [107] and Jourgholami et al. [108] concluded that the nutrients absorption capacity of a crop was essentially related to its ability for developing extensive rooting system to overcome mechanical impedance of soils. Besides, Olubanjo & Yessoufou [95] reported that increased soil compaction reduced the maize grain yield by 18.8% due to interference with uptake of macro and micro-nutrients (Figure 1). Generally, researchers concluded that adverse mechanical resistance created by increased compaction accounted for the decline in uptake of plant nutrients.

5. Crop Growth, Productivity, and Induced Mechanical Impedance

The changing climate has brought about the pressure of systematic and complex changes to agricultural soils, impinging on their edaphic functions [109]. Although the terms ‘soil quality’ and ‘soil health’ are still heavily debated [110,111], there is a wide acceptance for the need of a new approach for ensuring that agricultural soils are edaphically fit in the future [112]

Numerous researchers have carried out studies aimed at characterizing crop growth and productivity as affected by soil quality and health, such as induced mechanical impedance. Early studies by Masle & Passioura [113] showed that the leaf area, shoots, and crop root dry weights were negatively correlated with soil strength as indicated by penetrometer resistance. It is worth noting that mechanical impedance had less effect on root growth compared to the shoots, although higher soil strengths produced substantially smaller stomatal conductance [113]. However, the literature has shown that all the mechanical impedance effects on crop productivity were reportedly the same, regardless of the source of variation of soil strength or penetration resistance, i.e., whether soil water content or in bulk density. For instance, Olubanjo & Yessoufou [95] performed experiments aimed at identifying the effects of penetration impedance on biomass growth of maize crop at three levels of ρ (1.17, 1.37, and 1.45 g cm³) and three water content levels (0.12, 0.18, and 0.30 g/g soil). The researchers [114] reported a significant (p < 0.05) decrease in shoot elongation (27.1%) and leaf area (67.8%) at high penetration resistances. Further, there was a significant decrease in fresh and dry root masses by 39.1% and 37.8%, respectively, at high penetration impedance levels. In a study aimed at evaluating how mechanical impedance would interact with nutrient stress, Wang et al. [115] found that even under a sufficient supply of phosphorus required by plants to promote healthy root growth and early shoot emergence, mechanical impedance reduced shoot and root growth significantly.

According to Lynch et al. [112], native soils mediate mechanical impedance to rooting growth by low resistance root ways formed by biopores, soil aggregates, and high organic matter. Contrastingly, mechanized and conventionally tilled soils are associated with thinner epipedon of low organic matter content, hence less water-holding capacity and greater susceptibility to soil hardening and nitrate leaching. This is due to soil drying, fewer low-resistance pathways from soil structure, and biopores associated with plow pan from vehicular traffic as shown in Figure 2.

According to Głąb [116], the effect of compaction induced penetration resistance using annual compaction by tractor passes was significant to cropping patterns. Researchers recorded data on root morphology, in terms of root length density, mean root diameter, specific length, distribution of roots DM, as well as crop yield. It was reported that soil compaction increased the proportion of DM in the roots and there was more root density in the upper soil horizons (0–10 cm). However, there was a decrease in the root length density and a general thickening of the roots in compacted treatments even though it was observed that increased wheeling and bypasses decreased in the DM of lucerne herbage of the second and third yields each year [116].

Researchers recommended reduction of machinery wheel passes and, if possible, restrict wheel passage to fixed permanent strips along the fields to reduce impedance related effects on crop growth and yield. According to Ahmadi & Ghaur [117], soil wheeling from traffic inversely affected percentage corn seedlings emergence, root mass, and plant dry mass at maturity, as shown in Figure 3 and Figure 4.

Summarized research findings by Olubanjo & Yessoufou [95] on the correlation of root production, biomass, and shoot elongation with soil strength showed that increased mechanical impedance was detrimental to maize biomass and resulted in low yield. The researchers concluded that high penetration resistance negatively influenced crop performance and productivity due to the inability of absorbing minerals from structurally perturbed soils [95,114,118,119]. Researchers have reported the need to avoid soil compaction as a strategy, enhancing nitrogen use efficiency, and for maintaining yields [118]. Further, Olubanjo & Yessoufou [95] studied the effect of mechanical impedance on plant height and leaf area index during crop growth stages, as shown in Figure 5. They reported that plant heights and leaf area indices were lower and decreased significantly under treatments of higher compaction levels compared to soils with lower penetration resistances after 30 days. Treatments with a 1.8 MPa and 5.2 MPa penetration resistances produced the highest (131.8 cm) and lowest (85.6 cm) plant heights, respectively [95].

These researchers have reported the highest value for leaf area indices under 1.5 MPa compaction treatment and the lowest in the 5.2 MPa compaction treatment [95]. Similar studies were carried out by Igon & Ayotamuno [120]. A higher percentage of early shoot emergence in plots with lower bulk densities were reported than those with higher bulk densities [120]. The researchers have forwarded a significant effect of bulk densities on the crop emergence and growth (

Table 1. Plant height at various stages of growth and different compaction treatments [120].

Plant Height (m)
Plot No.	3 WAP	4 WAP	5 WAP	6 WAP	7 WAP	8 WAP	9 WAP	10 WAP	11 WAP	12 WAP	13 WAP	14 WAP
1	0.360	0.470	0.560	0.722	0.752	0.790	0.843	0.874	0.879	0.898	0.930	0.941
2	0.376	0.572	0.710	1.020	1.023	1.030	1.296	1.280	1.272	1.440	1.380	1.380
3	0.330	0.500	0.631	0.790	0.800	0.810	0.837	0.855	0.741	0.863	0.872	0.872
4	0.291	0.393	0.494	0.610	0.650	0.693	0.745	0.810	0.855	0.900	1.124	1.146
5	0.290	0.331	0.470	0.514	0.763	0.766	0.880	0.913	0.934	1.131	1.143	1.402

WAP—Weeks After Planting; Plot treatments; 1—untilled and uncompacted (Natural state), 2—tilled and uncompacted, 3—tilled and compacted, 4—tilled and compacted, 5—tilled and compacted.

and

Table 2. Effect of soil compaction on leaf area index at various soil compaction treatments [120].

Plot No.		Leaf Area (m2)
	1 MAP	2 MAP	3 MAP
1	0.0527	0.0840	0.1150
2	0.0998	0.1097	0.1974
3	0.0667	0.0863	0.1112
4	0.0502	0.0735	0.1490
5	0.0446	0.0757	0.1561

MAP—Months After Planting; Plot treatments: 1—untilled and uncompacted (Natural state),2—tilled and uncompacted,3—tilled and compacted, 4—tilled and compacted, 5—tilled and compacted.

). Lowest bulk densities recorded the highest emergence levels and stiff competition of growth in proceeding weeks after planting compared to the high ρ plot counterparts [95].

From the foregoing, it is evident that there exist significant efforts among researchers to understand the response effects of arable soils and edaphic oriented crop development to mechanical perturbation from machinery wheeling and soil-tool interactions.

Specific research has focused on establishing the effects of soil perturbation on mechanical impedance, soil characteristics, and edaphic function in crop morphological development and yield. For instance, Chen et al. [93] reported lower root biomass in compacted soil cores. Olubanjo & Yessoufou [95] conducted experiments to determine the wet and dry mass of the maize crop roots after harvest. The highest wet and dry root masses (15.7 g and 13.6 g) were, respectively, recorded in treatments with 2.0 Mpa and lowest values (7.5 g and 5.9 g), respectively, in the 5.2 Mpa compaction readings of the soil penetrometer [95].

According to Botta et al. [92], topsoil compaction was a more limiting factor to root growth than the subsoil compaction. Taylor & Brar [121] reported that the root DM and diameter increased linearly with increase in soil penetration resistance. According to Chen et al. [93], higher root biomasses were observed in un-compacted soils compared to soils with higher mechanical impedance. Kumar et al. [122] reported incongruencies of rooting depth with penetration resistance, as shown in Figure 6. This was regarded and evidenced as a compensatory response of plants to increased penetration resistance that led to reduced total ε and aeration due to mechanical impedance [93]. Some researchers have, however, reported mechanical impedance led to significant greater root biomass up to some reasonable depth [123,124,125,126,127]. This suggests that increase in ρ as a result of soil compaction may sometimes have a favorable effect on root biomass (Figure 6). This is probable because compaction may induce vigorous root proliferation and growth as evident in more and relatively thicker roots in bulk soils.

Numerous researchers have conducted studies to establish the influence of soil compaction specifically on grain yield. For instance, Ramazan et al. [96] studied the impact of soil compaction on the yield of maize and reported significant results. In their initial findings Ramazan et al. [96] established that ρ of the soil was congruent with number of tractors passes. Further, the researchers established a decrease in plant height and grain yield with increase in tractor passes and concluded that soil compaction reduces grain yield [96]. Research conducted by Ahmad et al. [28] showed an inverse relationship between ρ and concentration and uptake of NPK, DM accumulation, and grain yield. According to Ahmad et al. [28], the compacted treatments decreased the DM accumulation in the range of 2 to 9%, whereas grain yield showed a reduction of 5 to 48%. In a separate study aimed at investigating the effect of compaction on yield of silage maize, Altuntas et al. [128] reported that soil compaction treatments negatively affected DM yield and caused a reduction on DM yield by 16%. Furthermore, DM yield of silage maize was negatively affected by the press wheel compaction treatments [128]. Conversely, while not impeding root growth, Taylor & Brar [121] verified that values of topsoil penetration resistance between 1.03 and 5.69 MPa affected the root system morphology, reducing the crop yield by 2.581 Mg ha⁻¹.

Bergamin et al. [91] studied the relationship between physical quality parameters of soil and maize yield under treatment levels of induced of mechanical impedance using a randomized complete block design with five replicates. The researcher reported a significant correlation (p < 0.01) between all soil physical attributes under study. Grain yield was positively correlated to macro porosity (r = 0.41) and negatively to penetration impedance (r = −0.42), while the emergence speed index, stem diameters, plant heights, and grain mass yield decreased with an increase in mechanical impedance of soil [91]. From their study the researchers concluded that increased soil macro porosity and reduction of penetration resistance benefited maize yield. It was argued that soil physical attributes viz. aggregate stability, macro porosity, and penetration impedance indices were responsive and closely correlated with maize grain yields and were sensitive enough to be adopted as indicators of soil physical quality [91].

In a different experiment Onwualu & Anazodo [129] studied three levels of soil compaction viz. heavy, medium, and no compaction and their effects on maize production under various tillage methods in a loamy sandy soil derived savannah zone of Nigeria. Results showed that soil moisture content and ε decreased with an increase in soil compaction, while ρ and soil resistant to penetrometer pressure increased. However, maize emergence increased with an increase in soil compaction, but the highest yield was obtained under medium compaction. Thus, deep tillage may not be of any special benefits when a loamy sandy soil is compacted [129]. Similarly, Sivarajan et al. [97] established the impact of soil compaction due to wheel traffic on corn growth, development, and yield. The yield data showed a significant difference between the soil transects, but no difference was observed between most trafficked (MT) and least trafficked (LT). A review by Kumar et al. [72] reported decreased crop production with an increased draft of tillage operations. The dry fresh and dry weights of the plants were reduced due to a delay in growth processes caused by compaction of the soil, which ultimately led to poor yield [72]. Igoni & Jumbo [94] carried out field investigations aimed at predicting the effects of soil compaction on the growth and yield of maize on a sandy loam soil in tropical climates. The researchers reported a statistically significant effect of soil compaction at p < 0.05 on the growth and yield of maize in a tropical sandy loam soil and the model prediction model correlated with experimental data up to about 99.5% [94].

Having known the effect of reduced plant growth and yield as a result of soil resistance, Wang et al. [115] studied the interaction of mechanical impedance with nutrient stress on wheat seedlings growth under contrasting phosphorus (P) availability. The study was carried out in a sand culture allowing for independency of mechanical impedance from water and nutrient supply [115]. Under sufficiently supplied P, the researchers reported significantly reduced growth, shoot, root biomass, total root length, and leaf area attributed to mechanical impedance. Contrastingly, low P supply under mechanical impedance had no effect on biomass, leaf length, nodal root number, and tiller number. It was thus adverted that growth restriction by mechanical impedance depended on P supply and was significant on most plant traits to evidence a physical, structural, and nutritional interaction of arable soils [115].

Cairns et al. [130] screened two rice varieties for root growth under droughted and irrigated treatments and reported high mechanical impedance as a more fundamental constraint on root growth than soil water availability during drought. Researchers reported that varietal ability difference for soil penetration could be very important for drought avoidance in soils [130]. Mechanical impedance of soils developed rapidly near the surface as drought increased. As such, drying stresses and mechanical impedances nearly stopped the elongation of crop roots [131]. Since drought led to a reduction in soil water content, which increased mechanical impedance of arable soils, researchers have emphasized incorporation of root penetration ability in developing drought resistant superior cultivars [130,132].

Compaction free soil structure improves the ability to hold and drain water, avail nutrients, and provide aeration required by plant rooting activities. Moreover, impedance free soils ensure good water retention and drainage for plants roots to draw in all the available nutrients. According to Ahmad et al. [28], soil compaction was the major cause of decreased crop yield. Continuous plowing at constant depth and associated tractor wheeling was the most significant cause of soil compaction that affected ρ, ε, and root proliferation, consequently hindering nutrients availability and uptake by plants [28]. Increased soil density inhibited root growth, even though researchers have reported that soil strength-controlled soil root penetration more than ρ [133].

Mechanical impedance of soils varies during the crop growing season and this effect and associated crop yield has been investigated under different tillage methods [134]. Numerous researchers have reported a significant influence of soil compaction on the yield in various cropping systems. Researchers have argued that soil penetration resistance restricts deep root growth and consequently limits plant access to subsoil water and soil nutrients [95]. Cairns et al. [130] reported an insignificant effect of water regime compared to penetration resistance that led to physiological stress of two rice varieties. Experimental results indicated higher mechanical impedance fundamentally constrained root growth compared to water supply during drought [130]. Researchers revealed that varietal variations in root penetration capability could be very important for easing adverse effects and drought avoidance in mechanically impenetrable soils [130,132,135]. However, there was little detectable effect of water regime on root distribution, but evidence of lower root numbers at depths below 20 cm in the higher penetration resistance (PR) site was revealed [130,132,135].

Whereas increased intensity of mechanical soil loading reduced the total grain yield, Gronle et al. [136] went ahead to study the effect of mechanical compaction on yield composition and reported a significant decline in crude protein content in peas and oats because of higher mechanical soil loading. Moreover, there were reports of limiting root and plant growth for bulk density values of 1.75–1.80 g cm³ and 1.60–1.70 g cm³ under loams and sandy loams, respectively [137,138].

Some researchers have associated penetration impedance with critical limits for plant rooting [139,140,141,142,143]. Cone penetrometer indexes exceeding 2–3 MPa were considered critical limits and restricted crop root development in arable soils [136,143,144,145,146,147]. However, some researchers have argued that these limits were dependent on plant species. This is because the rooting of certain crops seemed to be restricted by values above such general limits [136]. For instance, experiments by Ehlers et al. [148] reported that rooting in oats was limited to a penetration resistance of 3.6 and 5.1 MPa as opposed to peas (0.06–1.8 MPa). Apart from cone index, soil response to penetration impedance was found to be specific to crop species. According to Bengough & Young [149], and as reported by Gronle et al. [136], root elongation of peas was 55% (under 1.8 MPa cone index and 1.5 mgm³ bulk density) of that of peas variety grown under 0.06 MPa and 0.85 Mg m³ loamy sand.

Some of the responses of plant rooting systems to mechanical impedance have been associated with morphological modifications viz. decreased root sizes, radial increments, and swelling of the cortex and sloughing of the cap cell [79,150,151,152,153,154]. However, different plants responded differently to temporal and spatial changes to impeded external impedance pressure, thus, to the extent of this review, there was no generalized and universal root morphological response to externally applied mechanical impedance pressure. Further studies response of root anatomy and plants rooting biomaterial viz. cell wall structure and root tissue responses to soil mechanical properties are encouraged.

6. Conclusions and Recommendation

Field crop rooting in arable soils suffer a combination of physio-mechanical and nutritional stresses. The edaphic factors of soil are severely impinged in tilled arable soils compared to naturally pristine conditions in untilled states.

There is need for new insights on soil disturbance by integrating both physio-mechanical and ecological nutritional stresses in tillage practice. The future direction of soil disturbance in tillage and associated field scale response of crop root and yield to soil management should focus on exploiting both the physio-mechanical and biological soil health.

The focus of soil working in tillage ought to be the soil-root system with greater interactions with soil microorganisms and minimal soil fracturing to further contribute to the quantity of carbon in the soil as opposed to conventional nutrition.