Can a Change in Agriculture Management Practice Improve Soil Physical Properties

Abstract

Soil conventional tillage has been associated with deterioration of its characteristics, while organic farming has been promoted as an approach to conserve a favorable soil environment. With the interest in nominating the tillage strategies without ploughing for maintaining long-term soil quality and subsequently increasing yields, this study set to identify if and how conservation tillage practices in organic management (OM) do improve soil physical properties compared to conventional management (CM). This study was conducted on matched field pairs in Baden-Württemberg, Germany. The conservation tillage treatment effects of OM (superficial tillage using chisel at 10 cm depth) was compared with conventional tillage practices CM (mouldboard ploughing at 30 cm depth). The field pairs were homogenous in most respects that would reflect tillage impacts. Measurements included soil infiltration capacity, saturated hydraulic conductivity, penetration resistance, and effective bulk density. Infiltration rate, measured using a hood infiltrometer at 10 parcels, was computed using Wooding’s analytical method, while Gardner’s equation was used to calculate the saturated hydraulic conductivity (Ks). The steady infiltration rate qs (h) was two times higher under OM than under CM with an average of 624 mm/h and 303 mm/h, respectively. Penetration resistances of OM were lower than under CM irrespective of the clay content. The degree of compactness (effective bulk density) was greater under CM than OM. That small change in soil compactness affects the water infiltration rate and the hydraulic properties rather than intrinsic soil matrix such as texture. Numerical model Hydrus-1D results were more representative for simulating the soil water transfer and hydraulic parameters under tillage changes.

1. Introduction

The Global Assessment of Soil Degradation (GLASOD), conducted between 1988 and 1991, highlighted that the human-induced degradation of vegetated soils had strongly increased from 6% (in 1945) to 17% (in 1990) and estimated a further significant increase to 25% by 2025. Many global policy frameworks, including the United Nations Sustainable Development Goals (SDGs), are strongly directly and indirectly linked to land and soil, and cannot be achieved without healthy soils. Therefore, updated and reliable information on soil conditions along with quantitative analyses on the impact of management practices are extremely important and needed to improve process-based modelling and facilitate the assessments of soil ecosystem services. This is of primary importance from the local up to the global scale, particularly for resources such as soil that are non-renewable in the short term and difficult to reclaim when degraded.

Improved practices of agricultural soil management as well as the quantification of their expected benefits are mandatory today, also considering that increasing population and other demands place a greater strain upon the soil. Soil management can not only protect soil and enhance its performance, but also generate environmental and economic benefits. Up to now, there are two main different soil management approaches, (i) conventional management (CM) and (ii) organic management (OM) which are fundamentally different [1,2,3,4,5]. Conventional tillage includes the removal of crop residues from soils and leads to negative impacts, such as damaging soil aggregates, a loss of mechanical stability, and compaction [1]. Soil organic management is often viewed as a more relevant farming system [2].

Conventional management (CM) incurs monoculture cultivations and intensive use of agricultural large machinery, including the usage of chemical fertilizers and pesticides, and the removal of crop residues. Organic management (OM) is often a more complex approach; it depends on less use of agricultural machinery, performing of manures, extensive cultivations, and leaving of crop residues.

However, the long-term intensive conventional tillage has pronounced negative effects on soil physical characteristics, especially on the soil structure that would reflect on the crop yields [6,7]. In contrast, [8] reported higher soil porosity and organic matter content, and lower penetration resistance could be created by applying organic farming compared to conventional management.

Organic farmers, mainly in Europe, revealed their interest to apply conservation practices to organic management [7,9] and to adopt conservation tillage practices using a chisel from 10 to 20 cm soil depth or seeding without any prior cultivation [10]. These techniques are used to stop or reduce plowing and to conserve and maintain the soil surface from soil water erosion, especially in heavy clay soils, by keeping crop residues at the soil surface [3,10,11,12,13]. Nevertheless, several soil types such as the clay, loamy clay, and loamy soils [14,15] showed no/small differences between the conventional and organic management on the soil characteristics and soil quality. For a valid comparison of tillage system impact, analyses on matched field pairs can be useful [16]. Analyses that were conducted on the soil physical characteristics [17] revealed that conservation tillage does enhance the soil aggregate stability.

Investigations [18] on soil physical quality in contrasting tillage systems in organic and conventional farming practices mentioned that conservation tillage in OM had a pronounced effect on soil quality with better soil penetration resistance compared to the CM. Therefore, investigations are needed to assess the impact of merging the conservation tillage into organic management on soil physical properties [4,10,19]. Furthermore, still today, soil hydraulic properties under different tillage practices have got little attentions due to the difficulties in preparing different soil physical measurements.

Several field and laboratory methods for the assessment of soil hydraulic parameters are expensive and time consuming [20]. Laboratory methods that are based on the direct solution of Darcy’s law (and the approximation of Richards’ equation) are considered less reliable and realistic than field measurements which retain the soil continuity [21]. For these reasons, to investigate the soil hydraulic parameters, (especially for estimating the preferential water flow pathways under various managements [22,23,24]), in situ field techniques such as, for example, those based on the Hood infiltrometer/tension infiltrometer, have received increasing attention. This is also based on the fact that compared to the pressure/ring infiltrometer, the tension infiltrometer requires a low amount of water to investigate and process the infiltration trials, and is suitable for the assessment soil hydraulic properties in spatial variation [25].

Among the advantages of the in situ analyses, it must be considered that data of both soil hydraulic properties and infiltration can be promptly used for assessing the effect of soil tillage on the soil water flow [26,27]. Moreover, by minimizing soil disturbance, in situ techniques allow the characterization of the soil hydraulic parameters dependency on land use change [19,23,28], tillage practices [29], and plant roots [30].

Several investigations [31,32] have also highlighted that soil hydraulic properties can be simulated for field experiments to estimate soil water transfer on a plot scale. HYDRUS-1D code was performed to simulate water flow in variably saturated media and the model was solved by the numerical method of [32,33].

The aim of the current work is: (a) to describe in situ how conservation tillage practices in organic management (OM) do improve soil physical properties compared to the conventional management (CM) and (b) to simulate soil water flow within the soil matrix at a plot scale under the different tillage systems.

2. Materials and Methods

2.1. Description of Study Area

Figure 1 shows the study was investigated in Brehmen village, (3537244, 5494266, Gauß Krüger, Zone 3) in the catchment area of the river Tauber. The test site is situated east of Tauberbischofsheim, in the northeastern district of Baden-Württemberg, Germany. The studied areas are located 310–430 m above sea level. The annual average temperature and precipitation are about 8.6 °C and 732 mm, respectively (DWD, 2015). The soil is a luvisols that developed mainly on limestone material. In the investigated location, the field pairs were homogeneous in most respects (irrigation, climate, agronomical practices, and crop type), that small differences among the investigated fields would refer to the management practices. One of the two fields was under conventional management (CM) of monoculture cultivations (mouldboard ploughing at 30 cm depth) and removal of the crop residues after harvest. The other field was under organic farming management (OM) of no-tillage management (superficial tillage using a chisel at 10 cm depth) with turning over crop residues on the soil surface after harvest. Such a particular field was converted from CM to OM for 8 successive years (

Table 1. Physical properties of the investigated sample locations for the different management practices.

Sample Location	Crop	Soil Management	Particle Size Distribution			Texture	Bulk Density Mg m−3
			Sand %	Silt %	Clay %
1	L	Organic	2.1	52.1	45.8	Clay	1.22
	L	Conventional	2.2	51.5	46.3	Clay	1.48
2	WW	Organic	11.3	52.1	36.5	Silty Clay Loam	1.23
	WW	Conventional	11.3	58.5	30.3	Silty Clay Loam	1.29
3	E	Organic	3.3	44.3	52	Clay	1.2
	E	Conventional	3	60.8	36.2	Silty Clay Loam	1.28
4	SE	Organic	3.3	63	33.7	Silty Clay Loam	1.02
	SE	Conventional	6.1	56.6	37.4	Silty Clay Loam	1.37
5	WW	Organic	1.2	60.9	37.8	Silty Clay Loam	1.14
	WW	Conventional	1.9	54.9	43.5	Silty Clay Loam	1.36

E: Erbse; L: Luzerna; WW: Winter Wheat; SE: Black emmer.

).

2.2. Soil Water Transfer Measurements

The infiltration rate of water through the soil surface was investigated using the hood infiltrometer device [26]. This instrument allows measuring the steady state infiltration rate (q_s). The amount of water infiltrating into the soil surface every 30 s was monitored versus time until reaching the infiltration steady state (Figure 2).

During the process, the last five readings at the steady state were averaged and the steady flow rate was recorded. Compared with a disc infiltrometer, a hood infiltrometer did not require preparation of soil surface, so no contact material and/or perforated plate is required on the infiltration soil surface (Figure 2). Thus, measurements using the hood infiltrometer would reflect many physical properties of the soil surface such as the soil compaction or silting [22,23]. For all the investigated sites, the steady state infiltration rate was measured at a pressure head of −1 cm, and samples that were taken for measurements were in 3 replicates and the average of the three replicates were used. Data that were collected in the field were as flow rates (Q_s) and converted into steady infiltration rate (q_s) according to [34] as follows:

$$q_s=\frac{Q_s}{\pi \cdot r^2}$$

(1)

where Q_s is steady flow rate [L³ L⁻¹], r is radius of the hood (L), and q_s is steady state of the infiltration rate [L.T⁻¹]. Infiltration data using the hood infiltrometer were based mainly on the Wooding’s analytical method [26]. Thus, with the determination of Q_s, the unsaturated hydraulic conductivity of the investigated soil was computed according to [35]:

$$\frac{Q_s}{\pi \cdot r^2}=K_u\cdot \left(1+\frac{4}{\pi \cdot r\cdot \alpha }\right)$$

(2)

where K_u is unsaturated soil hydraulic conductivity [L.T⁻¹], and α is sorptivity number of the soil [L⁻¹]. The soil sorptivity number (α) is a constant that is set equal to the reference value of 0.12 cm⁻¹ for agricultural soils [36]. With the computing of unsaturated hydraulic conductivity using the Wooding’s equation, saturated hydraulic conductivity (K_s) was calculated according to Gardner’s equation [37]:

$$K_s=\frac{K_u}{\exp \left(\alpha \cdot h\right)}$$

(3)

where K_s is saturated soil hydraulic conductivity [L.T⁻¹], and h is pressure head [L].

2.3. Degree of Compactness Calculations

2.3.1. Effective Bulk Density

The soil dry bulk density (BD) and initial water content (near to the hood place) were measured by collecting the undisturbed soil samples from 0–10, 10–20, and 20–30 cm soil depths [38]. Composite soil samples were collected from 5 locations from the surface layer 0–10 cm close to the infiltration test location and analyzed for soil texture and BD (Table 1), according to [39]. Heavy clay soils are affected by swelling and shrinking processes. Thus, BD is not a relevant tool for reflecting the soil compactness degree. [40] stated that although BD of a certain value may be critical in a fine textured soil, it would not be so crucial in a coarse-textured one. This could be attributed to the dependence of BD on many factors other than texture, including soil organic matter content and the clay mineralogy type. Therefore, the “effective bulk density” (eBD) considers these factors [23,40,41]. The eBD is computed according to the following equation [41]:

$$eBD\left(\mathrm{Mg}\mathrm{m}{}^{-3}\right)=BD+0.009\cdot \mathrm{Cc}$$

(4)

where BD is the bulk density (Mg m⁻³), and Cc is the clay content (kg 100 kg⁻¹).

2.3.2. Penetration Resistance

Soil penetration resistance was investigated using a penetrologger UGT-Art.-Nr.: 5060407 Eijkelkamp. The penetrologger was prepared for measurements to a depth of 70 cm. At the investigated site, the probing rod that ended with proper cone at the defined point was propelled down into the soil. The soil resistance values to probing rod penetration at each soil layer of the ground profile were measured and saved in the data-logger. The measurements were performed with 10 replicates at each investigation site and the measurement point.

2.4. Simulation of Water Flow

To simulate the water transfer in the variably saturated soil, a HYDRUS-1D model was applied in the current work which was numerically provided by the method of Simunek et al. [42]. In addition, the HYDRUS-1D code mainly depends on the Richards equation for the one-dimensional water flow. The flow equation was explained as:

$$\frac{\partial \theta \left(h,t\right)}{\partial t}=\frac{\partial }{\partial z}\left[k\left(h\right)\left(\frac{\partial h}{\partial z}+1\right)\right]$$

(5)

where $\theta$ is volumetric water content, h is soil water pressure head, k(h) is unsaturated hydraulic conductivity, z is vertical coordinate at the soil surface (negative downward), and t is time. The initial condition and upper boundary conditions for the studied experiment were:

h(0,t) = h₀

(6)

h(z,0) = h_i(z)

(7)

where h0 is the water potential at upper soil surface, and h_i(z) is the initial water pressure head within the soil column. Van Genuchten–Mualem model was utilized for the selected areas [43]:

$$\frac{\theta -\theta r}{\theta s-\theta r}=\left(1{\left|\alpha h\right|}^{\mathrm{n}}\right){}^{-\mathrm{m}}h>0$$

(8)

$$\theta =\theta sh\le 0$$

(9)

where $\theta$ is volumetric water content (cm³ cm⁻³), h is soil pressure head (cm), $\theta s$ and $\theta r$ are the saturated and residual water contents (cm³ cm⁻³). While $m$, $n$, and $\mathsf{\alpha }$ are empirical parameters and the m value calculated using van Genuchten-Mualem model with m = 1–1/n. The HYDRUS-1D code connected mainly with the ROSETTA model of [44]. For the boundary conditions in this investigation, we used the van Genuchten–Mualem model with no-hysteresis to study the soil hydraulic properties for all the sites. The upper boundary conditions for the investigated sites were in pressure head −1 cm, which were applied during the infiltration rate experiment using the hood infiltrometer, while the lower boundary condition was free drainage.

3. Results and Discussions

3.1. Infiltration Measurements

The infiltration capacity was approximately two times higher under the organic management (OM) field compared with that of the conventional management (CM) field with an average of 624 mm/h for the former field and 303 mm/h for the latter one. The physical soil properties showed that, in comparison with the CM system, the OM system showed an improved steady state infiltration rate (Figure 2). The results agree with the findings of [45,46] who revealed that organic management improved the soil structure and increased the soil biological activity through increasing the infiltration rate of heavy soils. Where the conversion to OM had been established for 8 years, the differences in steady state infiltration were scale-dependent to the tillage regime (Figure 3). This was supported by the Ks that were computed under different tillage systems.

Comparisons of the two systems regarding hydraulic parameters are displayed in

Table 2. Impact of land management on certain soil physical and hydraulic properties.

Sample Location	Crop	Soil Management	eBD	qs (h)	Ks
			Mg m−3	(cm h−1)	(cm h−1)
1	L	Organic	1.63	54.6	33.17
	L	Conventional	1.9	30.6	18.59
2	WW	Organic	1.55	47.4	28.79
	WW	Conventional	1.56	25.8	15.67
3	E	Organic	1.67	78.6	47.75
	E	Conventional	1.6	43.8	26.61
4	SE	Organic	1.32	64.2	38.99
	SE	Conventional	1.71	24.6	14.94
5	WW	Organic	1.48	67.2	40.82
	WW	Conventional	1.75	27	16.4
St. dev.			0.15	19.49	11.78

eBD: Effective bulk density; qs: steady state infiltration rate; K_s: saturated hydraulic conductivity; E: Erbse; L: Luzerna; WW: Winter Wheat.

. The results reveal higher K_s values under OM compared with CM. Saturated hydraulic conductivity reflects land management independent of the intrinsic soil matrix and clay content (Table 2). There was a higher frequency of the bio-pores and/or good efficient macro-pores [46] under OM than CM, which would increase water preferential flows. These results are in agreement with the results of [2,23,40], who mentioned that soil hydraulic characteristics are related to the applied management practices. [47] noticed that organic farming systems have higher hydraulic conductivity compared to conventional farming practices due to greater values of pore roughness (i.e., fractal dimensions) that are connected with greater biological activity [46]. The frequent disturbance of soil structure due to conventional tillage operations contributes to the lower efficiency of soil physical properties in conventional farming systems [5,10]. Organic farming, on the other hand, integrates with greater cultivation requirements than conventional farming. [48] reported that organic farming improves soil structure. However, they reported that, on some soil types, decades of organic farming may not be appropriate to create good conditions for the soil structure by extensive cultivations. Ref. [49] reported that intensive tillage practices could lead to greater oxidation of organic matter, hence decreasing aggregate stability.

3.2. Degree of Compactness

The values of effective compactness eBD were associated with the bulk density values that were lower under OM than CM (Table 2). Such results are in harmony with the results of infiltration capacity under different management systems (Figure 3). This might be due to the high frequency of soil micro-pore creation as a result of organic farming. The impact of OM at Location 3 (long-term organic farming) was the same as the other locations, there were even differences in soil texture type in the OM and CM, and the results revealed greater infiltration rate on OM than CM (Figure 3). In this particular location, the eBD for the OM was slightly higher than that for the CM. This might be due to slightly higher compactness in the organic field due to the high clay content (Table 2). Values that were obtained using the penetrologger associated with the hood infiltrometer device in Location 3 indicate that penetration resistances for OM within the soil profile 3 were lower than for CM (Figure 4), and the steady state infiltration rate for OM was 80 % higher than for CM. These results indicate that tillage in this location could be the main factor causing soil compaction. The results are in agreement with [3,45], who revealed that organic matter improves soil structure. Therefore, the CM would show more compaction. Ref. [50] stated that soil vulnerability to compaction would decrease the soil macro-pores, and hence increase the soil dry bulk density and penetration resistance, resulting in reducing the water infiltration rate.

The OM showed lower penetration resistance, especially in the upper 0–10 cm layer (Figure 4), while the CM soils at 10–70 cm depth showed higher penetration. These results show that when the soil is converted to OM, more compactness could appear in the deep soil. This could lead to a decrease in macro-pores and bio-pores. These results agree with [14,18], who found variable differences between organic and conventional management practices. In addition, Ref. [51] noticed that BD values were not significantly different among the soil depths lower than 10 cm for the cropland.

3.3. Simulation of One-Dimensional Water Flow

Hydrological parameters are shown in

Table 3. Hydraulic parameters of the investigated soil under different land-management.

Field Management	Soil Type	eBD	Theta r	Theta s (cm3 cm−3)	Alpha (1/cm)	n	Ks
		Mg m−3	(cm3 cm−3)				(cm.h−1)
Conventional	Clay	1.9	0.095	0.46	0.012	1.367	0.184
Organic	Clay	1.63	0.102	0.535	0.014	1.374	0.803

Theta r (Q_r): Residual water content (cm³ cm⁻³); Theta s (Q_s): Saturated water content (cm³ cm⁻³); K_s: Saturated hydraulic conductivity (cm d⁻¹); Alpha: Sorptivity number (1/cm); n: pore-size distribution index.

. The hydrological parameters (Qr, Qs, α, n) were optimized when implementing the obtained field results of the particle size distribution, soil hydraulic parameters, and the effective bulk density into the Hydrus-1D model (Table 3). The obtained results are consistent with [31,52].

Figure 5 shows a logarithmic decrease for the saturated hydraulic conductivity with the pressure head distribution in the soil matrix (0–30 cm). This indicates that the impact of management was predominant. The analyzed K_s values that were obtained by the Hydrus-1D model revealed that the K_s of OM surpassed that of CM. Simulated K_s at a depth of 10 cm under CM was 0.01 and 0.02 cm/hour after one hour and two hours, respectively, compared with 0.13 and 0.26 cm/hour given by the OM soils (Figure 4). This reflects an increased soil wetting front, higher biological activity [45], more “bio-pores” of soil [46] under OM, and hence a higher infiltration capacity in comparison.

The cumulative infiltration curve was more abundant under OM than CM (Figure 6a). This relates to the effect of water retention during infiltration, where the infiltration rate for OM 624 mm/h and that for CM was 303 mm/h. The results of Figure 6a are in harmony with the results of K_s versus volumetric water content (Figure 6d) which shows increasing K_s under OM compared with CM. The soil required 4 h under OM to reach the constant storage capacity compared with 12 h for CM (Figure 6b). These results agree with [53], who found that a low soil micro-porosity negatively affected root development and water storage capacity. They also agreed with those of [29], who stated that soil hydraulic properties are affected by soil management.

Water retention curves are shown in Figure 6c. Distinct differences were detected where OM showed higher values than CM. At the field capacity level, the volumetric water content at was 0.312 cm³ cm⁻³ for CM compared with a higher value of 0.347 cm³ cm⁻³ for the OM. Differences were apparent with decreasing the water tension. These results agree with [2,15], who concluded that organic farming is a more adequate farming compared with conventional farming.

4. Conclusions

This study was focused on the numerical evaluation of the impact of organic management (OM) practice versus conventional management (CM) on the soil, to assess if and how conservation tillage practices in OM do improve soil physical properties compared to CM. Organic farming promotes more appropriate soil physical conditions for plant growth than conventional farming. Also, conservation tillage coupled with organic farming stratified soil quality in the upper soil layer. The results of the infiltration rate, hydraulic conductivity, and effective bulk density properties and other related parameters indicate such a conclusion. The obtained results revealed organic farming practices reduces the bulk density and increases infiltration. In addition, changing soil management from conventional to organic farming decreases penetration resistance, especially in the soil upper layers. However, with deeper soil, soil compaction was noticed using conservation tillage in OM, which could affect deep rooting. The numerical model Hydrus-1D proved useful in assessing the hydraulic parameters and simulated the water transfer in the soil matrix. We conclude that it will be essential to monitor and follow up this research with the soil depth to reflect the management practice impacts on soil compaction and hence the root distribution and potentiality on their crop yield.