Tillage Effects on Soil Hydraulic Parameters Estimated by Brooks–Corey Function in Clay Loam and Sandy Loam Soils

Abstract

Tillage practices can significantly impact soil structure and pore size distribution and connectivity, consequently affecting the shape of the soil water retention curve (SWRC) and its related estimated hydraulic parameters in the top layer of soil. This study investigated the effect of no-tillage (NT) and conventional tillage (CT) practices on SWRCs and their soil hydraulic parameters, estimated by the Brooks–Corey (BC) function at 0–15 and 15–30 cm depths within sugarbeet and corn planting rows in clay loam and sandy loam soils, respectively. Soil water retention curves were measured using the evaporative method (HYPROP). Measured SWRC results were modeled for both untilled and tilled soils using the BC function for each depth in both soils. In clay loam, results indicated that all soil parameters of the BC function, water contents at 330 (θ₃₃₀) and 15,000 (θ_15,000) hPa, and plant available soil water content (AW) were not significantly affected by the type of tillage at either soil depth. The lack of difference in results between NT and CT may be due to considerable soil disturbance, primarily by the harvest process of sugarbeet roots. However, in sandy loam, results indicated that differences occurred in SWRC’s estimated parameters between the NT and CT practices. Averaged across 4 years and two soil depths, the pore size distribution index (λ) and saturated water content (θ_s) were significantly larger under CT than under NT due to greater soil loosening and disturbance caused by multiple passes of the CT process, thereby developing more soil macroporosity. However, the θ₃₃₀ and AW were significantly larger in NT than in CT due to reduced soil disturbance and improved soil structure under NT compared to CT practices. Regardless of tillage, measurements of SWRC are important for determining better irrigation management practices, enabling producers to optimize crop productivity, while saving water and sustaining water quality.

1. Introduction

Tillage and soil disturbance have a direct and significant impact on the physical and hydraulic characteristics of soil, including soil water retention curves (SWRCs) and their estimated parameters [1,2,3,4,5,6]. The SWRC is the ability of soil to retain and release water due to changes in soil suction, that explains the relationship between soil water content and soil water potential [7,8], which can be used to estimate various physical and hydraulic properties under unsaturated soil conditions. The shape of the curve differs for various soils and is generally related to particle size, aggregate size, and pore size distribution in each soil type [1,5,7,8]. For example, clay-textured soils typically retain more water than sandy-textured soils at the same matric potential or suction force level.

Information derived from the SWRC and related properties are important for soil, irrigation, drainage, modeling, and hydrological studies and applications [5,6,7,8,9].

Through the years, numerous mathematical models and equations have been proposed to describe SWRCs and estimate their hydraulic and physical parameters [10,11,12]. Commonly used models include the van Genuchten, Brooks and Corey and Gardner models [12,13,14,15,16].

Pan et al. [16] evaluated and compared the van Genuchten, Brooks–Corey and Gardner models for their ability to determine the best fit model for SWRCs in silt loam and sandy loam soils in a degraded alpine meadow region. They found that SWRCs were best fit by the van Genuchten and Brooks–Corey models compared to the Gardner model for a wide range of soils. They also reported that residual water content and air-entry potential were lower when using the Brooks–Corey model than when applying the van Genuchten model.

Ahuja et al. [1] used the Brooks–Corey model to estimate SWRC parameters of tilled and untilled soil. They found that tillage did not significantly change the air-entry pressure head value in silty clay loam and clayey soils under field conditions.

To date, there is a lack of information on the effect of different tillage systems on SWRCs and their fitted parameters using the BC model. Therefore, the objective of this research paper was to evaluate the effect of no-tillage (NT) and conventional tillage (CT) practices on the estimated parameters of Brooks–Corey’s model (BC) for SWRCs at 0–15 cm and 15–30 cm depths in sugarbeet (Beta vulgaris L.) and corn (Zea mays L.) planting rows in clay loam and sandy loam soils, respectively. We hypothesized that estimated soil hydraulic parameters of BC equation for SWRC differed between NT and CT in clay loam and sandy loam soils due to more soil loosening and disturbance under CT.

The BC model is relatively simple and accurate and has been widely used to fit measured SWRCs and estimate soil hydraulic parameters for various soils and under different conditions [12,16,17,18,19,20,21,22,23].

2. Materials and Methods

The studies were conducted at two U.S. Northern Great Plain region sprinkler irrigated field sites with differing soil texture, one in western North Dakota and the other in North-Eastern Montana.

2.1. The EARC Research Site

The Montana site was located at the Montana State University Eastern Agricultural Research Center (EARC) near Sidney, MT, USA (47.7255° N, 104.1514° W, elevation 650 m) within a long-term tillage and crop rotation study (2018–2023). The soil series at this site is Savage clay loam (fine, smectitic, frigid Vertic Argiustolls) with 18.4, 18.5% sand; 35.1, 34.7% silt; and 46.5, 46.8% clay at 0–15 and 15–30 cm depths, respectively, that formed in alluvium parent material [6]. The average soil bulk density at a depth of 0–30 cm was 1.40 g/cm³ for NT plots and 1.43 g/cm³ for CT plots. Organic matter content and pH were 15.3 g/kg and 6.80, respectively, in the top 20 cm of soil.

During the 2019 sugarbeet growing season, the crop received 446 mm of rainfall and 203 mm of irrigation, while experiencing 513 mm of water loss through evapotranspiration (ET).

The main plots (14.7 m wide × 30.5 m long) for the original cropping system study of a 3-year sugarbeet–dry pea–spring wheat rotation and three tillage treatments of no-tillage (NT), strip tillage (ST), and conventional tillage (CT) were arranged in a balanced randomized complete block design with five replications [6]. The ST results and information were not included in this manuscript to be consistent with two tillage treatments at the Nesson location.

2.2. The Nesson Research Site

The North Dakota site was located at a North Dakota State University Williston Research Extension Center irrigated research farm (Nesson) in western North Dakota (48.1640° N, 103.0986° W. elevation 584 m), approximately 37 km east of Williston, USA. The soil at the site was Lihen sandy loam (sandy, mixed, frigid Entic Haplustoll), consisting of very deep, well-drained soil that formed in sandy alluvium, glacio-fluvial, and eolian deposits in places over till or sedimentary bedrock. Sand, silt, and clay amounts were 70, 16, and 14%, respectively, at the 0–15 cm depth and 74, 13, and 13%, respectively, at the 15–30 cm depth. The average soil bulk density for NT plots was 1.65 g/cm³ and 1.59 g/cm³ for CT plots at 0–30 cm depth. Soil organic matter content was 18.7 g/kg in the top 20 cm depth and pH was 6.82 at 0–25 cm depth.

During the 2014–2017 growing seasons, the average total crop water requirement (evapotranspiration, ET) for corn was 493 mm, while the average rainfall was 214 mm and the average supplemental irrigation provided was 348 mm.

The experimental design at the Nesson site is a randomized complete block design with 5 replications into plots, each 11 m wide × 24 m long. Treatments consisted of 2-year corn–soybean and two tillage practices NT and CT [5].

Each phase of the tillage and rotation occurred every year at both locations and all plots were irrigated using a linear-move overhead sprinkler system to maintain soil moisture content above 50% of maximum plant available water-holding capacity. The measured field water capacity for sandy loam and clay loam soils was approximately 0.228 and 0.344 cm³ cm⁻³, respectively.

More information regarding tillage operations, planting, fertilizer applications, corn and soybean varieties, irrigation amounts, weed control, and other farming activities were provided by [5,6,24,25].

2.3. Soil Core Sampling and Preparation

Intact soil cores were sampled using stainless steel cylinders (8 cm diameter × 5 cm height) from 0 to 15 cm and 15–30 cm depths in sugarbeet and corn rows at one sample per plot in the spring of 2019 for the EARC site and in the fall of 2014, 2015, 2016, and 2017 for the Nesson site, respectively [5]. Soil cores were saturated by capillary action prior to developing SWRCs using the evaporation HYPROP method [26] as shown in Figure 1. More information regarding soil sampling, sample preparation, and laboratory procedures were provided by Jabro and Stevens [5], Jabro et al. [6], and Schindler et al. [26].

2.4. Brooks–Corey Model

The Brooks–Corey (BC) model [14] was used to define SWRC in unsaturated soils under two tillage systems at both locations. The soil hydraulic parameters according to BC model are given as follows:

$$\mathsf{\theta }(\mathrm{h})={\mathsf{\theta }}_{\mathrm{r}}+\left({\mathsf{\theta }}_{\mathrm{s}}-{\mathsf{\theta }}_{\mathrm{r}}\right){\left({\displaystyle \frac{\mathrm{h}}{{\mathrm{h}}_{\mathrm{a}}}}\right)}^{-\mathsf{\lambda }} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{h}\le {\mathrm{h}}_{\mathrm{a}}$$

(1)

$$\mathsf{\theta }\left(\mathrm{h}\right)={\mathsf{\theta }}_{\mathrm{s}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{h}>{\mathrm{h}}_{\mathrm{a}} \mathrm{w}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e} {\mathrm{h}}_{\mathrm{a}}={\displaystyle \frac{1}{\mathsf{\alpha }}}$$

(2)

Then, Equation (1) can be expressed in terms of α as

$$\mathsf{\theta }\left(\mathrm{h}\right)={\mathsf{\theta }}_{\mathrm{r}}+\left({\mathsf{\theta }}_{\mathrm{s}}-{\mathsf{\theta }}_{\mathrm{r}}\right)(\mathsf{\alpha }\mathrm{h}{)}^{-\mathsf{\lambda }} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{h}>{\displaystyle \frac{1}{\mathsf{\alpha }}}$$

(3)

$$\mathsf{\theta }\left(\mathrm{h}\right)={\mathsf{\theta }}_{\mathrm{s}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{h}\le {\displaystyle \frac{1}{\mathsf{\alpha }}}$$

(4)

where θ is the volumetric water content (%); h is the matric potential or matric suction (cm or hPa); θ_s and θ_r represent the saturated and residual water contents (%), respectively; α is a parameter that is inversely related to the air-entry potential value h_a (cm⁻¹ or hPa⁻¹); and λ is a dimensionless index related to soil pore size distribution.

2.5. Data Analysis

Tillage effects on estimated parameters by BC model for SWRC were evaluated using the mixed model of SAS, version 9.2 [27]. Tillage was considered as the fixed effect and replication as the random effect for the EARC location.

For the Nesson location, tillage effects on hydraulic parameters for SWRC were determined using the mixed model with repeated measures analysis using the year as a repeated measure interval. Tillage type was considered as the fixed effect and replication as the random effect. The least significant differences at p < 0.05 were used to compare between tillage treatment means.

3. Results and Discussion

3.1. EARC Site

The measured and estimated SWRCs using the BC function for clay loam soil at 0–15 cm and 15–30 cm depths are displayed in Figure 2 for NT and Figure 3 for CT.

The estimated α, λ, θ_r, and θ_s of the BC equation, volumetric water content at 330 hPa matric potential (θ₃₃₀), volumetric water content at 15,000 hPa matric potential (θ_15,000), and plant available water capacity (AW) parameters were not significantly affected by tillage (

Table 1. Analysis of variance for estimated parameters of soil-water retention curves for EARC (clay loam) and Nesson (sandy loam) based on the Brooks-Corey (BC) equation; α is an inverse matric potential at air-entry value, λ is a parameter related to pore-size distribution, θ_r and θ_s are the residual and saturated water contents, respectively, θ₃₃₀ and θ_15,000 are volumetric water contents at 330 and 15,000 hPa matric potentials, respectively, and AW (θ₃₃₀–θ_15,000) is available water content.

Location	Source	α	λ	θr	θs	θ330	θ15,000	AW (θ330–θ15,000)
EARC	Tillage	NS	NS	NS	NS	NS	NS	NS
Nesson	Tillage	NS	0.0103	NS	0.0056	0.0184	NS	0.0244

NS indicates not significant at p < 0.05.

).

The results of analysis of variance for soil hydraulic parameters at 0–15 cm, 15–30 cm, and across both depths under NT and CT are listed in

Table 2. Estimated Brooks–Corey (BC) model parameters of the soil water retention curve for clay loam (EARC) at 0–15 and 15–30 cm depths under no-tillage (NT) and conventional tillage (CT) practices. The α parameter is an inverse matric potential at air-entry value (cm⁻¹), λ is an index of pore size distribution, θ_r and θ_s are the residual and saturated water contents (%), respectively, θ₃₃₀ and θ_15,000 are volumetric water contents at 330 hPa and 15,000 hPa matric potentials, respectively, and AW (θ₃₃₀–θ_15,000) is available water content.

Depth	Tillage	α cm−1	λ	θr	θs	θ330	θ15,000	AW (θ33–θ15,000)
%
0–15	NT	0.1368 (0.05)	0.1436 (0.04)	9.98 (0.06)	44.48 (0.01)	33.06 (1.9)	25.04 (3.1)	7.99 (1.2)
	CT	0.1473 (0.02)	0.1582 (0.02)	10.46 (0.02)	48.26 (0.01)	31.55 (0.7)	22.21 (1.2)	9.34 (0.5)
15–30	NT	0.1688 b (0.05)	0.1074 (0.01)	10.94 (0.02)	43.48 (0.01)	32.61 (1.3)	25.27 (1.5)	7.34 (0.3)
	CT	0.1242 a (0.03)	0.1270 (0.03)	12.50 (0.02)	42.74 (0.02)	34.11 (1.4)	26.60 (1.4)	7.60 (0.3)
Average across 2 depths
0–30	NT	0.1528	0.1255	0.1091	43.98	32.68	25.00	7.68
	CT	0.1191	0.1443	0.1148	45.50	32.64	24.09	8.55

Different letters indicate significant effects at p < 0.05. Values between parentheses represent one standard error.

. Results indicated that the estimated parameters of the BC equation and soil hydraulic properties were not affected by tillage except for the α parameter at 15–30 cm depth (Table 2).

The lack of tillage effect on soil hydraulic properties could largely be associated with soil disturbance and loosening during the sugarbeet root harvest, because soil under NT was considerably disturbed by the harvest operation in the fall of 2018. Apparently, soil disturbance by the harvest process of sugarbeet roots increased total soil porosity and altered pore size distribution and soil structure, consequently affecting soil water retention curves and their estimated hydraulic parameters [6]. This harvest-related soil disturbance by roots diminished the expected benefits of NT system.

However, the α parameter was significantly greater in NT than in CT at 15–30 cm depth (Table 2). This could indicate that untilled soil under NT at this depth held more water at low matric potential than tilled soil under CT. However, tilled soil usually has a smaller air-entry value than untilled soil, because tillage disrupts soil structure, creating larger pores which allow air to enter easily at a lower matric potential, resulting in a larger α value compared to untilled soils [5,28].

Findings from this study suggested that conservation tillage practices such as NT can improve soil water retention, particularly in clayey soils, leading to better plant AW capacity compared to CT practices.

Recently, Hashimi et al. [29] reported that soil field capacity and plant AW content were significantly greater under NT practices compared to moldboard plow tillage systems at the 0–30 cm layer in Andosols soil.

3.2. Nesson Site

The measured and fitted SWRCs using the BC function for sandy loam are presented only for 2014 because the year effect was not significant for all parameters. However, the fitted parameters for SWRCs based on the BC function and their statistics for 2014, 2015, 2016, and 2017 are presented in this manuscript. The measured and fitted SWRC under NT and CT at 0–15 cm and 15–30 cm depths for 2014 are depicted in Figure 4 and Figure 5, respectively.

The results of the analysis of variance for the estimated parameters of the BC function and other derived hydraulic parameters are listed in Table 1 and

Table 3. Estimated Brooks–Corey (BC) model parameters of the soil water retention curve for sandy loam (Nesson) at 0–15 and 15–30 cm depths for 2014, 2015, 2016, and 2017 and their averages under no-tillage (NT) and conventional tillage (CT). The α parameter is an inverse matric potential at air-entry value (cm⁻¹), λ is an index of pore size distribution, θ_r and θ_s are the residual and saturated water contents (%), respectively, θ₃₃₀ and θ_15,000 are volumetric water contents at 330 hPa and 15,000 hPa matric potentials, respectively, and AW (θ₃₃₀–θ_15,000) is available water content.

Year	Depth cm	Tillage	α cm−1	λ	θr	θs	θ330	θ15,000	AW (θ330–θ15,000)
%
2014	0–15	NT	0.0680	0.4096	6.59	39.26	16.64	8.94	8.02
		CT	0.0839	0.4206	6.78	41.56	15.95	8.82	7.13
	15–30	NT	0.0644	0.2982 a	5.01	37.04	17.80	8.62	9.18
		CT	0.0536	0.4266 b	7.00	38.90	16.98	9.17	7.81
2015	0–15	NT	0.0700	0.3198 a	2.50	38.44	17.71	6.63	11.08
		CT	0.0548	0.4738 b	5.93	42.34	15.55	5.40	10.16
	15–30	NT	0.0648	0.2970	3.82	35.56	16.92	8.09	8.83
		CT	0.0625	0.5230	8.63	39.56	16.08	8.16	7.66
2016	0–15	NT	0.0556	0.4254	7.41	38.12	16.92	8.97	7.94
		CT	0.0597	0.5288	7.72	41.58	15.96	9.04	7.26
	15–30	NT	0.0686	0.2322 a	2.33	35.60	18.49 b	8.61	9.87 b
		CT	0.0615	0.5112 b	7.16	39.70	14.88 a	7.98	7.07 a
2017	0–15	NT	0.0624	0.4394	5.80	38.82	16.21	8.46 b	8.57 b
		CT	0.0510	0.5588	6.68	41.12	14.45	7.69 a	6.77 a
	15–30	NT	0.0567	0.3140	5.40	36.90	17.39	7.69	9.70
		CT	0.0537	0.4196	4.70	39.84	15.87	7.19	8.67
Average across 4 years
	0–15	NT	0.0636 (0.01)	0.4046 (0.04)	5.92 (0.6)	38.70 a (0.6)	16.80 b (0.6)	8.33 (0.4)	8.77 (0.4)
		CT	0.0624 (0.01)	0.4980 (0.03)	6.86 (0.4)	41.61 b (0.6)	15.36 a (0.3)	7.74 (0.4)	7.62 (0.4)
	15–30	NT	0.0634 (0.0)	0.2855 a (0.02)	4.54 (0.7)	36.26 a (0.4)	17.66 (0.5)	8.28 (0.5)	9.37 (0.3)
		CT	0.0578 (0.0)	0.4700 b (0.03)	6.58 (0.6)	39.46 b (0.7)	16.05 (0.5)	8.16 (0.5)	7.87 (0.5)
Average across 4 years and 2 depths
	0–30	NT	0.0638	0.3400 a	5.16	37.47 a	17.31 b	8.28	9.21 b
		CT	0.0601	0.4852 b	6.75	40.57 b	15.66 a	7.94	7.82 a

Different letters indicate significant effects at p < 0.05. Values between parentheses represent one standard error.

. The λ, θ_s, θ₃₃₀, and AW parameters were significantly affected by the type of tillage; however, α, θ_r, and θ_15,000 parameters were not different between the two tillage systems (Table 1).

Averaged across 4 years, λ was significantly larger under CT compared to NT at 15–30 and θ₃₃₀ was significantly greater in NT than in CT at 0–15 cm depth. However, θ_s was significantly larger under CT than under NT at both depths (Table 3). These variations could be associated with an increased total porosity and less soil compaction in CT plots compared to NT system, particularly in the topsoil layer.

When averaged over a 4-year period (2014–2017) and two depths (0–15 and 15–30 cm), the parameters λ and θ_s were statistically greater in soils under CT than soils under NT. The higher λ and θ_s values in CT plots were likely related to soil loosening and disturbance caused tillage, thereby creating larger soil pores and a large proportion of macroporosity [5].

On the other hand, θ₃₃₀ and plant AW were significantly larger in untilled soils than in tilled soils (Table 3) due to reduced soil disturbance, better soil structure, and increased organic matter content under NT, resulting in a greater soil water retention compared to CT practices. The NT practices tended to increase θ₃₃₀ and plant AW amounts by approximately 10.5% and 17.8%, respectively, due to greater microporosity and minimal soil disturbance under NT practices compared to CT. These results agreed with those found by Jabro and Stevens [5], estimated by the van Genuchten function [15] using the same dataset. Furthermore, Hashimi et al. [29] and Alam et al. [30] concluded from their studies that NT practices increased soil field capacity and plant AW contents compared to moldboard plow and conventional tillage practices.

Blanco-Canqui and Ruis [31] reviewed 14 studies on tillage effects on soil water retention and plant AW. In their paper, they reported that NT increased plant AW in seven studies, had no effect on AW in five studies, and decreased AW in two studies compared to moldboard plow and conventional tillage systems. In another long-term study, Blanco-Canqui et al. [32] revealed that NT farming had no positive impact on soil water retention and AW compared to CT practices. Further, they concluded that in addition to tillage type, other soil properties such as organic matter, soil texture, pore size distribution, and soil structure could greatly impact the ability of soils to hold water [5,7,8,33,34].

4. Conclusions

Soils under NT system were greatly disturbed due to the sugarbeet root harvest process in the fall of 2018. Therefore, the effect of different tillage systems on SWRC’s estimated parameters becomes less relevant under highly disturbed conditions in clay loam soil at the EARC research site.

Soils under CT practices tend to have greater number of macropores due to the tillage process, resulting in a smaller soil water-holding capacity and AW content compared to the NT system in sandy loam soil at the Nesson site.

Soil water retention curves under any tillage practice have been considered an important attribute in estimating water movement in soils under saturated and unsaturated conditions, in determining irrigation thresholds, and in interpreting other hydrological processes and soil studies. This soil–water interaction information is extremely helpful for determining effective irrigation management practices that ensure efficient use of water to sustain crop yields and to minimize adverse environmental impacts.

More comprehensive field research is required to fully understand how different tillage practices influence soil physical and hydraulic properties as well as soil health across diverse soil types and cropping systems.