1 Introduction

Soil is one of the most precious natural resources and its proper management is a key for sustaining agricultural production. Agriculture production has primarily focused on increasing yields and ensuring sustainability. However, Ethiopia's highlands are prone to soil erosion due to deforestation, overgrazing, cultivation of unsuitable slopes, and farming practices without conservation measures [2].

Degraded soils are also a significant constraint to agricultural production and food security in the Southern Ethiopian highlands. Since, the greatest concern for soil degradation is erosion caused by water and the potential for erosion of a specific soil type largely depends on the severity of the slope, the crops that are grown, and the number and types of tillage operations [14]. The annual soil loss rate in Northern Ethiopia has been estimated at 25 tons per hectare per year [11] and at 18 tons per hectare in Southern Ethiopia, which is above the acceptable range for the country [25]. Therefore, to combat this challenge, crop residue management, crop rotation, minimized contour tillage, grass waterways, terraces, and conservation structures are some of the approaches. Since, the methods used must guarantee the long-term productivity of the land, be ethical in terms of the environment, and of course be profitable [4]. Among these, crop residue management and minimized contour tillage are regarded as effective, affordable solutions to lower soil erosion and preserve production [19], while, the use of agricultural mechanization is considered the main factor contributing to the total energy inputs in the agricultural system. Half of the operations carried out annually in the agriculture fields are related to tillage. Consequently, there is a probable to reduce energy inputs and production expenses by decreasing tillage [20].

Tillage manages plant residue by incorporating it into the soil or retaining it on the top layer to reduce erosion, to improve the physical condition of the soil so that rainwater can be adsorbed easily and soil erosion can be minimized to permit root elongation and proliferation due to low-density soil, etc. [16]. A reduced conservation tillage system does not include traditional tillage techniques that bury crop residues and turn the soil upside down. Zero-tillage (only soil disturbing for crop planting), strip tillage, and reduced tillage (two times tillage) are types of conservation tillage systems [10].

Therefore, conservation tillage is very important to combat soil erosion because, it reduces the effect of raindrop impact and formation of hardpan layers on the soil surface, increases infiltration of water into the soil, slows the breakdown of organic matter in the soil, provides a better soil environment for crop growth, labor-saving and increases soil organic matter, and it creates conditions that save susceptible soils from compaction or crusting [16].

In Ethiopia, particularly in the Southern region, various researches were conducted on the impact of tillage methods on soil availability for plants [3]. However, the impact of reduced tillage practice on soil erosion control by saving runoff on-farm and preventing loss of soil material from farmlands has not been studied. Therefore, the present study was initiated to evaluate the impact of tillage practice on some selected soil physicochemical properties, soil loss, and yield and components of maize in the South Ari District.

2 Material and methods

2.1 Study area description

The experiment was carried out in the South Ari District of the South Omo Zone, Southern Ethiopia, for two consecutive years (2019 and 2020). (Fig. 1). The study site is geographically located in the ranges of 05°51′–05°88′ N and 36°32′–36°58′ E with an elevation of 1404–1597 mean above sea level. It is located 542 km from Hawassa; it is the capital city of former SNNPR, Ethiopia.

Fig. 1
figure 1

Location map of the study area

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The study site experiences two distinct seasons of rainfall: the shortest, which runs from March to May, and the longest, which runs from August to November. There is 1272.4 ± 250.7 mm of rainfall in total each year. The average annual temperature is 16.3 ± 0.9 °C for the minimum and 27.7 ± 1.4 °C for the maximum. Because weather data was collected solely during the experiment season, there are missing values for certain months of the year; as a result, the monthly average values for those climate elements were taken into consideration as the only available records.

2.1.1 Treatments and experimental design

A randomized complete block design (RCBD) was used to set up the experiments on five farmers’ fields’ (with four treatments and a farmer as replication) (Fig. 2). The study site was selected on farmland under 15–20% slope where, erosion occurrence, the similarity of farming systems, rain-fed and soil types were considered. A local plow operated by oxen (Maresha) was used to prepare the trial field There were four (4) tillage treatments; zero tillage (the only described to plant the maize), two times tillage (reduced tillage), strip tillage (only the strip that was used as seedbeds 25 cm were plowed and the strip between the tilled rows were left under zero tillage) and conventional tillage (farmers practice means four-times plowed).

Fig. 2
figure 2

Schematic diagram of field experimental layout

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The plot size of (10 m*10 m) and 1 m spacing between plots, also spacing between rows 75 cm and between plants 25 cm was used. 100 kg of nitrogen, phosphorous, sulfur and boron NPSB was applied at planting time but, hundred Kg of Urea fertilizer was split twice; half was applied at planting and the remainder was applied after 35 days of planting. NPSB and Urea were used as sources of Nitrogen, Sulphur, Boron and Phosphorous. All agronomic practices were considered in the trial period. Weeds in reduced tillage methods (two times and zero tillage) were managed by application of 4 l/ha round-up chemical herbicide (Glyphosate) and applied 15 days before sowing maize (Zea mays L.). But, for other treatments, all management practices such as; weeding, pest control, etc. were applied as per research recommendations for maize (Zea mays L.).

2.2 Data collection

2.2.1 The soil sampling and laboratory analysis

Before experimentations, five composited surface and sub-surface soil samples were collected separately from five farmers' farmlands, whereas, after maize crop harvesting sub-surface soil samples were collected separately from each farmer’s field as well as from each plot of tillage practices by zigzag movement from five spots to the depth of 0 to 30 cm using an augur. All collected soil samples were air-dried and grounded, and then physicochemical parameters (soil pH, organic-carbon concentration (OC), total nitrogen concentration (TN), availability of phosphorus and exchangeable acidity) were analyzed at the soil laboratory of Jinka Agricultural Research Center, Jinka, Ethiopia. Soil pH was measured with a combination of electrodes in a 1:2.5 (volume/volume) soil-to-water suspension. Available P was analyzed by sodium bicarbonate (NaHCO3)-extractable P according to the standard method described by Olsen [18]. OC was determined according to the Walkley and Black method [23] while total N was measured following the semi-micro Kjeldahl method [5]. Cation exchange capacity (CEC) was determined titrimetrically by distillation of ammonium displaced by sodium [6].

Soil texture was determined quantitatively by using the hydrometer method. The soil moisture status on different tillage methods was monitored by collecting soil samples at the planting of the test crop, 65 days after planting (development stage) and after crop harvest for consecutive two years. However, for bulk density undisturbed soil samples while for available water disturbed soil samples were taken by using core samplers at a depth of 0–30 cm from each treatment at three stages: at planting of the test crop, after 65 days (at development stage) and after 95 days (at harvesting). Initially, fresh soil was weighed and oven-dried at 105 °C for 24 h, and then dry soil was weighed. Then, the bulk density of the soil was calculated by the following formula.

$$BD=\frac{DSW}{SV}$$

where BD = bulk density (g/cm3), DSW = dry soil weight (g) and SV = soil volume (cm3).

Also, the percentage of soil moisture content was calculated by using the gravimetric method [21] and then the average means were used for interpretation.

$$SMC=\frac{Ww-Wd}{Wd}* 100$$

where SMC = Soil moisture content dry base (%), Ww = Weight of the wet soil (gm) and Wd = Weight of the dry soil (gm).

To measure the soil loss amount catch pit was set up under each pilot 10 m × 1 m × 0.5 m and the amount of the soil sediment in the catch pit was measured after every rainfall. The highest comparative soil loss was recorded on conventional tillage practice (X2) whereas, the other compared tillage treatment (X1). Then, the measured amount of the soil sediment was computed to compare each treatment performance of soil loss reduction by using the following formula.

$$SLR=100-\left[\frac{X1}{X2}*100\right]$$

where SLR = Soil loss reduction (%), X2 = conventional tillage practice and X1 = other comparable tillage treatment.

2.2.1.1 Agronomic data

Data for yield and yield components of maize crop (Fig. 3) were collected as per the procedures mentioned as follows. For plant height, five plants from the entry rows of each treatment were randomly selected and measured. Then the mean value was computed and recorded as plant height. Hundred seed weight (gm): 100 sun-dried seeds were randomly taken from the seed lots of each plot by using a seed counter machine and then weighed by using a sensitive balance. Both grain and biomass yields were converted into kilograms per hectare after harvesting from a 16 m2 net plot area of 13 rows. Grain yield was adjusted to 12.5% moisture level; whereas biomass was weighed after leaving it in the open air until dried (for 2 weeks).

Fig. 3
figure 3

Field pictures at A maize crop development and B at crop maturity stage

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2.3 Economic analysis

Grain yield data were economically analyzed using partial and marginal analysis for the feasibility of tillage practices. A treatment was deliberated worthwhile to farmers when its minimum acceptable rate of return (MAR) is 100% [8], which is recommended to be convincing.

2.4 Statistical data analysis

The gathered information was analyzed using R software version R × 64 3.4.1 to perform a one-way analysis of variance (ANOVA). Tukey's HSD (honestly significant difference) was then utilized to separate means at p ≤ 0.05 levels of significance. The goal of comparing the fifth treatment to the pre-experiment soil samples was to assess the impact of conservation tillage practices on the physicochemical characteristics of the soil.

3 Result and discussion

3.1 Effects of tillage methods on soil physicochemical properties

After the experiment (Table 1), the soil physical and chemical properties showed slight differences among treatments at (p < 0.05). The soil pH value revealed a significantly increasing difference from the initial pH of the soil compared to after the experiment and among treatments. The highest soil pH was observed in reduced tillage while the lowest was in conventional tillage. This study indicated that the experiments were carried out on clay loam soil which is very strongly acidic to moderately acidic. Pre-tillage results of soil analysis revealed strong acidic as compared to the results after experimentation. In terms of soil organic carbon and total nitrogen; conservation tillage methods have slight effects. The soil organic carbon in zero tillage, reduced tillage, strip tillage and conventional tillage is increased from 1.15 to 1.17, 1.15 to 1.18, 1.15 to 1.18 and 1.15 to 1.21 respectively as compared to before experimentation even though there is no statically significant difference. This might be due to increased decomposition through tillage in the form of physical weathering. In line with this result, the study result of Andrade [1], revealed soils under the tilled layer have greater organic carbon than the untilled layer. In contrast, the study results of Thomas [26] where soils under no-till generally contain greater organic carbon than the conventional tillage practice. However, the organic carbon values were lower or constant under the conservational tillage method and the frequency of tillage accelerated the loss of organic matter in the soil. Relatively, according to Melero [17], conventional tillage (CT) promotes the loss of the soil organic matter, which leads to a disruption of soil aggregates and contributes to soil erosion. In terms of soil bulk density, there is no statistically significant difference among treatments at all stages. However, slightly higher bulk densities were observed in zero tillage and as well as before the experimentations. This may be due to minimized soil disturbance and also there is higher compactness of the soil. Similar to this study result, the study result of Deen and Kataki [9] and Sessiz [24] shows a greater bulk density in the untilled soil layer than the tilled layer.

Table 1 Soil properties before and after the experiment
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The soil moisture status on different tillage methods was monitored by collecting soil samples at the planting of the test crop, 65 days after planting (development stage) and after crop harvest for consecutive two years. In the first year, the initial SMC (%) of the experimental site was 14.3% (Fig. 4). The result of soil moisture content at the crop development stage shows differences. The soil moisture content at the crop development stage of all tillage methods except strip tillage was higher than the control.

Fig. 4
figure 4

The average soil moisture contents analyzed for both years

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In the second year, the average initial soil moisture content of the experimental site was 20%. At the crop development stage, SMC (%) between tillage methods was monitored accordingly. The soil moisture on zero tillage, two times tillage and strip tillage was by far higher relative to the control.

The SMC (%) of the first and second years on average showed an increasing trend from conventional tillage (13.73%), zero tillage (15.08%), reduced tillage (15.11%) and strip tillage (18.61%). In general, Zero tillage, two times tillage and strip tillage had 9.25%, 10.05% and 35.54% soil moisture-conserving advantages, respectively as compared to conventional tillage/control.

Comparatively, at crop development stage zero tillage, two times tillage and strip tillage had 29.81%, 70.36% and 39.75% soil moisture conserving advantage, respectively over the control. A similar trend was observed after crop harvest, the soil moisture on conservation tillage methods was higher as compared to the control. The soil moisture content of all tillage methods at the crop development stage was higher than the control. In both years, the soil moisture content after planting maize was higher compared to the initial stage, which could be due to low evaporation and high infiltration rate in conservation tillage methods. This could be due to low runoff through a high infiltration rate on conservation tillage methods. According to Kovaci [13], improving the rate of infiltration has bigger benefits and influence on soil moisture balance. The result of Chen [7] also implies among conservation tillage zero tillage has an advantage in reducing soil moisture loss as compared to other tillage methods.

3.2 Effect of conservation tillage methods on soil loss

The result in Table 2: Soil sediment of different conservation tillage methods) above describes the soil loss of each conservation tillage method. In both years, changes were observed in soil loss between the conventional and conservation tillage methods. In the first year, no-till, strip and two-times tillage minimized soil loss by 46.10%, 35.44% and 21.83% as compared to the conventional tillage methods, respectively. But in the second year, zero tillage, two times tillage and strip tillage reduced soil loss by 43.63%, 35.57% and 32.20% as compared to the conventional method, respectively. The combined year soil loss result reveals that zero tillage reduced the severity of soil loss by 44.84% followed by two strip tillage (35.5%) and two times tillage (27.17%). In both years, conventional tillage methods aggravated soil loss as compared to conservation tillage methods. Rockström [22] also confirm that the conservation tillage store more moisture reduces surface runoff, and benefits the crop in arid and semiarid areas by reducing drought risk and increasing grain yield. This implies that reducing the frequency of tillage affects reducing soil loss which in turn boosts crop productivity.

Table 2 Soil sediment of different conservation tillage methods
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3.3 Effect of conservation tillage method on maize yield and yield components

The combined analysis result in Table 3 depicted that there were statistically significant differences in yield and yield components except hundred seed weight between tillage treatments at (P < 0.05). Despite significant yield differences between strip tillage and reduced tillage, two times tillage and conventional tillage have yield advantages over the strip and zero tillage method. Even though a high soil loss has been recorded under conventional tillage as compared to other conservation tillage, it is not sustainable and economic practice. The zero tillage method has recorded the highest soil loss reduction; however, it has the lowest crop maize grain yield as compared to other tillage methods. This might be due to high soil infiltration rate and moisture conservation. This finding agrees with the result of Bekele [3], which indicated that there was low maize grain yield in the zero tillage method as compared to others. Similarly, the findings of Lundy [16] reveal that the yield of zero tillage was smaller as compared to conventional tillage. Lampurlanes [15] also reported that conservational tillage can reduce the yield but it offers more protection against soil degradation and more improvement in the quality of soil in the long term.

Table 3 Combined maize data analyzed the result of the conservation tillage practice
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Regarding the germination rate, there is the highest germination rate was recorded at zero and two-times tillage as compared to strip and traditional tillage. This might be due to optimum soil moisture in the soil in zero tillage and reduced tillage as well as manual planting system. In terms of plant height, zero tillage is the lowest as compared to other methods. These results are in line with that of Kayode and Ademiluyi [12] who observed that the shortest maize plant in the zero tillage plots is in contrast with that in the tilled plots.

3.4 Economic evaluation of tillage methods

Tillage has various roles like; reducing the severity of soil loss, improving soil fertility through the addition of organic matter, and improving the structural organization of soil. Therefore, the long and short-term tillage advantages should be studied. In this study, partial budget analysis was a reminder that not all production costs were included in the budget those that were affected by the alternative tillage treatments were considered to select cost-effective conservation tillage methods.

As shown in Table 4, due to an economic point of view except for conventional tillage methods, all tillage treatments are economically important. The marginal rates of return of the changes from zero tillage to strip tillage and from strip tillage to reduced tillage are 9543.9% and 958.4%, respectively, well above the 100% minimum. Therefore, even though there is a highly comparable net benefit recorded in conventional tillage, it was dominated by the other alternative treatments, because of higher investment costs. Therefore, they were removed from the recommendation. The result in Table 4 clearly shows that conventional tillage has less value than the other tillage treatments. This guides the farmers to select cost-effective and highest-beneficiary tillage methods for sustainable crop production. However, if preferences are required reduced tillage is better than zero tillage and strip tillage concerning cost-effectiveness. However, zero tillage has saved more soil loss than reduced tillage from the farmland and contributes to protecting the topsoil productivity which in turn protects the structural stability of the soil and crop productivity in a sustainable way.

Table 4 Partial budget analysis for different tillage methods for the two-year average yield
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4 Conclusion and recommendation

This study revealed that the two-times (reduced) tillage method was the right approach in improving yield and yield components of the crop as well as reducing soil erosion from the farmland by increasing soil infiltration and reducing the runoff through modifying the structural organization of the soil. The 2-year combined analysis result depicted that there were statistically significant yield differences among all tillage treatments. The reduced tillage and convention tillage gave better results in terms of grain yield and yield components. Even though the maize grain yield recorded is low in the zero tillage practices as compared to the other tillage methods; however, a very good soil loss reduction advantage was observed. Based on the current findings, it has been concluded that the in-year period conservation tillage practices are more effective than the conventional type in maintaining better soil moisture at various stages of crop growth, including planting, development, and post-harvest. This is primarily due to the high infiltration rate and reduced runoff, which greatly benefits and contributes to a better soil moisture balance.

Therefore, adopting reduced tillage increases maize yield and economic benefits to farmers. Although, adopting strip tillage and zero tillage gives less grain yield but has a higher economic benefit to farmers and improves sustainable productivity in the long run as compared to conventional tillage. The economic evaluation recommends reduced, strip, and zero tillage. If reference is needed to select the best of the best, reduced tillage is the best to get economic benefits without compromising the soil erosion issue. Most of the time, the impacts of the conservation tillage method on soil and water balance are long-term. Therefore, to get a change in soil and water balances, yield and other physicochemical properties, more than two years period at a permanent field plot should be considered for the future.