Field experiments were conducted at the Research Farm of the Deendayal Upadhyaya Centre of Excellence for Organic Farming, CCS Haryana Agricultural University, Hisar, Haryana, India. The soil texture at the experimental site is classified as sandy loam with no previous crop residue present on the surface. The objective of the field tests was to analyze the freshly induced soil compaction beneath the tillage working depth purely due to the tilling action under different active tillage treatments. Field tests were completely randomized with four treatments at three different test sites. The active tillage implements considered in this study for different treatments were rotavator, power harrow, and PTO-operated disc tiller. The different tillage treatments considered were T1 (1 × rotavator), T2 (1 × power harrow), T3 (1 × PTO-operated disc tiller), and T4 (1 × Cultivator + 2 × Disc harrow). T4 was a conventional tillage treatment generally followed by the farmers to prepare the seedbed which involves a single pass of a cultivator followed by two passes of an offset disc harrow. Before initiating the field tests, the soil parameters such as moisture content, CI, BD, and porosity were measured in each experimental plot at two soil profile layers i.e. 0-120 mm and 120-220 mm depth ranges, to ensure the uniformity of the test sites for conducting the experiments. The criteria for selecting the 0-120 mm depth range was based on the maximum achievable depth under different tillage treatments which were observed to be 120 mm.

The major specifications of the different active tillage implements used in the study are given in Table 1. The rotavator was equipped with L-shaped blades having an average working width of 1524 mm with a total of seven rotor flanges (horizontal axis). The power harrow had seven rotor flanges (vertical axis) with an overall working width of 1710 mm. Each rotor flange was equipped with two vertical tine blades having lengths and thicknesses of 285 and 12 mm, respectively. The PTO-operated disc tiller consisted of eight actively rotating plain concave discs mounted in a single gang with spacing between adjacent discs of 215 mm. The cultivator was a rigid-tyne type having nine furrow openers at 235.5 mm spacing. The trailed type offset disc harrow consisted of eight concave discs in each gang with spacing of 225 mm between adjacent discs.

The cultivator and offset disc harrow were successfully operated in transmission gear L3, while the rotavator, power harrow, and PTO-operated disc tiller required transmission gear L1 for satisfactory operation during tillage. The treatment T1 comprised a single pass of the rotavator; its actual forward speed of operation and rotor speed were 2.23 km h^-1 and 250 rpm, respectively. T2 involved a single pass of power harrow at forward speed and rotor speed of 2.11 km h^-1 and 215 rpm, respectively. T3 applicated a single pass of PTO-operated disc tiller at forward speed and rotational speed of the front gang axle of 2.19 km h^-1 and 170 rpm, respectively. In T4, the seedbed was prepared by operating a single pass of cultivator followed by the two passes of offset disc harrow to achieve the desired tilth for seedbed preparation; their actual forward speeds achieved in transmission gear L3 with the cultivator and disc harrow were 5.47 and 6.33 km h^-1, respectively. The implement weight per unit width of operation for rotavator, power harrow, PTO-operated disc tiller, cultivator, and offset disc harrow were 269.7, 318.7, 282.5, 134.2, and 187.2 kg m^-1, respectively.

The residue of various cereal and pulses crops can be effectively mixed with soil using active tillage rotary implements in contrast to passive type implements, which generally bury it into the soil. Recently, the development of machinery such as Super Seeder, allows residue incorporation and seeding operations in one go, and make the residue incorporation method more feasible. Super Seeder is equipped with a rotavator fitted with ‘LJF’ blades at the front, followed by sowing and compaction units at the rear. The shape of ‘LJF’-type blades gives the benefit of gradual increment in bite-width as opposed to constant bite-width in traditional ‘L’-type blades in rotavators during their impact on the ground (Kumar et al., 2023b & 2023c). As the compaction tests were carried out in plots having no previous crop residue present on the surface, we did not use Super Seeder in any treatment. However, it could be considered for future studies.

The moisture content of the soil before the tillage operation was determined by the standard oven drying method. For each treatment, the soil samples were collected from two soil profiles (i.e. 0-120 mm and 120-220 mm) at three random locations within a particular strip. The wet weight of each sample was recorded. Thereafter, the samples were placed in the oven for 24 hours at 105 ºC and weights of dried samples were recorded.

Measurement of soil cone index (CI)

The soil CI was measured with the help of a tractor hydraulic-assisted embedded microprocessor-based cone penetrometer (Fig. 1), capable of recording the data of soil CI and depth of penetration with less human engagement and errors. The penetrometer consisted of a driving system, a sensor unit for measuring the force required to push the probe into the soil and depth of penetration, and a data logging system (Nisha et al., 2023). The selected control valves installed between the tractor hydraulic system and the hydraulic cylinder helped to achieve a desired penetration speed of 30 mm s^-1. A standard cone penetrometer probe with a cone having base diameter 20.30 mm, shaft diameter 15.90 mm, cone base area 323 mm², and an apex angle of 30° was used. The sensing unit of the developed system consisted of an industrial S-type load cell of 500-kg capacity fitted between the threaded end of the cylinder rod and cone penetrometer rod to measure the penetration resistance. The depth of penetration was measured using an ultrasonic sensor mounted beneath a circular plate fixed in the cylinder rod. The ultrasonic sensor had a sensing range of 20 to 4000 mm with a sensing angle of 15° and was accurate to the nearest 30 mm. A WiFi module transmitted the acquired data with the help of the microprocessor unit to a mobile application developed in the Android platform.

The accuracy and non-linearity of the soil penetration resistance measurement unit were 1.85% and 0.69%, respectively. The accuracy and non-linearity of the penetration depth measurement unit were 1.90% and 0.55%, respectively (Liu et al., 2018; Hensh et al., 2021b; Upadhyay et al., 2022). The repeatability of the soil penetration resistance and penetration depth measurement units was 0.37% and 0.52%, respectively (Liu et al., 2018; Hensh et al., 2021b; Upadhyay et al., 2022).

The average CI of the soil beneath the working depth (i.e. 120-220 mm depth range) was measured before and after each tillage treatment. For each tillage treatment, soil CI was measured at five random locations, and the average was calculated for the two considered depth ranges. The percentage increment in soil CI beneath the working depth was calculated with respect to soil CI of the test strip before tillage, which indicates any freshly induced soil compaction beneath the working depth purely due to the tilling action of the implement.

Measurement of soil bulk density (BD)

The soil BD was determined with the help of a core cutter sampler of diameter 100 mm and height 130 mm. The core sampler was inserted vertically into the soil and a core was carefully extracted without disturbance. Further, the core samples were placed in a 105 °C hot air oven for 24 hours until thoroughly dried, after which these samples were weighed (ASA, 1965). The resulting weight was used for calculating the dry BD of the soil. The average BD of the soil beneath the working depth (i.e. 120-220 mm depth range) was measured before and after each tillage treatment. For each tillage treatment, soil BD was measured at three random locations, and the average was calculated for the two considered depth ranges. The increment in soil BD beneath the working depth was calculated with respect to BD of the test strip before tillage, which indicates any freshly induced soil compaction beneath the working depth purely due to the tilling action of the implement.

Measurement of soil porosity

The porosity of the soil or pore space is defined as the volume of soil voids that can be filled by water and/or air. It has an inverse relationship with BD. Porosity is expressed as a percentage of the total volume of the soil. Many studies on the impact of the tillage traffic compaction on the porosity and structure of various soils have found a strong link between soil porosity and penetration resistance (Pagliai & De Nobili, 1993; Marsili et al., 1998). The average porosity of the soil beneath the working depth (i.e. 120-220 mm depth range) was measured before and after each tillage treatment. For each tillage treatment, soil porosity was measured at three random locations, and the average was calculated for the two considered depth ranges. The % decrement in soil porosity beneath the working depth was calculated with respect to porosity of the test strip before tillage, which indicates any freshly induced soil compaction beneath the working depth purely due to the tilling action of the implement. The particle density was determined using the pycnometer method. According to Carter & Gregorich (2007), the average particle density of agricultural soils is about 2.65 g cm^-3.

Measurement of soil pulverization

Soil pulverization was quantified in terms of the mean weight diameter (MWD) of soil aggregates using a mechanical sieve shaker, following the methods prescribed by Smith et al. (1994). A set of 11 sieves of size 31.5, 19.0, 13.2, 9.5, 8.0, 5.6, 4.75, 3.35, 2.0, 1.4, and 1.0 mm were used for the test. Sieve analysis was carried out by carefully collecting soil samples from randomly selected areas of 1×1 m up to the tillage working depth in collecting bags to find the MWD of the clods. Before performing the sieve analysis, the samples were naturally dried for 24 hours and a column of sieves with a gradual decrease in mesh sizes was introduced on a mechanical sieve shaker. The mean size of clods retained on the largest aperture sieve was determined by measuring the clod’s dimensions in principal planes. The weight of the soil retained on each sieve was measured with the help of an electronic balance. Three replications were taken for each tillage treatment within the test run, and the average MWD of soil clods in mm was computed.

To determine the forward speed of operation, a length of 25 m was marked with the ranging rods, and the tractor with the implement was operated in the test run. A stopwatch was used to record the time for the operation to traverse the marked run and the forward speed of the operation was computed (Rasool et al., 2017; Upadhyay et al., 2017). The depth of the operation under the different treatments was calculated by measuring the distance between furrow sole and ground level using a scale along the furrow wall at an interval of about 3 m along the length of the furrow. The average of five readings was calculated to determine the depth of the operation under the different treatments. The rotational speed of PTO shaft was measured with the help of a laser tachometer. A reflecting paper was placed on the PTO shaft and the tractor engine was operated. The tachometer sensed the reflection from the reflector and gave the rotational speed of the PTO shaft. The rotational speed of the tillage tool was determined after considering the gear ratio from PTO input shaft to the rotor shaft.

All the experiments were conducted with a 29 kW 2WD tractor (New Holland 4010) at three different test sites: Site 1 (29.13654° N, 75.69817° E), Site 2 (29.13644° N, 75.69817° E), and Site 3 (29.1504° N, 75.7057° E). The experimental layout is presented in Fig. 2. Each test site was divided into 12 strips (four treatments and three replications) for the comparative study of different tillage treatments. A completely randomized block design was followed for carrying out the soil compaction experiments. Each test was conducted over an approx. 30 m test strip. Before conducting each test, soil samples were randomly collected from the test strip to determine the moisture content, BD, porosity, and CI of the soil before tillage operation. The moisture content of the soil was measured to ensure the uniformity of different test strips for conducting the compaction tests.

Thereafter, each implement in the respective treatment was operated at a suitable forward speed and recommended rotor/gang axle speed. The forward speed of the tractor was varied by selecting the appropriate transmission gear and the throttle position of the tractor was set to maintain a PTO speed of 540±40 rpm during all the tests. The average operating depth under all treatments was maintained as 120±10 mm. After the tillage treatments, the soil compaction parameters such as CI, BD, and porosity of the soil were again measured beneath the working depth (i.e. 120-220 mm depth range) in that particular operation. The data were analyzed in terms of % increment or decrement in these parameters beneath the working depth with respect to their values before tillage, in order to identify any freshly induced soil compaction beneath the working depth purely due to the tilling action of the implement. The data should not include the tractor-imposed soil compaction under the tires, so we carefully measured the soil parameters about 200 mm distant from the inner ends of tractor tires. Further, the data regarding soil pulverization in terms of MWD of soil aggregates were also determined to assess the quality of the tilled soil under different tillage treatments. The graphical abstract of the soil compaction research is presented in Fig. 3.

Statistical analysis was also performed on the acquired data using SPSS 22.0 software. Analysis of variance (ANOVA) was carried out to check the significance of different tillage treatments on measured soil compaction parameters i.e. CI, BD, and porosity of the soil beneath the tillage depth at a 5% level of significance. Duncan’s multiple range (DMR) tests were also carried out to check the existence of significant treatment differences between the means of % increment in soil compaction beneath the working depth in terms of CI, BD, and porosity of the soil at a 5% level of significance.

The data on the moisture content of the soil measured at a depth of 0-120 mm and 120-220 mm before the tillage operation at different test sites are given in Table 2. The average moisture content of the soil profile at a depth of 0-120 mm varied from 11.84 to 12.15% (db), 12.10 to 12.96%, and 11.74 to 12.15% at test sites 1, 2, and 3, respectively. The results of DMR tests indicated that the differences in the moisture content of soil at a depth range of 0-120 mm under most of the tillage treatments (except in T4 at site 2) were statistically non-significant (p ≤ 0.05) for the three test sites.

The average moisture content of the soil at a depth range of 120-220 mm varied from 15.01 to 15.90% (db), 15.17 to 15.99%, and 15.19 to 15.72% at test sites 1, 2, and 3, respectively. Moreover, the results of the DMR tests indicated that the differences in the soil moisture content under the treatments (except in T1 at sites 1 and 3) were statistically non-significant (p ≤ 0.05) for the three sites.

All these data indicated uniform soil moisture profiles for conducting the compaction tests.

It is to be noted that the soil pulverization in terms of MWD of the soil aggregates was determined to assess the quality of the tilled soil under different tillage treatments to analyze the suitability of test beds for immediate sowing, and this parameter is not related to any induced soil compaction. The results on soil pulverization or MWD of the soil aggregates observed after the tillage treatments are presented in Table 3. It is evident from Table 3 that soil pulverization significantly varied with the tillage treatments followed. The smallest MWD of the soil aggregates or maximum soil pulverization was obtained in T1 (4.25 mm), followed by T2 (7.25 mm), T4 (11.95 mm) and T3 (13.80 mm) at site 1. Similar results were observed at the other sites (Table 3). DMR tests results indicate that the effect of different tillage treatments on soil pulverization were statistically significant at 5% level of significance for all three test sites.

The clod sizes should be the in the ideal range; too fine soil causes soil erosion and requirement of higher energy demand to pulverize the soil, while too large clods affect the sowing operations and harm plant growth. After following all the tillage treatments, the test bed was found to be suitable for sowing operation. Ahmad et al. (2010) analyzed the tillage quality of different tillage treatments i.e. conventional rotavator (T1), modified rotavator (T2), spade cultivator (T3), chisel plough + rotavator (T4), and chisel plough with modified rotavator (T5). They reported highest soil pulverization under treatment T4 followed by T5, T2, T1, and T3, indicating that the use of rotavator produces finer particles than spade cultivator. Bhusan (1971) reported that different tillage practices have an impact on the distribution of clod sizes. According to his study, disc and moldboard ploughs produced a higher percentage of clods with a diameter greater than 52 mm. Choudhary et al. (2021) assessed the tillage quality of different treatments i.e. T1 (1 × cultivator + 2 × offset disc harrow), T2 (1 × combined offset disc harrow), T3 (1 × cultivator + 1 × single rotor type rotavator), T4 (1 × double rotor type rotavator), and T5 (1 × power harrow). They reported highest soil pulverization under treatment T4 followed by T3, T5, T1, and T2.

The soil compaction induced under different tillage treatments was analyzed in terms of % increase in CI and BD, and % decrease in the porosity of the soil beneath the respective working depths. It is worth mentioning that the presented results on soil compaction are purely induced by the tilling action of different tillage treatments during multiple passes and do not include the tractor-imposed soil compaction under the tires. The results of tillage-induced soil compaction under different tillage treatments are presented in the following sub-sections.

The readings on soil CI were taken before and after the tillage operation beneath the working depth (120-220 mm) at three different test sites. For each test site, soil CI readings were measured and averaged for five random locations under each treatment and the results are given in Table 4. The average CI of the soil beneath the working depth (120-220 mm) before the tillage operation varied from 2670.89 to 2806.86 kPa, 2317.21 to 2574.89 kPa, and 2810.40 to 2915.05 kPa at test sites 1, 2, and 3, respectively. Further the results of DMR tests indicate that the differences in the CI of soil beneath the working depth before the tillage operation under most of the tillage treatments were statistically non-significant (p ≤ 0.05) for the three sites indicating uniform field conditions for carrying out the compaction tests. ANOVA tests (Table 5) carried out on the data of soil CI observed beneath the working depth (120-220 mm) after tillage operation with different treatments, indicated that at all test sites, the CI beneath the tillage depth observed after tillage operation was significantly affected by the tillage treatments.

The data were analyzed in terms of % increments in the CI beneath the tillage depth under different tillage treatments and the results are presented in Table 4 and plotted in Fig. 4(a). It is clear that, at test site 1, the maximum % increment in soil CI beneath the working depth (or soil compaction) was observed in T1 (6.79 ± 0.32%) followed by T4 (5.17 ± 0.07%), T3 (4.57 ± 0.11%), and T2 (2.08 ± 0.14%) with respect to CI of the soil before operation. Similar results were observed at test sites 2 and 3 with average % increment in CI of 7.05 ± 0.21%, 2.36 ± 0.18%, 4.74 ± 0.11%, and 5.29 ± 0.35% at test site 2; and 6.67 ± 0.31%, 2.24 ± 0.16%, 4.29 ± 0.22%, and 4.97±0.19% at test site 3 under treatments T1, T2, T3, and T4, respectively. DMR tests indicated that the effect of different tillage treatments on soil CI were statistically significant 5% level of significance for the three test sites.

The higher CI values observed after tillage operation under all treatments indicate the possibility of freshly induced soil compaction beneath the tilled layer due to the tilling action of the tools and the weight of the implement. The possible explanation for T1 showing a higher % increase in soil CI beneath the tillage depth could be due to the greater impact force exerted by the rotation of the blades of the rotavator on the soil while travelling horizontally at the bottom of their stroke, as stated by Whitefield (2004), Ahmad et al. (2010) and Upadhyay & Raheman (2019). Treatment T2, which includes a single pass of power harrow, shows the lowest increment in soil CI beneath the tillage depth. This could be due to the vertical rotation of the blades exerting less downward impact force on the soil profile. Similar results for the change in CI after tillage treatments were also reported in different studies conducted by Hamza & Anderson (2005), and Jabro et al. (2008). Upadhyay & Raheman (2019) observed the highest % increase in CI beneath the tillage depth with rotavator followed by powered and free rolling modes of a disc harrow with respect to CI of the field before operation. Their results indicate that active tillage machines cause more soil compaction for the same operating conditions as compared to conventional passive type tillage implements. According to Meyer et al. (1997), the use of a rotary cultivator caused compaction in the 100-200 mm and 200-300 mm soil layers beneath the tillage depth. Ahmad et al. (2010) observed that although rotavator produce fine soil tilth at top soil layers, the high speed rotating action of its blades compact the soil beneath its operational depth.

The treatment T4 showed next higher % increment in soil CI beneath the tillage depth after T1. The possible explanation for this behavior could be the multiple tillage passes (total three) required with the treatment T4 for preparing the seed bed suitable for sowing.

The readings on soil BD were taken before and after the tillage operation beneath the working depth (120-220 mm) at three different test sites. For each test site, soil BD was measured and averaged for three random locations under each treatment, and the results are shown in Table 6. ANOVA tests carried out on the data of soil BD observed beneath the working depth (120-220 mm) after tillage operation with different treatments (Table 7), indicated that at all test sites, the BD beneath the tillage depth observed after tillage operation was significantly affected by the tillage treatments at a 5% level of significance.

The data were analyzed in terms of % increments in the BD beneath the tillage depth under different treatments and the results are presented in Table 6 and plotted in Fig. 4(b). At test site 1, the maximum increment in soil BD beneath the working depth was observed in T1 (3.99 ± 0.09%), followed by T4 (2.42 ± 0.27%), T3 (1.76 ± 0.34%), and T2 (1.31 ± 0.13%) with respect to the BD of the soil before operation. Similar results were observed at test sites 2 and 3, with an average increment in BD of 4.06 ± 0.08%, 1.36 ± 0.10%, 1.71 ± 0.16%, and 2.37 ± 0.03% at test site 2; and 3.96 ± 0.06%, 1.40 ± 0.21%, 1.88 ± 0.16%, and 2.30 ± 0.07% at test site 3 under treatments T1, T2, T3, and T4, respectively. DMR tests were also carried out to check the existence of significant treatment differences between the means of increment in soil compaction beneath the working depth at a 5% level of significance. DMR tests indicate that the effect of different tillage treatments on soil BD beneath the tillage depth were statistically significant for all the test sites.

The higher BD values observed after tillage operation under all treatments indicate the possibility of freshly induced soil compaction beneath the tilled layer due to the tilling action of the tools and the weight of the implement. The BD of the soil was directly correlated with the soil CI value, as also propounded by Kumar et al. (2012). Therefore, the possible reasons behind the trend of higher or lower soil compaction in terms of BD under different tillage treatments are similar to the ones mentioned for soil CI in the above section. Similar results for a greater range of observations concerning BD, hydraulic conductivity, and particle density of the soil were also quoted by Alam et al. (2014), and Blanco & Ruis (2018). In a comparative study conducted by Ahmad et al. (2010), a minimum BD of 1.23 g cm^-3 was observed after the tillage operation with a spade cultivator at a depth of 200-300 mm, while maximum BD of 1.99 g cm^-3 was observed with a rotavator indicating higher soil compaction beneath the tilled layers with the use of rotavator. Their results were in agreement with the findings of Lampurlanés & Cantero‐Martínez (2003), and Jabro et al. (2008).

The porosity of the soil was calculated based on the relationship between BD and particle density (Danielson & Sutherland, 1986). The average particle density for sites 1, 2, and 3 was 2.42, 2.56, and 2.47 g cm^-3, respectively. The readings on soil porosity were taken before and after the tillage operation beneath the working depth (120-220 mm) at the three test sites. Table 8 shows that the average porosity of the soil beneath the working depth (120-220 mm) before the tillage operation varied from 36.91 to 37.62%, 37.80 to 39.31%, and 40.40 to 41.09% at test sites 1, 2, and 3, respectively. Further, the results of DMR tests indicate that the differences in the porosity of soil beneath the working depth before the tillage operation under most of the tillage treatments were statistically non-significant (p ≤ 0.05) for the three sites.

The data were analyzed in terms of decrements in the soil porosity beneath the tillage depth under different treatments and the results are presented in Table 8 and plotted in Fig. 4(c). Table 8 shows that at test site 1 the highest decrement in soil porosity beneath the working depth was observed in T1 (6.61 ± 0.16%) followed by T4 (4.04 ± 0.36%), T3 (2.94 ± 0.50%), and T2 (2.25 ± 0.20%) with respect to the porosity of the soil before operation. Identical results were also observed at test sites 2 and 3 with average decrement in porosity of 6.50 ± 0.14%, 2.09 ± 0.16%, 2.82 ± 0.23%, and 3.7 ± 0.08% at test site 2; and 5.67 ± 0.14%, 2.06 ± 0.32%, 2.74 ± 0.19%, and 3.37 ± 0.14% at test site 3 under T1, T2, T3, and T4, respectively.

DMR tests indicate that the effect of different tillage treatments on soil porosity beneath the tillage depth were statistically significant 5% level of significance for the three test sites. The decrement in soil porosity indicates the possibility of freshly induced soil compaction beneath the tilled layer. The trend of higher or lower soil compaction in terms of soil porosity under different tillage treatments is similar to that of BD. Many studies conducted on the impact of the tillage traffic compaction on the porosity and structure of various soils have found a strong relationship between soil porosity and penetration resistance (Pagliai & De Nobili, 1993; Marsili et al., 1998). Tarawally et al. (2004) stated that compacted soil can destroy the soil structure, reduce porosity, limit air and water infiltration, and increases soil BD and CI. Many researchers (Green et al., 2003; Bhattacharyya et al., 2006) reported higher porosity values with conventional tillage as compared to no-till practice under tilled conditions. This may be due to the loose-soft, and fluffy structure of the tilled layer. However, Pagliai & De Nobili (1993) found lower porosity under tilled soil conditions.