Effects of Biopesticides and Undersown Cover Crops on Soil Properties in the Organic Farming System

Abstract

The inclusion of undersown cover crops in crop rotations and the use of biopesticides are essential for the long-term sustainability of the agroecosystem in organic farming. We hypothesized that biopesticides and undersown cover crops (crimson (incarnate) clover (Trifolium incarnatum Broth.), hairy (winter) vetch (Vicia villosa Roth.), perennial ryegrass (Lolium perenne L.), and winter rye (Secale cereale L.)) are likely to have a positive impact on soil agrophysical and biological properties. Soil shear strength, soil aggregate–size distribution, plant root dry biomass and number and biomass of earthworms were determined at the end of the plant growing season. We concluded that the application of biopesticides and growing of undersown cover crops decreased soil shear strength and increased the percentage of macro-aggregates (0.25–10 mm) in the plough layer. In the plots with cover crops, plant root biomass was found to be significantly higher by a factor of 1.9 to 3.5, compared to the plots without cover crops. The application of biopesticides and undersown cover crops did not significantly affect the number and biomass of earthworms in the soil. The abundance of earthworms was more influenced by meteorological conditions.

1. Introduction

Agricultural activities affect soil, water, air and the environment around us. The larger and more intensive a farm is, the greater its impact on the environment. Increasingly intensive agricultural production encourages farms to focus on higher and higher yields of agricultural crops. This is achieved through the use of higher rates of pesticides and fertilizers (especially nitrogen fertilizers). This has a negative impact on soil biological activity, reducing soil organic matter content [1]. Organic farming is becoming increasingly popular worldwide, influenced by a variety of factors such as environmental concerns, human health and socio-economic issues [2]. Since its inception, organic farming has been considered as an alternative farming system that can help solve a wide range of environmental and food quality problems [3].

The world‘s organic oilseed rape area was estimated at 1.68 million ha in 2019 [4]. In Lithuania, 13,638.22 ha of oilseed rape were grown on organic farms in 2021, including 2378.32 ha of spring and 11,259.90 ha of winter oilseed rape.

Around the world, there is a growing emphasis on maintaining soil fertility and vitality. Cover crops can improve soil physical, biological properties and organic matter content, which can reduce soil loss and improve land productivity and environmental quality [5,6,7,8,9]. There has been an increasing emphasis on the fact that soil structure changes when organic matter content is reduced [10,11]. Sharma et al. [12] reported that soil organic matter under cover cropped plots in the 0 to 5 cm soil layer was 28% higher than the bare soil treatment. Integrating cover crops into cropping systems may reduce soil bulk density and improve soil structure [13]. With cover crops, concentrations of large and medium macroaggregates were twice that of the fallow [14]. The results of a study by Blanco-Canqui et al. [15] indicate that spring triticale and spring lentil cover crops increased soil aggregate–size distribution. Nouri et al. [16] found that hairy vetch cover crop increased the mean weight diameter of aggregates by promoting macro aggregation. According to Kemper et al. [17] and Ogilvie et al. [18], the improvement of soil physical properties is associated with the roots of cover crops. Saleem et al. [19] demonstrated that cover crop diversity improved belowground root coverage, which in return increased its positive effects on soil properties, such as the soil organic matter content and the composition of soil aggregate-size classes. Soil shear strength is closely related to soil organic matter content [20] and other soil agrophysical properties [21]. Holland et al. [22] report that winter cover cropping reduces soil shear strength compared to stubble.

Earthworms are a natural sign of healthy and fertile soil [23,24]. Long-term cover crop usage contributes to improving soil structure and increases earthworm population, which subsequently reduces nutrient and sediment losses in surface runoff. Cover crops such as pea and oat showed evidence for higher earthworm populations compared to bare fallow plots or the cover crop-spring barley crop rotation [25]. According to Bilalis et al. [26], significantly higher populations of earthworms were found under the legumes for green manure and no-tillage system. Brassica species such as mustard had large above ground biomass but less earthworm population [27]. According to De Notaris et al. [28], earthworm density was the greatest in the organic system with grass-clover, especially following the ley year, thanks to a combination of high quality plant input and reduced soil disturbance.

The use of biologicals instead of conventional chemical pesticides is one way to reduce the use of chemicals on farms, as well as for farmers to move towards greener farming and achieve the desired sustainable agricultural development [29]. Biopesticides, which are pest and pathogens management agents based on living microorganisms or natural products, offer great promise in controlling yield loss, reducing the demand for energy and restoring the efficiency of agroecosystems [30]. They are an alternative of agrochemicals (chemical fertilizers and pesticides) to improve soil fertility and control various insects, pests and diseases in almost all agricultural crops [31].

The research hypothesis is that biopesticides and undersown cover crops are likely to have a positive impact on soil agrophysical and biological properties.

The aim of this study was to investigate the influence of biopesticides and undersown cover crops on the soil agrophysical and biological properties of organic farming systems.

2. Materials and Methods

2.1. General Experimental Conditions

The field experiment was carried out in 2020 and 2021 at the Experimental Station of Vytautas Magnus University Agriculture Academy (VMUAA) (54°53′ N, 23°50′ E) (Figure 1). The soil of the experiment was Endocalcaric Amphistagnic Luvisol [32]. In 2020, soil agrochemical properties included: pH_KCl—6.5–6.9, total nitrogen content—0.107–0.144%, humus content—2.14–2.67%, mobile nutrients in soil: P₂O₅—226–305 mg kg⁻¹, K₂O—109–118 mg kg⁻¹. In 2021 soil agrochemical properties: pHKCl—7.0–7.5, total nitrogen content—0.131–0.168%, humus content—2.21–2.69%, and mobile nutrients in soil: P₂O₅—193–268 mg kg⁻¹, K₂O—110–122 mg kg⁻¹.

2.2. Experimental Design

The two-factor field experiment was set up using the SPLIT PLOT method. Experimental applications included: Factor A: biopesticides: (1) not used; (2) used. Factor B: undersown cover crops: (1) no cover crop; (2) crimson (incarnate) clover (Trifolium incarnatum Broth.) “Kardinal” (10 kg ha⁻¹); (3) hairy (winter) vetch (Vicia villosa Roth.) “Rea” (50 kg ha⁻¹); (4) perennial ryegrass (Lolium perenne L.) “Merkem” (10 kg ha⁻¹); and (5) winter rye (Secale cereale L.) “Elias” (50 kg ha⁻¹). The initial plot area was 72 m² and the reference plot area was 20 m². The experiments were carried out in 4 replicates.

2.3. Agro-Technologies of Experiment

In April 2020 and 2021, the experimental field was cultivated twice with a germinator KLG-4.0 (Company “Laumetris”, Keleriškiai, Lithuania). During the third ten-day period of April, spring oilseed rape (Brassica napus L. spp. oleifera biennis Metzg.) “Fenja” (7 kg ha⁻¹) was sown at 48 cm row spacing (sown in every fourth row, with 3 seed tubes shut for gaps). At the 2–3 leaf stage (BBCH 12–13), the inter-rows of oilseed rape were loosened with a soil loosener (KOR-4.2-01, Ukraine) and two rows of cover crops were undersown in the oilseed rape inter-rows. Biopesticide Recharge was sprayed twice: at sowing and one month after sowing (1.5 kg ha⁻¹). Biopesticide Fizimite (1.0 l ha⁻¹) was sprayed on the oilseed rape at the flower formation stage (BBCH 57–58). No mineral fertilizers were applied to the spring oilseed rape and no chemical plant protection products were used. The oilseed rape was harvested in August with a Wintersteiger Delta (Austria) combine harvester. After rapeseed harvesting, the cover crops were left to grow until the following spring.

Recharge (Russel IPM Ltd., Great Britain, UK) contains a wide range of beneficial soil micro-organisms. Recharge improves soil properties and naturally protects against the spread of pests and diseases. Recharge contains a range of bacteria and fungus Metarhisium anisopliae var. Anisopliae 4% (causes infection of insect pests on contact).

Fizimite (Russel IPM Ltd., Great Britain, UK) is a concentrated combination of surfactants and trace elements that nourish and protect plants. It strengthens plants, effectively cleanses the surface against harmful organisms, and naturally improves the plant’s defense system, allowing plants to defend themselves against diseases and pests. Fizimite contains copper (Cu) 0.7%, manganese (Mn) 0.7%, and zinc (Zn) 0.7%.

2.4. Meteorological Conditions

In 2020, plant growth resumed on 7 April. April was very dry (Figure 2), while May was warm and humid. The temperature in June was 2.9 °C above the multiannual average and HTC was 1.74 (excess humidity). The temperature in July was 1.3 °C below the perennial average, with HTC of 1.12 (optimum humidity). The temperature in August was 1.4 °C above the multiannual average, with HTC of 1.61 (excess humidity). The temperature in September was warm and dry, with HTC of 0.30 (very dry). October, November and December were warmer than normal, and precipitation was lower than normal.

In 2021, plant growth resumed on 11 April. The temperature during this month was 0.7 °C below the multiannual average (Figure 2). May was cool and wet. The monthly HTC was 4.04 (excess humidity). June and July were hot and dry. The HTC for these months was 0.69 (arid). August was cooler than normal, with 33.3 mm more rainfall than normal. The monthly HTC was 2.40 (excess humidity). September was cooler and drier than normal (HTC—1.05).

2.5. Research Methods

Soil agrochemical properties were determined before the installation of the experiment. Soil pH was determined potentiometrically in 1 n KCl extract, available phosphorus P₂O₅ and available potassium K₂O (mg kg⁻¹ soil) were estimated by using the Egner–Rim–Domingo (A–L) method, and the organic carbon (%) was calculated by incineration of samples at 900 °C using a Heraeus incinerator. The analyses were performed at the Agrochemical Research Laboratory of the Lithuanian Research Centre for Agriculture and Forestry.

Soil shear strength was determined at the end of the growing season using a penetrometer Geonor 72410 at 10 spots in each plot at a depth of 8–10 cm.

The soil aggregate–size distribution was determined using a Retsch sieving machine. In each plot, a soil sample of about 300 g was taken with a shovel from at least 5 spots at the 0–20 cm depth of the plough layer at the end of the plant growing season. A 200 g sample was taken, sieved for 2 min, and sieving amplitude was 60%. The percent of soil aggregate–size distribution was estimated at mega (>10 mm), makro (0.25–10 mm) ir mikro (<0.25 mm).

Total root biomass of plants (undersown plants and oilseed rape) was determined using the small monolith method (0.001 m³), sampling at two spots in each plot from the 0–10 and 10–20 cm soil layers, at the end of the plant growing season [33]. The roots were washed on sieves and oven-dried at 105 °C. The total dry biomass (t ha⁻¹) of plant roots from the 0–20 cm soil layer was calculated.

The number and biomass of earthworms in the soil was determined at the end of the growing season. In each field, 50 × 50 cm pits 20 cm deep were dug at four spots [34]. Earthworms were collected, counted and weighed. The number of earthworms (pcs. m⁻²) and biomass (g m⁻²) were calculated.

2.6. Statistical Analysis

A two-factor field experiment was established using a split-plot design. The data were statistically processed by qualitative traits’ two-factor analysis of variance. The significance of the differences between the treatments was estimated using the F criterion and the LSD test [35]. Statistical analysis of the data was performed using the computer program SPLIT PLOT from a package SELECTION [36]. The differences between averages of treatments, marked by different letters, are significant at a 95% probability level (p < 0.05). Bars indicate the standard error.

3. Results

3.1. Soil Shear Strength

In 2020, the soil shear strength was found to be significantly lower by 7.2 to 10.8% in the plots with hairy vetch undersown in spring oilseed rape and zero biopesticide application, compared to the plots with crimson clover, perennial ryegrass and winter rye (Figure 3).

The application of biopesticides and use of perennial ryegrass cover crop resulted in significantly lower soil shear strength of 10.0 and 14.6% compared to the plots with cover crops of hairy vetch and winter rye. Crimson clover and perennial ryegrass cover crops undersown in oilseed rape and spraying of biopesticides resulted in significantly lower soil shear strength of 10.2 and 18.9% compared to the unsprayed plots.

In 2021, the soil shear strength in the hairy vetch cover crop after biopesticide application was 16.1% lower than that in the plots without biopesticide application. The application of biopesticides in the winter vetch cover crop reduced the soil shear strength significantly by 15.0% compared to the unsprayed and the no cover crop plots. Although the soil shear strength was lower in the unsprayed winter rye cover crop compared to other unsprayed plots, there were no significant differences.

3.2. Soil Aggregate–Size Distribution

In 2020, the highest proportion of mega-aggregates was found in all plots tested (

Table 1. Soil aggregate–size distribution, 2020.

Undersown Cover Crops (Factor B)	Biopesticides (Factor A)	Soil Aggregate–Size Distribution, %
		Mega >10 mm	Macro 0.25–10 mm	Micro <0.25 mm
1. Without cover crop	−	68.5 ± 6.97 a	29.0 ± 3.39 b	2.44 ± 0.47 a
	+	59.4 ± 5.01 ab	38.0 ± 2.57 ab	2.54 ± 0.12 a
2. Crimson (incarnate) clover	−	66.6 ± 11.5 ab	30.6 ± 5.47 ab	2.86 ± 0.41 a
	+	52.4 ± 13.7 b	44.5 ± 6.79 a	3.15 ± 0.46 a
3. Hairy (winter) vetch	−	66.8 ± 18.3 ab	30.9 ± 8.73 ab	2.25 ± 0.50 a
	+	59.3 ± 8.56 ab	37.7 ± 4.21 ab	2.96 ± 0.21 a
4. Perennial ryegrass	−	60.9 ± 8.50 ab	36.7 ± 3.84 ab	2.36 ± 0.50 a
	+	51.8 ± 10.8 b	44.7 ± 5.06 a	3.44 ± 1.06 a
5. Winter rye	−	60.0 ± 9.59 ab	37.1 ± 4.63 ab	2.91 ± 0.67 a
	+	64.4 ± 5.19 ab	32.5 ± 2.60 ab	3.13 ± 0.54 a

Note. − without biopesticides, + with biopesticides. Differences between the averages of treatments marked with different letters (a, b) are significant (p < 0.05); Mean ± standard error.

). This was probably due to the lack of moisture during the autumn months (Figure 2). The highest levels of mega-aggregates (68.5%) in the plough layer were found in the plots without cover crops and with no biopesticide application. Significantly lower mega-aggregate levels of 23.5 and 24.4% compared to the above-mentioned plots were found in the plots with crimson clover and perennial ryegrass cover crops cultivated after spring oilseed rape harvesting and biopesticide application.

In the plots with crimson clover and perennial ryegrass cover crops and biopesticide application, the levels of macro-aggregates were found to be significantly higher by 53.4 and 54.1% compared to the plots with no cover crops and no use of biopesticides. The use of biopesticides tended to increase the level of macro-aggregates and to decrease the level of mega-aggregates in the plots studied, compared to the non-use of biopesticides, except for the winter rye cover crop.

There was no significant difference in micro-aggregate content in all the plots tested.

In 2021, macro-aggregates included the highest proportion, which was between 57.9% and 72.8% (

Table 2. Soil aggregate–size distribution, 2021.

Undersown Cover Crops (Factor B)	Biopesticides (Factor A)	Soil Aggregate–Size Distribution, %
		Mega >10 mm	Macro 0.25–10 mm	Micro <0.25 mm
1. Without cover crop	−	36.0 ± 2.75 a	59.8 ± 2.46 c	4.20 ± 0.68 b
	+	19.0 ± 2.19 b	76.4 ± 2.20 a	4.60 ± 0.22 b
2. Crimson (incarnate) clover	−	36.3 ± 5.50 a	57.9 ± 3.94 c	5.80 ± 1.78 ab
	+	23.7 ± 5.22 b	71.3 ± 4.80 ab	5.00 ± 0.48 ab
3. Hairy (winter) vetch	−	35.9 ± 7.49 a	60.5 ± 6.67 bc	3.60 ± 1.06 b
	+	20.9 ± 4.35 b	71.3 ± 4.74 ab	7.80 ± 2.02 a
4. Perennial ryegrass	−	25.6 ± 7.17 ab	68.2 ± 6.56 abc	6.20 ± 0.68 ab
	+	28.6 ± 3.36 ab	66.7 ± 3.13 abc	4.70 ± 0.28 ab
5. Winter rye	−	27.4 ± 6.10 ab	67.1 ± 5.54 abc	5.50 ± 0.67 ab
	+	21.0 ± 3.25 b	72.8 ± 3.11 a	6.20 ± 0.70 ab

Note. − without biopesticides, + with biopesticides. Differences between the averages of treatments marked with different letters (a, b, c) are significant (p < 0.05); Mean ± standard error.

). In the plots without undersown cover crops and without biopesticide use, significantly higher mega-aggregate content was found by a factor of 1.9 compared to the plots where biopesticides were applied. Biopesticides were found to have a positive effect and a tendency to reduce mega-aggregate levels by a factor of between 1.5 and 1.9 in all the plots studied, except for the plots with perennial ryegrass.

Macro-aggregate levels were found to be significantly higher in the plots without cover crops and in the plots with winter rye and biopesticide application compared to the plots with undersown crimson clover and hairy vetch without biopesticide application, and to the plots without undersown cover crops and without biopesticide application by 20.3 to 32.0%, respectively. The use of biopesticides, compared to the non-use of biopesticides, significantly increased the macro-aggregate content by 27.8 and 23.1% in the plots without cover crops and in the plots with undersown crimson clover.

The levels of micro-aggregates were found to be significantly higher by 1.9 and 1.7 times in the plots with hairy vetch and biopesticide application compared to the plots without undersown cover crops and with both non-use and use of biopesticides. The use of biopesticides, compared to the non-use of biopesticides, resulted in a significant 2.2 times increase in micro-aggregates in the plots with undersown hairy vetch.

3.3. Plant Root Dry Biomass

The highest plant root dry biomass in 2020 was found in the plots where perennial ryegrass cover crop was undersown in rapeseed and no biopesticides were applied (Figure 4). In the plots without cover crops, in the plots where hairy vetch and winter rye had been undersown in rapeseed with both non-use and use of biopesticides, and in the plots where crimson clover had been undersown in rapeseed and pesticides applied, the plant root dry biomass was found to be significantly lower compared to the above plots, ranging from 1.4 to 4.0 times, respectively. In the plots where crimson clover and hairy vetch cover crops had been undersown in rapeseed, the plant root dry mass was found to be significantly higher than that in the plots without cover crops by a factor of 2.0 to 2.3 without biopesticide application, and by a factor of 2.8 to 2.9 with the use of biopesticides.

In 2021, the dry roots biomass of plants was established to be from 1.3 to 3.2 times less compared to 2020. The influence on such a result was the high air temperatures in June and July (Figure 2). In 2021, the plant root dry biomass in the plots with perennial ryegrass cover crop, not sprayed with biopesticides, was, significantly, 1.6 to 5.4 times higher than that in the plots without cover crops, and in the plots with hairy vetch and winter rye cover crops. In the plots with the perennial ryegrass cover crop undersown in oilseed rape and sprayed with biopesticides, the plant root dry biomass was found to be 3.9 times higher than that in the plots without cover crop, which was also significant.

3.4. Number and Biomass of Earthworms

According to the data collected in 2020, the highest earthworm populations were found in the plots where winter rye cover crop had been undersown in oilseed rape and biopesticides applied (

Table 3. Number of earthworms.

Undersown Cover Crops (Factor B)	Biopesticides (Factor A)	Number of Earthworms, pcs. m−2
		2020	2021
1. Without cover crop	−	94 ± 33.6 a	122 ± 42.9 a
	+	100 ± 32.3 a	86 ± 26.2 ab
2. Crimson (incarnate) clover	−	85a ± 36.6 ab	88 ± 35.0 ab
	+	90 ± 17.6 a	79 ± 45.9 ab
3. Hairy (winter) vetch	−	57 ± 13.4 b	92 ± 39.3 ab
	+	92 ± 20.8 a	116 ± 23.9 a
4. Perennial ryegrass	−	68 ± 29.1 b	65 ± 32.3 b
	+	84 ± 17.7 ab	52 ± 11.2 b
5. Winter rye	−	60 ± 11.3 b	104 ± 47.1 ab
	+	104 ± 21.4 a	116 ± 43.8 a

Note. − without biopesticides, + with biopesticides. Differences between the averages of treatments marked with different letters (a, b) are significant (p < 0.05); Mean ± standard error.

).

The plots with hairy vetch undersown in spring oilseed rape, perennial ryegrass and winter rye and no biopesticide use had significantly lower earthworm numbers of 39.4, 27.7 and 36.2% compared to the plots without cover crops and without biopesticide use. There was no significant difference in earthworm abundance in the plots studied after the application of biopesticides. The number of earthworms in the soil was found to be 61.4 and 73.3% significantly higher in the oilseed rape crop after having undersown hairy vetch and winter rye and sprayed with biopesticides compared to the unsprayed crop.

An assessment of the data obtained in 2021 shows that in the plots without biopesticide application and no cover crops, the number of earthworms was found to be 1.9 times higher compared to the perennial ryegrass cover crop. In contrast, in the plots where biopesticides had been applied and hairy vetch and winter rye cover crops cultivated, the number of earthworms was found to be significantly higher by a factor of 2.2 compared to the perennial ryegrass cover crop.

The highest earthworm biomass in 2020 was found in the plots with no cover crop and no biopesticide application (

Table 4. Biomass of earthworms.

Undersown Cover Crops (Factor B)	Biopesticides (Factor A)	Biomass of Earthworms, g m−2
		2020	2021
1. Without cover crop	−	48.9 ± 16.7 a	47.4 ± 11.6 a
	+	33.3 ± 7.69 ab	38.0 ± 7.84 ab
2. Crimson (incarnate) clover	−	31.8 ± 13.0 ab	31.3 ± 4.42 ab
	+	47.0 ± 6.37 a	27.9 ± 8.76 ab
3. Hairy (winter) vetch	−	21.7 ± 2.41 b	34.8 ± 2.55 ab
	+	36.8 ± 8.02 ab	42.8 ± 3.86 ab
4. Perennial ryegrass	−	28.1 ± 12.9 ab	25.4 ± 7.24 ab
	+	27.1 ± 3.85 ab	23.1 ± 9.25 b
5. Winter rye	−	23.0 ± 3.31 b	41.5 ± 16.9 ab
	+	40.9 ± 10.5 ab	51.7 ± 16.5 a

Note. − without biopesticides, + with biopesticides. Differences between the averages of treatments marked with different letters (a, b) are significant (p < 0.05); Mean ± standard error.

). In the plots with hairy vetch and winter rye undersown in spring oilseed rape and without the use of biopesticides, the earthworm biomass was found to be significantly lower by a factor of 2.3 and 2.8 compared to the plots without cover crops and with no biopesticides. There was no significant difference in earthworm biomass in the plots studied after the application of biopesticides.

The research carried out in 2021 showed that the highest earthworm biomass in soil was found in the plots with undersown winter rye, where biopesticides had been applied. Without biopesticide application, there was no significant difference in earthworm biomass in the plots studied. In the plots with winter rye undersown in spring oilseed rape and biopesticide application, the earthworm biomass was found to be 2.2 times higher than that in the plots with perennial ryegrass cover crop.

4. Discussion

The results showed that soil shear strength was mostly reduced by undersowing hairy vetch and perennial ryegrass and by spraying with biopesticides. In the dry autumn period of 2020, the highest increase in soil macro-aggregates (0.25–10 mm) was observed when crimson clover and perennial ryegrass were grown as cover crops in combination with biopesticides. In 2021, crimson clover, hairy vetch and winter rye cover crops in combination with biopesticides increased the number of macro-aggregates in the soil. Restovich et al. [37] found in a long-term study that soil structure was improved with cover crops compared to the plots without cover crops. The data by Wei et al. [38] showed that winter cover crops influenced soil agrophysical properties. According to Calonego et al. [39], Demir et al. [40], Blanco-Canqui and Ruis [41] cover crops improve soil structure. Stegarescu et al. [42] point out that the incorporation of the shoots and roots of rye and barley resulted in a higher increase in aggregates stability. The results suggest that binding agents from cover crops such as roots increase the stabilization of large macro-aggregates [43].

The roots of cover crops are essential for soil ecosystem stability, carbon and nutrient cycling, and improving soil health [44]. According to Hudek et al. [45] the highest root biomass of cover crops was found in the top 0–15 cm, and with increasing soil depth root biomass decreased. A significantly higher total root biomass was measured for radish and mustard compared to buckwheat, vetch, oat, rye and phacelia. The results of our research showed that the plots with cover crops undersown in oilseed rape and with both non-use and use of biopesticides demonstrated significantly higher plant root biomass, compared to the plots without cover crops, by a factor of 1.9 to 3.5 and by a factor of 2.5 to 3.5, respectively. According to Redin et al. [46], Poaceae species in cover crops demonstrate higher root biomass compared to Fabaceae species. The oats cover crop showed the best results in the production of root biomass, while the vetch had the lowest productivity [47]. Vecchia et al. [48] note that Poaceae (rye and oats) had the highest root biomass compared to all the other species. Our results confirmed those findings. The highest plant root dry biomass was found in the plots with perennial ryegrass cover crop undersown in oilseed rape. Combining crimson clover with grass and certain brassica species can improve total root biomass production [49]. Biopesticide application did not have any significant effect on plant root dry biomass. Parewa et al. [31] indicate that biopesticides increase root branching, root number and accelerate root growth.

In agricultural systems, earthworm abundance has increased due to adequate soil moisture, food supply and reduced soil tillage [50]. According to Curry and Schmidt [51], the different cover crops provided food, coverage of the soil surface and influenced soil moisture. The main influence on earthworm numbers is the organic matter biomass left in the soil [52]. Our data showed that undersown cover crops did not significantly affect, or even significantly reduce, the abundance of (perennial ryegrass) earthworms in the soil compared to the plots without cover crops. Apparently, the abundance of earthworms in the soil was more influenced by meteorological conditions (Figure 2). According to Sánchez de Cima et al. [53], winter cover crops had no significant effect on the number and biomass of earthworms. This was connected with the intensive tillage carried out in the systems, the weather conditions and the characteristics of the organic amendments. Euteneuer et al. [54] state that cover crops can support earthworm development, but under field conditions, soil moisture is more important. A comparison between different cover crop species showed that earthworm abundance in soil increased the most while cultivating winter rye cover crop undersown in oilseed rape with biopesticide application. Rye planted in corn silage-soybean rotation resulted in 1.2 times higher earthworm population and 1.4 times higher biomass in cover crop plots compared to no cover crop plots [29]. According to Ahmadnia et al. [55], on average, earthworms in all cover crop treatments increased by 80.5%, relative to the control.

5. Conclusions

The application of biopesticides and growth of undersown cover crops decreased soil shear strength and increased the percentage of macro-aggregates (0.25–10 mm) in the plough layer. In the plots with cover crops, plant root biomass was found to be significantly higher by a factor of 1.9 to 3.5, compared to the plots without cover crops. The application of biopesticides and undersown cover crops did not significantly affect number and biomass of earthworms in the soil. The abundance of earthworms was more influenced by meteorological conditions. This result prompts further questions about the yet unknown long-term effects of biopesticides and undersown cover crops on soil properties in the organic farming system.