Effect of an Innovative Solarization Method on Crops, Soil-Borne Pathogens, and Living Fungal Biodiversity

Abstract

Recently, a new solarization method gained a great deal of attention thanks to various advantages in comparison with both the traditional one and soil fumigation (alternative soil treatment based on the use of chemical agents). This method implements traditional solarization by spraying a biodegradable black liquid over the soil surface before the application of a thermic film. This creates a thin black film that acts like a “black body”, significantly increasing soil temperatures at various depths. Thanks to higher temperatures, it is possible to eliminate most of the pathogens in shorter times compared to traditional solarization. In the present paper, the results of different trials carried out on green beans, Romanesco broccoli, and lettuce were reported. The aims of this work were to demonstrate the efficacy on soil borne pathogens, its lower impact on living soil fungal biodiversity and the agronomical performance of the new solarization method. All crops tested showed a significant yield increase when grown in soil treated with the innovative solarization method. Romanesco broccoli also exhibited improved inflorescence quality. Solarization had a positive impact on overall crop productivity: green beans showed a maximum yield increase of 165.3%, lettuce yields rose by 47.5%, and Romanesco broccoli yields were 111.5% higher compared to the non-solarized control. These results confirm that the new solarization method is more effective, as well as environmentally, economically, and socially sustainable compared to traditional methods.

1. Introduction

Reducing the use of agrochemicals to manage weeds, pests, and soil-borne pathogens represents one of the key challenges for the future of agriculture.

Growing concerns about environmental sustainability and human health are driving research toward agronomic practices that minimize reliance on chemical inputs such as fungicides, pesticides, and herbicides. In this context, there is a high priority to find innovative materials and alternative soil treatment methods that should be both cost-effective and environmentally friendly. Soil solarization stands out as a nonchemical method capable of controlling soil-borne pathogens in open fields, greenhouses, nurseries, and home gardens [1,2,3,4,5,6,7,8,9,10]. For the implementation of this simple and well-tested method, it needs to cover the soil surface for several weeks with a thin transparent plastic film, with special optical properties, to increase the accumulation of heat in the soil. The resulting high soil temperatures reduce most of the pathogens, but this action depends on the application times that normally should exceed 50–60 days in our environment to be effective. This extended period influences and limits the diffusion of this natural method for soil treatment.

An alternative to this eco-sustainable practice is fumigation, which is a soil treatment based on the use of chemical agents (fumigants) which, in the gaseous state, eliminate most of the pathogens, including soil-borne fungal, bacterial, and nematode ones accumulated in the soil. Fumigation can sterilize the soil quickly, depending on the type of gas used. However, it is often dangerous for the environment, harmful to human health, very expensive, and extremely aggressive toward beneficial microorganisms present in the soil [11,12,13,14,15,16,17,18].

Nevertheless, soil chemical fumigation is known to be a destructive and dangerous method non-selective to soil biological targets, resulting in different degrees of change in the diversity and abundance of soil microorganisms, as well as soil enzymatic activities [19,20].

Furthermore, it has been widely demonstrated that, after chemical treatments, different harmful elements, including uncontrolled molecules, remain in the soil and the residues, accumulated year by year, affect the nature of the soil and they could represent a serious problem for future crops [21].

Soil solarization is the best alternative to fumigation—it is a very efficient practice that respects the environment, it does not pollute, it preserves the quality of crops, and, ultimately, the costs of its application are cheaper than those of fumigation [22,23,24].

Because the plastic film used for solarization plays an important role, it needs to be chosen carefully with highly selective optical properties to obtain a very efficient soil treatment. These properties guarantee a dynamic heat accumulation in the soil, and this gives rise to high temperatures, which can control most pathogens. The only limit of solarization is the running time—two months is the time required for a satisfactory result with a Polyethylene plastic film. Unfortunately, in some areas with intensive cultivation, and in particular agricultural production contests, the interval between two crop cycles is shorter than the one necessary for a good solarization efficacy. With the aim of introducing the use of soil solarization even in these contexts, it was necessary to study further implementation to reduce the running times, further increasing soil temperatures. With this goal, a new methodological approach was studied, based on the combination of a solarizing film and a carbon powder-based black liquid to spray on the soil [25]. The new proposed model is the simulation of a thermal solar panel (used for home hot water production), and it behaves in the same way to increase the water temperatures in the soil. This innovative physical model permits the incident solar radiation to be absorbed on the soil’s surface (thanks to the black layer) and, at the same time, to entrap heat (due to the optical properties of the plastic film). This method generates higher soil temperatures with a satisfactory sterilization effect obtained in shorter times, in comparison with those obtained with the traditional method [26,27].

As a first approach to the study of the improved solarization, experimental tests were arranged to compare temperatures with the new method in comparison with those collected via the traditional one. Parcels were treated with vegetal carbon powder, working as a “black body”.

Trials were arranged, both in greenhouses and fields, to analyse the efficacy of the method on pathogens’ survival, on yield, and on quality results in the post-solarization phase.

The objectives of the present work are: (i) the comparison between traditional solarization and the innovative one, on temperature and the sum of hours at which the temperatures were above the maximum cardinal growth temperature of the fungal pathogens; (ii) the effect on the target pathogens of the crops grown after solarization; (iii) the evaluation of the impact on living fungal biodiversity using the traditional serial dilution method; (iv) the influence on production and quality on the three tested crops.

2. Materials and Methods

2.1. Protocol of Application of the New Solarization Method

The new soil solarization method reported was defined by two basic elements: black liquid and functionalized plastic film. To obtain satisfactory results, it was important to follow the right protocol of application, and materials must have the right thermal and dimensional characteristics. The first necessary step was to moisten the soil until the field capacity was reached before spraying the black liquid (125 kg ha⁻¹) uniformly on the soil surface using common machinery and then putting the thermal film firmly onto the soil. The black liquid sprayed on the soil surface is a biodegradable mixture based on vegetal carbon powder produced by the “slow pyrolysis process” through the pruning of forest trees, without any chemical contaminant, and labelled as “food grade”; to avoid any pipe clogging problems during the spraying operation, carbon powder should not exceed 10 µm. The applied plastic film, which was produced in Israel by Ginegar Plastic Products Ltd. (Jezreel Valley, Israel), was chosen for its optical properties able to enhance solarization effects and the anti-drip effect (Figure 1) [27]. In greenhouses after the film was applied, greenhouses were completely closed to decrease the thermal loss.

2.2. Treatments and Crops Compared and Experimental Sites

To evaluate the impact of the new solarization method on temperature, pathogens survival, fungal diversity, production, and quality on the crops, three biennial trials (2 in greenhouses and 1 in the field) were carried out in three farms in the Campania Region, Southern Italy (

Table 1. Information on sites in the Campania Region (Italy), crops, and farms where trials were carried out.

Farm	Trial Location	Type of Trail	Crop
D’Ambrosio	Giugliano in Campania (NA) 40°56′52.1″ N 14°03′56.4″ E	greenhouse	Lactuca sativa L. (Lettuce)
Palmieri Emilio	Mondragone (CE) 41°04′50.6″ N 13°55′12.2″ E	greenhouse	Phaseolus vulgaris L. (Green bean)
Natura Verde	Falciano del Massico (CE) 41°08′58.1″ N 13°57′59.6″ E	open field	Brassica oleracea L. var. italica (Romanesco broccoli)

). The solarization treatments compared in all sites and crops were: (i) innovative solarization (Solin^®) (S1); (ii) new solarizing film (Polysolar^®, Ginegar Plastic Products Ltd.) (S2); (iii) polyethylene film (S3), and (iv) a non-solarized control (US4). In the greenhouse, the duration of solarization for S1 and S2 treatments was 30 days, while for S3 and US4 it was 60 days, as is common practice. All trials were carried out over 60 days in open field sites. Moreover, all trials were arranged as a randomized block design with three replicates. Each treatment was applied on a surface area of 500 m².

2.3. Temperature Measurements

To compare the new solarization method with the traditional one, for each trial the temperatures and the sum of the hours at which the temperatures were above the maximum cardinal for growth of the tested pathogens were assessed. Soil temperatures were acquired with Nematool^® (Bayer Crop Science S.r.l., Milano, Italy) at 25-cm depths for all trials’ duration (Figure 2). Nematool^® is an electronic device that emits soil temperature information using GPRS technology. The device is in accordance with EU directives: 2014/53/EU—placing on the market of radio equipment and 2011/65/EU—restrictions on the use of certain hazardous substances in electrical and electronic equipment.

2.4. Effects of Solarization Treatments on Survival of Pathogens Propagules

To evaluate the impact of different solarization treatments on chosen pathogens, we used: pseudosclerotia for Rhizoctonia solani (Rs), conidia for Fusarium oxysporum f.sp. lactucae (Fol), and sclerotia for Sclerotinia sclerotiorum (Ss), which were mixed with sterile sand, put in bags (Sun Bag^® Merck, Darmstadt, Germany, Figure 3), and placed into the soil.

The pathogens reported above were chosen because they represent serious problems for growers and affect both the quality and yield of tested crops. The concentration of pathogens used in the bags is reported in

Table 2. Pathogens used for the preparation of fungal inoculum in Sun Bags^® and propagules’ concentration.

Pathogens	Abbreviation	Concentration (CFU · g−1 of Sand)
Fusarium oxysporum f.sp. lactucae	Fol	1 × 106
Rhizoctonia solani	Rs	2 × 103
Sclerotinia sclerotiorum	Ss	3 × 102

.

The inoculum of the three fungal pathogens was made using the appropriate propagules for each of them. For this purpose, Petri dishes containing Potato Dextrose Agar (PDA, BD Difco™, Franklin Lakes, NJ, USA) were inoculated by fungi; after one week, conidia of Fol, and after four weeks sclerotia of Ss and pseudosclerotia of Rs, were collected. Propagules were mixed with sterile sand at the appropriate concentration reported in Table 2 before filling the bags. Sun Bags^® with a 0.22-µm filter were put in the soil addressed for each treatment and fixed in each parcel at a 20-cm depth before solarization treatment. The survival of propagules of the three pathogens in the different plots was evaluated at the end of solarization treatment: 30–35 days after exposure for S1 and S2, and 60 days for S3 and US4. Ten grams of soil per bag were collected and analysed by the dilution method [28]. For each of the three replicates, the number of CFU per gram of soil was counted.

All the three tested pathogens inoculated in Sun Bags^® were put in the soil of all trials independently from the crop grown so that we had data of many replicates for each fungus (six and three per year for greenhouse and field trials, respectively).

In addition to assessing the survival of the three selected pathogens in the Sun Bags^®, the efficacy of solarization against naturally occurring soil-borne pathogens was evaluated by assessing disease incidence resulting from natural infection in tested crops. At harvest time for each crop, the incidence of diseased plants was evaluated following the formula: n° diseased plants/total of observed plants × 100. Efficacy was also calculated by Abbott’s index [29]. The assessments were made on three randomized areas of 10 m², consisting of 90 and 180 lettuce and green beans plants, respectively, while for Romanesco broccoli all plants in the plots were evaluated. The data reported for each crop are the mean values of a biennial trial.

2.5. Effect on Live Cultivable Fungal Diversity

The Gini-Simpson method was chosen instead of molecular analyses to avoid the over estimation of data due to the presence of DNA from death fungi.

To evaluate the effect on living and cultivable fungal biodiversity, at the end of solarization treatments soil samples were collected in clean polythene bags and labelled after sampling. For each of the three farms, three replicates of 200-g soil samples from 10 different points of the same field (at 20 cm depth) were sampled. The samples from the two greenhouse trials were pooled together, maintaining the three replicates for each treatment. Ten grams of soil per replicate were analysed by using the serial dilution method [30] using PDA amended with Rose Bengal as non-specific media. The inoculated Petri dishes were incubated at 24 ± 1 °C for 8 days. Colonies’ colour combined with morphology of conidia were used to discriminate fungal genera to estimate fungal diversity. The Gini-Simpson diversity index D [30] was calculated for each treatment; the two greenhouse trials were combined as one experiment. Relative abundance was measured as the Gini-Simpson D index that ranges from 0 to 1. The D index was calculated as

D = 1 − [ni(ni − 1)/N (N − 1)]

where “ni” is the abundance of individual species, and N = total number of organisms.

2.6. Assessment of Biometric, Yield Parameters of Tested Crops

Two greenhouses (

Table 3. Assessment of biometric, yield, and quality parameters of tested greenhouse crops.

Crop/Cv	Transplanting/Sowing Date		Harvesting Dates	Nr. of Plants ha−1	Assessed Parameters
Lettuce/Bretzel	Trial 1	10 July 2022	18 December	90,000	fresh and dry weight of 30 lettuce heads per replicate and yield on 5 m−2
	Trial 2	8 September 2023	9 November
Green bean/SV1545GA	Trial 1	8 September 2022	18 and 23 November	180,000	plant height, number of pods, weight of pods per plant, plant fresh and dry weight on 25 plants/replicates and yield on 5 m−2/replicate
	Trial 2	1 September 2023	5 and 9 November

) and one field trial (

Table 4. Assessment of biometric, yield, and quality parameters of tested open field crop.

Crop/Cv	Sowing Date		Harvesting Dates	Nr. of Plants ha−1	Assessed Parameters
Romanesco broccoli/Veronica	Trial 1	2 October 2022	21 February 2023	20,000	plant height, fresh weight, dry weight (g), inflorescence weight and diameter on 15 plants per replicate and yield on 5 m−2/replicate
	Trial 2	10 October 2023	20 February 2024

) were carried out in 2022 and repeated in 2023.

2.7. Quality of Romanesco Broccoli

2.7.1. Colour and Chlorophyll Content

The colour of Romanesco broccoli was evaluated using a digital colorimeter (Konica Minolta Model CR5, Konica Minolta, Chiyoda, Tokyo, Japan) to measure chromaticity values: L* (lightness/brightness, ranging from 0 = black to 100 = white), a* (negative values indicate green, positive values indicate red), and b* (negative values indicate blue, positive values indicate yellow). Chromaticity (C*) and the hue angle (H*) were calculated based on the a* and b* values, following the method described by McGuire [31]. Chlorophyll and carotenoid content was determined by grinding Romanesco broccoli in N, N-dimethylformamide (1:50) and centrifuging the samples at 10,000× g for 15 min. The absorbance of the supernatant was measured spectrophotometrically at 663, 647, and 437 nm. Photosynthetic pigments concentrations were calculated using the Wellburn formula [32], and the results were expressed as mg per 100 g of dry weight (DW).

2.7.2. Bioactive Compounds and Antioxidant Activity

The total phenolic content (TPC) was determined using the Folin-Ciocalteu method [33]. Approximately 1 g of Romanesco broccoli was homogenized in 5 mL of an 80:20 methanol-water solution and incubated overnight under agitation at 20 °C. The assay mixture was prepared by combining 20 μL of the supernatant with Folin-Ciocalteu reagent and 20% Na₂CO₃ in water, followed by incubation at 20 °C for 120 min. Absorbance was then measured at 765 nm on a UV–visible spectrophotometer (Jasco UV-VIS630, Milan, Italy). The results were expressed as mg of gallic acid equivalents per gram of dry weight (mg GAE 100 g⁻¹ DW) [34]. The flavonoid content (TFC) of methanolic extracts were measured using the aluminum-chloride methods described by Jia et al. [35]. A defined volume (0.2 mL) of the extract was added 5% w/v NaNO₂, 10% w/v AlCl₃, and 1 M NaOH, and the absorbance was measured at 510 nm. The results were expressed as mg of catechin equivalents (CE) 100 g⁻¹ DW.

The antioxidant capacity of the methanol extracts was assessed using the 2,2-azinobis-(3-ethylbenzothiazolin-6-sulphonic acid) (ABTS) assay, based on the method described by Cice et al. [36]. Results were reported as micromoles of Trolox equivalents (TE) g⁻¹ DW.

2.8. Statistical Analysis

For pathogens’ survival and incidence data, since the greenhouse trials among them and the two field trials showed similar variances, the trials were treated as replicates of the same trial. The mean percentages of surviving propagules were calculated by referring to US4 values. For biometric and productive parameters, the data were analysed separately for each trial. Statistics were performed using GraphPad InStat version 3.00 for Windows (GraphPad Software, San Diego, CA, USA) and SAS\v. 9.3 (SAS institute Inc.2010.SAS/STAT\ 9.22 User’s Guide. SAS Institute Inc., Cary, NC, USA). The differences between treatments were considered significant at p ≤ 0.05. All percentages were transformed by arcsine square root according to the formula Y = arcsine (√(x%/100)) before analysis, and were back transformed to percentages for the tables and figures. The data obtained were subjected to analysis of variance (ANOVA) for quantitative variables, and means were separated using the Tukey-Kramer test [37].

3. Results

3.1. Temperature Measurements

In the greenhouse experiments, S1 represented the best treatment. Nevertheless, it needs to be considered that S1 and S2 lasted for just 30 days, that was half the time of traditional solarization (S3), which remained covered for a longer period of 60 days. Although S2 and S3 showed similar results, Polysolar^® film reached the same level in half the time. The two years of trials were warmer, and 2023 had a higher temperature than 2022—this explains the best results obtained in 2023 both in field and greenhouse. Interesting data were the accumulation of hours above 38 and 40 °C, which showed in the greenhouse due to the greenhouse’s closure, and which had never been observed before in other experiments. Although F1 and F2 represent two farms that are almost 30 km away from each other, the results obtained were remarkably similar. The present experiment demonstrated that the sum of temperatures reached was efficient at reducing pathogens’ inoculum both in open field and greenhouse (

Table 5. Sum of the hours at which temperatures were above the maximum cardinal for growth of tested pathogens, recorded for each treatment in both the greenhouse trial in D’Ambrosio farm (Giugliano in Campania—NA) (F1) and in Palmieri farm (Mondragone—CE) (F2) between the 5th of July and the 10th of August in both 2022 and 2023.

Treatment	2022				2023
	∑ h > 38 °C		∑ h > 40 °C		∑ h > 38 °C		∑ h > 40 °C
	F1	F2	F1	F2	F1	F2	F1	F2
US4	168	228	152	191	270	261	98	129
S1	811	803	792	784	1235	1215	994	974
S2	633	798	612	772	1111	1098	938	925
S3	626	789	600	756	1110	1100	937	927

). In the open field, the measurements’ duration was 60 days (from July the 7th to September the 6th in both years) for all treatments; conversely, in the greenhouse it was 30 days (from the 1st to 31st July) for S1 and S2, and 60 days for US4 and S3 (from July the 1st to August the 30th in both years). The new solarization method (S1) showed the highest values, while in the non-solarized control (US4) temperatures never crossed 38 °C. The best results were obtained in 2023 because the temperatures in the two periods were higher than in 2022 (Figure 4). Dates of application and measurement were the same as in the open field (

Table 6. Sum of the hours at which temperatures were above the maximum cardinal for the growth of tested pathogens, recorded for each treatment in the open field trial in the Natura Verde farm (Falciano del Massico—CE) between the 6th of July and 12th of September 2023.

Treatment	Year 2022		Year 2023
	∑ h > 38 °C	∑ h > 40 °C	∑ h > 38 °C	∑ h > 40 °C
US4	0	0	0	0
S1	883	715	1425	1112
S2	665	318	1277	739
S3	620	239	1310	1025

).

3.2. Effect of Solarization Treatments on Survival of Pathogens Propagules

Regarding temperature, the combination of biodegradable black liquid and plastic films allowed faster and more uniform soil heating and higher thermal sums, respecting the common solarization, reaching lethal temperatures for pathogens in a shorter time. In S1, soil temperatures recorded with the new solarization were always higher than the maximums acquired with the traditional one.

Different trials conducted in different farms in the South of Italy confirmed the clear reduction in the survival level of pathogens, such as F. oxysporum, R. solani, and S. sclerotiorum in just 30 days [1], instead of the 60 normally required. Shorter treatment duration minimized resource consumption, including labour and energy inputs [39], and allowed the more efficient use of farmland with resulting economic benefits [24].

Table 7. Percentage of fungal propagules of three pathogens: F. oxysporum f.sp lettuce, R. solani, and S. sclerotiorum survived in the Sun Bag^® of the biennial greenhouse trial after treatment. Mean values with the same letter are not statistically different according to the Tukey-Kramer test.

Treatments	Propagules Survival (%)
	Greenhouse			Open Field
	Fol	Rs	Ss	Fol	Rs	Ss
US4	100 a	94.0 a	97.0 a	90.0 a	95.4 a	96.1 a
S1	0.8 d	3.0 d	0.0 c	0.0 d	1.4 d	0.0 d
S2	14.5 c	15.6 c	8.4 c	8.6 c	12.3 c	10.5 c
S3	28.0 b	22.6 b	19.0 b	32.9 b	28.4 b	29.3 b

reports the mean values of the percentage of pathogen propagules surviving in two biennial trials in the greenhouse and field. Data had similar variance so that they were treated as replicates of a single experiment. The results of the biennial open field trial showed that propagules’ survival in untreated plots (US4) remained close to the initial values (90–96.1%). In the S3 plots, survival significantly decreased to approximately 30%. A further substantial reduction was observed in the S2 plots, while in S1 the propagule achieved almost the total pathogen propagules’ mortality (0–1.4%). The results of greenhouse trials showed that the number of surviving propagules in untreated plots (US4) remained the same as the starting values (94–100%). In the S3 plots, the number of propagules that survived significantly decreased, ranging from 19% for Ss to 28% for Fol. A further significant reduction was observed in the S2 plots, where only the thermal film was used, with propagule survival ranging from 8.4% for Ss to 15.6% for Rs. The new solarization method resulted in a drastic decrease in propagule survival, reducing it to less than 3%, with no significant differences among the three pathogens tested.

Table 8. Disease incidence and efficacy of different solarization methods of the two biennial greenhouse trials on lettuce and green bean. Mean values with the same letter are not statistically different according to the Tukey-Kramer test.

Treatments	Fol on Lettuce		Ss on Green Bean
	Incidence (%)	Efficacy (Abbott)	Incidence (%)	Efficacy (Abbott)
US4	5.4 a	-	35.0 a	-
S1	0.1 c	98.1 a	0.0 c	100 a
S2	1.0 b	81.5 b	0.0 c	100 a
S3	1.5 b	72.2 c	5.0 b	85.7 b

presents data on the disease incidence and solarization efficacy of two crops at harvest time. They had been cultivated after the application of different soil solarization methods. A very low incidence of diseased plants (0.2%) was observed in Romanesco broccoli, making it unnecessary to assess field efficacy. On lettuce, the mean value of disease incidence in US4 was 5.4%, and the resulting efficacy in controlling Fol was 98.1 for the innovative solarization method (S1), and 81.5 and 72.2 using the new thermal film (S2) and the traditional method (S3), respectively. On green bean with a mean disease incidence of Ss in US4 of 35%, the efficacy was 85.7 in S3 and 100 in both S2 and S1 treatments.

3.3. Effect on Live Cultivable Fungal Diversity

To verify the effects of solarization methods on fungal diversity, the Gini-Simpson index was calculated. When the traditional solarization method (S3) was applied for 60 days in both field and greenhouse trials, a reduction in cultivable fungal diversity was observed. The traditional method needs at least 60 days of duration to be effective. This negatively affected fungal soil biodiversity both in the greenhouse and the field, reducing by 0.5 points the biodiversity index compared to the other treatments (Figure 5). S1 and S2 showed less impact on fungal biodiversity, highlighting that the reduced application time (30 days) is determinant for this aspect.

3.4. Assessment of Biometric, Yield Parameters of Tested Crops

3.4.1. Green Beans

Green Beans 2022

Regarding plant growth, in terms of height, S1 gave greater values than other treatments, especially compared to US4, but also compared with S2 and S3 values, and these last two treatments resulted in no statistical difference between them. This was even more obvious in fresh weight results, where the S1 weight was almost twice that of US4. Regarding dry weight, there were no significant differences. Concerning the number of pods per plant, this was higher in S1 than in other treatments, and the lowest number was obtained in US4. All these parameters, especially plant growth and pod number, resulted in an S1 yield four times higher than that of US4, and twice that of S2 and S3.

Green Beans 2023

In 2023, plant heights of S1, S2, and S3 treatments were not statistically different among them, but higher than US4 (Figure 6). Regarding the fresh and dry weight of plants, S1 and S2 showed similar results, and ones that were greater than those of US4 and S3. Concerning S3, fresh weight values were almost twice those of US4 ones, while their dry weights were very comparable. Regarding the number of pods per plant and the weight of pods, US4 showed the lowest values. Specifically, S2 and S3 showed the same number of pods per plant, lower than S1, while the S1 pods’ weight was not significantly different among treatments.

S1 yield showed the highest values, due to both the number and weight of pods, and it was twice the US4 yield, but not so far from the S2 and S3 results (

Table 9. Assessment of biometric and yield parameters of green bean one month before harvesting. Values followed by the same letter are not significantly different according to the Tukey-Kramer test.

	Plants						Pods
Treatments	Height (mm)		Fresh Weight (g)		Dry Weight (g)		Nr. Plant −1		Weight (g)		Yield (t ha−1)
	2022	2023	2022	2023	2022	2023	2022	2023	2022	2023	2022	2023
US4	563 c	277 b	24.2 c	28.2 c	10.5	15.9 b	9.9 c	9.0 c	2.2 ns	1.9 b	3.92 c	3.12 c
S1	699 a	510 a	43.1 a	62.5 a	12.9 a	18.7 a	15.4 a	15 a	2.6	2.4 a	19.2 a	6.48 a
S2	660 b	513 a	34.9 b	62.2 a	10.6 b	18.9 a	12.1 b	12 b	2.6	2.4 a	6.48 b	5.18 b
S3	676 b	529 a	32.9 b	55.0 b	9.8 b	15.4 b	13.1 b	12 b	2.5	2.0 b	6.36 b	4.32 b

).

3.4.2. Lettuce

S1 treatment showed the best results, especially for fresh weight and percentage of dry matter, in both years (

Table 10. Assessment of biometric and yield parameters of lettuce at the harvest. Values followed by same letter are not significantly different according to the Tukey-Kramer test.

Treatments	Head Fresh Weight		Head Dry Matter		Yield
	(g)		(%)		(t ha−1)
	2022	2023	2022	2023	2022	2023
US4	495 bc	468.8 b	3.0 b	3.1 c	25.7 c	18.9 d
S1	581 a	550.2 a	4.1 a	4.2 a	33.3 a	32.5 a
S2	527 b	499.1 b	3.6 b	3.8 b	30.1 b	28.4 b
S3	515 b	487.7 b	3.1 b	3.5 bc	29.5 b	23.3 c

). For the same parameters, S2, S3, and US4 were not statistically different. Regarding yield results, S1 had the highest productivity in both 2022 and 2023, obtaining a two-year mean increase of 47.5% compared with US4. S2 treatment registered the same yield results of S3 in 2022, while giving higher yield results than S3 in 2023.

3.4.3. Romanesco Broccoli

In Romanesco broccoli crops, 2022 and 2023 showed data with the same variance, so they can be considered as one trial, and the data of each year were considered as replicates of the same experiment (

Table 11. Assessment of biometric parameters in Romanesco broccoli plants three months after transplantation. The data represent the mean values of three replicates consisting of 25 plants ± SE. Values within columns followed by the same letter are not significantly different according to the Tukey-Kramer test.

Treatment	Plant Height (cm)	Fresh Weight (g)	Dry Weight (g)
US4	50.3 c	554.2 c	67.5 c
S1	67.5 a	1240.0 a	150.0 a
S2	60.5 b	1010.8 b	123.3 b
S3	62.2 ab	1054.2 b	119.2 b

). Regarding plant height, S1 showed the highest value, especially if compared with US4; fresh and dry weight plants (Figure 7) were both considerably higher in S1 plots, and more than double US4. Inflorescence diameter values of S1 and S2 were not different from each other but higher than S3 and US4, whereas inflorescence weights were greater in S1, and more than double than US4. These parameters reflected yield results, where S1, S2, and S3 treatments showed yield increases of 111, 82.7, and 45.5%, respectively, compared with US4 (Figure 8).

3.5. Quality of Romanesco Broccoli

Colour and Chlorophyll Content

Significant differences (p ≤ 0.05) were observed among treatments in the colour parameters of cauliflower florets, as measured in the CIELAB colour space (

Table 12. Effect of different soil solarizations treatments on colour parameters (Lightness (L*), Chroma (C*), and Hue angle (H*)) in Romanesco broccoli. Values within a column followed by same letter are not significantly different according to the Tukey-Kramer test.

Treatment	L*	C*	H*
US4	41.23 ± 2.34 a	36.86 ± 1.62 a	99.43 ± 0.41 a
S1	46.21 ± 2.80 b	43.66 ± 1.77 b	102.14 ± 0.56 c
S2	46.52 ± 2.96 b	43.46 ± 2.44 b	102.08 ± 0.62 c
S3	44.31 ± 2.95 b	38.73 ± 2.96 a	100.58 ± 0.49 b

). Lightness values ranged from 41.23 ± 2.34 in US4 to 46.52 ± 2.96 in S2, indicating that florets from S1 and S2 were significantly lighter compared to US4, with S3 showing intermediate lightness. Chroma, reflecting colour saturation or intensity, was significantly higher in S1 and S2, indicating more vivid floret coloration compared to US4 and S3, which exhibited duller tones. Hue angle, indicative of the actual perceived colour, also varied significantly across treatments. S1 and S2 showed the highest hue angles, denoting a slight shift towards a greenish tint, while US4 had the lowest value, suggesting a more yellowish hue. These variations suggest that solarization treatments had a measurable impact on the visual appearance and potential marketability of cauliflower florets.

As shown in Figure 9, chlorophyll and carotenoid concentrations in cauliflower florets varied significantly among treatments. Chlorophyll a content was highest in treatment S1, followed by S2 and S3, while US4 showing the lowest Chl a concentration. In contrast, more significant differences were observed in chlorophyll b and carotenoid contents. S1 displayed the highest Chl b level, whereas US4 had the lowest. Similarly, carotenoid content was greatest in S1 and S2, and lowest in S3 and US4. These results suggest that treatments S1 and S2 promoted a higher accumulation of both accessory pigments (Chl b and carotenoids), which may enhance the photoprotective capacity and visual appeal of the cauliflower.

In

Table 13. Effect of different solarizations on polyphenol, flavonoids content, and antioxidant activity in Romanesco broccoli cv. Veronica. Values within columns followed by same letter are not significantly different according to the Tukey-Kramer test.

Treatment	Polyphenol (mg GAE/100 g DW)	Flavonoids (mg CE/100 g DW)	Antioxidant Activity (µmol/g DW)
US4	8.78 ± 2.24 a	1.11 ± 0.19 a	39.20 ± 7.33 a
S1	13.02 ± 2.40 b	1.83 ± 0.05 c	56.42 ± 2.44 c
S2	13.72 ± 1.16 b	1.69 ± 0.11 c	53.49 ± 1.43 c
S3	9.27 ± 0.58 a	1.30 ± 0.22 b	48.06 ± 3.47 b

, the effects of different solarization treatments are reported on polyphenol content, flavonoid content, and antioxidant activity.

The S2 treatment resulted in the highest polyphenol concentration (13.72 ± 1.16 mg GAE/100 g DW), followed by S1 (13.02 ± 2.40 mg GAE/100 g DW). Both were significantly higher than US4 and S3, which had comparable and lower values (8.78 ± 2.24 and 9.27 ± 0.58 mg GAE/100 g DW, respectively). S1 and S2 showed the highest flavonoid content (1.83 ± 0.05 and 1.69 ± 0.11 mg CE/100 g DW, respectively), significantly higher than all other treatments, while USC4 had the lowest value (1.11 ± 0.19). S1 and S2 also led to the highest antioxidant activity (56.42 ± 2.44 and 53.49± 1.43 µmol/g DW), indicating a strong correlation with its higher flavonoid and polyphenol levels, while US4 showed the lowest value (39.20 ± 7.33 µmol/g DW).

Overall, the S1 and S2 treatments significantly enhanced the bioactive compound content and antioxidant activity compared to the control (US4), with S1 being the most effective across all parameters (Table 13).

4. Discussion

The plastic film used for solarization plays a fundamental role: it needs to be highly transparent to incoming solar radiation, while simultaneously limiting the heat loss from the soil. This mechanism, which relies on trapping energy within the soil, is further enhanced by applying the black liquid mentioned, leading to an additional temperature increase. The film that must be chosen for the trials, from a theoretical standpoint, should have the appropriate optical characteristics [27]. Experimental data confirmed the effectiveness of the new solarization method, as it consistently recorded a greater number of hours with temperatures exceeding 38 °C and 42 °C compared to the traditional approach. It improves all aspects of sustainability as reported below.

4.1. Impact on Pathogens

The direct comparison between the innovative solarization method and treatments based on traditional solarization methods clearly indicated the effectiveness of the new approach, and highlighted its key advantages. The new approach significantly increased soil temperatures and reduced the mortality of F. oxysporum f.sp. lactucae, R. solani, and S. sclerotiorum that was higher than 97% in greenhouse trials and was near-total (98.6–100%) in the field trials. Notably, the high mortality rates of sclerotia and pseudosclerotia were observed within just 30 days, confirming the method’s ability to sustainably and effectively reduce pathogen inoculum potential, in both greenhouse and open-field conditions, requiring significantly less time than traditional solarization. These findings are consistent with earlier studies highlighting the role of solarization in suppressing soil-borne pathogens [40] but further improve upon the efficiency by reducing the required treatment duration. Assessment of mortality were also consistent with data assessed in the Ss control on green beans and Fol on lettuce in greenhouse confirming also in vivo the higher efficacy of new method proposed.

4.2. Environmental Impact and Sustainability

The shorter duration of innovative solarization and the replacement of fumigation give direct benefits to the environment, soil, groundwater, and human health [18].

Innovative solarization offers a completely different situation because it eliminates, as Katan et al. demonstrated [3,4], just the pathogens preserving soil fungal biodiversity. In the present work, we chose to evaluate only the living fungal biodiversity, nevertheless, the NGs technique is the best technique to evaluate microorganism dynamics, but it overestimates biodiversity when applied just after the treatments because of the presence of DNA from death cells. The innovative solarization method is reported to enhance the antagonistic activity of native soil microflora, particularly thermophilic bacteria, by promoting the selection of strains with broad-spectrum biocontrol mechanisms such as antibiotic production, competition for nutrients, and pathogen suppression [23,41].

Nevertheless, it has been demonstrated that solarization affects microbial biomass [42], reducing the duration of solarization to 45 days allows microbial communities to recover faster [41]. The new Solin^® solarization method, that lasts only 30 days, minimizes the impact on fungal biomass while still effectively controlling soil-borne pathogens, supporting long-term soil health and sustainability and does not affect soil resilience.

4.3. Social, Health and Economic Benefits

In fumigation, it happens often that a part of the gas passes through the film, polluting not only the environment but causing serious problems for the health of workers, people around, and affecting the quality of air, which becomes unbearable. Even if in any country there are rules that impose limits on the use of fumigants, they are not always respected. This can be overcome thanks to the introduction of innovative solarization, which represents a major advance over conventional solarization and chemical fumigation, in line with recent studies on alternative soil disinfestation methods [43]. Moreover, the method has the advantage of being easy to apply and highly adaptable. In fact, it is designed to be user-friendly, requiring only standard agricultural equipment for its utilization [24]. Its adaptability to diverse climates, crops, and soil types, and Solin^®’s affordability (EUR 0.15/m²) compared to other treatments such as chemical fumigation (EUR 0.60 ÷ EUR 0.75/m²), ensure a widespread use. Shorter treatment duration, from 60 to 30 days, minimizes resource consumption, including labour and energy inputs [39], and allows the more efficient use of farmland with resulting economic benefits [24]. This means one month more in terms of soil use and better management by farmers who must organize cultural cycles in the short and long terms. This represents a real and significant positive impact on the economy of each farm.

4.4. Agronomical Performances Post-Solarization

It is well known that solarization produces a plant growth increase [44], and one of the most striking findings of this study was the significant improvement in crop productivity.

In our experiments carried out in Southern Italy on green beans, lettuce, and Romanesco broccoli, the S1 treatment demonstrated remarkable growth, leading to increased fresh weight, biomass accumulation, and yield in all three crops compared to both traditional solarization and control treatments.

Solarization positively influenced crop productivity. In green bean trials (S1, S2, and S3), yields increased, reaching the maximum increase of 165.3%, 65.6%, and 51.7%, respectively. Regarding lettuce, S1 and S2 plots showed yield increases of 47.5% and 31.2% compared to US4. Similarly, in Romanesco broccoli, crop productivity improved by 111.5% and 82.7% in S1 and S2 trials, respectively. These results align with previous studies demonstrating the efficacy of soil solarization in enhancing crop yield by reducing soil-borne pathogens and improving soil conditions, but again the higher increases was obtained by innovative solarization method.

The higher productivity observed in these crops was likely a result of improved soil health conditions and reduced pathogen pressure [40]. Moreover, it has been demonstrated that traditional solarization method results on crops productivity is directly proportional on its duration, 60 days gives better results than 30 days [45], but in this study, S1 gave the highest yield despite shorter time.

Solarization affects also the presence of minor pathogens and influences the activity of microbes related to nitrogen and other important elements [2,25]. Furthermore, biodiversity is better preserved and the lower impact assessed just after the innovative solarization application, could render soil more capable of maintaining homeostasis and resilience, allowing a faster return to optimal functional conditions.

In our field trial on Romanesco broccoli, we achieved higher yields in absence of pathogens. This suggests that applying solarization, even in absence of soil-borne disease, could also be beneficial. Romanesco broccoli is rich in health-promoting phytochemicals such as chlorophylls, carotenoids, polyphenols (including flavonoids, hydroxycinnamic acids, and anthocyanins), and glucosinolates, along with essential nutrients [46,47]. These polyphenols typically appear in complex glycosylated and acylated forms. Colour values (L*, C*, and hue angle) aligned with those of other Romanesco broccoli cultivars [48,49]. Increases in L* values under soil solarization matched trends seen in strawberries grown in similar conditions [50]. Total polyphenol content was consistent with previous findings in Romanesco broccoli [48]. However, these compounds showed sensitivity to environmental stress, particularly heat and drought during growth [46]. Soil solarization combined with calcium cyanamide has been shown to enhance the nutritional and bioactive profile of crops like strawberries [50]. Similarly, pumpkins grown in solarized soil, with or without organic amendments, showed increased antioxidant activity [51].

In conclusion, new solarization method enhance not only yield but also crop quality. In the case of Romanesco broccoli, better soil health and reduced pathogen pressure may contribute to higher levels of nutraceutical compounds, including antioxidants and other bioactive phytochemicals. These qualitative improvements are particularly relevant for functional food production and align with current trends in health-oriented agriculture.

5. Conclusions

This study highlighted the innovative sustainable solarization method that integrates a biodegradable black liquid and a functionalized plastic film. This approach represents a significant advancement in soil disinfection by achieving higher temperatures, reducing treatment time, and enhancing pathogen suppression more effectively than traditional methods. In addition, the method contributes to higher crop productivity and quality, making it a promising possibility for the sustainable agriculture.

Future studies should also optimize the application rate and explore its compatibility with other sustainable practices like organic mulching and cover cropping [52].