Wood-Vinegar-Added Biochar as a Soil Conditioner Enhances Safflower Performance in the Brazilian Semi-Arid Northeast

Abstract

Food security is threatened in the semiarid region of Brazil, which is susceptible to climate change and has low-fertility soils degraded by inadequate agricultural practices. This study aimed to evaluate safflower’s adaptation to the region and the benefits to the soil and crop of applying biochar and wood vinegar (WV). Biochar, pure or WV-added (Wv-biochar), was applied to the soil at doses of 3.0, 6.0, and 9.0 t ha⁻¹. Determinations performed in three harvests of safflower were plant height, number of capitula per plant, number of seeds per capitulum, mass of 1000 seeds, seed yield, and oil content. The maximum safflower yields (1818.52 kg ha⁻¹) and oil content (45.50%), and the average values of mass of 1000 seeds (35.55 g) were consistent with results reported in literature. Evidence of better performance of the variables under the effect of Wv-biochar than of pure biochar was observed, and, in general, the curves obtained showed quadratic behavior, with maximum values at intermediate doses. The seed yield and oil content achieved indicate that safflower is a promising crop for the region, particularly when more adapted genotypes and improved management practices are employed. The most pronounced effects on safflower production and oil content were observed at doses of 5 to 6 t ha⁻¹ of Biochar and Wv-biochar, which are economical and sustainable alternatives due to their use of organic waste and the benefits they provide for soil and food security.

1. Introduction

Current agricultural practices raise concerns about climate change and food security, as they are threatened by climate, soil, and water contamination and degradation, as well as increased production costs [1,2]. In the semiarid region of Brazil, the yield of the main crops is limited by high temperatures and evapotranspiration, and reduced and inconstant rainfall [3,4]. The soils of this region are subjected to excessive mobilization and the use of synthetic fertilizers, which result in low nutrient-use efficiency and contribute to soil and water pollution and greenhouse gas (GHG) emissions. The degradation of these soils is evidenced by the loss of organic carbon, nutrient depletion, loss of structure, clay dispersion, low water retention capacity, erosion, compaction, acidification or alkalinization, and salinization [2,3,5].

The adoption of safflower can contribute to food security, farmers’ income, and poverty alleviation, as it grows well in semiarid climates and is tolerant to water stress and salinity. Edible safflower oil is highly valued for its high quality and diverse industrial uses, including as a biofuel. The well-developed root system of safflower provides high water-use efficiency, allows the recycling of residual nutrients, and enables the recovery of NO₃⁻ leachate. Therefore, growing safflower in marginal areas subject to degradation avoids deforestation in new regions, reduces GHG emissions, and lowers irrigation and synthetic fertilizer costs [6].

Biochar and WV are obtained from the pyrolysis of organic waste that, if left in the environment, burned, or placed in landfills, causes pollution and gas emissions [7]. Biochar is a solid, rich in highly aromatic, stable carbon, with a porous structure, a large specific surface area, functional groups, and strong adsorption capacity. WV is liquid, composed of water, as well as condensed and highly oxygenated organic acids, such as acetic acid, as well as alcohols, aldehydes, phenols, esters, and ketones [8,9,10]. The application of biochar to soil helps mitigate climate change through carbon sequestration and reduced GHG emissions [11,12] and can help recover degraded soils [13]. Biochar contributes to crop yields by improving the physical, chemical, and biological properties of the soil, including soil structure, cation exchange capacity (CEC), water and nutrient retention, acidity, and the availability of nutrients such as potassium and phosphorus [14,15].

Agricultural productivity is enhanced by the safe application of WV, as functional organic substances, including phenolic compounds, confer antimicrobial, insecticidal, and herbicidal action, as well as growth-promoting effects. WV stimulates seed germination and rooting and benefits nutrient absorption. Its application as a soil conditioner increases P availability to plants, reduces soil acidity and salinity, inhibits NH₄⁺ loss, mitigates N₂O and CH₄ emissions, and increases microbial activity [8,9,10,14].

The combined application of biochar and WV can enhance the beneficial effects of each on soil and plant growth. This is due to increased soil fertility, enhanced crop resistance to stress, improved soil structure, remediation of salinity, and increased fertilizer and pesticide efficiency. This combination can reduce the application of synthetic fertilizers and, consequently, soil and water pollution [8,9,10,14]. The study hypothesizes that safflower can adapt to the climate and soils of the semiarid region of Brazil, resulting in satisfactory production, and that the WV-added biochar is effective as a new soil conditioner. Therefore, the study aimed to evaluate safflower production and oil yield over three harvests in a semiarid environment of the northeastern region of Brazil, under the effect of biochar application to the soil. No previous studies have reported the use of Wv-biochar on safflower under Brazilian semiarid conditions, highlighting the originality and relevance of this research and its contribution to advancing sustainable soil management strategies and enhancing crop diversification in marginal environments.

2. Materials and Methods

2.1. Local, Soil, and Climate

The research consisted of the cultivation of safflower (Carthamus tinctorius L.) in three harvests, in the years from 2023 to 2025, at the Experimental Farm of the Federal Rural University of the Semiarid Region (UFERSA), in Mossoró, state of Rio Grande do Norte, Brazil (−5.1833° S and −37.3333° W). According to Köppen’s classification, the climate of Mossoró is BSw’h’, hot, semiarid, with an irregular rainy season concentrated in the first months of the year and extending into autumn.

Table 1. Average meteorological data from the UFERSA automatic station during the study months for the years from 2019 to 2023.

	August	September	October	November	December	January	February
Tave (°C)	27.97 ± 0.36	28.57 ± 0.19	29.06 ± 0.19	29.24 ± 0.18	28.62 ± 0.23	28.81 ± 0.15	28.85 ± 0.19
Tmax (°C)	35.22 ± 0.90	36.47 ± 0.47	36.63 ± 0.45	36.00 ± 0.47	35.83 ± 0.47	34.87 ± 0.28	35.17 ± 0.31
Tmin (°C)	21.75 ± 0.80	22.58 ± 0.47	23.46 ± 0.42	24.49 ± 0.38	24.60 ± 0.36	24.73 ± 0.18	24.57 ± 0.17
RUmax (%)	85.28 ± 2.20	83.68 ± 1.66	85.03 ± 1.66	86.06 ± 1.66	86.84 ± 1.70	89.89 ± 1.05	92.55 ± 0.88
RUmin (%)	35.89 ± 4.06	33.33 ± 1.82	34.66 ± 2.35	39.19 ± 2.25	39.94 ± 2.16	46.14 ± 1.52	46.58 ± 1.61
Rac (mm)	2.77	0.25	1.46	5.84	31.81	55.43	141.77
ET0 (mm day−1)	4.79 ± 0.53	5.82 ± 0.27	6.05 ± 0.23	5.79 ± 0.25	5.49 ± 0.28	4.77 ± 0.18	4.71 ± 0.19
Insol (h day−1)	10.10 ± 0.74	11.21 ± 0.02	11.32 ± 0.07	11.42 ± 0.05	11.44 ± 0.08	11.40 ± 0.08	11.33 ± 0.05

T_ave = average temperature; T_max = maximum temperature; T_min = minimum temperature; RU_max = maximum relative humidity; RU_min = minimum relative humidity; R_ac = accumulated rainfall for the month; ET₀ = reference evaporation; Insol = insolation.

presents average data from the Automatic Meteorological Station of UFERSA (−5.2134° S; −37.3121° W) for the study months.

The area is located in the Caatinga dry forest biome and the soil was classified as Typic Rhodustults [16], with a sandy texture in the 0 to 20 cm layer (sand—921 g kg⁻¹, silt—20 g kg⁻¹, and clay—59 g kg⁻¹), and loam sand in the 20 to 40 cm layer (sand—726 g kg⁻¹, silt—24 g kg⁻¹, and clay—200 g kg⁻¹). According to Rêgo et al. [17], the chemical characteristics of the soil in the 0 to 20 cm layer were: pH—5.2; organic matter—14.9 g dm⁻³; P—1.9 mg dm⁻³; K⁺—0.10 cmol_c dm⁻³; Ca²⁺—0.66 cmol_c dm⁻³; Mg²⁺—0.8 cmol_c dm⁻³; (H + Al)—2.84 cmol_c dm⁻³; CEC—3.68 cmol_c dm⁻³.

2.2. Biochar and Wood Vinegar

The biochar was produced by carbonizing Eucalyptus wood in a rectangular masonry kiln equipped with a tubular metallic condenser. The carbonization temperature was 450 °C, and after 30 h of cooling, the coal was removed and ground. The condensed liquids were decanted, and the supernatant was distilled twice at 100 °C to obtain oil- and tar-free WV. A part of the biochar was mixed with bi-distilled WV in a 3:1 mass ratio (WV-added biochar) and dried in an oven for 24 h at 60 °C. The biochar contained 50 g kg⁻¹ moisture, 730 g kg⁻¹ C, 3.2 g kg⁻¹ N, 2.5 g kg⁻¹ P, 4.6 g kg⁻¹ K, and a pH of 8.8. The WV had a pH of 2.85, and its composition was described by Pimenta et al. [18], with a predominance of guaiacol, phenol, cresol, and furfural.

2.3. Organization of the Experimental Area

The experiment consisted of three safflower harvests conducted in adjacent areas, with plots cultivated after a single application of biochar. The experimental design was set up as an entirely randomized block design, with three blocks defined by harvests. Treatments consisted of a control (no conditioner), three doses of pure biochar (3.0, 6.0, and 9.0 t ha⁻¹), and three doses of WV-added biochar (Wv-biochar) at the same rates. All treatments were replicated five times in each harvest. These conditioners, with a particle size of less than 0.25 mm, were applied to the plots by topdressing and incorporated into the 0 to 10 cm soil layer using a disc harrow. Each plot consisted of 4 rows of safflower spaced 0.5 m apart and 4 m long. The sampling area consisted of the two central rows, each 2.5 m long (2.5 m²).

2.4. Crop Management

The safflower was sown manually in October 2023, August 2024, and October 2024, at a depth of 0.05 m, with a spacing of 0.5 m between rows and 15 seeds per meter of row. Mineral fertilization was carried out in the furrow of planting, when 20 kg ha⁻¹ of N, 70 kg ha⁻¹ of P, and 40 kg ha⁻¹ of K were applied. Irrigation was carried out using a drip irrigation system, with emitters spaced 0.20 m apart and a flow rate of 1.6 L h⁻¹.

2.5. Determinations Made

Harvests were carried out in February and November 2024 and January 2025. The plants of the sampling areas were cut close to the ground, and all the capitula were removed and threshed manually. The determinations made were:

• Average height of the plants (HEIGHT—cm).
• Average number of capitula per plant (NCAP).
• Number of seeds per capitulum (NSEED).
• Average mass of 1000 seeds (THSEED).
• Grain yield (YIELD—kg ha⁻¹) at 13% moisture.
• Oil content (OIL—%): the oil was extracted from 10 g of seeds, by means of a Soxhlet system with an oil bath in a cycle of 95 °C and 3 h. 150 mL of N-hexane solvent was used, which was subsequently separated from the oil using a rotary evaporator.

2.6. Statistical Procedures

Initially, the data were analyzed using descriptive statistics, the Shapiro–Wilk test (p > 0.05) for normality, the Levene test (p > 0.05) for homogeneity of variances, and the Interclass Correlation Coefficient (ICC) for variance components. The dose of biochar and Wv-biochar was treated as a continuous variable using quadratic regression, according to the Model: Y = β₀ + β₁ × Dose + β₂ × Dose², adjusted for dose. The Likelihood Ratio Test compared the biochar and Wv-biochar curves. In the Linear Mixed Models (LMM) procedure, the harvests were used as a random effect, according to the model: Y = β₀ + β × Treatment + (1|Harvest) + ε. The software STATISTICA version 13 [19], the R environment version 4.4.2 [20], and Microsoft Excel 2019^® were used.

3. Results

3.1. Descriptive Statistics

The summary of descriptive statistics for the variables measured across the three safflower harvests is presented in

Table 2. Descriptive statistics of production parameters of safflower in three harvests in the semiarid region of Northeast Brazil.

	Harvest	N	Mean	SD	SE	Minimum	Maximum
HEIGHT	1	34	48.2	5.35	0.92	38.80	58.25
	2	25	40.6	5.12	1.05	29.65	48.05
	3	35	47.6	4.17	0.7	38.29	57.12
NCAP	1	34	8.4	1.90	0.33	5.05	10.05
	2	25	7.00	1.52	0.32	3.55	10.25
	3	35	8.00	2.10	0.35	4.06	13.92
NSEED	1	34	12.2	2.01	0.35	8.30	18.50
	2	25	25.3	4.17	0.87	16.40	33.32
	3	35	24.2	3.15	0.53	18.75	29.80
THSEED	1	34	38.0	4.73	0.81	28.89	46.06
	2	25	34.2	2.99	0.62	30.8	42.8
	3	35	34.1	2.90	0.49	30.80	39.70
YIELD	1	34	1109	325	55.75	576	1818
	2	25	839	193	39.07	467	1218
	3	35	866	178	30.07	478	1315
OIL	1	21	21.9	1.40	0.33	18.45	24.00
	2	21	37.2	8.56	0.47	26.63	56.99
	3	21	29.9	6.53	0.97	18.45	45.50

HEIGHT—plant height; NCAP—number of capitula per plant; NSEED—number of seeds per capitulum; THSEED—mass of thousand seeds; YIELD—seed yield; OIL—oil content of seeds. N—number of observations; SD—standard deviation; SE—standard error.

. In general, the values of the statistical parameters did not show evident differences from one harvest to another, except for NSEED and OIL, which presented the lowest means in the first harvest, in which the highest mean of YIELD was observed. The averages per harvest presented the following ranges of values: HEIGHT—40.6–48.2 cm; NCAP—7.0–8.4; NSEED—12.2–24.2; THSEED—34.1–38.0 g, YIELD—839–1109 kg ha⁻¹, and OIL—21.9–37.2%. The variability of the data is evidenced by the standard deviation and amplitude, especially for NCAP, YIELD, and OIL. This variability may be caused by factors inherent to the soil and the genotype.

3.2. Linear Mixed Models with Harvest as a Random Effect

The block effect (harvest) explains a substantial fraction of the total variability, especially for NSEED and OIL, which is evidenced by a high ICC. The high p-values (p > 0.05) of the coefficients associated with the different treatments in HEIGHT, NCAP, THSEED, and YIELD, after adjusting for the harvest effect, suggest a statistically weak impact of the treatment (combinations of doses and type of biochar) and that the numerical differences between them are not statistically detectable. Some levels of Wv-biochar in NSEED showed positive, significant coefficients, indicating an increase in the number of seeds per chapter relative to the reference level. There is evidence of substantial increases in THSEED and OIL, especially at intermediate WV-biochar doses. However, only OIL showed a significant treatment effect (p = 0.045) in some models and in specific doses.

3.3. Comparison of Response Curves

In view of the low LR statistics and the p-values much higher than 0.05 of the Likelihood Ratio Test (LRT) in the mixed quadratic models of HEIGHT, NCAP, THSEED, YIELD, and OIL, there is no statistical evidence that the dose–response curves of Biochar and Wv-biochar are different (α = 0.05). A trend of difference between the NSEED curves is indicated by the p-value close to the significance threshold (p = 0.061), so that the shape of the NSEED response to dose may be slightly different between types. The application of Wv-biochar tends to present numerically superior responses in some cases, such as in NSEED, THSEED, YIELD, and OIL. Still, these differences occur within the same functional curve shape.

The direct comparison of the coefficients (β₀, β₁, β₂) of the two curves (not presented) showed that the Wv-biochar coefficients lead to slightly higher optimal values of NSEED, THSEED, YIELD, and OIL and to somewhat different optimal doses. In addition to the fact that the differences in intercept (β₀) were minor, the fact that differences in β₁ (linear effect) and β₂ (quadratic effect) modulate the intensity of gain with increasing dose and the speed of saturation/decline after the optimal point is agronomically interesting, but does not produce statistical significance in LRT in most cases. It is observed that Wv-biochar causes the approximation of the optimal points for a similar range of doses (≈5 t ha⁻¹), but with slightly higher yield, THSEED, and OIL at the apex of the curve.

3.4. Dose-Adjusted Quadratic Regressions

The doses were transformed into continuous variables using quadratic regressions based on the means per dose (

Table 3. Means per dose and type of biochar of the variables of production and oil content of safflower in the semiarid region of Northeast Brazil.

Dose t ha−1	HEIGHT		NCAP		NSEED		THSEED		YIELD		OIL
	B	WV	B	WV	B	WV	B	WV	B	WV	B	WV
0	45.76	45.76	7.28	7.28	19.63	19.63	34.23	34.23	844.61	844.61	27.85	27.84
3	44.45	45.83	7.54	8.18	18.38	21.34	34.82	35.38	886.52	1015.75	28.71	28.99
6	45.65	45.18	7.96	7.78	20.31	21.69	36.83	37.43	1012.78	970.28	29.99	31.91
9	47.04	47.05	7.51	8.19	19.15	20.41	35.44	34.32	903.81	962.92	27.83	28.68

B—pure biochar; WV—wood vinegar-added biochar; HEIGHT—plant height; NCAP—number of capitula per plant; NSEED—number of seeds per capitulum; THSEED—mass of thousand seeds; YIELD—seed yield; OIL—oil content of seeds.

).

The quadratic response of the data of HEIGHT (Figure 1a) showed that the intermediate doses of the two conditioners harmed HEIGHT, whose minimum value corresponded to the dose of 3.38 t ha⁻¹ of pure biochar (44.78 cm) and 3.43 t ha⁻¹ of Wv-biochar (45.33 cm). The curves indicate that at intermediate doses, Wv-biochar provided numerically superior responses. In both cases, the highest estimated HEIGHT corresponded to the higher dose (9.0 t ha⁻¹), but differences in relation to the control were minimal.

On the other hand, NCAP, NSEED, THSEED, YIELD, and OIL exhibited quadratic responses, with an inflection point at intermediate values, yielding their maximum values. In general, the curves for these variables corresponding to Wv-biochar also showed numerically superior responses at intermediate doses than those of the pure biochar curves. The WV biochar provided the highest estimated NCAP (8.12) at the dose of 7.42 t ha⁻¹ (Figure 1b), surpassing the control (7.38) by approximately 10%. On the other hand, the doses of pure biochar that provided the highest NCAP (7.81) were 5.43 t ha⁻¹, which was 8% higher than the control (7.23).

With respect to NSEED (Figure 1c), the dose of 5.04 t ha⁻¹ of Wv-biochar provided the highest value (21.72), which was 10.8% higher than the control (19.61). However, when pure biochar was applied, the differences in NSEED values between doses were negligible. In the case of THSEED (Figure 1d), the application of Wv-biochar yielded the highest value (36.69 g) at a dose of 4.80 t ha⁻¹, surpassing the control (33.92 g) by 8.2%. In turn, the dose of 6.20 t ha⁻¹ of pure biochar yielded the highest THSEED value (36.11 g), representing a 6.25% increase relative to the control.

Figure 1e clearly illustrates the numeric superiority of the response in YIELD provided by the application of Wv-biochar in relation to pure biochar. In addition, the percentage differences between the maximum YIELD values and the control were much higher than those observed for the other variables. These maximum values, obtained at doses of 5.54 and 5.71 t ha⁻¹ for WV-biochar (1010 kg ha⁻¹) and pure biochar (965 kg ha⁻¹), respectively, were higher than those of the controls by 17.85 and 16.4%, respectively. In relation to the oil content of safflower seeds (Figure 1f), the highest values (30.79 and 29.55%) were obtained with the doses of 5.24 t ha⁻¹ of Wv-biochar and 4.74 t ha⁻¹ of pure biochar, respectively, and provided increases of 12.2 and 6.8% in relation to controls.

3.5. Gains Provided by Doses and Types of Biochar in Relation to Controls

Although the effect of harvest has been verified, the inclusion of the quadratic term enables quantification of the dose at maximum technical efficiency for each type of biochar. Regardless of the type of conditioner, there is a relatively consistent optimal dose range of 5.0–6.0 t ha⁻¹ that maximizes THSEED, YIELD, and OIL. The application of Wv-biochar tends to provide slightly higher maximum values than pure Biochar for NSEED, THSEED, YIELD, and OIL.

The comparison between the numerical responses for each dose of each biochar type and the control revealed relevant percentage differences in some cases. The smallest percentage differences were observed with the application of pure biochar, among which the increases of 19.9% in YIELD and 9.2% in NCAP stood out, achieved with the dose of 6 t ha⁻¹. On the other hand, the variable most favored by the application of Wv-biochar was YIELD, whose increases in relation to the control were 20.3, 14.9, and 14% when 3, 6, and 9 t ha⁻¹ were applied, respectively. In addition to YIELD, the application of Wv-biochar increased NCAP by 12.2% and 12.4% relative to the control at doses of 3 and 9 t ha⁻¹, respectively. In comparison, OIL and NSEED showed increases of 14.6 and 10.5% at the dose of 6 t ha⁻¹.

4. Discussion

4.1. General Data Behavior

The mean (45.47 cm) and maximum value (58.25 cm) of HEIGHT, in the three safflower harvests, are lower than the reported values of 70 to 120 cm [21] and 65.31 to 131.07 cm [22]. The advantage of taller plants is that they form more branches and, consequently, more capitula and higher seed yield [23]. Thus, lower plant development would be responsible for the mean (7.79) and maximum (13.92) NCAP values being lower than those reported in the literature. For example, 9.3 to 12.5 [21], 4.18 to 19.81 [22], and 8.2 to 15.6 [24]. In turn, the mean (20.57) and maximum value (33.32) of the NSEED are within the range observed by other authors, such as 15 to 60 [25] and 20 to 100 [26].

In addition to safflower’s tolerance to various types of stress, the maximum yield obtained (1818.52 kg ha⁻¹) indicates the crop’s potential for the study region, since yields between 1000 and 3300 kg ha⁻¹ are typical in semiarid areas of the world [27,28]. The average productivity of 938.00 kg ha⁻¹ over three harvests is lower than the values of 2068 to 3820 kg ha⁻¹ [21] reported in southern Brazil, indicating the influence of climate and genotype. The results of THSEED and OIL obtained in this study were consistent with those reported by other authors. In this regard, it is known that unfavorable conditions can alter the dynamics of safflower plants, so that seeds are prioritized in the partitioning and translocation of photoassimilates [23]. The mean and maximum THSEED values obtained (35.43 and 46.06 g) are comparable to values of 20 to 55 g reported by [25,26], while values ranging from 48.4 to 68.7 g were also reported [21].

The mean (29.14%) and maximum OIL values in the range of 40–50%, obtained in this study, stand out in relation to values found in the literature, such as 23.5 to 24.2% [21], 29 to 33% [29], and 27 to 32% [25]. This is a compelling argument for the adoption of safflower in the region, as the extraction of edible oil from the seeds is one of the main reasons for its cultivation worldwide [6]. The high temperatures and minimal variation in sunstroke (Table 1) may have led to anticipatory flowering and the shortening of the plants’ growth stages, thereby harming some aspects of safflower development and yield. This problem can be solved by selecting the most suitable genotypes for the region, since this species has wide genetic variability [21,22,23,27].

4.2. Effects of Biochar and of Wood-Vinegar-Added Biochar on Safflower

The application of specific doses of pure biochar and Wv-biochar resulted in consistent numerical increases in most variables; however, the weak influence of the treatments and the significant impact of the harvest effect were evidenced, which suggests that the results were influenced in a combined way by climate, management, and soil conditions in each year or season. In this context, the spatial variability of the study area can be accounted for, as confirmed by studies on soil resistance to penetration and by the growth of Crotalaria juncea and maize [30,31].

Despite the weak effect of the treatments in this study, the positive effects of biochar application on crop yield in semi-arid environments are well documented. In this regard, a review of many studies indicates an average increase of 11% in crop yields [32]. The application of biochar, produced from available and inexpensive organic waste, can store carbon, recover degraded soils, improve soil fertility and structure, and increase biological activity. Some mechanisms by which biochar promotes the correction of acidic soils and increases soil water storage, CEC, and nutrient retention include its alkalinity and porosity, as well as its large specific surface area with oxygen-containing functional groups and negative charges [33,34]. Some studies in an arid region with low-fertility soil showed that safflower yield and growth benefited from the application of 3.0 t ha⁻¹ of biochar and a small amount of chemical fertilizer. The leading causes were reduced water stress and increased K release in the soil. Higher doses of biochar (1.5 and 3.0% w/w) increased the dry mass of safflower under water stress by increasing soil nutrient availability and plant uptake. The application of biochar also promoted the availability of P and K in the soil, both those contained in it and those released by the action of solubilizing bacteria and fungi [33,35].

This study provided evidence that the application of Wv-biochar can promote effects superior to those of pure biochar in some parameters of safflower production and oil content, depending on the dose applied. This soil conditioner represents a novelty for semiarid regions. It is inspired by studies in other areas and crops, which also indicate that WV increases the effectiveness of mineral fertilizers, manure, biochar, compost, and pesticides [36,37]. Confirmation of this superiority would be especially relevant in semiarid conditions, where soil fertility is often low and access to fertilizers is limited. Wv-biochar represents a low-cost, waste-based alternative that can improve soil conditions while reducing dependence on external inputs.

The complex composition of WV and its direct effects on plant metabolism would be responsible for the intensification of the effects of biochar, which acts as a conditioner, improving the physical and chemical properties of the soil [38]. Therefore, the improvement in plant performance provided by Wv-biochar can be attributed to an even greater increase in nutrient availability and water retention than that observed with pure biochar, as reported in the literature [8,33,35]. In plants, WV acts as a biostimulant, growth promoter, metabolism regulator, and pesticide. The rich chemical composition of WV, which includes humic substances, enhances the soil’s physical and chemical properties by increasing organic matter and microbial activity. The remediation of alkaline and saline soils is due to the acidity of WV, which promotes the leaching of soluble salts from the soil, the dissolution of minerals and carbonates, and the extraction and release of K. At the same time, its organic acids solubilize the P of the soil, making it more available to plants [9,10,14,38]. WV-added biochar is a convenient and cost-effective soil conditioner, which can enhance the benefits of the two materials for the soil–plant system and reduce the need for synthetic fertilizers.

4.3. Effects of the Dose of Biochar and Wv-Biochar

In general, the variables of production and oil content of the safflower crop showed promising positive numerical results compared to the control, obtained with low doses of conditioners, compared to other studies. Under the conditions of the present study, the variables exhibited quadratic behavior as a function of dose, and the highest values of most variables were obtained in the dose range between 5.0 and 6.0 t ha⁻¹. This confirms that, depending on soil and crop conditions, there is an optimal dose above which the application of biochar is no longer advantageous. This dose range is consistent with that used in another study [35], in which an increase in safflower yield was observed. Furthermore, when Wv-biochar was applied, the doses in the range of 5 to 6 t ha⁻¹ consistently stood out numerically over the application of pure biochar. This suggests that this level may represent an optimum application rate under semiarid conditions. On the other hand, the quadratic behavior of the data showed that the highest dose (9 t ha⁻¹) did not lead to proportional gains, indicating that excessive application may not be agronomically efficient. High doses of biochar can harm soil and crop growth and yield. Among the causes are the priming effect of biochar on soil organic carbon; N immobilization, by the increased C/N ratio; toxicity to biota; alteration in the balance of nutrients; obstruction of soil pores, causing an increase in soil density and reduction in water retention capacity; and soil salinization, due to the salts contained in the feedstocks [33,35].

5. Conclusions

The differences between harvests were the main determinants of the variables’ behavior. Despite the numerical discrepancies between doses and biochar types, their significance was verified in terms of safflower oil content, the variable most influenced by biochar and WV-added biochar. In addition, the superiority of means and optimal values observed in the response curves of the WV-added biochar over the pure biochar was only numerical. Despite this, there is evidence that WV-added biochar can provide additional gains compared with pure biochar and is a technically safe and efficient strategy that reconciles production and oil content, especially at doses between 5 and 6 t ha⁻¹.

The novelty of the study lies in evaluating the safflower crop and applying Wv-biochar in Brazilian semi-arid conditions. The crop’s adaptation to the region and the feasibility of the WV-added biochar are supported by promising data on seed production and safflower oil content. Biochar produced from organic waste is a cost-effective and sustainable alternative for smallholder farmers, promoting soil health and food security. Future studies should be conducted to consolidate safflower culture and to incorporate biochar into WV as sustainable alternatives for agriculture in the region, focusing on the adoption of more adapted genotypes, improved soil management practices, and the long-term effects on soil properties.