3.1. Height, Fresh Biomass, Essential Oil Content and Yield
For both varieties of
O. basilicum L., the characteristics of the organo-mineral fertilizer allowed us to obtain satisfactory results for biomass and essential oil yield in comparison to other fertilizers. An increase in the plant biomass and essential oil production of basil in two consecutive cultivation years (2013 and 2014) was observed by [
24], in which organo-mineral fertilizer (carbonized biomass and chemical fertilizers) provided a nutrient supply for a longer period.
Other authors also found interesting results when they added organic fractions to nutrient management strategies [
25,
26]. As occurred in the work of [
25], which showed higher essential oil yields when applying chemical fertilizer (41%), biofertilizer (33%) and combinations of biofertilizer and 50% chemical fertilizer (38%).
Our study is in discordance to [
15], which obtained the highest fresh weight from the plants grown with conventional fertilizer at a rate of 250 kg N ha
−1. For us, unsatisfactory results for both cultivars were obtained with exclusively mineral fertilization, not favoring fresh biomass and essential oil yield. In the early stages of crop development, there is still no fully formed root system for the absorption of nutrients already available in the soil solution. As a result, in the period of the greatest need for absorption, with greater metabolism by plants, the content of nutrients (especially N and K) in the soil is possibly lower. At that time, slow-release fertilizers have advantages for greater crop performance, specifically because such fertilizers break down over time, releasing nutrients gradually. This helps prevent the leaching of nutrients into the environment, which can otherwise harm nearby water systems. It also ensures that the nutrients are available to plants when they need them, rather than all at once [
27].
Organic fertilizers have also led to satisfactory results for both cultivars. Organic sources, when compared to chemical fertilizers, have lower nutrient contents and are slow to release their nutrients, but are as effective as chemical fertilizers [
28], especially for the activation and conservation of the soil microbiota responsible for the release of nutrients to plants.
The superiority of using the required N in biologic form and 75% in mineral form to improve vegetative growth may be due to the favorable effect of chemical nitrogen application on the activity of microorganisms responsible for biofertilizer decay in the soil, which increases the available N in soil and N-uptake, consequently encouraging the vegetative growth of the plant [
26].
The protected cultivation showed that the sources of organo-mineral and organic fertilizers led to the best performance of “Sweet Dani” cultivar. Greenhouse cultivation favored the growth of plants because the plants were protected from adverse conditions of the climate and soil, evapotranspiration, solar radiation, the direct effects of rain, and leaching of nutrients. Our findings agree with [
13], as the experiment was carried out in the rainy season. Plastic protection and controlled irrigation inside the greenhouse influenced the release of the nutrient supply from fertilizers, and in this case mineral fertilization took place more quickly due to its formulation. Furthermore, the high photosynthesis potential under organic fractions is probably due to the stimulated activity of beneficial soil microorganisms, enhancing the chlorophyll content of the plants [
29].
Under field cultivation, organo-mineral and organic fertilizers had better production of fresh biomass and essential oil yield, probably because these fertilizers provided greater nutrient availability during the time of the experiments. The high soil humidity caused by the rainy period certainly led to greater leaching of the nutrients of more soluble sources, and organic formulations kept a larger quantity of nutrients around the root systems of the basil plants.
According to [
30], combining organic and mineral fertilizers could result in increasing crop yields and quantities of antioxidant compounds (phenol and flavonoids) with fewer resources, including being able to reduce the rate of nutrient application without causing damage to production.
These results should be available to the extension section and pioneer farmers to consider as alternative nutrient sources in support of environmental health, especially in the production of high-quality crops such as medicinal plants.
Organic practices have been considered a good strategy considering the terms of economics, plant growth and environmental aspects, especially because by-products of cropping or animal production systems represent cheap, biodegradable and fully environmentally friendly materials for the production. Moreover, organic matter and plant residues can also decrease plants’ exposure to various pathogens and suppress some types of nematodes in the soil [
31].
Organic fractions added to soils significantly improve the microbial community compared to the application of inorganic fertilizers alone. The presence of life in the soil strongly affects crop growth and soil quality, which has become an indicator of soil fertility and health. Because microorganisms are the dominant players in restoring soil functions and beneficial microorganisms can suppress soil-borne diseases, their presence enhances resistance to environmental stress [
32].
The application of fractions of organic matter such humic acid was effective on the growth and yield parameters of field-grown plants, and the highest values were obtained from the highest dose of humic acid [
33].
The benefits of this fertilizer segment bring new horizons to nutrient management, such as the application of organic fractions associated with nutrients and microorganisms, expanding the possibilities in management aimed at improving the development of basil varieties. These combinations can be very different, and the results reveal substantial increases, as reported by [
34], who found root weight doubled or tripled compared to control plants when the plants received microorganisms associated with plant growth, chlorophyll and mineral uptake.
The essential oil and its chemical constituents vary drastically over the environment and season in the
Ocimum plant [
35,
36,
37]. It is always essential to determine the most suitable varieties for each region according to its climatic conditions. The variation between basil cultivars for herb growth and oil yield are usually related to genetic diversity due to hybridization, targeted cultivation, and breeding practices for desired morphochemotypes [
38].
Regarding essential oil content, our findings are in accordance with [
3], which studied the influence of nitrogen fertilizer on the performance of basil cultivars, observing essential oil contents between 0.8 and 2.07%.
Optimal and balanced mineral fertilization of aromatic plants, adjusted to their nutritional requirements and growing conditions, is an important cultivation factor determining the quantity and quality of the essential oil [
39].
In our experiment, essential oil content presented similar results for both cultivars and agricultural systems, with small but not significant differences between fertilizers. Compost application on growing basil contributes to an increase in essential oil content [
40]. According to [
25], the highest essential oil content was obtained in chemical fertilizer and combinations of biofertilizer and 50% chemical fertilizer.
3.2. Essential Oil Composition
The constituents of basil essential oils across countries can be very different due to environmental and genetic factors, different chemotypes, harvest time, climate and the use of fertilizer. In northeast India, [
41] reported camphor (42.1%), followed by limonene (7.6%) and β-selinene (5.6%), was the major component of the oil, while [
42] found linalool (66.9–84.2%) and geranial (1.1–9.0%). In Turkey, methyl eugenol (78.02%) stood out against α-cubebene (6.17%) [
43]. In China, [
44] found 5-allylguaiacol, accounting for 50.2%, while [
45] found methyl eugenol (12.93–25.93%) and eugenol (17.63–27.51%).
In [
46], the authors identified seven chemotypes: linalool, methyl chavicol, citral/methyl chavicol, eugenol, methyl cinnamate/linalool, linalool/methyl eugenol, and methyl chavicol/linalool after characterizing the chemical diversity of Turkish basil (
O. basilicum L.). Our study is in accordance with this study and [
15], even though linalool was the dominant constituent for the “Cinnamon” cultivar.
The percentage of the dominant compounds linalool and geranial can also vary in relation to the season of harvest, despite the influence of the fertilizers and agricultural systems that are the objects of study in the present work. Knowledge regarding the occurrence of seasonal variability in the production of active ingredients is one of the main parameters to be considered in planning the harvest of medicinal and aromatic plants when the objective is the quality of the raw material and the presence of the active ingredients of interest [
47].
In the present study, again, organic fertilizers provided better results for major compounds of essential oil. According to [
48], organic fertilizer affects the essential oil composition in
O. basilicum L. in a way similar to that of inorganic fertilization (ammonium nitrate). The authors of [
3] related that the highest concentrations of the main constituents of essential oil (with the exception of methyl chavicol), namely 1,8-cineole and methyl cinnamate, were obtained under treatment without nitrogen fertilizer, as compared to treatments with nitrogen chemical fertilizers. For [
40], compost application also increased the concentration of linalool and borneol in the oil, with a simultaneous decrease in the content of methyl chavicol and 1,8-cineole.
The higher rate of nitrogen fertilization causes an increase in volatile oil content and yield in some plants, as well as increasing the percentage of methyl chavicol and b-asarone and decreasing the linalool concentration in the oil [
39]. The same author also reports that foliar feeding of nitrogen in the form of urea contributes to an increased concentration of linalool and epi-cadinol as well as a decreased content of 1,8-cineole, geraniol and eugenol in the oil of
O. basilicum.
The different fertilizer sources improved micro-elements content due to enhancing the cation exchange capacity of the soil, the gradual release of nutrients, and the biological activities and physicochemical properties of the soil [
49]. It is worth mentioning that the supply of micronutrients through organic fertilization possibly led to the higher production of compounds of interest in the essential oil, since nutrients such as zinc and manganese stimulate the route of production of secondary metabolites in secondary plant metabolism [
13].
Increasing the efficiency of nutrient use in medicinal plants such as basil is a relevant factor because the metabolization of elements throughout development can affect the synthesis of secondary metabolite products, as reported by several researchers working with medicinal plants [
50,
51,
52,
53,
54,
55].
Due to the characteristics of the fertilizer (composition and combination with organic fractions), we observe that sources with organic components positively affect development. Therefore, the form of fertilization can affect the absorption of nutrients and their translocation in plants [
56]. Working with
Echinacea purpurea L., [
57] observed maximum values in plant growth, total phenolics, total flavonoids and essential oil content in nutritional management with controlled-release fertilizer and greenhouse conditions. The results of this study suggested that the use of organo-mineral fertilizer has a beneficial effect on the biomass and essential oil composition of
O. basilicum cultivars.
Foliar spraying of organic components during development has also been a subject of study by researchers, with encouraging results for the yield and production of medicinal plants [
45]. Works like this and new formulations and forms of application are important to support and encourage new studies and new possibilities to supplement medicinal plants and make the valuable by-products of these species more accessible and of better quality.