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Introduction
Oregano is one of the most important aromatic species in the world, due to its uses in food, in the manufacture of cosmetics, drugs and liquors, and as a source of antioxidants (Said-Al Ahl, Ayad, & Hendawy, 2009; Sarikurkcu et al., 2015). Mexico is an important producer of this crop (Flores-Hernández et al., 2011), and its production represents a viable agricultural alternative from an economic point of view (Juárez-Rosete et al., 2013), due to its proximity to the main importing country of this crop, which is the United States. Additionally, the demand it has in specialized markets and its low production costs represent advantages for Mexican producers (Bonfanti et al., 2012).
In the international market, Turkey, Mexico and Greece are the main suppliers of processed and non-processed dry oregano (Bonfanti et al., 2012); the first two contribute 65 and 31 % of production, respectively. On the other hand, Peru stands out for its growth in production, harvested area and yield, as well as for exports to countries in South America and Europe (Salas-Portugal, & Alagon-de la Sota, 2016). In Mexico, the species of oregano grown for fresh consumption is Origanum vulgare L. spp. hirtum, and 90 % of its national production is for the export market (Aguilar-Murillo, Valle-Meza, González-Rosales, & Murillo-Amador, 2013).
The current growth of the market for aromatic and medicinal plants demands the study of the factors that affect their yield and quality (Yeritsyan & Economakis, 2002), in order to improve production systems. In this context, the main problem in aromatic herbs is the lack of precise information on nutrition management (Pedraza & Henao, 2008); moreover, because they are crops in which the aerial part is harvested and in a commercial manner, successive harvests of sprouts are made. This makes it a crop with a high capacity to extract nutrients from the soil.
Among the nutritional requirements of oregano, it is known that N, P and K play a fundamental role due to their functions in plant metabolism (Said-Al Ahl et al., 2009); in addition, they are necessary elements for optimal plant development, mainly for sprouting after harvest (Aguilar-Murillo et al., 2013). For optimal nutrition management, it is necessary to know the nutritional requirement of the crop, which is expressed in kilograms of nutrients per ton of product, in this case fresh product. This data is key in the estimation of nutrient demand, an indispensable parameter to estimate the appropriate fertilization dose for crops.
Without information on the nutritional requirement of this crop, achieving sustainable fertilization management is difficult. Similarly, in hydroponic systems, the nutrient solution (NS) concentration that should be used for oregano production is not defined. The existing studies on fertilization in this species have been conducted in Europe and Asia in the open field, in plantations of different subspecies of Origanum vulgare L., such as O. vulgare L., O. vulgare ssp. hirtum (Link) Ietswaart, O. vulgare ssp. viridulum Nyman, and O. vulgare ssp. virens (Hoffmansegg & Link) (de Falco, Rozigno, Landolfi, Scandolera, & Senatore, 2014; Pal, Adhikari, & Negi, 2016; Said-Al Ahl et al., 2009; Sarikurkcu et al. 2015; Sotiropulou & Karamanos, 2010). Hydroponic systems allow more precise control in nutrition studies to determine the nutritional requirement of the crop.
Therefore, the objective of this research was to determine the optimal Steiner nutrient solution (NS) concentration for biomass production in oregano (Origanum vulgare spp. hirtum [Link] Ietswaart), and to quantify its nutritional requirement of N, P and K. This information is useful to support fertilization programs in commercial oregano farms, both in soil production systems and under hydroponic conditions.
Materials and methods
The research was carried out under tunnel-type shade house conditions in the Agricultural Academic Unit of the Universidad Autónoma de Nayarit, located at 21° 25' North latitude and 104° 53' West longitude, during the 2013 spring-summer period. In the crop cycle, a maximum temperature of 35 °C and a minimum of 8 °C were recorded, with minimum relative humidity of 56 % and maximum of 88 %.
The plants were obtained from oregano seeds of the commercial Greek variety (O. vulgare spp. hirtum), which were placed in 200-cavity polystyrene containers with peat moss as a germination substrate. The seeds germinated 10 days after sowing and were watered until reaching a height of 5 cm.
The transplant was made in black polyethylene bags (20 x 20 cm) containing 1.5 L of basaltic volcanic rock (red tezontle) as substrate, with particle size from 0.3 to 1.0 cm, bulk density of 0.83 g·cm-3, aeration capacity of 21.33 % (v/v), moisture retention capacity of 12.79 % (v/v) and total pore space of 32.33 % (v/v). Irrigation was applied daily, where 300 mL of NS were supplied in the morning and 150 mL of water in the afternoon to avoid the accumulation of salts in the substrate.
Experimental design and evaluated variables
A completely randomized design was established with five treatments and five replicates. The treatments were different concentrations of the universal NS proposed by Steiner (1984): 25, 50, 75, 100 and 125 % (Table 1). Daily, 300 mL of NS per bag were manually supplied. The NS was prepared with soluble fertilizers: potassium nitrate, potassium sulfate, calcium nitrate, magnesium sulfate and monopotassium phosphate; as a source of micronutrients, 0.025 g·L-1 of Ultrasol® microMix, a commercial mixture, was used. During the entire growth cycle, the pH of the NS was adjusted between 5.5 and 6.0 with H2SO4. Osmotic pressure remained at -0.018, -0.036, -0.054, -0.072 and -0.090 MPa in the treatments, respectively. The experimental unit consisted of five bags with substrate, with one plant in each bag, giving a total of 125 plants.
Table 1 Ionic composition of Steiner nutrient solution at different concentrations.
| Treatment | Anions (meq·L-1) | Cations (meq·L-1) | Osmotic potential (MPa) | Electrical conductivity (dS·m-1) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| NO3 -2 | H2PO4 -2 | SO4 -2 | K+ | Ca2+ | Mg2+ | |||||
| 1 | 3 | 0.25 | 1.75 | 1.75 | 2.25 | 1 | -0.018 | 0.5 | ||
| 2 | 6 | 0.5 | 3.5 | 3.5 | 4.5 | 2 | -0.036 | 1.0 | ||
| 3 | 9 | 0.75 | 5.25 | 5.25 | 6.75 | 3 | -0.054 | 1.5 | ||
| 4 | 12 | 1 | 7 | 7 | 9 | 4 | -0.072 | 2.0 | ||
| 5 | 15 | 1.25 | 8.75 | 8.75 | 11.25 | 5 | -0.090 | 2.5 | ||
During the experiment, four harvests (H) were carried out at 30, 60, 90 and 120 days after transplanting, and the following variables were quantified:
Production of fresh and dry matter per plant: The accumulated aerial fresh matter was weighed at the time the stems reached commercial size (15 cm). The cut was made 5 cm above the surface of the substrate to allow resprouting. The amount of biomass per plant was obtained from the average of the experimental unit. Subsequently, the fresh matter was dried in an oven (EI45-AIA, Novatech®, Mexico) at 70 °C to constant weight to record the accumulated aerial dry matter and to know its water content. Total fresh and dry matter production was estimated by adding the amount obtained in the four harvests and considering 15 plants·m-2.
Concentration of nitrogen, phosphorus and potassium (%): A wet digestion extraction was carried out according to the procedures described by Alcántar-González and Sandoval-Villa (1999). In the case of N, a mixture of sulfuric acid with salicylic acid was used, while for P and K a mixture of nitric acid with perchloric acid was used. Total N content was determined by the semi-microkjeldahl method, P by colorimetry with a spectrophotometer (DR2800, Hach®, USA) and K with a flame photometer (Flame Photometer 410, Sherwood®, Great Britain).
Nutritional requirement
Once the NS concentration that induced the highest production of fresh matter was identified, the nutritional requirement of N, P and K for the crop was calculated with the nutritional concentration data obtained. From the values of fresh and dry matter per plant, the amount of dry matter needed to produce a ton of fresh matter was calculated, and based on its nutritional concentration the amount of nutrients needed to produce that amount of fresh matter was calculated.
Statistical analysis
With the results obtained from each harvest, an analysis of variance and a Tukey mean comparison test (P ≤ 0.05) were performed using Statistical Analysis System software (SAS Institute, 2004). In addition, a quadratic adjustment was made between the NS concentration and the production of fresh matter.
Results and discussion
The five evaluated variables presented significant and highly significant statistical differences among the four harvest dates (HD) (Table 2).
Table 2 Mean squares and level of significance of the variables evaluated in Greek-type Origanum vulgare ssp. hirtum (Link) Ietswaart for each harvest date.
| Variable | 1 HD 1 | HD2 | HD3 | HD4 |
|---|---|---|---|---|
| Fresh matter per plant (g) | 5.13** | 49.28** | 49.44** | 349.06** |
| Biomass per plant (g) | 0.10** | 0.97** | 0.62** | 12.82** |
| Nitrogen concentration (%) | 0.41* | 0.50* | 0.80** | 0.67** |
| Phosphorus concentration (%) | 0.03** | 0.006ns | 0.01** | 0.02** |
| Potassium concentration (%) | 0.18ns | 0.48* | 1.94** | 8.11** |
1HD = harvest date; * = significance at 5 %; ** = significance at 1 %; ns = not significant.
Fresh and dry matter
In terms of biomass production, NS treatments at 50 and 75 % had the highest total fresh and dry matter production (P ≤ 0.05), while those at 25 and 125 % had the lowest biomass production (Table 3). Both fresh and dry matter are the product of economic interest in the crop, and these results allow us to define that the 50 and 75 % Steiner NS concentrations are optimal for oregano production in hydroponic systems; however, the 50 % NS concentration represents a lower use of fertilizers compared to the 75 % one.
Table 3 Effect of Steiner nutrient solution concentration on fresh and dry matter production in Origanum vulgare ssp. hirtum (Link) Ietswaart, on four harvest dates.
| Steiner NS concentration (%) | 1 HD 1 (30 dat) | HD2 (60 dat) | HD3 (90 dat) | HD4 (120 dat) | TW (m2) |
|---|---|---|---|---|---|
| Fresh weight (g) | |||||
| 25 | 2.16 ± 0.20 cz | 12.02 ± 0.83 c | 12.10 ± 0.77 c | 29.62 ± 0.82 c | 838.8 ± 34.7 c |
| 50 | 3.44 ± 0.31 b | 18.15 ± 0.29 ab | 18.15 ± 0.30 ab | 42.353 ± 2.09 b | 1231.5 ± 30.6 ab |
| 75 | 4.81 ± 0.78 a | 19.73 ± 1.96 a | 19.75 ± 1.98 a | 52.231 ± 5.59 a | 1448.0 ± 125.7 a |
| 100 | 2.59 ± 0.35 b | 16.16 ± 2.51 abc | 16.88 ± 3.31 abc | 45.04 ± 5.16 ab | 1217.8 ± 157.1 b |
| 125 | 2.59 ± 0.36 bc | 13.76 ± 4.10 bc | 13.76 ± 4.11 bc | 38.38 ± 4.13 b | 966.3 ± 252.3 bc |
| LSD | 0.84 | 4.46 | 4.82 | 55.19 | 275.97 |
| CV | 13.92 | 14.77 | 15.79 | 9.63 | 12.78 |
| Dry weight (g) | |||||
| 25 | 0.41 ± 0.05 cz | 1.75 ± 0.19 b | 2.37 ± 0.17 ab | 4.09 ± 0.47 d | 25.91 ± 2.11 c |
| 50 | 0.61 ± 0.02 b | 2.80 ± 0.09 a | 3.00 ± 0.02 a | 6.48 ± 0.37 b | 38.72 ± 1.36 ab |
| 75 | 0.80 ± 0.07 a | 2.41 ± 0.20 ab | 2.51 ± 0.05 ab | 7.99 ± 1.19 a | 41.21 ± 4.06 a |
| 100 | 0.55 ± 0.08 b | 2.14 ± 0.68 ab | 2.47 ± 0.64 ab | 5.44 ± 0.32 bc | 31.87 ± 4.82 bc |
| 125 | 0.50 ± 0.07 b | 1.79 ± 0.63 b | 2.01 ± 0.50 b | 4.39 ± 0.67 c | 24.63 ± 6.83 c |
| LSD | 0.11 | 0.82 | 0.70 | 1.29 | 40.7 |
| CV | 10.41 | 19.97 | 15.09 | 11.99 | 13.25 |
1HD = harvest date; dat = days after transplanting; TW = total weight considering 15 plants per m2; NS = nutrient solution; LSD = least significant difference; CV = coefficient of variation. zMeans with the same letter inside each column do not differ statistically (Tukey, P ≤ 0.05).
The results found coincide with those reported by Carrasco, Ramírez, and Vogel (2007), who indicated that the highest biomass production was achieved with the NS that had an electrical conductivity (EC) of 1.5 dS·m-1, which is similar to that of Steiner's NS at 75%. However, these results contrast with those obtained by Juárez-Rosete, Olivo-Rivas, Aguilar-Castillo, Bugarín-Montoya, and Arrieta-Ramos (2014) in the cultivation of mint, another aromatic species of commercial importance established under the same production conditions used in the present study. These authors obtained the highest weight gain in aerial biomass with the NS at 125 %.
The significant decrease in biomass production on the four HDs with the NS at 25 and 125 % indicates, on the one hand, that the concentration of nutrients in the case of NS at 25 % does not supply the nutrient needs of the crop, mainly N. On the other hand, the NS at 125 % is excessive and induces nutrient absorption problems because it has a high osmotic potential, which limits the absorption of ions by the root (Trejo-Téllez & Gómez-Merino, 2012).
Biomass production is a function of time, which has facilitated the successful prediction of dry matter production throughout the growth cycle in crops such as corn, wheat and some olericultural species such as lettuce, radish and Chinese cabbage (Bugarín-Montoya et al., 2011). However, in the case of resprouting leaf species, biomass production has a different dynamic, because after each harvest there is an increase in the number of stems that resprout.
In general, when oregano is cultivated for fresh consumption, harvests are made every 30 days (Juárez-Rosete et al., 2013), and if it is for the production of dry leaves, the plants are not harvested until floral buds are formed. In order to quantify the nutritional requirement, it is important to know the amount of nutrients extracted by the crop at the time of harvest, when its growth and development have taken place under optimal conditions. In this sense, it was observed that the highest biomass production was achieved with the Steiner NS at 50 and 75 %; however, the nutritional requirement calculations of the crop were made with the SN at 50 %, for representing savings in the use of fertilizers.
Nitrogen
The N concentration shows significant statistical differences among treatments on the four HDs. The highest N concentration was recorded in the NS with 100 and 125 % concentration (from 3.06 to 3.46 % N); however, these treatments did not have higher biomass production, indicating that these N are excessive for this crop. The levels of this element obtained by the NS at 50 and 75 % are in an interval of 2.55 to 3.16 %, which can be considered as optimal (Table 4).
Table 4 Effect of Steiner nutrient solution concentration on nitrogen content (%) in biomass of Origanum vulgare ssp. hirtum (Link) Ietswaart, on four harvest dates.
| Steiner NS concentration (%) | 1 HD 1 (30 dat) | HD2 (60 dat) | HD3 (90 dat) | HD4 (120 dat) |
|---|---|---|---|---|
| 25 | 2.59 ± 0.08 bz | 2.79 ± 0.01 ab | 2.19 ± 0.14 b | 2.98 ± 0.28 ab |
| 50 | 2.69 ± 0.03 ab | 2.92 ± 0.33 ab | 2.66 ± 0.10 ab | 3.16 ± 0.12 ab |
| 75 | 2.79 ± 0.01 ab | 2.57 ± 0.63 b | 2.85 ± 0.69 a | 2.55 ± 0.72 b |
| 100 | 3.07 ± 0.01 ab | 3.39 ± 0.08 a | 3.06 ± 0.01 a | 3.41 ± 0.20 a |
| 125 | 3.28 ± 0.01 a | 3.16 ± 0.08 ab | 3.23 ± 0.06 a | 3.46 ± 0.10 a |
| LSD | 0.59 | 0.68 | 0.60 | 0.69 |
| CV | 10.80 | 12.18 | 3.2 | 11.75 |
1HD = harvest date; dat = days after transplanting; NS = nutrient solution; LSD = least significant difference; CV = coefficient of variation. zMeans with the same letter within each column do not differ statistically (Tukey, P ≤ 0.05).
As the NS concentration increased (100 and 125 %), so did the N content. Azizi, Yan, and Honermeier (2009) indicated that excess N supply has other repercussions on the oregano crop, such as a significant decrease in the concentration of essential oils, which is another aspect of interest in the production of this crop. In this sense, it is essential to identify the NS concentration that does not cause excessive nitrogenous nutrition, and that also allows for the highest biomass production.
N is one of the nutrients responsible for vegetative growth; however, it does not follow an infinite linear trend, as crops reach a saturation point of the element (Alejo-Santiago et al., 2015), which leads to a decrease in biomass production. One explanation for this behavior is the dynamics that N follows in the plant, since once it is absorbed and reaches the leaves it undergoes a conversion to ammonium to continue its route towards protein formation. This N reduction implies carbon oxidation (Xu, Fan, & Miller, 2012), which is a process of energetic wear that affects the accumulation of biomass. Barreyro, Ringuelet, and Agrícola (2005) also found a yield-decreasing effect by raising the N dose from 80 to 120 kg·ha-1 in oregano production in the field; this confirms that an excess of nitrogen fertilizer affects yield.
Phosphorus
The P concentration in biomass showed statistically significant differences on three HDs. The NS that caused the highest concentration of this element was 125 %, while the NS at 25 % had the lowest value. The 50 and 75 % NS concentrations had P values between 0.35 and 0.48 % (Table 5).
Table 5 Effect of Steiner nutrient solution concentration on phosphorus content (%) in biomass of Origanum vulgare ssp. (Link) Ietswaart, on four harvest dates.
| Steiner NS concentration (%) | 1 HD 1 (30 dat) | HD2 (60 dat) | HD3 (90 dat) | HD4 (120 dat) |
|---|---|---|---|---|
| 25 | 0.36 ± 0.004 cz | 0.40 ± 0.016 a | 0.31 ± 0.028 c | 0.35 ± 0.024 c |
| 50 | 0.42 ± 0.004 bc | 0.48 ± 0.008 a | 0.35 ± 0.008 bc | 0.45 ± 0.036 ab |
| 75 | 0.40 ± 0.101 c | 0.44 ± 0.114 a | 0.39 ± 0.095 abc | 0.37 ± 0.091 bc |
| 100 | 0.50 ± 0.008 a | 0.47 ± 0.004 a | 0.42 ± 0.028 ab | 0.45 ± 0.028 ab |
| 125 | 0.56 ± 0.287 a | 0.49 ± 0.004 a | 0.46 ± 0.008 a | 0.51 ± 0.012 a |
| LSD | 0.08 | 0.097 | 0.08 | 0.08 |
| CV | 9.97 | 11.16 | 11.82 | 10.87 |
1HD = harvest date; dat = days after transplanting; NS = nutrient solution; LSD = least significant difference; CV = coefficient of variation. zMeans with the same letter within each column do not differ statistically (Tukey, P ≤ 0.05).
Although there is a significant statistical difference in the P concentration due to treatment effects, the levels fluctuated between 0.31 and 0.51 %, which are considered optimal in most crops. One of the factors that most influence P absorption in crops is the pH of the NS, and in this research it remained between 5.5 and 6.0 during the production cycle, so an increase was observed in the absorption of the nutrient as the NS concentration increased. Pal et al. (2016) obtained a P concentration of 0.25 % with a 125-250 kg·ha-1 dose of this element and a yield of 18.60 t·ha-1 of fresh matter. In this sense, it is inferred that the P concentration in the present research was in the sufficiency ranges.
Oregano is a crop that can withstand high P levels without presenting phytotoxic problems. In this regard, Karagiannidis, Thomidis, Lazari, Panou-Filotheou, and Karagiannidou (2011) achieved P concentrations of 0.90 % by incorporating mycorrhizal fungi (G. etunicatum and G. lamellosum) in the management of this same crop, while without mycorrhizae they obtained a concentration of 0.10 %. These values, apparently high, did not negatively affect biomass production, although in the present experiment the P concentration was not increased to such levels.
The P concentration that occurs in biomass can be explained by the function that this element plays in plants and the fact its route ends in the seed, when the organ of interest is seed production, as reported by Dordas (2009), who indicates that there is a strong translocation of P and N from vegetative tissues to grain development. In the case of oregano, the economic interest is the production of fresh or dry matter; therefore, the plant does not complete its life cycle and does not reach the stage of flower and fruit production, which causes an accumulation of P in the tissue.
Potassium
The K concentration showed significant statistical differences due to treatment effects on HD3 and HD4, while in the first two harvests there was no statistical difference. The NS at 75, 100 and 125 % presented the highest K concentration (Table 6). This probably happened because as the NS concentration increased, the concentration of ions increased, including K, which favored higher absorption by the plant. According to Marschner (2012), two mechanisms can operate in the absorption of K, depending on the K concentration on the outside of the roots. Mechanism I occurs when the K concentration in the medium in which the roots grow is less than 0.5 mmol·L-1, and the absorption of K is more selective. For its part, mechanism II operates in concentrations greater than 50 mmol·L-1, and the absorption process is less selective.
Table 6 Effect of Steiner nutrient solution concentration on potassium content (%) in biomass of Origanum vulgare ssp. (Link) Ietswaart, on four harvest dates.
| Steiner NS concentration (%) | 1 HD 1 (30 dat) | HD2 (60 dat) | HD3 (90 dat) | HD4 (120 dat) |
|---|---|---|---|---|
| 25 | 2.85 ± 0.012 az | 3.05 ± 0.376 a | 1.79 ± 0.162 b | 2.70 ± 0.170 c |
| 50 | 3.27 ± 0.008 a | 3.20 ± 0.213 a | 1.64 ± 0.027 b | 4.12 ± 1.024 b |
| 75 | 2.82 ± 0.702 a | 2.50 ± 0.772 a | 2.63 ± 0.628 a | 5.28 ± 0.107 a |
| 100 | 2.95 ± 0.043 a | 2.56 ± 0.107 a | 2.78 ± 0.055 a | 5.73 ± 0.451 a |
| 125 | 3.14 ± 0.004 a | 2.95 ± 0.051 a | 3.04 ± 0.111 a | 5.57 ± 0.170 a |
| LSD | 0.59 | 0.75 | 0.55 | 0.97 |
| CV | 10.44 | 13.98 | 12.41 | 10.96 |
1HD = harvest date; dat = days after transplanting; NS = nutrient solution; LSD = least significant difference; CV = coefficient of variation. zMeans with the same letter within each column do not differ statistically (Tukey, P ≤ 0.05).
Economakis (1993) reported that a K concentration ranging from 150 to 450 mg·L-1 in the NS did not have a significant effect on plant growth; although the author does not report the nutrient concentration, the study was carried out with the same K concentrations in the NS as in the present research. This information allows inferring that K can accumulate in the biomass, but it does not precisely lead to a significant increase in vegetative material production.
Nutritional requirement
The production of fresh matter showed a significant difference (P ≤ 0.05), and the NS at 50 and 75 % had the highest total biomass production compared to the other treatments. The nutritional requirement was estimated with the treatments of greatest total fresh matter production and their concentration of N, P and K; in this case, the concentrations that stood out were those of 50 and 75 % with significant coefficients (Figure 1). The 50 % NS was considered for the calculation of nutritional requirement since it is more economical as it uses less fertilizer. On the other hand, the amount of dry matter in a ton of fresh matter is equivalent to 167.24 kg, due to the fact that the tissue consists of 83.28 % water. The concentration of N, P, and K in dry matter for this treatment was 2.85, 0.42 and 3.05 %, respectively (Table 4, 5 and 6); therefore, the nutritional requirement of oregano was estimated at 4.76 kg of N, 0.70 kg of P and 5.10 kg of K, per ton of fresh matter.
Figure 1 Effect of Steiner nutrient solution concentration on total fresh biomass production in Origanum vulgare ssp. hirtum (Link) Ietswaart. Pr > t (x = 0.03; x2 = 0.03). Vertical lines indicate standard errors (n = 5). zMeans with the same letter among concentrations do not differ statistically (Tukey, P ≤ 0.05).
Conclusions
The highest yield in fresh and dry matter in oregano was obtained with the Steiner NS at 50 and 75 % concentration. However, considering the cost of fertilizers, it is recommended to use the NS at 50 % for the commercial production of oregano in hydroponic systems, since it guarantees an optimal nutritional supply. The nutritional requirement of the crop to produce one ton of fresh matter is 4.76 kg of N, 0.70 kg of P and 5.10 kg of K.
