Introduction
Tomato (Solanum lycopersicum L.) is the most widely grown vegetable under greenhouses and shade houses in Mexico (Servicio de Información Agroalimentaria y Pesquera [SIAP], 2021). With the conventional management practiced by most companies in the country (population density of 2.5 to 3 plants∙m-2 of greenhouse, a single crop cycle per year and intermediate-level technology), an average of 300 t∙ha-1∙year-1 are produced (Castellanos & Borbón-Morales, 2009). In contrast, in Northern Europe and Canada, with hydroponic technology and high-tech greenhouses, yields exceeding 500 t∙ha-1∙year-1 have been reported (Cheiri et al., 2018; Heuvelink et al., 2018), although with high production costs.
At the Universidad Autónoma Chapingo, Mexico, an alternative system for managing hydroponically-grown greenhouse tomatoes was developed, in which the time from transplanting to the end of harvest lasts four months, thus obtaining three crop cycles per year. To do this, transplanting is done with 45-day-old seedlings, 8 plants∙m2 of greenhouse are established and the plants are tipped two leaves above the third inflorescence (Sánchez-del Castillo et al., 2012). This system obtains yields of 16 kg∙m-2 in one crop cycle (Moreno-Pérez et al., 2021), which is equivalent to almost 500 t∙ha-1∙year-1 if all three crop cycles are established.
In hydroponic systems that use substrate as a growing medium, drip irrigation is applied with a nutrient solution containing the essential minerals for optimum plant growth and development. The fertilizers used, especially when preparing concentrated nutrient solutions, are usually highly soluble and expensive; in fact, the cost has increased significantly in recent years. To reduce the cost of production in this regard, without reducing yield and quality, there are two alternatives: a) prepare the nutrient solution with less soluble fertilizers, with the same composition and nutrient concentration, but cheaper, and b) reduce the concentration of the nutrient solutions during some stage of the crop cycle, although this aspect has been little studied.
In the initial growth stage of a crop, the main cause of water loss is evaporation (López-López et al., 2009), due to the low leaf area index of the plant at that time (Sánchez-del Castillo & Moreno-Pérez, 2017). Some substrates, such as tezontle sand, have a rough structure (Ponce-Lira et al., 2013) and a high surface area exposed to air and sun, which increases evaporation. If these types of substrates are not irrigated constantly and uniformly over their entire surface (as is often the case with drip irrigation), areas with greater evaporation remain, leaving them dry or partially dry. This causes the localized accumulation of precipitated or fixed nutrients from the nutrient solution in significant quantities, which reduces the efficiency of nutrient uptake by the plant (Sánchez-del Castillo et al., 2021). In this regard, it is necessary to develop research that explores nutrient dynamics in tomato plants to optimize the use of water and fertilizers, and reduce production costs.
Based on the above, this study aimed to: evaluate water and macronutrient consumption in tomato plants; compare the yield obtained by using a nutrient solution made with poorly soluble, low-cost fertilizers versus a conventional solution formulated with highly soluble fertilizers of higher cost; and compare the use of a conventional nutrient solution at 100 % concentration against one at 80 %, and another with a variable concentration of nutrients according to the phenological stage of the plant.
Materials and methods
The experiment was established in a greenhouse belonging to the Postgraduate Program in Horticulture of the Universidad Autónoma Chapingo, located in Texcoco, State of Mexico. The Harris Moran seed company’s 'El Cid' F1 cultivar, which has a saladette-type tomato and an indeterminate growth habit, was used. Sowing was done in 60-cavity trays with a volume of 250 cm3 per cavity. The substrate used was a mixture of peat moss and perlite (1:1 by volume). During the first eight days after sowing (das), it was irrigated with water; subsequently, irrigation was carried out with 50 % nutrient solution until 45 das. The 100 % nutrient solution contained the following elements and concentrations (mg∙L-1): N = 200, P = 50, K = 250, Ca = 230, S = 150, Mg = 50, Fe = 2, Mn = 1, B = 0.5, Cu = 0.1 and Zn = 0.1, as proposed by Sánchez-del Castillo et al. (2012) for tomato cultivation.
Transplanting was carried out at 45 das in cultivation trays made of galvanized sheet metal measuring 20 cm wide × 1 m long and 24 cm deep, previously lined on the inside with 150 µm thick black plastic and filled with red tezontle sand with particle size between 1 and 3 mm in diameter. Each tray was fitted with an outlet pipe to collect the excess nutrient solution after each irrigation.
A randomized complete block experimental design with six replications was used. Six treatments were compared, resulting from the combination of three concentrations of the nutrient solution (100 %, 80 % and one with a variable concentration according to the phenological stage) and two fertilizer formulations (conventional and alternative). The treatments were applied from transplanting to the end of harvest. The 100 % conventional formulation contained the macro and micronutrients in the concentrations indicated above, provided by calcium nitrate, 85 % phosphoric acid, potassium sulfate, magnesium sulfate, ferrous sulfate, manganese sulfate, sodium tetraborate, copper sulfate and zinc sulfate. The alternative formulation was prepared with the same concentrations of macronutrients, but from less soluble and cheaper fertilizer sources: ammonium nitrate, simple calcium superphosphate, potassium chloride and calcium chloride, while the sources of magnesium and micronutrients were the same as in the conventional nutrient solution.
Treatments with variable concentrations of the nutrient solution (conventional and alternative) depending on the phenological stage were applied as follows: from transplanting to anthesis (50 % of the flowers of the first cluster), a 60 % concentration of all macronutrients was used, except N, which was applied at 100 %. From anthesis to fruit set of the third cluster, an 80 % concentration of all macronutrients was used, keeping N at 100 %, and from this stage until the end of harvest, all nutrients were applied at a 100 % concentration. Micronutrients were supplied at 100 % throughout the cycle.
The experimental unit consisted of eight plants established in two contiguous cultivation beds. Four plants were established in each tray at a distance of 20 cm between plants and 25 cm between rows. There were 50 cm aisles between the sets of trays, resulting in a population density of 9 plants∙m-2 of greenhouse.
For each nutrient solution formulation, a 200 L plastic barrel was used. Irrigation was carried out using drip tape with integrated drippers every 20 cm. Throughout the crop cycle, three to five irrigations were applied per day depending on the environmental conditions and the phenological stage of the plants. Over-irrigation was used to allow drainage of between 10 and 20 % of the volume of the solution applied during the day to maintain a constant electrical conductivity (EC) in the rhizosphere (Sánchez-del Castillo & Moreno-Pérez, 2017). The nutrient solution that drained off due to over-irrigation was collected in plastic containers, and its volume, pH and EC were measured daily. With this information, a weekly count was made of the nutrient solution supplied and the volume of nutrient solution drained from the trays of each treatment throughout the crop cycle.
The plants were trained to a single stem, and the tipping (removal of the terminal bud) was done two leaves above the third inflorescence in order to harvest only three clusters per plant.
Variables evaluated
1. Morphological variables. At the end of the crop cycle (130 das), the following were measured: plant height (cm) from the base to the tipping height (which was two leaves above the third inflorescence), stem diameter (mm) between the first and second inflorescence, and leaf area per plant (cm2). This last variable was obtained by measuring the area of each leaf in a leaf area integrator (L-3000, Li-Cor ®, USA). Three plants per experimental unit were used to determine these variables.
2. Total dry weight per plant (g). It was obtained at the end of the crop cycle from a destructive sampling of three plants per experimental unit (the same ones used for the morphological variables).
3. Yield (kg∙plant-1) and its components (number of fruits per plant and average fruit weight [g]).
4. Macronutrient balance in the system. At the end of the cycle (130 das), chemical analyses were performed on one plant per experimental unit of each replication to determine the amount of each macronutrient absorbed and its percentage of absorption with respect to the total supplied with the nutrient solution. Likewise, the amount and percentage of macronutrients leached and retained (fixed) by the substrate until the end of the crop cycle, as well as those resolubilized or volatilized, were determined.
5. Water use efficiency (WUE). This variable was determined based on the amount (L) of water required (applied) to produce 1 kg of fruit (ratio of L of water applied per plant/yield expressed in kg per plant).
6. Nutrient absorption efficiency (NAE). This was obtained by calculating the amount (g) absorbed of each macronutrient per kilogram of fruit produced (ratio of nutrients absorbed by the plant/yield expressed in kg per plant).
Statistical analysis
The data obtained were subjected to analysis of variance tests and Tukey's comparison of means (P ≤ 0.05) using SAS statistical package ver. 9.1 (SAS Institute Inc., 2002). WUE and NAE were determined only for the 80 % conventional nutrient solution formulation.
Results and discussion
Morphological variables and dry weight
Both the analysis of variance (data not shown) and the comparison of means (Table 1) show that no morphological variable (plant height, stem diameter and leaf area), nor dry weight, presented significant statistical differences among treatments.
Table 1.
| Formulation | Leaf area/plant (cm2) | Plant height (cm) | Stem diameter (mm) | Dry matter weight (g) |
|---|---|---|---|---|
| Conventional at 100 % | 6,538 a | 107 a | 10.1 a | 144.2 a |
| Conventional at 80 % | 6,658 a | 107 a | 9.8 a | 170.5 a |
| Conventional variable | 5,593 a | 110 a | 9.7 a | 145.9 a |
| Alternative at 100 % | 5,349 a | 107 a | 10.2 a | 120.8 a |
| Alternative at 80 % | 5,874 a | 109 a | 10.3 a | 135.6 a |
| Alternative variable | 6,151 a | 110 a | 10.5 a | 150.5 a |
| HSD | 2,464 | 4.3 | 1.7 | 60.3 |
According to the leaf area obtained and the population density established, the leaf area index of the crop was between 4.7 and 5.8, which probably caused leaf shading. The index reached is slightly high for maximum photosynthesis and dry matter accumulation per day in tomato (Cheiri et al., 2018; Heuvelink & Dorais, 2005; Heuvelink et al., 2018). Dry matter weight per plant ranged between 120.8 and 170.5 g, with no statistical differences among treatments. Considering the above, it can be inferred that the change in fertilizer sources did not affect plant growth, and neither was it affected by decreasing the concentration by 20 % or by varying the concentration according to phenological stage. These results agree with those reported by Godoy-Hernández et al. (2009).
Yield and its components
Analysis of variance (data not shown) and comparison of means (Table 2) indicate that yield was significantly higher in plants grown with conventional formulations (2.1 kg∙plant-1 on average), compared to plants grown with alternative formulations (1.6 kg∙plant-1 on average). This difference was due to the higher number of fruits per plant obtained with conventional formulations (about four more fruits per plant), since the average fruit weight did not vary significantly among treatments. It should be noted that in this case only commercial fruits were considered, i.e., without damage from blossom end rot.
Table 2.
| Formulation | Fruits/plant | Average fruit weight (g) | Yield (kg∙plant-1) |
|---|---|---|---|
| Conventional at 100 % | 19.8 ab | 102 a | 2.03 ab |
| Conventional at 80 % | 19.8 ab | 107 a | 2.11 a |
| Conventional variable | 20.4 a | 106 a | 2.16 a |
| Alternative at 100 % | 16.2 abc | 98 a | 1.58 c |
| Alternative at 80 % | 15.7 bc | 107 a | 1.68 bc |
| Alternative variable | 15.0 c | 106 a | 1.58 c |
| HSD | 4.3 | 9.4 | 0.41 |
The lower number of commercial fruits obtained in plants grown with the alternative formulations was due to the presence of fruits affected by blossom end rot, a physiological disorder caused by a localized calcium deficiency. This probably occurred because calcium was supplied mainly in the form of simple calcium superphosphate (low-solubility fertilizer) (Sánchez-del Castillo & Escalante-Rebolledo, 1988), which resulted in non-commercial fruits. The calcium deficiency could also be explained by the use of N-NH4 + in 50 % of the N contribution in conventional formulations, which, due to its positive charge, competes strongly with Ca2+ uptake by the root (Resh, 2013; Sonneveld & Voogt, 2009; Wallace & Muller, 2008). Likewise, a high K:Ca ratio can negatively affect Ca uptake, since both cations compete during root uptake and K+, being monovalent, is absorbed faster than Ca2+, which is divalent (Malvi, 2011; Sonneveld & Voogt, 2009). In conventional formulations, calcium was supplied as calcium nitrate, a highly soluble fertilizer, so no blossom end rot was observed in the fruits.
Regarding the concentration of the nutrient solution (100 and 80 %) in both formulations (conventional and alternative), the results show that it is possible to use a solution with a lower concentration without affecting yield or average fruit weight, as long as the same fertilizer sources and the same daily volume of nutrient solution are maintained (Table 2), since yield was statistically equal with both concentrations. In a similar study, Suazo-López et al. (2014) also found no significant differences in concentrations at 75 % compared to 100 %.
From an economic point of view, the above translates into fertilizer savings for the producer. From an environmental perspective, it would increase NAE, reduce leaching losses (Rodríguez-Jurado et al., 2020) and, thus, decrease the pollution of the water table, particularly in open hydroponic systems, which are the most commonly used.
Nutrient solution consumed by plants
The volume of nutrient solution consumed (nutrient solution supplied minus that drained) during the 85 days from transplanting to the end of harvest was statistically the same in all treatments (Table 3). Water consumption ranged from 59 to 65 L∙plant-1, which is equivalent to an average daily expenditure of 0.73 L∙plant-1.
Table 3.
| Formulation | Water (L∙plant-1) | N (g) | P (g) | K (g) | Ca (g) | Mg (g) |
|---|---|---|---|---|---|---|
| Conventional at 100 % | 62 a | 12.4 a | 3.1 a | 15.5 a | 15.5 a | 3.1 a |
| Conventional at 80 % | 60 a | 9.5 b | 2.4 d | 11.9 b | 11.9 d | 2.4 d |
| Conventional variable | 64 a | 12.7 a | 2.7 bcd | 15.4 a | 13.4 bc | 2.7 bcd |
| Alternative at 100 % | 59 a | 11.8 a | 3.0 ab | 14.8 a | 14.8 ab | 3.0 ab |
| Alternative at 80 % | 62 a | 9.9 b | 2.5 cd | 12.4 b | 12.4 cd | 2.5 cd |
| Alternative variable | 65 a | 12.9 a | 2.7 bc | 15.5 a | 13.6 bc | 2.7 bc |
| HSD | 6.8 | 1.26 | 0.31 | 1.57 | 1.52 | 0.31 |
Since the 80 % formulations had lower nutrient concentrations than the 100 % formulations, N, P, K, Ca and Mg consumption was also lower. The consumption of P, Ca and Mg was statistically lower in the formulations that varied according to the phenological stage compared to the 100 % one, since during one third of the crop cycle they were managed at 50 % and another third at 80 % of their normal concentration.
With the conventional formulation at 100 %, the cost per m3 was $60.00 MXN, while at 80 % the cost was $48.00 MXN. If an expenditure of 0.67 m3 of nutrient solution per m2 per cycle is considered, under the system proposed by Sánchez-del Castillo et al. (2012) of three crop cycles per year (Table 3), at a commercial scale of 1 ha, the consumption would be 20,100 m3 of nutrient solution per year, with a fertilizer cost of $1,206,000.00 MXN. If the 80 % solution is applied, the same yield and average fruit weight values are obtained, but at a cost of $964,800.00 MXN; that is, it represents an annual saving of $241,200.00 MXN in fertilizers.
The 80 % alternative solution had a cost close to $30.00 MXN per m3, which meant a potential fertilizer saving of $600,000.00 MXN per hectare per year compared to the conventional formulation at the same concentration. However, the higher yield and quality obtained with the conventional formulation makes it economically more cost-effective.
Nutrient balance in the system
Table 4 shows the amounts of macronutrients (N, P, K, Ca and Mg) supplied with the 80 % conventional nutrient solution, which was the most economically advantageous. The nutrients absorbed by the plant, leached, resolubilized, and retained or volatilized are also reported. The table shows that, of the total N supplied, the plant absorbed about 30 %, 7 % was leached and 3 % was resolubilized. Part of the remaining 60 % of N was retained in the substrate and another part was lost through volatilization. Pineda-Pineda et al. (2011), in a similar study on tomato, recorded plant N uptake values of 25 % at 40 days after transplanting (dat), 47 % at 59 dat and 80 % at 74 dat (1.9 % stored in the substrate and 18 % lost to drainage). Sonneveld and Voogt (2009) reported that N uptake in a tomato crop in a greenhouse substrate, under free-draining conditions, can be up to 57 %.
Table 4.
| Nutrients | Contributed | Absorbed | Leached | Resolubilized | Retained or gasified |
|---|---|---|---|---|---|
| N | 11.1 (100) | 3.34 ab (30) | 0.83 bc (7) | 0.29 b (3) | 6.64 (60) |
| P | 2.8 (100) | 1.48 a (53) | 0.08 a (3) | 0.06 a (2) | 1.18 (42) |
| *K | 14.4 (100) | 6.70 a (47) | 2.16 d (15) | 1.70 c (12) | 3.84 (26) |
| *Ca | 15.81 (100) | 3.18 a (20) | 5.08 ab (32) | 1.98 c (12) | 5.57 (36) |
| *Mg | 3.8 (100) | 1.26 a (33) | 2.78 b (73) | 0.68 b (18) | -0.92 (+24) |
The relatively low plant N uptake (30 %) may be related to the third cluster tipping at 80 dat, since, as plant growth stops, nutrient uptake is reduced (Terabayashi et al., 2004). According to Bock (1984), up to 50 % of the N lost may be through leaching, denitrification, or volatilization of NH4 + to NH3. In the present study, it is possible that N losses from the substrate occurred through volatilization, a situation that can occur especially under alkaline conditions and high ambient temperatures (Mengel & Kirkby, 2001).
Van den Ende (1989) points out that there may be a percentage of N lost through denitrification if samples are left to air dry for a long period of time, a situation that occurred in this study, since the samples were left to dry for at least 20 days. In addition, plants can also release NH3 into the atmosphere through stomata and cuticle (Marschner, 2012) during crop growth, especially if photorespiration is high (Taiz et al., 2017).
Of the total P supplied to the system, about 53 % was absorbed by the plant, 3 % was lost in the drainage, 2 % was resolubilized from the substrate, and 42 % was precipitated or adsorbed in the substrate. Pineda-Pineda et al. (2011) recorded 38 % of P absorbed by the tomato plant during vegetative development, 54.8 % retained in the substrate and 6.8 % lost in the drainage. Silber et al. (1999) point out that tezontle has the ability to adsorb or release nutrients, especially P, during the growth period of the plants.
K uptake by the plant was 47 % of the total applied, 15 % was lost in the drainage, 12 % was resolubilized and 26 % was precipitated or adsorbed in the substrate. Pineda-Pineda et al. (2011) recorded 39.3 % of K absorbed by the plant, 35.1 % retained in the substrate and 25.5 % lost in the drainage.
Of the total amount of Ca supplied to the system, 20 % was absorbed by the plant, 32 % leached, 12 % resolubilized and 36 % precipitated or adsorbed by the substrate. It is likely that the Ca reacted with P and S to form chemical precipitates, so a good part could not be resolubilized. Similarly, Pineda-Pineda et al. (2011) recorded 24.3 % Ca adsorbed by the plant, 38.8 % retained in the substrate and 36.8 % lost in the drainage.
The Mg absorbed by the plant with respect to the total amount supplied was 33 %. According to the results (Table 4), 73 % of the Mg was lost in the drainage and 18 % was resolubilized from the substrate, which resulted in 24 % more Mg in the system. It is likely that the extra Mg is a contribution from the tezontle and the water with which the nutrient solution was prepared, since its analysis reported 14.7 mg∙L-1 of Mg. Due to its composition, tezontle is a substrate that corresponds to the group of ferromagnesian minerals, mainly constituted by O, Si, Al, Fe, Mn, Ca and Mg (Raviv & Lieth, 2008; Trejo-Téllez et al., 2013). In addition, it has cation exchange capacity (Ponce-Lira et al., 2013), which is why, depending on the pH, it can adsorb or release nutrients (Silber et al., 1999).
Water use efficiency and nutrient uptake
The amount of water and nutrients required to produce 1 kg of tomato fruit was analyzed; for this purpose, the conventional formulation of an 80 % nutrient solution was considered, which was the most cost-effective. The amount of water supplied to each plant through irrigation (nutrient solution) was 60 L (Table 3) and the yield obtained was 2.11 kg∙plant-1 (Table 2), so 28.4 L of water were required to produce 1 kg of tomato fruit.
To obtain 1 kg of tomato fruit, Heuvelink and Dorais (2005) needed 40 L of water in an unheated plastic greenhouse in Spain, 22 L in a climate-controlled greenhouse in the Netherlands, and 15 L when reusing the nutrient solution (closed hydroponic system). Cunha-Chiamolera et al. (2017) report a WUE of at least 20 L∙kg-1 of fruit. Rouphael et al. (2005) point out that the WUE in a soilless culture is related to the physical properties of the substrate, in particular to the readily available water content.
Although the WUE obtained in this study is within the range reported by other authors, the relatively low efficiency found may be due to the fact that tezontle, because of its high degree of roughness and particle size, has a large surface area (Ponce-Lira et al., 2013), which causes high evaporation rates. It should be noted that water is an increasingly limited resource for agricultural production, and in many regions of the world the limit of its use has been reached, so alternatives must be sought that make its use more efficient to avoid negative impacts on the environment (Salazar-Moreno et al., 2014).
According to the yield achieved (Table 2) and the amount of nutrients absorbed by the plant (Table 4) under the conventional system and the 80 % nutrient solution, it was found that to produce 1 kg of tomato the plants absorbed 1.58 g of N, 0.70 g of P, 3.17 g of K, 1.51 g of Ca and 0.60 g of Mg. Similar results were reported by Heuvelink and Dorais (2005) and Quesada-Roldán and Bertsch-Hernández (2013).
Conclusions
Nutrient solution formulations and concentrations had no effect on the morphological aspects or dry matter weight of the plants. However, the use of formulations with low-solubility fertilizer sources reduced the yield of commercial fruits per plant compared to nutrient solutions made with highly soluble fertilizers. In addition, it was shown that if the same fertilizer source and daily volume of nutrient solution are maintained, it is possible to use a concentration of 80 % without affecting fruit yield.
The variation in nutrient solution formulations based on phenological stage did not show significant fertilizer savings, nor did it have an impact on yield per plant or fruit size, compared to the 100 % conventional solution.
With the 80 % conventional nutrient solution, the nutrient uptake efficiency with respect to the total amount supplied from transplanting to the end of harvest (85 days) was 30, 53, 47, 20 and 33 % for N, P, K, Ca and Mg, respectively.
With the system at a high population density, tipping at the third cluster and use of highly soluble fertilizers at 80 % concentration, 28.4 L of water, as well as 1.58 g of N, 0.70 g of P, 3.17 g of K, 1.51 g of Ca and 0.60 g of Mg, were required to produce 1 kg of tomato.