Greenhouse lettuce production with and without nutrient solution recycling

 

Producción de lechuga en invernadero con y sin recirculación de la solución nutritiva

 

Esaú del C. Moreno-Pérez¹; Felipe Sánchez-Del Castillo¹; Jorge Gutiérrez-Tlaque¹; Lucila González-Molina²*; Joel Pineda-Pineda³

 

¹ Departamento de Fitotecnia. Universidad Autónoma Chapingo. Carretera México-Texcoco km 38.5. Texcoco, Estado de México, C. P. 56230, MÉXICO.

² Campo Experimental Valle de México. Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias. Coatlinchán. Carretera Los Reyes-Texcoco km 13.5. Estado de México, C. P. 56250, MÉXICO. Correo-e: lucilaag@colpos.mx tel.: 595 92 412805 ext. 162 (*Autor para correspondencia).

³ Departamento de Suelos. Universidad Autónoma Chapingo. Carretera México-Texcoco km 38.5. Texcoco, Estado de México, C. P. 56230, MÉXICO.

 

Received: December 10, 2013.
Accepted: March 19, 2015.

 

Abstract

Hydroponic systems with nutrient solution recycling provide more efficient water and fertilizer use, but over time it becomes difficult to sustain the nutritional balance and control diseases that attack plant roots, which eventually results in lower yield compared to systems where nutrient solution is not recycled. The aim of this study was to evaluate the yield and earliness of butter lettuce (Cortesana M1 variety) grown with different hydroponic systems and their efficiency in terms of water and nutrient use. A randomized block experimental design with four replicates and three treatments (T) was used: T1: tezontle substrate beds without recycling the drained nutrient solution; T2: tezontle substrate beds with recycling the drained nutrient solution; and T3: floating root system. Yield and earliness variables were measured and also the savings and efficiency in water and fertilizer use were estimated. Data were subjected to analysis of variance and Tukey's multiple comparisons of means test (P = 0.05). The best yield, earliness and efficiency in water and nutrient use occurred with the floating root system. There were no differences in yield or earliness between the substrate systems with or without nutrient solution recycling.

Keywords: Lactuca sativa L., soilless culture, hydroponics, substrate.

 

Resumen

Los sistemas hidropónicos con recirculación de la solución nutritiva, hacen un uso más eficiente de agua y fertilizantes, pero con el tiempo se hace difícil mantener el balance nutricional y controlar las enfermedades que atacan a la raíz de las plantas, lo que eventualmente repercute en menor rendimiento respecto a sistemas en donde dicha solución no se recircula. El objetivo de este estudio fue evaluar el rendimiento y precocidad de lechuga tipo mantequilla variedad Cortesana M1 cultivada con diferentes sistemas hidropónicos y la eficiencia de éstos en el uso de agua y nutrimentos. Se usó un diseño experimental de bloques al azar con cuatro repeticiones y tres tratamientos (T): T1: camas con sustrato de tezontle sin recirculación de la solución nutritiva drenada; T2: camas con sustrato de tezontle con recirculación de la solución nutritiva drenada; y T3: sistema de raíz flotante. Se midieron variables de rendimiento y precocidad, asimismo se estimó el ahorro y eficiencia en el uso del agua y fertilizantes. Los datos fueron sometidos a análisis de varianza y comparaciones de medias de Tukey (P = 0.05). El mejor rendimiento, precocidad y eficiencia en el uso de agua y nutrimentos se tuvo con el sistema de raíz flotante. Entre los sistemas con sustrato con o sin recirculación de la solución nutritiva no hubo diferencias en rendimiento ni en precocidad.

Palabras clave: Lactuca sativa L., cultivo sin suelo, hidroponía, sustrato.

 

Introduction

In Mexico the area planted for vegetable production under protected agriculture conditions has increased in recent years (Global Agricultural Information Network [GAIN], 2010; Servicio de Información Agroalimentaria y Pesquera [SIAP], 2010), reaching about 20,000 ha in 2013 (Asociación Mexicana de Horticultura Protegida A. C. [AMHPAC], 2013). The tomato (Solanum lycopersicum L.) is the most widely grown vegetable in this system; however, intensive production of this vegetable already faces the problem of market saturation, which results in a lower price for this product and therefore fewer economic benefits to the producer, making it necessary to diversify the species grown under this system. One alternative is the production of lettuce (Lactuca sativa L.) in a greenhouse using hydroponics ( production system in which plants grow in a nutrient solution with or without a substrate as a means of support), as soil often facilitates the establishment of pathogens after several crop cycles (Takahashi, 1984). Added to this, intensive soil management also results in salt accumulation that affects yield due to changes in the soil's chemical and physical properties (Zhang, Jiang, & Liang, 2006). For lettuce plants to grow without nutritional limitations, the nutrient solution should have a pH of 5.5 to 6.5, electrical conductivity (EC) of 1.5 to 2.5 dS∙m^-1 and the mineral nutrients must be dissociated into ionic form and in proportions and concentrations that avoid precipitates and antagonisms (Adams, 2004).

A hydroponic system is known as open when the drained nutrient solution is not reused and infiltration is allowed on the site or is conducted outside the greenhouse, and is called closed if the nutrient solution is collected for reuse in the culture, after sterilization and adjustment of pH, EC and nutrients.

Technological production packages that are being used in Mexico for protected agriculture consist of open systems, which have the disadvantage of high water and fertilizer consumption. It should be stressed that water is an increasingly scarce natural resource; also, fertilizers, which represent a significant percentage of the overall production cost in hydroponic systems, are expensive (Huang, 2009), making it necessary to look for crop production systems that are more efficient in terms of water and fertilizer use. In this sense, closed hydroponic systems have significant advantages in water and fertilizer savings compared to open systems (Dhakal, Salokhe, Tantau & Max, 2005; Sánchez del Castillo, González, Moreno, Pineda, & Reyes, 2014), as well as less environmental impact by avoiding infiltration of mineral salts that pollute rivers and seas (Massa, Incrocci, Maggini, Carmassi, Campioti, & Pardossi, 2010; Pardossi, Incrocci, Massa, Carmassi, & Maggini, 2009). However, these systems have the disadvantage of resulting in a gradual increase in the electrical conductivity of the nutrient solution over time, an imbalance in the nutrient solution and an increased risk of spreading diseases that attack the root (Tüzel, Tunali, Tüzel, & Öztekin, 2009; Massa et al., 2010).

The problem of high EC (five or more for a crop like lettuce) can be minimized by renewing the nutrient solution whenever it reaches a conductivity threshold value (Dasgan & Ekici, 2005), and by replenishing the transpired water with plain water and adding all nutrients based on estimated daily consumption (Nakano, Sasaki, Nakano, Suzuki, & Takaichi, 2010).

The imbalance in the nutrient solution is generated by excesses of the ions least consumed by the plant (normally SO₄^-, Ca²⁺ and Mg²⁺), which disrupts the balance of the nutrients and often increases the EC to levels that affect growth and yield, often forcing growers to discard the nutrient solution (Savvas, Sigrimis, Chatzieustratiou, & Paschalidis, 2009).

In the case of lettuce, although much is grown in soil and open field conditions, a management option that has been practiced in various parts of the world is the reuse of the nutrient solution, recirculating and collecting it for its reincorporation (closed system), after sterilization and adjustment of nutrients, pH and EC, using a substrate as a support. However, lettuce production can also be done by use of a floating root system in which substrate is omitted and the plants grow directly in a nutrient solution.

Based on the above, the aim of this study was to compare the components of yield and earliness achieved with different open and closed hydroponic lettuce production systems, as well as the expenditure and efficiency in the use of water and nutrients, under the hypothesis that with production systems with nutrient solution recycling and in particular with the floating root system, it is possible to obtain at least the same yield and earliness as in an open system with the advantage that it makes more efficient use of water and fertilizers.

 

MATERIALS AND METHODS

The experiment was conducted in a greenhouse at the Universidad Autónoma Chapingo, located in the county of Texcoco, State of Mexico, at 19° 31' north latitude and 98° 51' west longitude, and at an elevation of 2,240 m. Two independent experiments, one in the spring and the other in the summer of 2012, were established.

Butter lettuce (Cortesana M1 variety) obtained from the Hydro Environment company was used as plant material.

The treatments (systems) evaluated were:

Treatment 1: Growing bed with tezontle substrate without recycling drained nutrient solution. This treatment was used as the control.

Treatment 2: Growing bed with tezontle substrate with recycling drained nutrient solution.

Treatment 3: Floating root.

To form the first two treatments, wooden crates (growing beds) of 1.9 m long, 0.9 m wide and 0.30 m deep were used as containers and placed with a slope of 5 %; this was done in order to more easily capture the drainage of the nutrient solution. The wooden crates were lined inside with 1,000-gauge black plastic. They were subsequently filled with gravel and tezontle sand; first a 7-cm layer of gravel (particles of 2 - 2.5 cm in diameter) and then a second, 18-cm layer of sand (particles of 3 - 5 mm in diameter) were placed.

To capture the drainage in both systems with and without nutrient solution recycling, 2-mm holes were made at the base of the plastic every 25 cm. With the same black plastic, a chute leading the drained water to an 18-L plastic bucket covered by a mosquito net was made.

The irrigation system that fed the plants in the first two treatments consisted of three, 450-L water tanks as reservoirs; two of them provided normal nutrient solution and the other a recycled solution previously sterilized with 25-Watt UV lamps. To apply irrigation in the substrate beds, irrigation lines were placed across the width of the beds. Three 16-mm drip tapes, with emitters every 15 cm, were then placed on top of the lines.

To form the floating root treatment (T3), wooden containers with the same dimensions as those used in the other systems (450 liters) were used. At a height of 25 cm, a hole of 5 cm in diameter was made to regulate the level of the nutrient solution. In this case, instead of substrate, only nutrient solution was used and the plants were held in place with styrofoam plates. To provide oxygen to the root (trying to keep between 5 and 8 mg∙liter¹ of dissolved oxygen), AC-9602 model aquarium pumps and polyethylene tubing with a diameter of 5 mm were used.

A randomized complete block experimental design with five replicates was used. The experimental unit was 55 plants, which were established in a real planting arrangement with a distance between rows and between plants of 16 cm (a population density of 32 useful plants∙m^-2).

The variables evaluated were: fresh weight per plant, dry weight per plant, leaf length and width, number of leaves longer than 1 cm and days to harvest. In addition, an estimate of the savings in water and fertilizers (N, P, K and Ca) was made.

The seedlings were obtained in 200-cavity polystyrene trays. As substrate, a mixture of peat moss and perlite at a volumetric ratio of 1:1 was used. The transplant was done 30 days after sowing (das).

In the growing beds with substrate, the volume of irrigation with nutrient solution applied depended on weather conditions (light intensity, temperature and relative humidity) and the crop's phenological stage, but we sought to give an over-irrigation of 20 % above the required water volume. The nutrient solution used in all treatments contained the following nutrients (in mg∙liter^-1): Nitrogen, 140; Phosphorus, 40; Potassium, 175; Calcium, 140; Magnesium 40; Sulfur, 140; Iron, 1.5; Manganese, 0.5; Boron, 0.5; Copper, 0.1 and Zinc, 0.1. The fertilizer sources were: calcium nitrate, potassium sulfate, 85 % phosphoric acid, magnesium sulfate, iron chelate, manganese sulfate, sodium tetraborate, copper sulfate and zinc sulfate.

 

RESULTS AND DISCUSSION

Yield and morphological variables

First cycle (Spring, 2012)

The floating root system had significantly more fresh weight and dry weight per plant compared to the treatments with and without nutrient solution recycling in beds with substrate on all evaluated dates; between these last two treatments, no statistical differences were found (Table 1).

The increased fresh weight and dry weight and greater earliness achieved in lettuce plants managed under the floating root system can be attributed to the fact that in this system the water and the nutrient concentration for the root are kept more stable in the rhizosphere area throughout the crop cycle, whereas in other systems both water and nutrients vary considerably between one watering and another, which discourages absorption (Silber & Bar-tal, 2008).

In measuring the length, width and number of leaves per plant, differences were only found for the first variable between the floating root systems and the system without nutrient solution recycling (Table 1).

At 32 dat, both dry and fresh weights in the floating root system were about double that of the other two systems. In addition, butter lettuce plants established in the floating root system in this spring cycle reached their commercial size and weight (at least 180 g) a month after being transplanted, while an additional nine days were required under the other two systems.

The difference in dry weight and fresh weight in favor of the floating root system was observed from 16 dat and was maintained throughout the crop cycle, suggesting that the plants established in beds with substrate, either with or without nutrient solution recycling, had a delay in their growth due to some transplant stress such as uneven moisture, lower nutrient availability or changes in the EC, a situation that did not occur with the floating root system.

This effect can also be explained by the fact that with the floating root system the pH and EC of the nutrient solution were more stable throughout the crop cycle, because although the three treatments started with a solution with pH 5.6 and EC 1.8 dS∙m^-1, in the drained solution in the recycling and nonrecycling systems, the pH and EC gradually increased to values of 9.0 and 3.7, respectively, at the end of the crop cycle due to the influence of accumulated salts in the substrate (Castellanos & Borbón, 2009; Pineda et al., 2011); by contrast, in the floating root system, both parameters remained close to their initial values ( pH 6.0 and EC 2.2), since the water lost through evapotranspiration, which could eventually lead to salinization by consuming more water than salts, was replenished daily and only occasionally (when the EC decreased to levels of 1.2) were nutrients added to the water and the corresponding pH adjustment made.

 

Second cycle (Summer, 2012)

For this cycle, the behavior of the treatments was similar to that of the first cycle, in that the floating root system produced lettuce with higher fresh weight than the bed systems with substrate, although in dry weight, this difference occurred only with respect to the bed treatment with substrate but without recycling at 10 and 17 dat (Table 2).

Between the growing systems in substrate beds with or without nutrient solution recycling, there was no difference for any variable. Similar results were reported by Sánchez et al. (2014) in American cucumber cultivated in growing beds filled with tezontle as substrate without nutrient solution recycling.

In this second cycle, due to environmental conditions, especially the temperature in which the plants grew in the greenhouse (in spring the average temperature during plant development ranged from 17 to 24 °C and in summer from 23 to 29 °C, with maximum temperatures from 36 to 44 °C and 32 to 45 °C and minimums from 3 to 13 °C and 10 to 16 °C, respectively), the time to harvest was shorter than in the spring period as also indicated by Fallovo, Youssef, Rea, and Battistelli (2009); specifically, with the floating root system, the harvest was made at 24 dat, while with the crop in substrate beds, either with or without nutrient solution recycling, the harvest was at 29 dat. This again shows the greater earliness with which lettuce is obtained in the floating root system compared to the other two systems evaluated, as well as a more favorable environment for the root (uniform temperature, constant moisture and nutrient availability without drastic changes in electrical conductivity).

In the first cycle the temperature of the root environment, for systems with and without nutrient solution recycling, ranged from 14 to 28 °C with an average of 20 °C, while with the floating root it ranged from 18 to 24 °C with an average of 21 °C. According to Jaques and Hernández (2005), the optimal temperature for growing lettuce is from 18 to 23 °C, a condition given by the floating root system.

The same occurred in the second assessment cycle, only in this case the recorded temperatures were slightly higher compared to those of the first cycle. It is noteworthy that the temperature of the root in the floating root system was very uniform and favorable (daytime average of 20 °C) over both crop cycles.

Lettuce grown in the spring reached, at the end of the cycle, an average fresh weight of 187.3 g for the three systems evaluated, while the average summer weight was 222.5 g. The first weight was lower than that reported by Stepowska and Kowalczyk (2001) in butter lettuce grown in the spring under rockwool (236 g), but when the lettuces were established in the autumn the fresh weight was lower (136 g). According to Valverde, Chang, and Rodriguez (2009), this behavior is because the amount of incident light changes from cycle to cycle which affects the activity of enzymes such as nitrate reductase.

The differences in lettuce weight obtained in the two crop cycles also coincides with the findings of Fallovo et al. (2009), who established that plants growing in the spring season have less growth and yield compared to summer plants, but higher leaf quality is achieved; Cracker and Seibert (1983) grew lettuce under controlled conditions and found that the time required to obtain commercial-sized plants decreases with increasing incident solar radiation.

With the fresh weight per plant achieved in this second cycle (219 g per plant in the T1 and T2 treatments and 228 g per plant in the T3 treatment), and given the population density established (32 plants∙m^-2), a yield equivalent to 7.0 kg∙m^-2 is obtained in treatments with and without nutrient solution recycling over a 29-day period from transplanting to harvest end, compared to 7.3 kg∙m^-2 with the floating root system over a 24-day period. With the latter system, the harvest is achieved in less time, allowing more cycles per year and therefore greater annual productivity. Based on the observed changes in EC in the different treatments, it was expected that growth in the bed treatments with substrate would be affected (Savvas et al., 2009), but the final yield results show that this did not happen because the lettuce was harvested within 40 dat, which contributed to the EC of the nutrient solution not increasing to levels affecting growth.

 

Water and fertilizer expenditure and savings

With the nutrient solution recycling and floating root systems, there were significant savings in nutrient solution, which implies savings in water and fertilizers (Table 3).

In the crop established in the spring, the floating root treatment allowed a 40.7 % water savings and the substrate bed system had a 24.7 % savings, compared to the substrate bed system without recycling in which no savings were obtained (Table 3). Similar results were obtained by Dhakal et al. (2005) and Sánchez et al. (2014) in evaluating open and closed hydroponic systems.

In the summer, although water savings were lower, it was also the floating root system where a saving of 33.6 % was achieved as shown in Table 4.

In both crop cycles, the floating root system also had lower water consumption per plant and increased efficiency in terms of liters of water required per kilogram of fresh lettuce obtained.

The results of the use of nutrients show that the substrate bed system without recycling had an expenditure of 17.5, 14.0, 14.0 and 4.2 g∙m^-2 of potassium, calcium, nitrogen and phosphorus, respectively. With the recycling and floating root systems, the expenditure was lower compared to the non-recycling system, but the floating root system achieved the greatest savings and efficiency gains in terms of grams of nutrient required per kilogram of lettuce grown (Tables 5 and 6).

According to Gonella, Seiio, Conversa, and Santamarina (2003), systems based on the use of solid substrates lose water more rapidly than salts and as a result the pH increases, which promotes NO₃^- uptake. Fallovo et al. (2009) indicate that the consumption of nutrients by butter lettuce in a nutrient recycling system per day per plant throughout the year averages from 8 to 16 mg of NO₃^-1, from 2 to 5.5 mg of P₂O₅ and from 11.5 to 23 mg of K₂O⁺, results that partially match those of the present experiment.

The present results are important both economically and ecologically, especially considering that in intensive commercial lettuce production it is possible to obtain up to 10 production cycles per year, due to the crop's short cycle. Thus, in the substrate bed systems without nutrient solution recycling, about 10,000 m³∙ha^-1∙year^-1 of water is required, while in the floating root system only 6,000 m³ would be needed, which is a significant difference, especially in conditions where water is limited.

On the other hand, in the system without nutrient solution recycling, based on averaging the two cycles evaluated and only to provide potassium, approximately 3,725 kg per hectare of potassium sulphate per year is used. At a current cost of $20 MXN∙kg, an investment of $74,000.00 MXN is required, while the floating root system would use 2,186 kg of potassium sulphate, resulting in an investment of $43.720 MXN (a savings of $30,000.00 MXN∙ha^-1∙year¹ only in this nutrient). Doing a similar calculation to provide calcium, nitrogen and phosphorus, in addition to potassium, shows that a non-recycling system requires an investment of $139,000 MXN∙ha^-1∙year^-1 versus $52,000 MXN with the floating root system. In addition to the economic advantage achieved with the latter system, there is less contamination by salts that are deposited into the water table and eventually contaminate seas and rivers, as occurs when open systems are used (Massa et al., 2010).

Therefore, despite the technical difficulty and high initial cost involved in installing a floating root system, it offers important advantages that make it economically and ecologically viable for greenhouse lettuce producers.

 

CONCLUSIONS

The floating root system obtained better growth and earliness, and more efficient water and nutrient use compared to the growing bed systems with substrate.

Lettuce growth and yield were similar between substrate bed systems with and without nutrient solution recycling. The difference between them was the more efficient use of water and nutrients in the latter system.

 

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