Mineral Composition of Lettuce Grown in Hydroponic System With Wastewater

The use of treated effluents is not a new practice in agriculture, however, the optimization of wastewater was given with mineral fertilizers to grow lettuce in a hydroponic system, subject that is still barely studied. The objective of this study was to evaluate the mineral composition of three lettuce cultivars (Verônica, Vanda and Thais) in a hydroponic system using wastewater, well water and optimized nutrient solutions. The plants was grown in seven nutrient solutions, as S1 = Furlani solution; S2 = domestic wastewater; S3 = optimized domestic wastewater; S4 = well water; S5 = optimized well water; S6 = wastewater UASB and S7 = optimized UASB wastewater and the sub-plot for three lettuce cultivars. It was verified that the treatments S2, S4 and S6 when compared with the respective optimized solutions S3, S5 and S7 presented lower levels of nitrogen, potassium, calcium, zinc, copper and manganese in the two experiments. Same behavior was not observed for phosphorus and sodium. As for the cultivars, they presented, regardless of the experiment, mineral composition similar to each other when the same nutrient solution was used.


Introduction
Lettuce (Lactuca sativa L.) is the most widespread leafy vegetable currently cultivated in almost all countries.Because of its ease of cultivation, low production cost and ease of commercialization, lettuce is grown for both commercial and subsistence purposes (Correia, 2013).In Brazil, crisp-type lettuce cultivation leads the market with 70% of production, while the Iceberg type owns 15%, lettuce (10%, and the others correspond to 5% of the market (Sala & Costa, 2012) ,and occupies an area of approximately 35.000 hectares being as much by the intensive production, as by familiar producers (Sousa et al., 2014).
Lettuce is produced in hydroponic solution has advantage comparing with that produced in the open field, because the leaves are not irrigated, reducing the incidence of diseases and also because the water used in the nutrient solution has a simpler quality control (Lopes, Duval, & Reis, 2010).Some studies developed with NFT technique ("Nutrient Film Technique"), hydroponic system or in culture with inert substrate has been using wastewater mainly for lettuce.In hydroponics, plants receive a balanced nutrient solution containing water and all essential nutrients for plant development (Santos et al., 2012).
With the use of the treated sewage it is possible to make possible a new source of water and nutrients available for application in agriculture, even during drought periods (Shaer Barbosa et al., 2014).On the other hand, the safety of reuse of wastewater for irrigation purposes is a matter of caution.In general, the current regulations for the reuse of treated sewage specify the microbiological quality standards, considering the minimum treatments required for the effluents, the type of crop to be irrigated and the irrigation methods used.the water bodies, improving the potability conditions, allowing a more rational use of water resources, being an alternative source of available water (Martínez et al., 2013).Thus, the use of treated sewage in agriculture can become an important tool in water management.
It should be noted that the domestic sewage effluent contains high concentrations of macro and micronutrients.The availability of this material for the soil in the form of fertirrigation can supply part of the nutritional needs of the crops, reduce the fertilizer costs avoid the disposal in water bodies, besides allowing better use by plants,, due to the installment of fertilization (Souza et al., 2012;Andrade Filho et al., 2013).When assessing the agricultural potential of effluent domestic sewage, Carr et al. (2011) found that on average the effluent used on farms in Jordan replaces up to 75% of fertilizer needs.Many wastewater exhibit high concentrations of nitrogen, phosphorus and potassium and organic material (Oliveira, 2012).On the other hand, the excess of some elements in the effluent can also reduce crop productivity (Hanjraa et al, 2012).
In this context the use of wastewater in addition to minimizing environmental problems can reduce the use of mineral fertilizers and clean water in irrigated agriculture in semiarid regions.This reduction in the use of mineral fertilizers is possible due to the high concentration of nutrients in wastewater (Varallo et al., 2012), since it is handled judiciously (Matos et al., 2013).Giving this conjecture, the plants play a significant role, extracting macro and micronutrients provided by the wastewater, necessary for the growth avoiding accumulation, the consequent salinization of the soil and the contamination of surface and groundwater (Ribeiro et al., 2009).
The use of treated wastewater in agriculture is essential, not only to serve as an extra source of water, but also as a source of nutrients for crops.In this context, it was objectified with this work, evaluate the mineral composition of three lettuce cultivars (Verônica, Vanda and Thais) in a hydroponic system using wastewater, well water and optimized nutrient solutions.

Material and Methods
This experiment was conducted in a hydroponic system using Nutrient Film Technique (NFT), in protected environment, a Greenhouse of the State University of Paraíba, Campus II, in the municipality of Lagoa Seca-PB.
Regarding the experimental design: were used randomized blocks with treatments arranged in subdivided plots, with three replications.The plots were the hydroponic solutions with conductivity of 1.7 dS m -1 ; S 1 = Furlani solution; S 2 = domestic wastewater; S 3 = optimized domestic wastewater; S 4 = well water; S 5 = optimized well water; S 6 = wastewater solution from the Upflow Anaerobic Sludge Blanket (UASB) reactor and S 7 = optimized wastewater solution from the UASB reactor and the subplot for three lettuce cultivars; Verônica, Vanda e and Thais.Each subplot was composed of six plants (two plants of each cultivar) with spacing of 0.30 m × 0.30 m.
The seeds of lettuce curly cultivars were sowed in phenolic foam using a table for germination.After emergence of the seedling (ES), the supply water used in irrigation was gradually replaced by nutrient solutions (33.33%, 66.66% and 100% every four days).After 25 days of ES, the seedlings were transplanted to the definitive profiles (the gutters used for the hydroponic system. Optimized nutrient solutions were formulated with reference to the nutritional solution of Furlani 1999 (Table 1).-------------------------------------------g ----------------------------------------- The water used in the experiment came from rainwater stored in cistern (for the solution S 1 ), the raw sewage of the city of Lagoa Seca-PB, tubular well water from the rural area of the municipality Lagoa Seca-PB and wastewater from the UASB reactor of the Experimental Station of Biological Treatment of Sanitary Sewers (EXTRABES) Campina Grande-PB.They were sent for physical-chemical analysis in the Laboratory of Irrigation and Salinity (LIS/DEAg/UFCG), the physical-chemical characterization of the waters used in this work is shown in Table 2.
Optimized nutrient solutions S 3 , S 5 and S 7 were prepared according to methodology proposed by Monteiro Filho et al. (2014), in order to present chemical composition similar to the nutritional mineral solution of Furlani (1995).The amount of ingredients necessary for the preparation of such solutions is described in Table 3.The management of the nutritive solutions were performed daily through the replenishment of the water consumed, along with the monitoring of the electrical conductivity (EC) and pH keeping it close to neutrality, with the use of a solution of NaOH ou HCL (1 mol L -1 ).
During the conduction of the experiment the S 1 and optimized solutions were calibrated by conducting electrical conductivity readings (EC) and using a portable conductivity meter, in addition to a single parameter.EC was maintained with approximately 1.7±0.3dS cm -1 and the pH between 6.5±0.5.Nutrient solutions were renewed every seven days.
For the analysis of the mineral composition of the plant tissue of cultivars of crisp lettuce, composite samples were prepared considering six plants/plot, depending on the cultivars and solutions.
For the analysis of the mineral composition of the plant tissue of cultivars of crisp lettuce, composite samples were prepared considering six plants/plot depending on the cultivars and solutions.The nitrogen was analyzed by means of distillation in Kjeldhal Microdistiller, phosphorus spectrophotometry by Visible Ultraviolet Spectrophotometry (UV-VIS), Sodium and potassium using flame photometer and for the other minerals the readings were made through atomic absorption spectrometry (Agilent Technologies 200 series AA).The analyzes were carried out in the Laboratory of Analysis of Soil, Water and Plant of the Agricultural Research Company of Rio Grande do Norte S/A-EMPARN, determined through the methodology proposed by Embrapa (1999).

Results and Discussion
During the cultivation no symptoms of mineral deficiency were observed in lettuce plants.One of the benefits of the NFT system for plants is the passage of the nutrient solution only through the root zone, without causing wetting of the leaves.Macro and micronutrient contents of aerial part of cultivars of curly lettuce Thaís, Vanda e Verônica the 30 days after transplant (DAT) in the first experiment and the 35 days after transplant (DAT) in the second experiment submitted to the different treatments are presented in the following figures.

Table 1 .
Quantitative of the mineral fertilizers used in the preparation of mineral nutritive solutions

Table 2 .
Physical-chemical characterization of waters used in hydroponic irrigation

Table 3 .
Quantitative of the ingredients used in the preparation for the optimized nutrient solutions from the physicochemical characterization of the waters used in hydroponic irrigation