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Productive performance and quality of arugula (Eruca sativa) under different doses of cassava wastewater containing potassium source

Cassava wastewater has high organic load and concentration of nutrients and can cause damage to the environment when discarded indiscriminately. However, the residue can help agricultural production. Therefore, this study aimed to determine the appropriate doses of cassava wastewater to enable the highest production and quality of arugula. The experiment was conducted in a greenhouse, in a completely randomized design, consisting of eight treatments composed of the following doses: D1 = 0, D2 = 6.5, D3 = 13, D4 = 19.5, D5 = 26, D6 = 32.5, D7 = 39 and D8 = 45.5 m 3 ha -1 of cassava wastewater with four replicates each. We used the arugula cultivar “Cultivada”, and samples of plant height, fresh matter, and shoot dry matter were performed at 15 and 30 days after emergence. The quality of arugula was evaluated through pH, total soluble solids and titratable total acidity. Cassava wastewater had a significant effect on plant height, fresh matter and dry matter of above ground part, pH and total soluble solids of arugula. However, titratable total acidity was not affected by treatments. Our results show that cassava wastewater is a good alternative fertilizer for arugula cultivation but at proper doses up to 26 m 3 ha -1 applied through the crop cycle to avoid the deleterious effect of some nutrients in excess, such as potassium.


Introduction
Arugula (Eruca sativa) is an herbaceous hardwood crop of the Brassicaceae family (Silva et al., 2009), has rapid vegetative growth, short cycle, and high yield per area . This plant needs mild temperatures, varying from 15 to 25 ºC (Silva et al., 2009;Costa et al., 2011). The production period begins in the emergence of the seedlings and extends until the floral initiation, corresponding to its economically viable production (Jardina et al., 2017). Brazilian production of arugula in 2017 was 41,778 tons, and the state of São Paulo was the largest producer (18,548 tons), followed by Rio Grande do Sul (4,819 tons) and Paraná (3,276 tons) (IBGE, 2017). Production of the state of Alagoas was only 107 tons, that is, the 22nd producer of arugula in Brazil (IBGE, 2017). The state of Alagoas presents favorable edaphoclimatic conditions for the cultivation of arugula, mainly in the region "Zona da Mata", which has an annual average temperature of 25.4°C and an average rainfall of 1,800 mm year -1 (Magalhães et al., 2017). Therefore, the low production of arugula in Alagoas is probably related to the management conditions. Family farmers produce arugula and have low purchasing power and lack of access to more rational techniques of soil and water use. The proper fertilization in these systems can boost production. However, chemical fertilizers represent the main cost of production , and the use of fertilizers without proper management contributes to environmental pollution . Thus, the use of organic waste in agriculture seems an excellent alternative to mitigate these problems (Moura et al., 2018).
Cassava wastewater is one of the main residues obtained during the production of starch or flour from cassava (Manihot esculenta Crantz) (O-Thong et al., 2011;Dantas et al., 2017), contains high levels of carbohydrates, sugars, lipids, cyanuric acid, starches, and others substances (Dantas et al., 2015;Andreani et al., 2019). The residue is also rich in macro and micronutrients, essential for the growth and development of plants (Avancini et al., 2007;Moura et al., 2018). However, cassava wastewater should be used carefully since it is harmful to the environment due to the high organic load and linamarina, which is a cyanogenic glycoside of high toxicity (Shang et al., 2018). However, the proper use of the residue does not lead to environmental contamination and enables agricultural production (Bezerra et al., 2019). Thus, its use might be a source of organic fertilizer with high potential to replace synthetic fertilizers. The use of cassava wastewater as a source of nutrients has promoted promising results during the cultivation of several crops. For example, Santos et al. (2010) found an increase in the number of leaves and leaf area index of lettuce using wastewater cassava as fertilizer, and Duarte et al. (2012) found higher plant height, leaf area, and fresh and dry matter production. Barreto et al. (2013) observed an increase in the production of fresh matter, and nitrogen and potassium contents of the above ground part of maize plants fertilized with cassava wastewater, and Dantas et al. (2015) reported the increases in plant height, number of leaves, fresh matter and leaf dry matter in sunflower cultivation. To reduce the damages to the environment by the incorrect disposal of cassava wastewater, and to supply the nutritional needs of crops reusing cassava wastewater decreasing the production costs with the use of chemical fertilizers, this study aimed to determine the adequate doses of cassava wastewater to enable the highest production and quality of arugula.

Plant height
Cassava wastewater had a significant effect (P <.001 and P = .05) on plant height at 15 (15 DAE) and 30 days after the emergence (30 DAE). Figure 1 shows that the mean values of plant height under increasing doses of cassava wastewater showed a quadratic adjustment, and the plants that received this residue presented higher plant height, which ranged from 0.1 and 30.6% when compared to control treatment (0 m 3 ha -1 ). This behavior was probably due to the higher nutrient supply to the plants submitted to the treatments containing cassava wastewater (Avancini et al., 2007;Dantas et al., 2015;Araújo et al., 2017 ), respectively. The decreasing in plant height at doses above the inflection point may be related to the deleterious effect of some nutrient, such as potassium, since it is the element of greatest occurrence in cassava wastewater, and when in excess impairs the absorption of other essential nutrients by the plant (Duarte et al., 2012;Andreani et al., 2019).
The highest plant height (18.9 cm) was found at the dose 26.5 m 3 ha -1 30 DAE, which was higher than the value observed by Carvalho et al. (2012) (16.3 cm) in arugula crop under conventional fertilization, and close to the 21 cm found by Linhares et al. (2011) for the cultivation of arugula under organic fertilization. Duarte et al. (2012) evaluated different doses of cassava wastewater and found a higher height (18.7 cm) of lettuce at the treatment of 45 m 3 ha -1 , which was 31% higher than that observed in plants that did not receive this organic residue. Several authors have already noted the fertilizer effect of cassava wastewater in different crops (Santos et al., 2010;Duarte et al., 2012;Dantas et al., 2015Dantas et al., , 2017Bezerra et al., 2019).

Accumulation of fresh matter and dry matter of the above ground part
Statistical analysis revealed significant effects (P < 0.001) of cassava wastewater on fresh and dry matter of the above ground part of arugula. The regression equations adjusted to the experimental data of these variables presented quadratic behavior (Figs 2A and 2B). Our data show that the fresh matter increased with cassava wastewater doses up to the estimated limit of 22.9 m 3 ha -1 , a gain of 28.6 and 3.5% when compared to the treatments in the absence of the residue and the dose of 26 m 3 ha -1 (100% of the potassium recommendation). There was an increase of dry matter up to the estimated dose of 20.3 m 3 ha -1 of cassava wastewater, reducing dry matter at higher doses. We verified gains in the dry matter in the order of 22.9 and 7.5% concerning the control (0 m 3 ha -1 ) and the recommended dose (26 m 3 ha -1 ), respectively. The influence of cassava wastewater doses on the biomass production of arugula was expected, since the height of the plant was significantly affected by the treatments (Fig. 1), which would consequently influence the accumulation of fresh and dry matter. Cassava wastewater is rich in macro and micronutrients (Barreto et al., 2014;Shang et al., 2018), which are essential for the growth and development of crops. Our results agreed with Duarte et al. (2012) when they observed an increase in the fresh and dry above ground mass of lettuce as cassava wastewater doses increased, reaching 45 m 3 ha -1 and declining after that level. Also, for lettuce, Santos et al. (2010) observed similar behavior of biomass production increased up to the limit of 396 mL per vessel, decreasing after this value. The decreases in fresh and dry matter observed at the doses above the maximum response point may be related to the excess of potassium in the soil solution, promoted by the higher doses of the residue, and when this element is in excess, it compromises the absorption of the other essential elements, especially calcium, iron, magnesium, manganese and zinc (Avancini et al., 2007;Dantas et al., 2015). When that nutrient absorption is compromised, there is a reduction in the plant biochemical processes since they are associated with several enzymatic functions in plants, such as the synthesis of chlorophyll (Taiz et al., 2017). Therefore, our results confirm the fertilizer benefits of cassava wastewater due to the essential nutrients for plant development, such as potassium, magnesium, phosphorus, and calcium. According to Barreto et al. (2013), cassava wastewater has a high potential for reuse in agriculture in the form of fertilizer, but it should be used rationally since it can cause environmental degradation when in excess (Table  1).       ) compared to plants that did not receive the cassava residue. The increase in pH may have occurred due to the increase of the organic acids present in the plant, possibly related to the greater nutrient supply by the cassava wastewater, and consequently increasing in ions absorption by the plant (Santos et al., 2010;O-Thong et al., 2011;Moura et al., 2018). According to the regression analysis (Fig. 3B), the averages of total soluble solids (ºBrix) presented a quadratic adjustment with cassava wastewater doses. The estimated optimal dose occurred at 24.5 m 3 ha -1 promoting the highest value of TSS 2.84 (°Brix), with reductions at doses above this level. We observed an increase of TSS of 46.8 and 4.9% concerning the treatments without organic fertilizer and with the recommended dose (26 m 3 ha -1 ), respectively. Increases in total soluble solids up to the maximum response dose of cassava wastewater probably occurred due to the improvement of the physical, chemical and biological properties of the soil caused by this organic residue. In organic cultivations, soil fertility increases by the chelation effect of the organic molecule complex present in the organic fertilizer (in this case, cassava wastewater), resulting in greater solubilization of the nutrients and mobilization to the plant systems, consequently, the plants become nutritionally balanced Dantas et al., 2017;Bezerra et al., 2019). The effects of reductions from the optimal dose of cassava wastewater observed in the present study should be related to the increase of the microbial population, due to the greater availability of the organic residue, causing greater consumption of available nutrients and consequently reduction their availability for plants (Barreto et al., 2013;Araújo et al., 2017;Shang et al., 2018). And, the decrease in total soluble solids may have occurred in response to the toxic effect on plants, promoted by excess nutrients at higher doses of cassava wastewater (Avancini et al., 2007;Santos et al., 2010;Dantas et al., 2015;Andreani et al., 2019). Our results are close to the values found by Campos et al. (2013) when they observed pH of 4.8 and 4.6, total soluble solids of 3.72 and 3.26 ° Brix for arugula cultivations under organic and mineral fertilization, respectively. Nunes et al. (2013) found a pH of 5.76 and TSS of 3.32°Brix for arugula cultivated in organic systems. Jardina et al. (2017) found significant effects of pH values between arugula varieties in a semi-hydroponic system, with data varying from 5.66 to 5.81, but it was not observed a significant effect on total soluble solids, which varied from 3.45 to 3.54° Brix. Using mineral fertilization, Vasconcelos et al. (2011) found pH values and total soluble solids of 5.68 and 4.92°Brix for arugula "Cultivada", and TSS was 42.3% higher than that found in the present study.

Plant materials
The experiment was conducted from May to August 2018, in a protected environment at the Agricultural Sciences Center (CECA) belonging to the Federal University of Alagoas (UFAL), in Rio Largo, state of Alagoas, Brazil (09°28'02" S, 35°49'43" W and altitude of 127 m). The climate of Rio Largo -AL, according to the classification of Köppen, is tropical sub-humid, with two well defined seasons, dry and rainy periods, with an average annual temperature of 25.4 ºC and an average rainfall of 1,800 mm year -1 (Magalhães et al., 2017).

Treatments and experimental design
The treatments were determined taking into account the concentration of potassium (K) in cassava wastewater and soil, as well as the potassium requirement for arugula crop, which is 26 m 3 ha -1 (IPA, 2008). The treatments consisted of eight doses of cassava wastewater D 1 = 0, D 2 = 6.5, D 3 = 13, D 4 = 19.5, D 5 = 26, D 6 = 32.5, D 7 = 39, and D 8 = 45.5 m 3 ha -1 , corresponding respectively to 0; 25; 50; 75; 100; 125; 150 and 175% of K recommendation for arugula crop (IPA, 2008). Each treatment was replicated four times totalizing 32 experimental units. The experimental design was completely randomized. The treatments were added to the pots where the plants were cultivated using a test tube nine days before transplanting seedlings.

Conduction of study
We used a Coarse Yellow Latosol soil, collected at 0-40 cm depth. To carry out the chemical and physical characterization of the soil, ten samples were performed for the chemical and physical characterization of the soil, using an auger, air dried, smashed and sieved in a 2 mm mesh, then a composite sample was formed. The physical and chemical characterization of the soil (Table 2) was performed at the Laboratory of Soil and Plant Nutrition, at UFAL, according to the methodology proposed by Embrapa (1997). Plant cultivation was carried out in pots of 9L filled with 2 kg of gravel (number zero), covering the base of the pot, and a layer of soil above. The soil was placed in field capacity, using the capillary saturation method and free drainage. After sowing, daily irrigations were performed. The water volume applied obeyed the water requirement of the plant, determined by the water balance, the volume applied minus the volume drained in the previous irrigation. The variety used in the experiment was "Cultivada" arugula (Eruca sativa Miller), which is the main variety in Brazil and has high adaptability to the conditions of the Northeast region. Arugula was sown in double-row vessels with spacing of 5 cm between plants. All treatments received a covering of limestone to raise the saturation by bases (V%) to 80%, 400 kg of P 2 O 5 ha -1 (super triple) and 40 kg of N ha -1 (Ammonium Sulphate) at 10 and 15 days after germination. Cassava wastewater was provided by a flour house located in the area of São Sebastião, AL. After application of cassava wastewater, all the pots were incubated for 9 days. The physical and chemical characterization of cassava wastewater (Table 3) was performed at the Laboratory of Analysis of Agricultural Products (LAPA) of UFAL. The determination of the physical and chemical parameters was performed according to the methodology described by MAPA (1988).

Variables analyzed Plant height (PH)
Growth evaluations were performed by plant height in cm, at 15 and 30 days after emergence (DAE), measuring from the soil level to the leaf end.

Accumulation of fresh matter and dry matter of the above ground part
At 30 DAE the plants were harvested and the accumulation of fresh matter of the above ground part (FMA) in g was obtained from the plants collected from each vase, using a balance with digital precision. Afterwards, the material was placed in an air circulating oven for drying at 65ºC until obtaining a constant mass, obtaining the dry matter of the above ground part (DMA) in g.

Quality characteristics of arugula after the harvest
To evaluate the quality characteristics of arugula after the harvest, we measured the total soluble solids (SST) -by the refractive index in portable refractometer, total titratable acidity (ATT) by neutralization reaction with NaOH 0.1N and phenolphthalein as indicator solution; and pH by direct reading the juice through previously calibrated pH meter.

Statistical analysis
To test the difference among treatments, we used the analysis of variance by the F test (P ≤ 0.05), and in the case of significance, a polynomial regression analysis was performed to unfold the effects of cassava wastewater doses. We used the statistical software SISVAR (Ferreira, 2014).

Conclusion
Our results show that the use of cassava wastewater as an alternative to chemical fertilization for arugula cultivation increase the plant height, fresh and dry above ground matter, pH, and total soluble solids contents. The data also show that cassava wastewater is a good alternative fertilizer for arugula cultivation but at proper doses up to 26 m 3 ha -1 applied through the crop cycle to avoid the deleterious effect of some nutrients in excess, such as potassium.