Improving sweet pepper yield and quality by means of fertigation management

Fertigation can be managed using fixed fertilizer doses or by assessing the ionic concentration of soil solution throughout the growing period. This work studied how different fertigation management systems affected sweet pepper yield and quality. The experiment was carried out in greenhouse, in pots. Fertigation was managed according to the crop uptake rate (M1) or by monitoring either the electrical conductivity (M2) or the N and K concentration (M3) in the soil solution. Fertigation management was combined with six N and K doses (0, 50, 100, 150, 200 and 300% of the recommended dose for sweet pepper in the region), in complete blocks at random, with four replications, and treatments in 3x6 factorial. Dry matter accumulation, yield and fruit grading were evaluated. Fertigation managed by means of monitoring the soil solution improved the vegetative growth in up to 25% and increased yield in up to 20% when compared to management according to the uptake rate. Highest fruit yields in M1, M2 and M3 were achieved with N and K levels corresponding to 127.6% (1.33 kg/plant), 222.5% (1.60 kg/plant) and 215% (1.48 kg/plant), respectively. N and K can be supplied successfully to sweet peppers using electrical conductivity or concentration of ions in the soil solution to manage fertigation. These management systems resulted in high quality fruits and up to 47% increase in N and K use efficiency. N and K concentration equivalent to 200% of the recommended for growing sweet pepper in hydroponics should be taken as reference.


Research
Hortic.bras., v35, n.2, Apr.-Jun.2017 S weet peppers (Capsicum annuum)   are among the leading vegetables in Brazil, both in terms of production and consumption.Figures are around 13 thousand hectares used for production and an estimate of 290 thousand tons of sweet peppers harvested (Marouelli & Silva, 2012).The state of São Paulo is the main producer, with an annual estimate of about 80 thousand tons and an area of 2.4 thousand hectares (IEA, 2015).Sweet peppers are grown predominantly in the open field, but protected cultivation has been expanding practically all over the country (Oliveira et al., 2015).
Fertigation is very usual in protected cultivation, especially when localized irrigation systems are in place (Oliveira & Villas Boas, 2008).Fertigation scales up input use in many irrigated crops

Received on October 13, 2016; accepted on April 10, 2017
in aspects related to both productivity and product quality.Nitrogen and potassium are the nutrients more often applied via irrigation water, since both fit perfectly into the system due to their high mobility in the soil and solubility of almost 100% in water.Moreover, and specifically when sweet peppers are considered, N and K are the nutrients exported in the largest amounts (Marcussi et al., 2004;Fontes et al., 2005).N is related to photosynthesis, respiration, root development and activity, ionic uptake of other nutrients, and cell growth and differentiation.K improves fruit physical and chemical characteristics by optimizing processes such as transpiration and carbohydrate synthesis (Taiz & Zeiger, 2009).
For fertigation to be efficient, a balance between the amount of nutrients and the volume of water applied at each stage of the crop cycle must be found.Water and nutrient availability should be close to the optimal for crop development and yield (Blanco & Folegatti, 2002).Excessive application of fertilizers causes physiological disorders, such as drops in fruit yield and quality, nutritional imbalance and increases in osmotic pressure (Epstein & Bloom, 2006).
Fertigation is traditionally managed by supplying pre-established fertilizer amounts, split according to the crop uptake curve.It is not usual to monitor the ion concentration in the soil solution or the plant nutritional s t a t u s ( P a p a d o p o u l o s , 1 9 9 9 ) .Medeiros et al. (2012), working with tomatoes in protected cultivation, found that fertigation management as recommended by literature may lead to an overestimation of fertilizer requirements, drastically reducing the nutrient efficiency use by plants.
In addition to the adequate fertilizer dose, another key factor is how fertilizers are supplied throughout the crop cycle (Oliveira et al., 2013).Instead of pre-established doses, it is wiser to rationalize fertigation management by determining the electrical conductivity and/or partial concentration of ions in the soil solution, allowing for necessary adjustments along the crop cycle (Dias et al., 2005).In this framework, several researchers have been obtaining very satisfactory results using porous cup soil-water extractors, either to monitor the electrical conductivity (Medeiros et al., 2012;Silva et al., 2013) or the concentration of ions in the soil solution (Silva Júnior et al., 2010;Oliveira et al., 2013Oliveira et al., , 2015)).However, studies comparing these two fertigation management systems are scarce in literature.
Given the aforesaid, the present work was carried out to evaluate fertigation management in sweet peppers grown in protected cultivation, with emphasis on biomass production and fruit yield and grading.

MATERIAL AND METHODS
The experiment was carried out from June to November 2010, in greenhouse, in the Rural Federal University of the Semi-Arid, in the city of Mossoró, state of Rio Grande do Norte, Brazil (5º11'S, 37º20'W, altitude 18 m).Air temperature and relative humidity during the experimental period were measured with a digital thermo-hygrometer (Mod.30.5000.02,TFA Technology HK), installed 1.5 m from the ground, in the middle of the greenhouse.The experiment was set in complete blocks at random, with treatments in 3x6 factorial, and four replications.Experimental plots consisted of 25 L pots, one plant per pot.Pots were arranged over 10-cm high masonry bricks and distributed in four rows, with 1.5 m spacing between rows and 0.50 m between pots, resulting in a density equivalent to 13,333 plants/ ha.
Treatments consisted in combining three fertigation management systems (M 1 )= crop uptake rate (Table 1); (M 2 )= monitoring of the soil solution conductivity, and; (M 3 )= monitoring of the concentration of N and K ions in the soil solution and six N and K proportional levels (N 0 K 0 , N 50 K 50 , N 100 K 100 , N 150 K 150 , N 200 K 200 and N 300 K 300 ).In M 2 and M 3 management systems, N and K were supplied whenever electrical conductivity levels (M 2 ) or N and K concentration (M 3 ) were reduced by 20% in relation to initial values.For the M 1 management system, N 100 K 100 level corresponded to the N and K recommended doses for sweet pepper in the region of Mossoró, namely 215 and 314 kg/ha of N and K, respectively, distributed throughout the cycle (Freitas, 2009).For M 2 and M 3 management systems, N 100 K 100 level corresponded to the recommended N and K concentration for sweet peppers in NFT hydroponic system, namely 152 and 245 mg/L for N and K, respectively (Castellane & Araújo, 1994).
Levels of electrical conductivity (M 2 ) and N and K (M 3 ) refer to the concentration of these nutrients in the soil solution, collected through porous cup extractors.Adjustment curves obtained in previous calibration (Oliveira et al., 2013) were used to correlate both the N and K doses to be used in M 2 and M 3 management systems to electrical conductivity values.The following levels were established: 0.6; 1.6; 2.2; 3.1; 3.8 and 4.4 dS/m, for N and K (N 0 K 0 , N 76 K 123 , N 152 K 245 , N 228 K 368 , N 304 K 490 and N 456 K 736 , mg/L).
The sweet pepper hybrid Atlantis (Topseed), one of the most popular in the region, was used.Hybrid Atlantis is adapted to both field and greenhouse conditions, has high yielding potential, 120-day average cycle and tolerance to the Pepper Yellow Mosaic Virus (PepYMV).Fruits are rectangular, dark green before maturation and red-intense when ripe (Agristar).We planted 35-day old seedlings, purchased from a nursery in the city of Icapuí, in the neighboring state of Ceará.
The drip irrigation system consisted of 1.000 L water tank installed on 1.0 m high wooden bench, polyethylene lateral lines (16 mm), manual shut-off valves, 0.5 m long microtube (spaghetti) emitters, 1 mm internal diameter, and 1.76 L/hour (± 0.1 L/hour) average flow rate.The volume of water used for irrigation was determined from the soil-water tension, measured by 15 cm depth tensiometers.Soil-water tension was converted to humidity using a soil-water retention curve [U = 124.53T -0.9419 , where U= current soil moisture (kg/kg) and T= soil-water tension (kPa)].Irrigation was carried out applying the volume necessary to raise soil moisture to field capacity.In addition to tensiometers, 15 cm depth solution extractors were also installed in each pot to collect and monitor soil solution.Nutrient concentration in the solution collected by extractors was corrected to restore the same moisture as at field capacity.
Top fertilizations were carried out via fertigation throughout the crop cycle.For management M 1 , fertigation took place every four days, according to the crop uptake rate.Fertilizer solutions were applied individually in each pot, without wetting plant stems, using a 500 mL beaker and the same volume in all pots.Nutrient sources were calcium nitrate, potassium chloride, potassium monobasic phosphate, potassium nitrate, sodium nitrate, calcium chloride, magnesium sulfate and phosphoric acid.In management systems M 2 and M 3 , fertigation were carried out according to the analysis of the solution collected with extractors.Fertigation took place whenever electrical conductivity (M 2 ) or N and K concentration (M 3 ) were reduced by 20% in relation to the initial value, applying the volume necessary to raise soil moisture to field capacity.P, Ca, Mg and S were applied regularly in the control treatment, without N and K (N 0 K 0 ), to limit plant response to variations in N and K levels.Accumulated N and K amounts (g/plant), corresponding to the applications during and until the end of the experiment are presented in Table 2.
Six harvests were carried out during the experiment, the first 60 and the last 120 days after planting (DAT), with harvests occurring as fruits reach maturation.Fruits were harvested when bright dark green and at commercial size (CEAGESP, 2014), i.e., length and diameter greater than 60 and 40 mm, respectively.In all harvests, fruits were graded by length (cm): class 4 (from 4 to 6 cm), class 6 (from 6 to 8 cm), class 8 (from 8 to 10 cm), class 10 (from 10 to 12 cm), class 12 (from 12 to 15 cm), class 15 (from 15 to 18 cm), class 18 (from 18 to 21 cm), class 21 (from 21 to 24 cm ) and class 24 (greater than 24 cm) (CEAGESP, 2014).Fruit length and diameter were measured with digital caliper.At the end of the experiment (120 DAT), plants were collected, separated in stems, leaves and fruits, and dried in oven with forced air circulation, 65°C, until constant mass.Dry mass of each part was determined using a precision scale (0.01 g) and total dry mass was calculated by summing all parts.
Data were submitted to analysis of variance, applying F test and unfolding the significant interactions.Regression analysis was applied to the quantitative factor, N and K levels.For the qualitative factor, fertigation management systems, means were separated by Tukey test, p>0.05.Fruit grading was analyzed by descriptive statistics, studying the distribution frequency in classes.Statistical analyzes were performed using the statistical software Sisvar 4.1 (Ferreira, 2008).

RESULTS AND DISCUSSION
F e r t i g a t i o n m a n a g e m e n t b y monitoring either the electrical c o n d u c t i v i t y ( E C ) o r t h e i o n concentration in the soil solution (treatments M 2 and M 3 , respectively) resulted in higher stem, leaf, fruit and total dry mass when compared to the management based on the crop uptake rate (treatment M 1 ), mainly in the higher N and K levels (Table 3).
T h e s u p e r i o r i t y i n b i o m a s s production observed in M 2 and M 3 over M 1 shows the importance of monitoring soil solution, either through the concentration of N and K ions or through the electrical conductivity.Both strategies allow a better ionic balance during crop cycle than fertigation management through the absorption rate (M 1 ), which does not monitor, nor controls salt concentration in the soil.Other authors also verified more biomass accumulation when managing fertigation by monitoring soil solution in other crops, such as tomato (Medeiros et al., 2012) and eggplant (Silva et al., 2013).
Using the regression equations for total dry mass in each fertigation management (M 1 = -0.0035**X 2 + 0.994**X + 93.44, R 2 = 0.63*; M 2 = -0.0040**X 2 + 1.5685**X + 76.33, R 2 = 0.96**; M 3 = -0.0039**X 2 + 1.428**X + 88.57, R2 = 0.79**), the highest total dry mass values were estimated for N and K levels corresponding to 142.0; 196.0 and 183.1%, with total mass accumulation corresponding to 164.0; 230.0 and 219.0 g/plant, for fertigation management systems M 1 , M 2 and M 3 , respectively (Table 3).As observed for stem, leaf and fruit dry mass, more expressive response in total dry mass were observed for M 2 (201.3%) and M 3 (147.6%),while M 1 resulted in lower gain (75.5 %) when estimates were compared to the total dry mass observed in the absence of N and K fertilization (Table 3).Medeiros et al. (2012) observed similar results in tomatoes grown in protected cultivation, also observing quadratic effects on the accumulation of biomass in response to increases in ion concentration in the soil solution.
Fertigation management systems M 2 (monitoring of the electrical conductivity) and M 3 (monitoring ion concentration in the soil solution) resulted in the highest fruit yield per plant at high N and K levels.Even though, the fertigation management system requiring the highest N dose was M1 (crop uptake rate) (Table 4).Thus, monitoring and controlling either the electrical conductivity or the ionic concentration of the soil solution may have allowed better nutritional conditions for plants, leading to higher yields, as also seen by Medeiros et al. (2012) and Silva et al. (2013).
Comparing the N and K levels which, by estimation, would result in the highest sweet pepper yields, to the doses applied in the experiment (Table 2), those top-yielding levels would be equivalent to doses of approximately (in g of N and K per plant, respectively) 14 and 18 (M 1 ); 15 and 23 (M 2 ) and; 10 and 16 g (M 3 ).It can be seen that management M 3 , besides outyielding M 1 , had a higher ratio yield: N and K  (Freitas, 2009); 2 M 1 = fertigation managed according to the crop uptake rate; M 2= fertigation managed by monitoring the electrical conductivity of the soil solution; M 3= fertigation managed by monitoring N and K ionic concentration in the soil solution.
dose (102 and 69 g/fruit per g of N and K, respectively), in other words, managing fertigation through monitoring the ionic concentration of the soil solution was more efficient than through the crop uptake rate.In addition, it was observed that, although management M 2 required the highest N and K doses to achieve maximum yield (140 and 89 g/fruit per g of N and K, respectively), management M 2 was more efficient than M 1 (95 and 72 g/fruit per g of N and K, respectively).Sweet pepper yield responded more intensely to N than to K, confirming reports from Aragão et al. (2011), who consider N as the most important nutrient for sweet peppers, since N directly influences plant and fruit growth.Oliveira et al. (2015), using the same fertigation management systems adopted here, found fertigation management based on ionic concentration in the soil solution provided adequate plant nutrition and high fertilizer savings.R e g a r d l e s s o f f e r t i g a t i o n management, most fruits were 8 to 12 cm long (class 8) (Figure 1).The lowest frequency of longer fruits, class 12 (length between 12 and 15 cm), was observed when fertigation was managed according to the crop uptake rate (M 1 ) (4.7%), while the lower frequencies of small fruits, class 6 (length between 6 and 8 cm) were observed for management based on monitoring the electrical conductivity, M 2 (8.9%), and the ion concentration of the soil solution, M 3 (11.3%).
In fertigation management M 1 , increases in fertilization levels increased also the frequency of fruit class 6 and reduced frequencies of fruit classes 8 and 10 (Figure 1A).On the other hand, the highest frequency of fruit class 12 (11%) was observed in the highest N and K levels.However, this was very likely due to the reduced number of fruits observed at this level, decreasing fruit competition for photoassimilates.Matos et al. (2012) state fruit demand for photoassimilates rises when there is a high number of fruits per plant, installing a strong fruit competition for assimilates.
For fertigation management M 2, the highest frequencies of fruit class 6, 8, 10 and 12 occurred at 50, 0, 150 and 300% of the recommended N and K doses for sweet pepper in Mossoró (Freitas, 2009), with an increase in frequency of fruit class 12 in the highest N and K doses (Figure 1B).In M 3 , the increasing N and K doses raised frequencies of fruit classes 6, 10 and 12, but reduced the number of fruits in class 8.The highest frequencies of classes 6, 8, 10 and 12 occurred at 200, 0, 150 and 300% of N and K, respectively (Figure 1C).
In summary, production of large fruits (class 12) was higher when fertigation was monitored by means either of the electrical conductivity (M 2 ) or the concentration of ions in soil solution (M 3 ).Reports on effect of N and K applied together, as in here, are scarce in literature.More abundant are studies evaluating the isolate effects of N and K, as Araújo et al. (2009) and Melo et al.   (Freitas, 2009); 2 M 1 = fertigation managed according to the crop uptake rate; M 2 = fertigation managed by monitoring the electrical conductivity of the soil solution; M 3= fertigation managed by monitoring N and K ionic concentration in the soil solution.
Improving sweet pepper yield and quality by means of fertigation management (2009).These authors, working with N and K fertilization, respectively, found that adequate N and K doses increase production of large fruits.
The results obtained in the present work show that, in addition to the N and K applied dose, application management throughout the growing cycle is fundamental to improve yield and fruit quality in sweet pepper, as had been previously indicated by Oliveira et al. (2013).In the present case, managing fertigation by monitoring either the electrical conductivity (M 2 ) or the ionic concentration of the soil solution (M 3 ) resulted in better plant development and higher fruit yield and quality, with lower N and K demands.The control of N and K concentration in the soil solution, keeping them at 304 and 490 mg/L of N and K, respectively, which corresponds to 200% of the recommendation for NFT hydroponic cultivation (152 and 245 mg/L of N and K, respectively), showed to be the most efficient for sweet pepper protected cultivation.

Figure 1 .
Figure 1.Fruit grading for sweet pepper grown in protected cultivation with different fertigation management systems and nitrogen and potassium levels.Figure1A, M 1 = fertigation managed according to the crop uptake rate; Figure1B, M 2= fertigation managed by monitoring the electrical conductivity of the soil solution, and Figure1C, M 3= fertigation managed by monitoring N and K ionic concentration in the soil solution.Grading according to fruit length, as follows: class 6 (from 6 to 8 cm), class 8 (from 8 to 10 cm ), class 10 (from 10 to 12 cm), and class 12 (from 12 to 15 cm).Mossoró, UFERSA, 2010.

Table 1 .
Nitrogen and potassium supply by fertigation according to the crop uptake rate.Mossoró, UFERSA, 2010.

Table 2 .
Amounts of nitrogen and potassium (g/plant) supplied in the three different fertigation management systems.Mossoró, UFERSA, 2010.
1100% equals to 215 and 314 kg/ha of N and K, respectively

Table 3 .
(Freitas, 2009uit and total dry weight of sweet pepper grown in protected cultivation with different fertigation management systems and nitrogen and potassium levels.Mossoró, UFERSA, 2010.Means followed by same letters in the line do not differ significantly from each other, Tukey test, p>0.05; 1 100% equals to 215 and 314 kg/ha of N and K, respectively(Freitas, 2009); 2 M 1 = fertigation managed according to the crop uptake rate; M 2= fertigation managed by monitoring the electrical conductivity of the soil solution; M 3= fertigation managed by monitoring N and K ionic concentration in the soil solution.
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