African Journal of
Agricultural Research

  • Abbreviation: Afr. J. Agric. Res.
  • Language: English
  • ISSN: 1991-637X
  • DOI: 10.5897/AJAR
  • Start Year: 2006
  • Published Articles: 6712

Full Length Research Paper

Potassium fertilization of lettuce in potassium-rich Eutrudox soil

A. B. Cecilio Filho*
  • A. B. Cecilio Filho*
  • Universidade Estadual Paulista, UNESP, Via de Acesso Prof. Paulo Donato Castellane, s/n, 14884-900, Jaboticabal, São Paulo State, Brazil.
  • Google Scholar
G. D. Bonela
  • G. D. Bonela
  • Universidade Estadual Paulista, UNESP, Via de Acesso Prof. Paulo Donato Castellane, s/n, 14884-900, Jaboticabal, São Paulo State, Brazil.
  • Google Scholar
M. C. P. da Cruz
  • M. C. P. da Cruz
  • Universidade Estadual Paulista, UNESP, Via de Acesso Prof. Paulo Donato Castellane, s/n, 14884-900, Jaboticabal, São Paulo State, Brazil.
  • Google Scholar
J. W. Mendoza-Cortez
  • J. W. Mendoza-Cortez
  • Universidade Estadual Paulista, UNESP, Via de Acesso Prof. Paulo Donato Castellane, s/n, 14884-900, Jaboticabal, São Paulo State, Brazil.
  • Google Scholar
C. R. P. Toscano
  • C. R. P. Toscano
  • Universidade Estadual Paulista, UNESP, Via de Acesso Prof. Paulo Donato Castellane, s/n, 14884-900, Jaboticabal, São Paulo State, Brazil.
  • Google Scholar

  •  Received: 22 April 2014
  •  Accepted: 17 December 2014
  •  Published: 22 January 2015


Soils intensively cultivated with vegetable crops may show potassium accumulation and this may cause damage to the environment and the plant. Viewing to evaluate the response of lettuce ‘Amanda’, of the crisphead group, to five doses of potassium (0, 25, 50, 75, and 100 kg/ha K2O) in a potassium-rich Rhodic Eutrudox soil, two experiments were carried out at two different planting times from 05/05 to 07/23/2008 (PT1) and from 03/24 to 05/14/2010 (PT2), in Jaboticabal, state of São Paulo, Brazil. The experimental units were distributed in the field, according to a randomized complete block design with four replications. Potassium content in the leaves (KL), number of leaves (NL), leaf area (LA), the plant aerial part fresh weight (APFW), the plant aerial part dry weight (APDW), and potassium content in the soil (KS) were determined. Significant interactions of the factors K rates and PT were not observed upon evaluated characteristics. The PT affected all characteristics. In PT1, the plants showed larger values of NL, LA, KL, and KS and in PT2, larger values of APFW and APDW. The KL adjusted to quadratic model with maximum (61.8 g/kg) obtained with 55 kg/ha K2O, while KS after experiments adjusted to linear model, showing 1.5 mmolc/dm3 when the crop was fertilized with K and 1.9 mmolc/dm3 when fertilized with 100 kg/ha de K2O. The potassium fertilization had no influence on NL, LA, APFW, and APDW, and therefore, it was not recommended to fertilize lettuce with K in soils with high content of K.  Aiming to keep the K content in the soil in the initial levels, the amounts of K accumulated by lettuce crop should be applied, in this case, 100.8 and 202.7 kg/ha K in the first and second planting time, respectively.


Key words: Growing season, growth, Lactuca sativa, production.


Lettuce became the most important leafy plant consumed by man since its domestication. In the state of São Paulo, Brazil, the cultivated area with lettuce is 9508 ha with an annual production of 6 697 934 crates, each one capable of boxing 9 dozens of plants (IEA, 2012). The crisphead type of lettuce is predominant in Brazil, with 70% of the market. The American group has 15% of the market, the group of smooth leafed plants has 10%, while the other  groups (Red, Mimosa, and Romaine) have the remaining 5% (Sala and Costa, 2005).
Mineral fertilization is one of the most influential practices affecting yield since horticultural crops are highly demanding of nutrients (Filgueira, 2008).
Unfortunately, most of the horticultural crops growers mismanage this practice and apply too large amounts of soil fertilizers frequently, resulting in accumulation of nutrients in the soil. One of the main reasons for this is that, usually lettuce growers do not have their soil chemically analyzed before planting the crop, that is, the amounts of fertilizers they apply are not based on results of chemical analyses and these are usually large amounts of fertilizers. This plus the fact that lettuce is a short cycled crop usually results in high rates of fertilizers in soils for horticultural production.
Although, potassium plays several functions in the plant (Epstein and Bloom, 2006) and is required by the plants in levels superior to the other nutrients (Beninni et al., 2005; Grangeiro et al., 2006; Sanchez, 2007; Kano et al., 2010) if supplied in excess to the plants, it becomes a cause of yield reduction in addition to increasing costs and negative effects on the environment (Reis Júnior and Monnerat, 2001). According to Padilla (1998), when potassium content in the soil is high its absorption by the plant may be four times that of nitrogen, this being characterized as luxury consumption. In excess, K may unbalance lettuce plant nutrition characterized by a reduction in calcium and manganese absorption (Malavolta, 2006). This unbalance may cause necrosis in the leaf edges, a physiological disorder known as tipburn and other negative implications mainly in enzymes activity. There is divergence of recommendation of K to lettuce in soils with high content of the nutrient. In this condiction, Fontes (1999) and Vilela et al. (2004) did not recommend applying K while Trani et al. (1997) recommend 50 kg/ha K2O, Comissão de Química e Fertilidade do Solo (2004) and 120 kg/ha K2O.
In the literature, results about soil fertilization with potassium for lettuce in potassium-rich soils are not supported in these recommendations. Thus, the objective of this research was to evaluate doses of potassium on the growth, nutrition, and production of lettuce in a potassium-rich soil.


Growing conditions
Two experiments were conducted in two planting times (PT), one during Autumn/Winter (from 05/05 to 07/23/2008 – PT1), and another during Autumn (from 03/24 to 05/14/2010 - PT2), in Jaboticabal, state of São Paulo, Brazil, at latitude of 21°15’22’’ S, longitude of 48°18’58’’ W and a mean altitude of 575 m above sea level. The soil of the experimental area is typical Rhodic Eutrudox (Soil Survey Staff, 1999). Previously to the experiments installation, the soil was cultivated with lettuce crops. The soil was sampled at 0-20 cm layer, and before the first experiment the soil chemical characteristics were: pH(CaCl2) 5.8; organic matter = 24 g/dm3; P = 136 mg/dm3; K+ = 3.4 mmolc/dm3; Ca2+ = 43 mmolc/dm3; Mg2+ = 8 mmolc/dm3; CEC = 106 mmolc/dm3, and soil base saturation = 79%; and before the second experiment were: pH(CaCl2)  5.6; organic matter = 22 g/dm3; P = 272 mg/dm3; K+ = 3.1 mmolc/dm3; Ca2+ = 47 mmolc/dm3;  Mg2+ = 18 mmolc/dm3;  CEC = 99 mmolc/dm3, and soil base saturation = 70%. Available P and exchangeable K+, Ca2+ and Mg2+ were extracted with resin method (Raij et al., 2001). K content larger than 3.0 mmolc/dm3 is considered high content (Raij et al., 1997). During the experiments, the mean maximum, minimum, and mean temperatures were 27.1, 13.5, and 19.2 (PT1) and of 29.2, 17.6, and 22.1 (PT2) respectively.  Mean relative humidity was of 68.9% in the first experiment and 79.4% during the second. Mean pluvial precipitation was 28.1 mm in the first and 96.4 mm during the second experiment.
Treatments and experimental design
Potassium doses in both experiments were 0, 25, 50, 75, and 100 kg/ha K2O, as potassium chloride. The doses were based on the recommendation by Trani et al. (1997) to cultivate lettuce in potassium-rich soils (levels above 3.0 mmolc/dm3 K), according to Raij et al. (1997). The experimental plots were distributed according to a randomized complete block design with four replications. The experimental plot (1.75 m2) constituted of 28 plants formed by four lines with seven plants each. The area in which measurements were made was of 1.25 m2 (1.00 x 1.25 m) since the plants at the beginning and at the end of each line as well as those of the lateral lines were not used for measurement purposes.
Plant material and crop management
Based on the soil chemical analyses it was decided not to apply lime since the soil saturation bases was close to the ideal value for lettuce cultivation, according to Trani et al. (1997). The application of organic fertilizer was not made either. At planting, in both experiments 40 kg/ha N, 200 kg/ha P2O5, and 1 kg/ha B as urea, single superphosphate, and boric acid were applied.  ‘Amanda’ lettuce seedlings were produced in 200 cell polypropylene trays in a commercial substratum (Plantmax HA™). Lettuce seedlings, having four leaves, were transferred on June 6 of 2008 and on March 24 of 2010 in the first and second experiments, respectively. The distance between rows and that between plants were both of 25 cm. Soon after transplantation, the seedlings were irrigated by the conventional sprinkler system. At first the water lamina was of 2 mm/day and then, when the plant cycle was reaching its end, it went up to 5 mm/day. In both experiments, 90 kg/ha N was side dressed to the plants 10, 20, and 30 days after transplantation (DAT), with basis on recommendation by Trani et al. (1997). Insecticides and fungicides were applied to the plants according to recommendations. Weed control took place whenever necessary. Harvesting took place when 80% of the plants of the best treatment reached the commercial point, that is, on 07/23/2008 in the first and on 05/14/2010 in the second experiment, 47 and 51 DAT, respectively.
In both experiments, the following variables were evaluated: a) potassium level in the diagnostic leaf: when the plants reached two thirds of their cycle, a sample of the recently developed leaf was taken (Trani and Raij, 1997). Leaf sampling was accomplished at the beginning of the day, between 6 and 7 A.M. Right after collection, the leaves were taken to the laboratory and washed in running water. After that, they were quickly dipped in a deionized water solution of hydrochloric acid and then dipped again in deionized water.  After removal of the water excess with the help of paper towel, the leaf samples were placed in paper bags and taken to a forced ventilation oven at 65°C, till a constant weight was reached. After that, each sample was ground in a Willey mill. The extract in which K content was determined by atomic absorption spectrometry was prepared according to instructions found in Carmo et al. (2000); b) Plant aerial part fresh weight (APFW): 6 plants were, between 6 and 7 A.M., randomly taken from the central part of each plot and  weighed, after dead and senescent leaves were removed; c) Number of leaves (NL): the number of leaves per plant was determined in the plants collected to determine APFW; d) Leaf area (LA): leaf area was measured in leaves of the plants collected to determine APFW, with the help of an electronic measuring device (LICOR, model 3100); e) Plant aerial part dry weight (APDW): the plants used to determine APFW, NL, and LA were washed and placed inside paper bags to dry in a forced ventilation oven at 65°C till a constant weight; f) Potassium content in the soil: immediately after harvest of lettuce, soil samples (0-20 cm) were obtained from 8 subsamples per plot, and exchangeable K was determined using resin method (Raij et al., 2001).
Statistical analysis
The experimental data were submitted to variance analysis (F test). The planting times means were compared by the Tukey’s test at the 5% level of probability, and the K rates by polynomial regression.


Potassium content in the diagnostic leaf (KL) was individually influenced by K doses and by the time of cultivation (Table 1). The means of the K content in the diagnostic leaf showed an adjustment to the quadratic model (Y = 55.19 + 0.2425x – 0.00222x2; R2 = 0.97*) as the doses of K increased.  Even in the soil with high content K, 3.1 and 3.4 mmolc/dm3 (Raij et al., 1997), the leaf K content increased from 55.2 g/kg (0 kg/ha) to 61.8 g/kg, with 55 kg/ha  de  K2O.  With  rates  higher  than  55 kg/ha K2O, the leaf K content decreases achieving close to that obtained without fertilization with potassium. Neuvel et al. (2000), evaluating the effects of doses of K between 0 and 480 kg/ha K2O during three cropping cycles and in different planting times in sandy and clayish soils, verified increasing levels of K in the plant tissue of two lettuce cultivars. Potassium content in the leaves as K dose increased from 0 to 100 kg ha-1 were verified to be within the limits of adequacy proposed by Trani and Raij (1997) for lettuce, that is, between 50 and 80 g kg-1.
With regards to planting time, the largest potassium leaf content was found in the first experiment, that is 63.2 g/kg followed by the value found in the second experiment - 54.8 g/kg (Table 2). The high level of K in the soil, associated with the potassium fertilization of up to 100 kg/ha K2O, resulted in no visual symptoms in the lettuce plants of deficiency of the other mineral nutrients, specially Ca, for, according to Malavolta (2006), K, when in high levels in the soil, has a relation of competitive inhibition with Ca. Probably the absence of the nutritional disorder tipburn resulted from the fact that the environmental conditions while the experiment was installed and conducted were favorable for lettuce growth. According to Wissemeier and Zühlke (2002), environmental conditions (temperature, duration and luminous intensity) favoring plant growth also favor the probability of disorder occurrence. 
NL was significantly influenced only by planting time (Table 1). At the Autumn/Winter time the number of leaves was larger than that of the second experiment (Autumn) with a difference between them of 4.2 leaves plant (Table 2). This difference is thought to be due to the low levels of precipitation during the first experiment  water in excess reduces leaf formation, a fact also verified by Purquerio et al. (2007) in a study in which arugula was sown at the same times of this experiment. Feltrim et al. (2009) reported to have lettuce plants of different cultivars growing in a K-rich soil with a mean number of leaves of 30.5, that is, higher than the values found in this work. Radin et al. (2004), verified in an experiment conducted during Autumn time NL values lower than those were reported: 16.5 leaves/plant in the cultivar Verônica and 15.4 leaves/plant in the cultivar Marisa, both lettuce cultivars of the crisphead group.
Similarly to what was observed for NL, LA was influenced only by planting time (Table 1). The largest LA was verified in the first experiment (3227.45 cm2/plant); in the second experiment LA was of 2187.66 cm2/plant, resulting from the higher NL of that planting time (Table 2). The results reported by Feltrim et al. (2009), similarly to those verified for NL, indicate that the highest LA of the different cultivars was verified in the Winter time (5100.22 cm2/plant). Differently from the results of this work were the data published by Radin et al. (2004) – the lowest LA were observed for crisphead lettuce cultivars (a mean of 999.1 cm2/plant) grown during Autumn time.  Fresh weight of plant aerial part (APFW) was significantly influenced only by planting time (Table 1). The largest value was verified during the second experiment – the plants of this experiment showed a difference of 82.32 g/plant in comparison with those of the first (Table 2), that is, a difference of 26%.  It was observed that the highest values of LA and NL verified during the first experiment did not result in larger APFW values. This result may be attributed to the presence of the stem in the composition of the weight of the shoot, and that in the second planting time (autumn) is usually larger than the first planting time (autumn-winter) because their growth is favored by higher temperatures, as reported by Silva et al. (1999) and Luz et al. (2009). 
The lack of response by lettuce plants growing in a potassium-rich soil (4.3 mmolc/dm3) to which up to 120 kg/ha K2O had been supplied was also reported by Sousa and Grassi Filho (2001). Similar results were also reported by Hoque et al. (2010). Neuvel et al. (2000) reported no effect of potassium fertilization on lettuce plants growing in a sandy soil but significant effects when the plants grew in a clayish soil. Cancellier et al. (2010) reported no morphophysiological (specific leaf area, liquid assimilation rates, absolute and relative growth, foliar, stem, and root mass ratio) responses of lettuce plants when the soil was fertilized with up to 300 kg/ha. On the other hand, Madeira et al. (2000), Mota et al. (2003), and Koetz et al. (2006) reported positive responses to doses of K2O applied to soils with low to medium levels of potassium.
Similarly to what was observed for LA, NL, and APFW, dry weight of plant aerial part (APDW) was influenced only    by the time of cultivation (Table 1).  The  largest APFW was attained in the second experiment. In Winter cultivation, Feltrim et al. (2005) found an average of 21.43 g/plant in their second experiment, a value larger than that observed in their first experiment although the APDW reported by Feltrim et al. (2005) was smaller than that found in the second experiment. Potassium content in the soil (KS) values, evaluated after the experiments were finished, similarly to what was observed for KL, were significantly influenced by both K doses and time of cultivation in an individual form (Table 1). The estimated values ??of K in the soil ranged from 1.5 to 1.9 mmolc/dm3, and the increase of KS in response to the increased K rate (0-100 kg/ha K2O) to the culture adjusted to a linear model (Y = 1.51 + 0.0036x; R2 = 0.95*).
Potassium levels in the soil, following both experiments, were found to be lower than the values found (3.4 and 3.1 mmolc/dm3 respectively) previously. After the experiments, potassium levels obtained (1.5-1.9 mmolc/dm3) in the soil became medium levels, according to classification by Raij et al. (1997). Potassium reduction in the soil may be explained by the high levels of K demanded by lettuce plants (Beninni et al., 2005; Grangeiro et al., 2006; Sánchez, 2007; Kano et al., 2010). Considering the lack of response of the lettuce plants to the potassium fertilization, it is hypothesized that a potassium-rich soil is capable of supplying all the potassium needed by the plants, that is, adding potassium fertilizers to this type of soil is not needed. But, due to the high level of K consumption by the lettuce plants (at the end of the experiments, potassium levels in the soil had fallen to 1.5 and 1.9 mmolc/dm3, when potassium rates were 0 and 100 kg/ha, respectively) and also to the fact that the majority of lettuce growers do not have their soils chemically analyzed for the cultivation of the crop immediately following that of lettuce, the most advisable procedure would be that of supplying the soil with a potassium rate sufficient to be able to preserve the levels of potassium in the soil. For this fertilization, the amounts of K to be applied may correspond to that accumulated by the lettuce plants and determined by the measurements of APDW and KL in each planting time (Table 2). 


In this research work, considering 100,000 plants per hectare, the lettuce crop accumulated 100.8 and 202.7 kg/ha K or 121.5 and 244.3 kg/ha K2O in the first and second planting time, respectively. On the other hand, it is very important to know which group the lettuce cultivars belong to so as to make a more appropriate fertilization of the soil - different cultivars will export different amounts of K due to their differences in mass and the levels attained by that nutrient in the foliar tissue. In K-rich Eutrudoxs, fertilizing the soil with this nutrient is not necessary for growth and production of lettuce plants of the crisphead group. A maintenance fertilization following lettuce cultivation is, nonetheless, recommended viewing to preserve soil fertility.


The author(s) have not declared any conflict of interests.


To FAPESP (The State of São Paulo Research Funding Foundation) for the scholarship granted to the second author, and to CNPq for the scholarship granted to the first author.


Beninni ERY, Takahashi HW, Neves CSVJ (2005). Concentração e acúmulo de macronutrientes em alface cultivada em sistemas hidropônico e convencional. Semina: Ciênc. Agrárias 26(3):273-282.
Cancellier LL, Adorian GC, Rodrigues HVM, Siebeneichler SC, Leal TCA B (2010). Doses de potássio nas respostas morfofisiológicas de alface. Rev. Caatinga 23(4):21-27.
Carmo CAFS, Araújo WS, Bernardi ACC, Saldanha MFC (2000). Métodos de análise de tecidos vegetais utilizados na Embrapa Solos. Rio de Janeiro: Embrapa Solos (Circular Técnica P. 6).
Comissão de Química e Fertilidade do Solo - RS/SC (2004). Manual de adubação e calagem para os Estados do Rio Grande do Sul e Santa Catarina. 10. Ed. Porto Alegre.
Epstein E, Bloom AJ (2006). Nutrição mineral de plantas: princípios e perspectivas. Trad. Maria Edna Tenório Nunes. Londrina: Editora Planta.
Feltrim AL, Cecílio Filho AB, Rezende BLA, Branco RBF (2009). Produção de alface-crespa em solo e em hidroponia, no inverno e verão, em Jaboticabal – SP. Científica 37(1):9-15.
Feltrim AL, Cecílio Filho AB, Branco RBF, Barbosa JC, Salatiel LT (2005). Produção de alface americana em solo e em hidroponia, no inverno e verão, em Jaboticabal, SP. Rev. Bras. Eng. Agríc. Ambient 9(4):505-509.
Filgueira FAR (2008). Novo manual de olericultura: agrotecnologia moderna na produção e comercialização de hortaliças. Viçosa, Minas Gerais: Universidade Federal de Viçosa.
Fontes PCR (1999). Alface. In: Ribeiro AC, Guimarães PTG, Alvarez VVH. (Eds). Recomendações para o uso de corretivos e fertilizantes em Minas Gerais. Viçosa: Comissão de Fertilidade do Solo do Estado de Minas Gerais. P. 177.
Grangeiro LC, Costa KR da, Medeiros MA de, Salviano AM, Negreiros MZ, Bezerra N F, Oliveira SL (2006). Acúmulo de nutrientes por três cultivares de alface cultivadas em condições do semi-árido. Hortic. Bras. 24(2):190-194.
Hoque MM, Ajwa H, Othman M, Smith R, Cahn M (2010). Yield and postharvest quality of lettuce in response to nitrogen, phosphorus, and potassium fertilizers. HortScience 45(10):1539-1544.
IEA - Instituto de Economia Agrícola (2012). Estatísticas de produção da agropecuária paulista 2012. Disponível em:
Kano C, Cardoso AII, Villas Bôas RL (2010). Influencia de doses de potássio nos teores de macronutrientes em plantas e sementes de alface. Hortic. Bras. 28(3):287-291.
Koetz M, Coelho G, Costa CC, Lima EP, Souza RJ (2006). Efeito de doses de potássio e da freqüência de irrigação na produção da alface americana em ambiente protegido. Eng. Agríc. 26(3):730-737.
Luz AO, Seabra Jr S, Souza SBS, Nascimento AS (2009). Resistência ao pendoamento de genótipos de alface em ambientes de cultivo. Agrarian 2(6):71-82.
Madeira NR, Yuri JE, Freitas SAC, Rodrigues Júnior JC (2000). Fornecimento de nitrogênio, potássio e cálcio para alface americana via fertirrigação. Congresso Brasileiro de Olericultura, 40. Anais... São Pedro: SOB/FCAV-UNESP. pp. 841-842.
Malavolta E (2006). Manual de nutrição mineral de plantas. São Paulo: Ceres.
Mota JH, Yuri JE, Freitas SAC, Rodrigues Jr JC, Resende GM, Souza RJ (2003). Avaliação de cultivares de alface americana durante o verão em Santana da Vargem, MG. Hortic. Bras. 21(2):234-237.
Neuvel JJ, Weijers G, Titulaer HHH (2000). Saving on potassium fertilizer in butterhead lettuce. PAV Bulletin Vollegrondsgroenteteelt 4:12-16.
Padilla WA (1998). Segundo curso internacional de fertirrigación en cultivos protegidos. Quito, Ecuador: Universidad San Francisco de Quito.
Purquerio LFV, Demant LAR, Goto R, Villas Boas RL (2007). Efeito da adubação nitrogenada de cobertura e do espaçamento sobre a produção de rúcula. Hortic. Bras. 25(3):464-470. 
Radin B, Reisser Júnior C, Matzenauer R, Bergamaschi H (2004). Crescimento de cultivares de alface conduzidas em estufa e a campo. Hortic. Bras. 22(2):178-181.
Raij B, Andrade JC, Cantarella H, Quaggio JA (2001). Análise química para avaliação da fertilidade de solos tropicais. Campinas, São Paulo: Instituto Agronômico & Fundação IAC.
Raij B, Quaggio JA, Cantarella H, Abreu CA (1997). Interpretação de resultados de análise de solo. In: Raij B van, Cantarella H, Quaggio JA, Furlani AMC (Eds). Recomendações de adubação e calagem para o Estado de São Paulo. Campinas, São Paulo: Instituto Agronômico & Fundação IAC. (Boletim Técnico 100). pp. 8-13.
Reis Jr RA, Monnerat PH (2001). Exportação de nutrientes nos tubérculos de batata em função de doses de sulfato de potássio. Hortic. Bras. 19(3):360-364.
Sala FC, Costa CP da (2005). 'PiraRoxa': cultivar de alface crespa de cor vermelha intensa. Hortic. Bras. 23(1):158-159.
Sanchez SV (2007). Avaliação de cultivares de alface crespa produzidas em hidroponia tipo NFT em dois ambientes protegidos em Ribeirão Preto (RP). Jaboticabal, São Paulo: Faculdade de Ciências Agrárias e Veterinárias, Universidade Estadual Paulista. (Dissertação de Mestrado).
Silva EC, Leal NR, Maluf WR (1999). Avaliação de cultivares de alface sob altas temperaturas em cultivo protegido em três épocas de plantio na região norte-fluminense. Ciênc. Agrotec. 23(3):491-499.
Soil Survey Staff (1999). Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys. Washington DC, USA: Natural Resources Conservation Service, United States Department of Agriculture (Handbook 436).
Sousa LMA de, Grassi Filho H (2001). Avaliação da fertirrigação potássica na produção e qualidade da alface (Lactuca sativa L.) americana em estufa. Irrigation 6(1):12-28.
Trani PE, Passos FA, Azevedo Filho JA (1997). Alface, almeirão, chicória, escarola, rúcula e agrião d'agua. In: Raij B van, Cantarella H, Quaggio JA, Furlani AMC (Eds). Recomendações de adubação e calagem para o Estado de São Paulo. Campinas, São Paulo: Instituto Agronômico & Fundação IAC (Boletim Técnico 100). . P. 168.
Trani PE, Raij B (1997). Hortaliças. In: Raij B van, Cantarella H, Quaggio JA, Furlani AMC (Eds). Recomendações de adubação e calagem para o Estado de São Paulo. Campinas, São Paulo: Instituto Agronômico & Fundação IAC. (Boletim Técnico 100). pp. 155-164.
Vilela L, Sousa DMG, Silva JE (2004). Adubação potássica. In: Sousa DMG, Lobato E (Eds). Cerrado: correção do solo e adubação. Goiás: Embrapa Cerrados. pp. 169-183.
Wissemeier AH, Zühlke G (2002). Relation between climatic variables, growth and the incidence of tipburn in field-grown lettuce as evaluated by simple, partial and multiple regression analysis. Sci. Hortic. 93(3):193-204.