Introduction

In 2014, in Mexico were planted 16 902 ha of cucumber (Cucumis sativus L.), with a production of 707 632 tons, of which 43% were produced in the state of Sinaloa, in an area of 4 203 ha (^{SIAP, 2014}). In Sinaloa cucumber is grown in open field and under protected conditions (greenhouse and shade house). In both production systems apply SN containing dissolved fertilizer to supply nutrients to the plant.

Of the 17 essential nutrients, usually vegetable producer applies 12 (N, P, K, Ca, Mg, S, Fe, Cu, Zn, Mn, B and Mo), since C, H, O, Cl and Ni, are not applied, because the first four plants obtain them from water or air and Ni is not used because its impact is unknown in vegetables or because it is considered that the plants obtain Ni from fertilizer or water irrigation; Ni is found as pollutant in fertilizers (^{Brown et al., 1987}). Of these nutrients N and K are used in higher amounts because are important for plant yield and the interaction N-K usually occurs in agricultural ecosystems (^{Johnston and Milford, 2009}).

N is a DNA, RNA, proteins and enzymes, chlorophyll, ATP, auxins and cytokinins component (^{Hawkesford et al., 2012}), and can be supplied to plants in four ways: nitric, ammoniac, ureic and amino acids, although the nitrate form is preferentially absorbed by plants grown in soils (^{Masclaux-Daubresse et al., 2010}), which is the most widely used in horticultural crops. K participates in the electric potential gradient in cell membranes, turgor, enzyme activation, photosynthesis, sugars and starch metabolism, protein synthesis, opening and closing of stomata, stabilization cellular pH and cell cation-anion balance (^{Zhang et al., 2010}).

An adequate supply of K is associated with increased yield and fruit size of many horticultural crops (^{Kanai et al., 2007}). A high concentration of NO₃ ^-, regarding H₂PO₄ ^- and SO₄ ^2- anions, and a low concentration of K ⁺, compared to Ca²⁺ and Mg²⁺ cations in the SN indicates a high ratio of NO₃ ^-/K⁺, also known as the N/K ratio. ^{Steiner (1961)} mentions that many people consider important the N/K ratio; however, this importance is currently subject to discussion by the lack of studies. In Mexico and other countries there are few studies of N/K ratio in SN, using NO₃ ^-/ anions and K⁺/ cations ratios, as to define these ratios is necessary to consider the methodology proposed by ^{Steiner (1984)}, however this technique is not common knowledge or has no general acceptance.

The objective of this study was to determine the effect of three NO₃ ^-/anions (NO₃ ^-, H₂PO₄ ^- and SO₄ ^2-) and three K⁺/ cations (K⁺, Ca²⁺ and Mg²⁺) ratios, based on the universal Steiner SN on growth, mineral composition and yield of a hybrid cucumber grown under greenhouse conditions.

Materials and methods

The study was conducted in a greenhouse from to the Faculty of Agriculture of the Autonomous University of Sinaloa, located at 24° 37' and 24.40' north latitude and 105° 24' and 107° 26' and 35.69' west longitude, to an altitude of 38 m. The experiment consisted of two stages: stage 1 from sowing to transplanting, with a duration of 38 days and stage 2 from transplantation to harvest, lasting 78 days. In stage 1, cucumber seeds (Cucumis sativus L. cultivar Luxell) American type, indeterminate growth, were sown in polystyrene trays of 128 cavities with individual volume of 35 cm³ containing coconut fiber as substrate, previously washed with distilled water.

The experimental design was completely randomized with a factorial arrangement 3², and four replications, for a total of 36 experimental units, where each experimental unit consisted of 30 plants. The factors and levels evaluated were: 1) NO₃ ^-/anions ratios (NO₃ ^-, H₂PO₄ ^-, SO₄ ^2-) (40/100, 60/100 and 80/100); and 2) K⁺/cations ratios (K⁺, Ca²⁺, Mg²⁺) (15/100, 35/100 and 55/100). By combining the three NO₃ ^-/anions with three ratios K⁺/cations ratios result in nine SN, which were designed from modifications of the universal SN (^{Steiner, 1984}) and consisting of varying the concentration of NO3^- in relation to H₂PO₄ ^- and SO₄ ^2-, thus K⁺ concentration regarding Ca²⁺ and Mg²⁺. The chemical composition of SN was calculated and adjusted to an osmotic potential of -0072 MPa (Table 1), according to ^{Steiner (1984)} proposal.

The SN were prepared with inorganic salts reagent grade and distilled water and the following micronutrient concentrations (mg L^-1) were added: 2.5 Fe, 0.5 Mn, B 0.5, Cu 0.02 and Zn 0.05 (^{Parra et al., 2010}). Fe was provided as Fe-EDTA and SN pH was adjusted to 5.5 ± 0.1 with 1N HCl or 1N NaOH. Eight days after planting, the application of SN started at 50% of its concentration for 10 days, the remaining 20 days, SN were applied at 100%. The plants were watered daily at 08:00 and 14:00 h spraying the foliage with atomizers (1 L capacity) until runoff of the solution through lower holes of the cavities. To evaluate the effect of the factors on the growth variables (height, stem diameter and dry weights of leaves, stems and plants) six plants were selected per treatment, and to determine the mineral composition of the stem, 20 plants were selected, 38 days after planting, four composite replications were integrated, each with five plants, whose shoots were fractionated on leaves and stems. A chemical analysis was performed to this organs to determine the concentrations of N, P, K, Ca and Mg according to methodologies proposed by ^{Motsara and Roy (2008)}.

In stage 2, cucumber plants of 38 d after sowing were used, from stage 1, previously subjected to the treatments described in Table 1. The plants were transplanted in a subirrigation closed circuit hydroponic system, consisting of two plastic containers with capacity of 20 L each, painted black and interconnected with rubber hose of 1.25 cm in diameter. One of the containers contained 18 L of coconut fiber as substrate, previously washed with distilled water; coconut fiber had a bulk density of 0.09 g cm^-3, a real density of 0.051 g cm^-3, and total porosity of 82.4% of volume, aeration porosity 19.8% of volume and water retention capacity 62.7% of volume.

In a bowl two cucumber plants were placed, led to a stem and on the other 10 L of SN of the corresponding treatment (Table 1) to perform two daily irrigations to the coconut fiber, the first at 7 am and the second at 12 pm; evapotranspired water was recovered daily through mark with distilled water and pH of the SN was adjusted to 5.5 ± 0.2 with HCl or NaOH 1N; SN were renewed every 15 d and prepared as described in stage 1. The treatments were distributed in a completely randomized design with factorial arrangement of treatments 3² and five replications. The harvest period lasted 37 d and the variables evaluated were yield (kg ha^-1), number of fruits per plant, average fruit weight, fruit length and nutrient concentration in leaves, to which are quantified the concentrations of N , P, K, Ca and Mg with the methods described in stage 1. Analysis of variance of response variables was performed by evaluating the main factors from the factorial design: 1) NO₃ ^-/ anions (NO₂ ^-, H2PO₄ ^- and SO₄ ^2-) ratio, and 2) K⁺/ cations (K⁺, Ca²⁺, Mg²⁺) and determined the interaction of these factors.

Results and discussion

Stage 1

Growth variables. The analysis of variance showed no significant effect on the interaction NO₃ ^-/ anions x K⁺/cations in any of the variables of seedling growth, indicating that N/K ratio did not affect growth, 38 d after sowing. However NO₃ ^-/anions ratio was significant (p≤ 0.05) for plant height and K⁺/cations ratio was for stem dry weight (PST) (Table 2). The greatest height was obtained with 80/100 NO₃ ^-/anions in SN (14.2 at 16.1 mol NO₃ ^- m^-3) and the lowest with 40/100 NO₃ ^-/anions (7.93 at 9.14 mol NO₃ ^- m^-3). This result agrees with that reported by ^{Preciado et al. (2002)}, who found that high doses of nitrogen (12 at 14 mol NO₃ ^- m^-3) promoted the growth of stem in melon (Cucumis melo L.), while in Cucumber (Cucumis sativus L.), ^{Moreno et al. (2011)} mention that 12.3 mol NO₃ ^- m^-3 increased stem length. The biggest PST (0.22 g).

Table 2. Effect of NO₃ ^-/anions and K⁺/cation ratio in nutrient solution in height (A), stem diameter (DT), dry weight of leaves (PSH), stems dry weight (PST) and plants dry weight (PSP) of cucumber, 38 days after planting. 

Factor	A cm	DT mm	PSH g	PST g	PSP g
Relación porcentual de NO3 -/aniones
40/100	27.5 b	4.8 a	0.37 a	0.19 a	0.56 a
60/100	30.4 a	4.7 a	0.39 a	0.18 a	0.57 a
80/100	30.7 a	4.7 a	0.41 a	0.2 a	0.61 a
Relación porcentual de K+/cationes
15/100	28.9 a	4.6 a	0.42 a	0.22 a	0.58 a
35/100	29.6 a	4.8 a	0.37 a	0.18 b	0.59 a
55/100	29.9 a	4.9 a	0.38 a	0.17 b	0.56 a
NO3 -/aniones X K+/cationes	ns	ns	ns	ns	ns

It was obtained with the lower K⁺/ cations (15/100) ratio, possibly this concentration covered K demand of plants and promoted the absorption of N, P, Ca and Mg in stem (Table 5), as these nutrients had the highest concentrations in this organ, while the lowest PST (0.17 g) was obtained with higher K⁺/cations (55/100) ratio, which had the highest concentration of K in stems and lower concentrations of N, P, Ca and Mg, perhaps because the ratio 55/100 K⁺/ cations exceeded K requirements by plants and high absorption of K reduced the concentrations of N, P, Ca and Mg. This result agrees with that mentioned by ^{Fageria (2001)}, in rice (Oryza sativa L.), where high concentrations of K in SN inhibited the absorption of P, Ca and Mg.

Table 3. Effect of NO₃ ^-/anions and K⁺/cation ratio in nutrient solution and the concentration of N, P, K, Ca and Mg in leaves of cucumber plants, 38 days after planting. 

Factor	N (%)	P (%)	K (%)	Ca (%)	Mg (%)
Relación porcentual de NO3 -/aniones
40/100	6.21 a	0.99 a	3.71 a	2.2 b	0.69 b
60/100	6.6 a	0.78 b	3.6 a	2.44 a	0.91 a
80/100	6.24 a	0.61 c	3.74 a	2.39 ab	0.95 a
Relación porcentual de K+/cationes
15/100	6.28 ab	0.82 a	2.43 c	2.89 a	0.95 a
35/100	6.12 b	0.75 a	3.72 b	2.37 b	0.86 b
55/100	6.66 a	0.81 a	4.9 a	1.72 c	0.75 c
NO3 -/aniones X K+/cationes	ns	**	**	ns	**

Table 4. Effect of the interaction NO₃ ^-/anions x K⁺/cations in nutrient solution in the concentration of P, K and Mg in cucumber seedlings leaves, 38 days after planting. 

Factor NO3 -/aniones x K+/cationes	P (%)	K (%)	Mg (%)
40/100 x 15/100	1.23 a	2.89 de	0.96 ab
60/100 x 15/100	0.65 bc	2.1 f	1.09 a
80/100 x 15/100	0.57 c	2.29 ef	0.81 bc
40/100 x 35/100	0.87 bc	3.14 d	1.05 a
60/100 x 35/100	0.79 bc	3.89 c	0.84 bc
80/100 x 35/100	0.6 bc	4.13 bc	0.7 cd
40/100 x 55/100	0.87 bc	5.1 a	0.84 bc
60/100 x 55/100	0.89 b	4.8 ab	0.81 bc
80/100 x 55/100	0.67 bc	4.79 ab	0.54 d

Table 5. Effect of NO₃ ^-/anions and K⁺/cation ratio in nutrient solution and the concentration of N, P, K, Ca and Mg in cucumber seedling stems, 38 days after planting. 

Factor	N (%)	P (%)	K (%)	Ca (%)	Mg (%)
Relación porcentual de NO3 -/aniones
40/100	4.87 b	1 a	6.18 a	1.13 a	0.64 a
60/100	6.13 a	0.81 b	6.17 a	1.21 a	0.6 ab
80/100	5.82 a	0.68 c	6.51 a	1.21 a	0.48 b
Relación porcentual de K+/cationes
15/100	6.11 a	0.92 a	4.48 c	1.52 a	0.65 a
35/100	5.45 b	0.79 b	6.8 b	1.11 b	0.61 a
55/100	5.18 b	0.78 b	7.57 a	0.82 c	0.42 b
NO3 -/aniones X K+/cationes	ns	**	ns	ns	ns

Nutritional analysis on leaves

The NO₃ ^-/anions ratio was different (p≤ 0.05) for Ca concentration in leaves (Table 3); the highest concentration of Ca (2.44%) was obtained with the ratio 60/100 NO₃ ^-/anions and the lowest (2.20%) was obtained with 80/100 NO₃ ^-/anions. According ^{Fageria (2001)}, when a nutrient is in an excessive concentration in the growth medium can affect the absorption of another, so in the present study a ratio of 80% in NO₃ ^- in SN, inhibited Ca absorption. N concentrations in leaves were affected (p≤ 0.05) by K⁺/cations ratio, the lowest value of N (6.12%) was obtained with 35/100 K⁺/cations and the highest value (6.66% N) was obtained with 55/100 K⁺/ cations (Table 3).

^{Zhang et al. (2010)} mention that an optimal application of K is favorable for proper nutrient management in agriculture, in this sense the high dose of K promoted the accumulation of N in leaves. The concentrations of Ca in the leaves were reduced (p≤ 0.05) with increases in K⁺/ cation ratio in SN (Table 3), by antagonism of K with Ca (^{Fageria, 2001}), which was reflected in increased absorption K and less of Ca. The interaction of the factors was significant for concentrations of P, K and Mg in leaves (Table 3), P concentrations obtained with the three NO₃ ^-/anions ratios were similar for ratios 35 and 55/100 K⁺/cations, however, for the ratio 15/100 K⁺/cations had variation (Table 4). The highest concentration of P (1.23%) was obtained with 40/100 NO₃ ^-/ anion and 15/100 K+ anion / cation and the lowest (0.57%) was obtained with 80/100 NO3- / K + 15/100 anion / cation , which can be attributed to the relationship 40/100 NO_3-/ anions had a concentration of 1.71 mol m^-3 H₂PO₄ ^-, compared to 0.5 mol m^-3 of H₂PO₄ ^- from the ratio 80/100 NO₃ ^-/anions (Table 1). Regarding K, the highest concentrations (5.1, 4.8 and 4.79% of K in leaves) were obtained with the combination of the ratio 55/100 K^+/ cations and 40, 60 and 80/100 NO₃ ^-/ anions (Table 4), attributed to higher concentration of K in these solutions, favoring their absorption and accumulation. Mg concentrations in leaves obtained with three ratios NO₃ ^-/anions were different (p≤ 0.01) for all three ratios K⁺/cations (Table 4), where combinations 80/100 NO₃ ^-/anions with 15, 35 and 55/100 K⁺/ cations had the lowest concentrations of Mg (0.81, 0.7 and 0.54%, respectively) due to antagonism of K on Mg (^{Gransee and Führs, 2013}), because at greater proportion of K in SN, lower concentration of Mg in leaves.

Nutritional analysis in stems

The interaction of NO₃ ^-/anions x K⁺/ cation factors was significant for P concentration (Table 5) where P concentrations obtained with the three NO₃ ^-/anions ratio were different (p≤ 0.05) for the ratio 55/100K⁺/cations (Figure 1), since in this ratio the highest concentration of P (1.25%) was obtained with 40/100 NO₃ ^-/anions, and the lowest (0.44% P) was obtained with 80/100 NO₃ ^-/anions (Table 5), as explained in the concentration of P in leaves. N and Mg concentrations were affected (p≤ 0.05) by the ratio NO₃ ^-/anions, whereas concentrations of N, K, Ca and Mg were different (p≤ 0.05) for the K⁺/ cations ratio (Table 5).

Figure 1. Effect of the interaction NO₃ ^-/anion x K⁺/cation in nutrient solution and the concentration of P in stems of cucumber plants cv. Luxell. Points with different letters in a column and in a line are statistically different (p≤ 0.05). 

With the ratio 60/100 NO₃ ^-/ anions the highest concentration of N in stems (6.13%) was obtained, while the lowest (4.87%) was obtained with 40/100 NO₃ ^-/anions; the highest concentration of Mg in stems (0.64%) was obtained with 40/100 NO₃ ^-/anions and the lowest (0.48% Mg) with 80/100 NO₃ ^-/anions. With ratio 15/100 K⁺/cations the highest concentration of N (6.11%) was obtained in stems, while the lowest (5.18%) was obtained with 55/100 K+/cations. K concentrations in stems had a direct relationship with the K+/cations ratio in SN, since increasing these relationships increased concentrations of K in that organ, whereas the concentrations of Ca and Mg in stems the relationship was inverse, because increasing the ratio K⁺/cations in SN the concentrations of Ca and Mg decreased by antagonism from K on Ca and Mg (^{Fageria, 2001}) and by the lower concentration of Ca and Mg in SN (Table 1).

Conclusions

Growth of cucumber seedlings and yield were not affected by the interaction NO₃ ^-/anion x K⁺/cations, although the nutrient concentrations in leaves and stems from seedlings and plants showed differential responses to combinations NO₃ ^-/ anions x K⁺/cations. In cucumber production, in a closed hydroponic system and coconut fiber as substrate, the NO3-/anions ratio should not be greater than 60/100, whereas K+/cation ratio should not exceed 35/100; at higher ratio NO₃ ^-/anions (80/100) and K⁺/ cations (55/100) cucumber yield reduced 9.98% and 31.84% respectively due to alterations in the concentrations of N, P, K, Ca and Mg in leaves.