RESEARCH ARTICLE

Potassium–calcium antagonistic interaction under tomato magnesium deficiency and magnesium fertiliser regulation in solar greenhouse

B. Yan1,2,3,4,5, Y.Y. Sun1,3,4,5 and Y. Wei1,3,4,5

1Shaanxi Provincial Land Engineering Construction Group Co. Ltd., Xi’an 710075, China; 2College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, Shaanxi, China; 3Institute of Land Engineering and Technology, Shaanxi Provincial Land Engineering Construction Group Co. Ltd., Xi’an 710075, China; 4Key Laboratory of Degraded and Unused Land Consolidation Engineering, the Ministry of Natural Resources, Xi’an 710075, China; 5Shaanxi Provincial Land Consolidation Engineering Technology Research Center, Xi’an 710075, China

Corresponding Author: B. Yan, [email protected]

Received: 8 March 2020 / Accepted: 29 June 2020 / Published: 25 July 2020

Abstract

This study sought to clarify the antagonistic interactions of potassium (K) and calcium (Ca) to magnesium (Mg) under a deficiency of Mg in tomato. Tomato leaves and soil samples that had differing levels of Mg deficiency were collected to study the relationship between symptoms of Mg deficiency and contents of soil K and Ca. Four different Mg fertiliser treatments were conducted to analyse the regulation of Mg for soil K, Ca and Mg. The results showed the following: (1) The yield of tomatoes decreased significantly with an increase in Mg deficiency, and the yield of tomatoes with moderate (MD) and severe (SD) Mg deficiency decreased by 38.02% and 59.53%, respectively, compared with treatments without Mg deficiency (ND). (2) The cation saturation ratio of K+ (CSRK+) was significantly higher with MD and SD compared with ND, while the CSRMg2+ was lower. The soil K/Mg and Ca/Mg values were higher than the critical value of imbalance. (3) The soil exchangeable K, CSRK+, Ca/Mg and K/Mg under SD increased significantly when compared with that under ND. (4) The content of Mg in tomato leaves and its yield were significantly negatively correlated with soil exchangeable K, CSRK+ and K/Mg. (5) With the increase in application of Mg fertiliser, the soil exchangeable K content, K/Mg and CSRK+ decreased significantly, while the Ca/K increased. The soil exchangeable K content, K/Mg and CSRK+ with 90 kg/ha MgSO4 and 234 kg/ha K2O applied (M2K1 treatment) were the lowest among all treatments. (6) The yields of tomatoes and uptake of Ca and Mg increased as supply of Mg increased. (7). Reducing the application of K was a much more efficient way to decrease soil K/Mg and restore cation imbalance than providing Mg fertiliser in calcareous soil.

 

1. Introduction

Magnesium (Mg) is one of the essential nutrients for plant growth that accounts for approximately 0.05 to 0.7% of their dry weight. It is an important component of chloroplasts. With a sufficient supply of Mg, approximately 20% of the total Mg is combined with chlorophyll (Marschner, 2012). As a central atom of the chlorophyll molecule (Huber and Maury, 1980), Mg plays an important role in photosynthesis, carbohydrate formation and protein synthesis (Guo et al., 2016). The application of Mg fertiliser helps to increase chlorophyll content of plants. Chapagain and Wiesman (2004) stated that the chlorophyll content of tomato leaves with application of potassium chlorate and magnesium chloride was significantly higher than that with potassium chlorate, suggesting that chlorophyll molecules can absorb quantum light only when combined with Mg ions (Mg2+). Essentially, a few molecules of Mg2+ (at a concentration of 5 mmol/L) induce the thylakoid membrane to form granules that are more conducive to capturing light energy and transferring energy (Kaftan et al., 2002). In addition, Mg2+ may act on reaction centres and antenna pigments as well as some electron carriers on the membrane at the molecular level (Genty et al., 1989; Gupta and Berkowitz, 1989). Similar studies have shown that under the radiation of light, Mg2+ on the chloroplast thylakoid membrane can be exchanged with hydrogen ions in the matrix, leading to an increase in the concentration of Mg2+ and pH value in the matrix, an activation of ribulose bisphosphate carboxylase, and an increasing Michaelis–Menten constant (Km) and maximum velocity of enzymatic reaction (Vmax) (Cacco et al., 2010; Keitaro et al., 2014). It is more conducive to the fixation and assimilation of carbon dioxide (Tatagiba et al., 2016).

Mg deficiency may cause the disintegration of plant chloroplasts, a reduction in the photosynthetic rate, limitation of transport of assimilation products and a decrease in crop yield and quality (Verbruggen and Hermans, 2013). When Mg becomes deficient, plant transpiration and photosynthesis are immediately inhibited (Hermans and Verbruggen, 2005; Hermans et al, 2004; Kobayashi et al., 2013). Crop yield and biomass are usually substantially reduced when the level of Mg is low. Moreover, deficiency in Mg is significantly correlated with the nutrient uptake of plants (Huang et al., 2019; Jin et al., 2016).

A deficiency of Mg corresponds to a direct lack of Mg (Aitken et al., 1999), but the antagonistic interaction can also be caused by an imbalanced input of fertiliser (Hermans et al., 2004). Potassium (K+) and calcium ions (Ca2+) are related to the absorption of Mg2+ (Chapagain and Wiesman, 2004; Gransee and Führs, 2013). When a large amount of potassium (K) is supplied, it decreases the content of Mg in plant, particularly in plant roots (Ding et al, 2006; Omar and Kobbia, 1966). In the past, experimental antagonistic interactions between K+ and Mg2+ occurred during the transport from roots to shoots (Ohno and Grunes, 1985). A high availability of Ca2+ is sensitive to Mg uptake (Marschner, 2012). Although there is some information on the interaction between K, Ca and Mg in forests, grasses and arable crops, there is currently no information regarding vegetables such as tomato (Guo et al., 2016).

The application of fertiliser is an effective way to relieve the deficiency of Mg. Similar results have shown that the application of gray Mg limestone on acidic soils in Malaysia can effectively increase the content of Mg in soil and rice yield, and in addition reduced the toxicity of iron and aluminum (Panhwar et al., 2014). However, fewer studies have been conducted in calcareous soils enriched in Mg.

In this study, tomato leaves and soil with different levels of Mg deficiency were collected to clarify the K–Ca–Mg antagonistic interaction in calcareous soil in a greenhouse. In addition, a test of Mg fertiliser was conducted to evaluate the effect of Mg fertiliser on the balance of K, Ca and Mg in soil.

2. Methods and materials

Study site

The effect of Mg deficiency on K and Ca antagonism mechanism

The study was conducted at solar greenhouses of Dazhai Town in Yangling Agricultural High-Tech Industry Demonstration Zone, Shaanxi Province, China (latitude 34° 16’ N, longitude 108° 4’ E). The greenhouses were built in 2009 and characterised by calcareous soil. Tomato plants were grown each autumn and winter, and each cropping cycle was separated by plantings of tomato, cucumber or watermelon during the spring.

The experiment was conducted on 5 August, the sowing date of tomato. Samples were collected on 15 November when the Mg deficiency became apparent on tomato leaves during fruit period. According to different symptoms shown in leaves in different greenhouses, the level of Mg deficiency were divided into three parts as follows: (1) no Mg deficiency (ND): no symptoms of Mg deficiency in leaves were observed; (2) moderate Mg deficiency (MD): only half of the leaves showed symptoms of Mg deficiency; the upper leaves showed no symptoms and chlorosis; (3) severe Mg deficiency (SD): the upper and lower part of leaves showed symptoms of Mg deficiency, and some leaves had died. Based on the classification of Mg deficiency, three greenhouses were selected with which different symptoms in leaves were observed. Each treatment was repeated for three times. The total amount of fertiliser and irrigation are shown in Table 1.

Table 1. Fertilisers and irrigation with different levels of tomato Mg deficiency in solar greenhouse.
Levels of Mg deficiency Organic fertiliser (t/ha) Chemical fertiliser (kg/ha2) Total irrigation (m3/ha)
N P2O5 K2O
ND (n = 3) No deficiency 19 ± 6b 240 ± 68b 287 ± 126a 310 ± 118b 1,447 ± 351a
MD (n = 3) Moderate deficiency 14 ± 4b 281 ± 69b 239 ± 70a 305 ± 70b 1,401 ± 186a
SD (n = 3) Severe deficiency 34 ± 7a 410 ± 21a 396 ± 82a 546 ± 50a 1,350 ± 464a
n is the number of greenhouse soil samples.
Different letters indicated that the difference was significant at P < 0.05.

The effect of Mg fertiliser on soil K, Ca and Mg

The study on the effect of Mg fertiliser was conducted from September to March of the next year in the solar greenhouse No. 11 of Wuquan Town, Shaanxi Province. China (latitude 34° 16’ N, longitude 108° 4’ E), where severe symptoms of Mg deficiency appeared. The ground was covered with a polyethylene film (PE), and the front crop was tomato with a severe deficiency of Mg.

The experiment was conducted using two types of fertilisers: Mg and K. There were three levels of Mg fertiliser as follows: (1) M0 at a rate of 0 kg/ha; (2) M1 at a rate of 45 kg/ha and (3) M2 at a rate of 90 kg/ha. There were two levels of K fertiliser as follows: (1) K1 at a rate of 234 kg/ha, and (2) K2 at a rate of 414 kg/ha. M1and K2 were established on the basis of local farming practices. K1 was established on the basis of local recommended amount. Four treatments – Mg0K2, Mg1K2, Mg2K2 and Mg2K1 – were arranged in a completely randomised design with three repetitions. The size of plot was 15.08 m2. The tomato plants were grown on 9 September. The soil properties before planting are shown in Table 2. Before planting, 135 kg/ha N, 405 kg/ha P2O5 and 135 kg/ha K2O were applied. Top dressing was applied on 6 November, 18 November and 2 December with the same amount. The total amount of fertilisers applied is shown in Table 3. The plants were harvested on 15 March of the next year.

Table 2. Soil properties before planting in greenhouse.
Soil layer (cm) Nitrate nitrogen (mg/kg) Available phosphorus (mg/kg) Available potassium (mg/kg) Exchangeable potassium (mg/kg) Exchangeable calcium (mg/kg) Exchangeable magnesium (mg/kg)
0–20 83.90 98.30 259.50 318.00 3993.75 289.94
20–40 46.87 68.13 126.11 134.00 4100.00 211.94

 

Table 3. Treatments of experiment and fertiliser rate.
Treatment N (kg/ha) P2O5 (kg/ha) K2O (kg/ha) MgSO4 (kg/ha)
Base fertiliser 135 405 135 0
Top dressing Mg0K2 216 57 279 0
Mg1K2 216 57 279 45
Mg2K2 216 57 279 90
Mg2K1 216 57 99 90
Mg0K2: 0 kg/ha MgSO4, 414 kg/ha K2O; Mg1K2: 45 kg/ha MgSO4, 414 kg/ha K2O; Mg2K2: 90 kg/ha MgSO4, 414 kg/ha K2O; M2K1: 90 kg/ha MgSO4, 234 kg/ha K2O.

Sampling and analysis

Soil and leaf samples were collected to study the changes in K, Ca and Mg under different symptoms of Mg deficiency. Soil samples for each greenhouse were collected from a depth of 0 to 0.2 m, and tomato leaves from both the bottoms and tops were cleaned and oven-dried at 60 °C on 15 November.

To analyse the effect of Mg fertiliser on soil chemical properties, the soil samples were selected from a depth of 0 to 0.2 m and the tomato samples were selected and divided into root, stem, upper leaf, lower leaf and fruit after the tomatoes were harvested on 15 March.

The exchangeable K, Ca and Mg contents of soil samples were extracted using a modified version of the sodium acetate (CH3COONa) method at pH 8.2 (Bao, 2005). The exchangeable K was determined using a flame photometric detector (FPD) (Taomsun-6400A, Shanghai, China), and the exchangeable Ca and Mg were measured by atomic absorption spectrophotometry.

The contents of K, Ca and Mg in both upper and lower leaves of tomato were extracted using the dry-ash method. The amount of K was measured using FPD, and the amounts of Ca and Mg were measured by atomic absorption spectrophotometry.

Statistical analysis

Since the exchangeable cations of calcareous soil are primarily K+, Ca2+, and Mg2+, the cation saturation ratios (CSR) of K+, Ca2+, Mg2+ were expressed as follows:

CSRK+ = m(K+) / m(K+Ca2++Mg2+)

CSRCa2+ = m(Ca2+) / m(K+Ca2++Mg2+)

CSRMg2+ = m(Mg2+) / m(K+Ca2++Mg2+)

The data were analysed using a one-way ANOVA variance by SAS 8.1. The Tukey test was used to compare means when there was a significant factor effect on the analysis of variance. The significance levels were fixed at P < 0.05. Graphs were drawn using MS Excel.

3. Results

The effect of a deficiency of Mg on mechanism of K and Ca antagonism

A deficiency of Mg affected not only the contents of K, Ca and Mg in leaves but also the yield of tomatoes. The concentrations of K, Ca and Mg in both upper and lower leaves were significantly reduced with MD and SD in comparison to ND, but there was no difference between MD and SD (Table 4). The yield of tomatoes decreased significantly when Mg deficiency became more serious. Compared with ND, the yield of tomatoes under MD and SD was decreased by 38.02% and 59.53%, respectively (Figure 1).

Table 4. The nutrient content in (a) upper and (b) lower tomato leaves displayed at different levels of Mg deficiency.
Mg deficiency level Leaf weight (g/leaf) K Ca Mg
Concentration (g/kg) Accumulation (mg/leaf) Concentration (g/kg) Accumulation (mg/leaf) Concentration (g/kg) Accumulation (mg/leaf)
Upper leaves ND 2.98a 44.97a 134.90a 65.49a 196.47a 7.18a 21.75a
MD 4.05a 32.13b 128.79a 42.82b 172.95a 4.81b 19.57a
SD 3.79a 27.22b 102.44a 49.55b 185.67a 4.04b 15.35a
Lower leaves ND 5.52a 30.73a 172.20a 91.17a 502.94a 10.07a 55.87a
MD 5.27a 24.82b 127.68a 70.41b 372.23a 7.93b 42.08a
SD 5.88a 21.55b 126.53a 64.48b 381.55a 6.95b 41.18a
ND, no Mg deficiency; MD, moderate Mg deficiency; SD, severe Mg deficiency.
Different letters indicated that the difference was significant at P < 0.05.



Fig 1

Figure 1. The tomato yields affected by different levels of Mg deficiency.
ND, no Mg deficiency; MD, moderate Mg deficiency; SD, Severe Mg deficiency. Different letters indicated the difference was significant at P < 0.05.

Figure 2 shows that the soil exchangeable K content and CSRK+ approximately correlated with enhanced Mg deficiency. The soil exchangeable K content under SD was significantly higher than that with ND (+69.94%) and MD (+35.78%), respectively. The soil CSRK+ under MD and SD increased by 21.74% and 55.73%, respectively, when compared with those under ND. In contrast, the soil exchangeable Mg content and CSRMg2+ were significantly lower under SD compared with those of ND and MD. However, there was no significant difference among the soil exchangeable Ca content and CSRCa2+ under different levels of Mg deficiency.



Fig 2

Figure 2. (a) Exchangeable content and (b) cation saturation ratios of soil K, Ca and Mg under different levels of Mg deficiency.
ND: no Mg deficiency; MD moderate Mg deficiency; SD: Severe Mg deficiency. Different letters indicated that the difference was significant at P < 0.05.

With the enhancement of Mg deficiency, the soil K/Mg (coml/cmol) and Ca/Mg (coml/cmol) increased (Figure 3), while the soil Ca/K (coml/cmol) decreased. The K/Mg of soil under ND, MD and SD were 0.72, 0.88 and 1.35, respectively. The Ca/Mg of soil with SD was significantly higher than that of others. The Ca/K values of soil under ND, MD and SD were 10.65, 8.61 and 6.76, respectively. The Ca/K of soil under MD and SD were 19.15% and 36.53%, respectively, significantly lower than that under ND.



Fig 3

Figure 3. The ratios of soil K/Mg, Ca/Mg and Ca/K under different levels of Mg deficiency.
ND, no Mg deficiency; MD, moderate Mg deficiency; SD, Severe Mg deficiency. Different letters indicated that the difference was significant at P < 0.05.

The correlation between yield, K, Ca and Mg of tomato and soil properties is shown in Table 5. The content of K and concentration of Mg in both upper and lower leaves were negatively correlated with soil exchangeable K, CSRK+ and K/Mg and positively correlated with CSRCa2+and Ca/K. The content of Ca in lower leaves was negatively correlated with soil exchangeable K, CSRK+ and K/Mg but positively correlated with CSRCa2+ and Ca/K. The content of K in both upper and lower tomato leaves was correlated with Ca/K in 99% cases, and the content of K in lower leaves was correlated with CSRK+ in 99% cases. The concentration of Mg in tomato leaves was correlated with CSRK+ and Ca2+/K+ in 99% cases. The tomato yield was negatively correlated with K, CSRK+ and K/Mg in 95% and positively correlated with Ca/K in 99% cases.

Table 5. Correlation between yield, K, Ca, Mg of tomato and soil exchangeable ions and cation saturation ratios.
Soil properties Upper leaves Lower leaves Yield (kg/ha)
K (g/kg) Ca (g/kg) Mg (g/kg) K (g/kg) Ca (g/kg) Mg (g/kg)
Exchangeable K+ (mg/kg) −0.74* −0.37 −0.83** −0.84** −0.72* −0.78* −0.73*
Exchangeable Ca2+ (mg/kg) −0.08 −0.07 −0.34 −0.30 0.10 −0.12 −0.15
Exchangeable Mg2+ (mg/kg) 0.50 −0.02 0.40 0.27 0.47 0.44 0.60
CSRK+ (%) −0.78* −0.40 −0.82** –0.84** –0.80** –0.82** −0.76*
CSRCa2+ (%) 0.72* 0.44 0.69* 0.78* 0.82** 0.77* 0.63
CSRMg2+ (%) 0.53 0.15 0.66 0.56 0.40 0.53 0.63
Ca/Mg (cmol/cmol) −0.40 −0.06 −0.53 −0.42 −0.25 −0.39 −0.51
Ca/K (cmol/cmol) 0.86** 0.56 0.89** 0.93** 0.83** 0.85** 0.83**
K/Mg (cmol/cmol) −0.73* −0.30 −0.80** −0.78* −0.72* −0.78* −0.75*
*Correlation was significant in 95% cases; **correlation was significant in 99% cases.

The effect of mg fertiliser on soil chemical properties

The application of Mg fertiliser caused a decrease in soil content of exchangeable K (Figure 4). Compared with the Mg0K2 treatment, the content of soil exchangeable K under Mg1K2 and Mg2K2 treatments decreased significantly by 15.78% and 27.74%, respectively. There was no significant difference in the soil exchangeable Ca content under different fertiliser treatments. With the raise of application of Mg fertiliser, the content of soil exchangeable Mg increased insignificantly. In comparison with the Mg0K2, Mg1K2 and Mg2K2 treatments, the exchangeable K content under Mg2K1 treatment decreased significantly by 15.78, 14.20 and 4.72%, respectively. There were no differences in exchangeable Ca content under different fertiliser treatments.



Fig 4

Figure 4. (a) Exchangeable content and (b) cation saturation ratios of soil K, Ca and Mg under different Mg fertiliser treatments.
Different letters indicated that the difference was significant at P < 0.05.

As the Mg fertiliser increased, the soil CSRK+ decreased, while the soil CSRMg2+ and CSRCa2+ increased. The soil CSRK+ under Mg1K2, Mg2K2 and Mg2K1 treatments was 13.39, 27.21 and 29.12% lower than that under Mg0K2 treatment, respectively, and the difference between Mg2K2 and Mg2K1 treatments and Mg0K2 treatment was significant. The soil CSRCa2+ of Mg1K2, Mg2K2 and Mg2K1 treatments increased but were not significant (Figure 4).

Compared with Mg0K2, the soil Ca/Mg under Mg2K1 treatment decreased by 16.89% (Figure 5). However, there were no significant differences among the four treatments. Mg fertiliser had no significant effect on soil Ca/Mg. Reducing the application of K fertiliser at the same time with the application of Mg fertiliser can greatly reduce soil Ca/Mg. The soil K/Mg decreased with an increase in application of Mg fertiliser. The soil K/Mg in Mg2K2 treatments was 26.61% lower than that in Mg0K2, while no differences were observed between the soil K/Mg in M1K2 and Mg0K2. The soil K/Mg in Mg2K1 treatment was the lowest among four treatments and was 41.94% and 20.88% lower than that of Mg0K2 and Mg2K2, respectively. With increase in the Mg fertiliser, the soil Ca/K increased. The soil Ca/K in Mg1K2, Mg2K2 and Mg2K1 increased by 17.25, 42.39 and 42.84%, respectively, when compared with those in Mg0K2 treatment.



Fig 5

Figure 5. The ratio of soil K/Mg, Ca/Mg and Ca/K under different fertiliser treatments.
ND, no Mg deficiency; MD, moderate Mg deficiency; SD, severe Mg deficiency. Different letters indicated that the difference was significant at P < 0.05.

The effect of fertiliser on tomato yield and Mg, K and Ca contents

A deficiency of Mg was observed in every treatment. As shown in Table 6, the dry matter of tomato stems and fruits increased, and the dry matter of tomato leaves decreased with the increase in Mg fertiliser. The dry matter of tomato fruit increased by 1.19, 3.19 and 2.55% in M1K2, M2K2 and M2K1 treatments, respectively, compared with that of M0K2. However, the differences were not significant among treatments.

Table 6. Effect of different fertiliser rates on tomato dry matter (kg/ha).
Treatments Roots Stems Leaves Fruits
M0K2 241.43a 799.38a 1087.45a 3213.42a
M1K2 188.49a 837.79a 1081.03a 3251.55a
M2K2 223.59a 841.36a 1076.82a 3315.82a
M2K1 213.36a 808.36a 1076.59a 3295.31a
M0K2: 0 kg/ha MgSO4, 414 kg/ha K2O; M1K2: 45 kg/ha MgSO4, 414 kg/ha K2O; M2K2: 90 kg/ha MgSO4, 414 kg/ha K2O; M2K1: 90 kg/ha MgSO4, 234 kg/ha K2O.
Different letters indicated that the difference was significant at P < 0.05.

When M2K1 was compared with M2K2 (Table 7), the Mg content of tomato fruit decreased by 31.45%, and the K supply decreased by 43.48%. The K content of tomato stem increased as supply of Mg increased, while the K content of lower leaves of tomato decreased. When M2K1 was compared with M2K2, the K content of tomato stems, leaves and fruits decreased significantly with decrease in supply of K. The Ca content of tomato roots and upper leaves decreased, while those of tomato stems and fruit increased as supply of Mg increased. The Ca content of tomato fruit increased by 9.78, 54.46 and 35.11% under M1K2, M2K2 and M2K1 treatments, respectively. There was no difference in the Mg content of tomato roots, stems and leaves among the four treatments. The Mg content of tomato increased as amount of Mg fertiliser increased. The Mg content increased by 30.83, 94.37 and 33.24% under M1K2, M2K2 and M2K1 treatments, respectively, compared with that of M0K2.

Table 7. K, Ca, Mg contents of tomato at different treatments (cmol/kg).
Nutrient Treatment Root Stem Upper leaves Lower leaves Fruits
K Mg0K2 53.72a 71.71b 79.53ab 71.49a 109.45a
Mg1K2 47.24a 81.22a 88.62a 59.24b 101.30a
Mg2K2 48.39a 80.78a 88.87a 59.73b 107.24a
Mg2K1 53.89a 68.55b 75.44b 55.61b 84.65b
Ca Mg0K2 192.64a 199.11b 358.2a 380.27a 18.71d
Mg1K2 155.42b 206.05ab 294.63b 380.32a 20.54c
Mg2K2 167.2b 240.37a 316.74b 401.87a 28.90a
Mg2K1 170.6ab 227.63ab 357.4a 339.96b 25.28b
Mg Mg0K2 17.29a 29.24a 11.11a 24.78a 3.73b
Mg1K2 15.63a 28.26a 10.76a 22.48a 4.88b
Mg2K2 15.39a 31.67a 12.51a 24.04a 7.25a
Mg2K1 15.32a 29.34a 13.78a 23.77a 4.97b
M0K2: 0 kg/ha MgSO4, 414 kg/ha K2O; M1K2: 45 kg/ha MgSO4, 414 kg/ha K2O; M2K2: 90 kg/ha MgSO4, 414 kg/ha K2O; M2K1: 90 kg/ha MgSO4, 234 kg/ha K2O.
Different letters indicated that the difference was significant at P < 0.05.

The results in Table 8 show that there was no difference in K and Mg uptake of tomato roots, stems, leaves and Ca uptake of tomato roots and leaves among the treatments as supply of Mg increased.

Table 8. K, Ca and Mg uptake of tomato in different treatments (kg/ha).
Nutrient Treatments Roots Stems Leaves Fruits Aboveground Total
K M0K2 5.19a 22.37ab 31.36a 136.67a 190.40a 195.59a
M1K2 3.49a 26.48a 26.58ab 128.01ab 181.06a 184.55a
M2K2 4.25a 26.51a 27.09ab 138.12a 191.72a 195.97a
M2K1 4.56a 21.65b 24.99b 108.41b 159.61b 159.61b
Ca M0K2 9.19a 31.81b 85.41ab 11.79c 129.19b 138.39b
M1K2 5.86a 34.54b 87.31ab 13.31c 135.16b 141.01b
M2K2 7.47a 40.36a 92.77a 19.06a 152.19a 159.66a
M2K1 7.39a 36.49ab 78.59b 16.60b 131.68b 139.06b
Mg M0K2 1.03a 5.67a 6.78a 2.90c 15.35b 16.38b
M1K2 0.72a 5.75a 6.26a 3.82b 15.83b 16.55b
M2K2 0.83a 6.44a 6.73a 5.80a 18.97a 19.80a
M2K1 0.80a 5.77a 6.70a 3.95b 16.42ab 17.22ab
M0K2: 0 kg/ha MgSO4, 414 kg/ha K2O; M1K2: 45 kg/ha MgSO4, 414 kg/ha K2O; M2K2: 90 kg/ha MgSO4, 414 kg/ha K2O; M2K1: 90 kg/ha MgSO4, 234 kg/ha K2O.
Different letters indicated that the difference was significant at P < 0.05.

There was no difference in the uptake of K by tomato roots, stems and leaves as supply of Mg increased among treatments. When the supply of K decreased by 43.48%, the uptake of K decreased in tomato aboveground parts (16.75%), such as the stems (18.33%), leaves (7.75%) and fruit (21.51%) when M2K1 was compared with M2K2.

The uptake of total Ca by tomato aboveground parts, such as the stems, leaves and fruit increased as supply of Mg increased. The uptake of Ca by tomato aboveground parts in the total plant was the highest under M2K2 treatment and was significantly higher than those under M0K2, M2K2 and M2K1 treatments. The uptake of Ca by tomato decreased as supply of K decreased.

The uptake of Mg by tomato fruit increased significantly by 31.72, 100 and 36.21% under M1K2, M2K2 and M2K1 treatments, respectively, compared with that of M0K2 treatment. There was no significant difference in the uptake of Mg by tomato roots, stems and leaves among the four treatments.

4. Discussion

Magnesium affected by the potassium–calcium antagonistic interaction under a deficiency of magnesium in tomato

The yield and content of K, Ca and Mg were reduced because of Mg deficiency, which is similar to the results of other research. It was reported that the initial physiological response of Mg deficiency for plants was the accumulation of carbohydrates in leaves, and the suppression of supply of carbohydrates to roots (Cakmak, 2013; Farhat and Bouraoui, 2013), which affected root growth and nutrient absorption (Cakmak and Kirkby, 2008). An Mg deficiency may induce an imbalance of active oxygen metabolism in plants by suppressing photosynthetic carbon assimilation and causing a decrease in yield (Cakmak and Kirkby, 2008).

In this study, the contents of soil Mg with ND, MD, SD were 2.25, 2.31 and 2.05 cmol/kg, respectively. The content of soil Mg was four times greater than the established Mg deficiency threshold of Chinese soil (0.5 cmol/kg) (Yuan, 1983). It is clear that there was no deficiency of Mg in the first 3–5 years before experiment. One could hypothesize that calcareous soil could provide enough Mg nutrients for plant growth. A lack of application of Mg was not the reason for Mg deficiency.

The soil exchangeable K and CSRK+ increased, and the soil exchangeable Mg and CSR Mg2+ decreased as the deficiency of Mg became more serious. Similar results were described by Chen et al. (2013). CSRK+ with ND and MD was 7.68% and 9.35% lower, respectively, and CSRMg2+ with ND and MD was 10.66% and 10.61% greater, respectively, compared with SD. White (2006) has proved that the appropriate ratio of cation saturation in soil colloids should be 80% of Ca2+, 15% of Mg2+ and 5% of other cations such as K+, Na+ and NH4 +. As compared with ND, the soil CSRK+ under a deficiency of Mg was much higher, and the CSRMg2+ was much lower. The changes might be caused primarily by the change in soil exchangeable K.

The soil K/Mg with ND, MD and SD was 0.72, 0.88 and 1.35, respectively. The soil K/Mg with MD and SD was 22.22% and 87.50% higher, respectively, than that with ND. It was reported that when the K/Mg ratio was more than 0.4~0.7 (Chapman, 1966; Kochian, 1995), the K will significantly inhibit absorption of Mg, which confirms the results of this study. The soil K/Mg without a deficiency in Mg was closer to the limit value, and the soil K/Mg with MD and SD was much higher than the limit value. The soil Ca/Mg with ND, MD and SD was 7.66, 7.55 and 8.98, respectively. Those were all larger than the limit values (<7.0) provided by Chapman (1966). The results described above indicate that Ca was one of the reasons for deficiency of Mg.

The contents of K, Ca and Mg in leaves and yield of tomato were negatively correlated with soil exchangeable K, CSRK+ and K/Mg but positively correlated with CSRCa2+and Ca/K. Since soil exchangeable Ca and Mg changed so little, we can be confident of the result: the deficiency of Mg was caused by an increase in K. The increase in the content of soil K could affect the imbalance of ion ratio and induce deficiency in Mg and reduction in yield (Cakmak, 2013; Cakmak and Kirkby, 2008; Mengel and Kirkby, 2001; Römheld and Kirkby, 2007; White, 2006). Farmers provided much more K fertiliser for tomato with Mg deficiency, which was not necessary according to the research done of Miller and Donahue (1995). Their research showed that the appropriate available K for vegetables was 150–250 mg/kg. The available K was 295.50 mg/kg in this study. The antagonism between K+ and Mg2+ is related to their similar chemical properties (Viadé et al. 2011). It was reported that some Mg transporters might also transport K. Hence, a high availability of K in the soil/rhizosphere blocks these nonspecific Mg transporters for Mg uptake (Gransee and Führs, 2013). However, there were studies that showed that cations, such as H+, Na+, NH4 + and Al3+, could also affect the absorption of Mg (Mengel and Kirkvy, 2001). Additional studies should be conducted to focus on the mechanism and correction of Mg deficiency in calcareous soils.

The corrective effect of supply of magnesium and decrease of potassium on soil chemical properties, tomato yield and nutrient uptake

Supplying Mg and decreasing K did not correct the Mg deficiency of tomato. The results were the same as those identified by Wang et al. (2017). Supplying Mg and decreasing K caused a decrease in the soil K/Mg. The K/Mg of soil in M0K2, M1K2, M2K2 and M2K1 was 1.24, 1.06, 0.91 and 0.72, respectively. However, the soil K/Mg was larger than the limit of Mg deficiency as described previously (Chapman, 1966; Kochian, 1995). Hence, the Mg deficiency was observed in every treatment. The soil K/Mg was the lowest under M2K1 treatment, indicating that reducing the application of K was a quick way to restore cation balance.

In this study, the dry matter of tomato fruit increased by 1.19% and 3.19% with 45 kg/ha MgSO4 and 90 kg/ha MgSO4 supplied by topdressing, respectively. Wang et al. (2017) found that the yield was improved by 8.0, 8.9 and 5.3% when 0.2% MgSO4·7H2O, 0.4% MgSO4·7H2O and 0.2% Mg (NO3)2·6H2O were supplied by foliar sprays, respectively. The difference could be caused by the manner in which Mg fertiliser was supplied. Foliar spraying could be a more effective mode to supply fertiliser for tomato growth.

The differences in K, Ca and Mg contents and uptake were primarily observed in tomato fruit. The content and uptake of Ca and Mg increased as supply of Mg increased significantly, and the content and uptake of K, Ca and Mg decreased as supply of K decreased. There was no significant difference in the content and uptake of Mg by tomato leaves among the four treatments. Perhaps the transport of Mg from leaves to fruit was one of the reasons that supplying Mg and decreasing K supply was not effective at correcting deficiency of Mg. In addition, Wang et al. (2017) observed the same results. The content of Mg in tomato leaves decreased by 17.4–35.1% with tomato fruit growth.

5. Conclusions

A deficiency in Mg had a negative influence on tomato growth. The yield of tomatoes decreased significantly by 38.02% and 59.53% with MD and SD, respectively, compared with that of ND. The increase in content of soil K that caused an imbalance in ion ratio was the main reason for Mg deficiency of tomato in calcareous soil. The amount of soil exchangeable K, CSRK+ and K/Mg increased, and the soil exchangeable Mg and CSR Mg2+ decreased as deficiency of Mg grew more serious. The K/Mg was higher than the limit value of Mg deficiency. Supplying Mg and decreasing the supply of K did not correct Mg deficiency of tomato during the first year. However, the yield of tomatoes increased, and the K/Mg of soil decreased as supply of Mg increased. Additional studies should be conducted to verify these results. The soil K/Mg under M2K1 treatment was the lowest (0.72) among treatments and could provide enough nutrition for tomato growth, indicating that a reduction in the supply of K while supplying Mg was a much more efficient way to decrease the soil K/Mg and restore cation imbalance than providing Mg fertiliser in calcareous soil.

Acknowledgement

The authors would like to thank the farmers for generously sharing greenhouse and tomato. And Thanking Y. Hou for her help with the grammar and spelling of this article.

Conflict of interest

The authors have declared no conflict of interest in this article.

Compliance with ethical standards

This article followed all ethical standards for a research without direct contact with human or animal subjects.

Funding

This research was supported by the project of Shaanxi Provincial Land Engineering Construction Group Co. Ltd. (DJNY 2019-17).

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