Magnesium Fertilizer Application and Soil Warming Improves Magnesium Absorption and Yield of Tomatoes in Greenhouse

Background: Magnesium (Mg) is an essential macronutrient that plays an important role in numerous physiological and biochemical processes in plants. However, Mg deciency is common worldwide, especially in greenhouse vegetable systems, due to the overuse or misuse of fertilizers. This study aimed to investigate different Mg application strategies for alleviating Mg deciency in tomatoes grown in a greenhouse vegetable system. Methods: Six eld treatments were used: the farmer conventional fertilization practice (CK), CK + soil application of Mg fertilizer (MgS), CK + foliar spray of Mg fertilizer (MgF), CK + the combination of soil and foliar spray of Mg fertilizer (MgSF), CK + soil warming (T), and MgSF + soil warming (MgSFT). Results: The results showed that the foliar spray of Mg fertilizer treatments (MgF, MgSF, and MgSFT) increased the total Mg uptake and Mg content of functional leaves in both winter-spring and autumn-winter seasons. Soil warming treatments (T and MgSFT) were also benecial for Mg uptake and chlorophyll biosynthesis compared with no-warming treatments (CK and MgSF), especially in the autumn-winter season. Additionally, Mg fertilizer application and soil warming increased tomato yields; the MgSFT treatment had the highest increase in yields compared with the CK treatment. Conclusion: Therefore, foliar Mg fertilizer application combined with soil warming, while considering seasonal variation, is feasible for reducing Mg deciency in tomatoes under greenhouse vegetable systems.


Background
The areas with low available soil Mg content in China are mainly located south of the Yangtze River; it is generally believed that the soil in the northern Chinese region is rich in Mg because of low rainfall and weak leaching [1,2]. However, Mg de ciency in greenhouse vegetable crops in northern China has occurred frequently in recent years, leading to a decrease in crop yield [3][4][5].
The main reason for Mg de ciency is that the Mg absorbed by crops from the soil is not replenished, and too much nitrogen, phosphorus, and potassium are applied to greenhouse elds, resulting in an imbalance in the soil nutrients, especially the excessive K/Mg ratio in the soil, leading to a Mg de ciency in crops [3,[6][7][8]. For example, the unbalanced nutrients in greenhouse soils in Shouguang, Shandong Province, China, are re ected as a surplus of nitrogen, phosphorus, and potassium, which has reached the following values: 1668 kg ha −1 N, 1801 kg ha −1 P 2 O 5 , and 20 kg ha −1 K 2 O [9]. In addition, according to our preliminary investigation, Mg de ciency was more likely to occur in the autumn-winter season during the greenhouse tomato cultivation in Shouguang, Shandong Province, China. Solar greenhouses, which are the dominant type of vegetable production system covered with polyethylene foliage in China, are typically 70-100 m long and 7-12 m wide. During winter, greenhouses are additionally covered with carpets made of straw at night but do not have any heating function, and this low soil temperature can limit crop nutrient absorption [8, 10,11]. Many studies have shown that applying Mg fertilizers can improve Mg absorption in tomatoes, which is re ected by the content increase of chlorophyll and Mg in the tomato leaves as well as the growth of tomato yield; however, most of these reports are substrate, pot, or hydroponic cultures, and there are scarce results from eld experiments, especially in eld under greenhouse cover [4,12,13]. Therefore, this study was carried out to investigate the effects of different methods of Mg fertilizer application, including soil application, foliar spray, soil warming, and their combination on improving Mg nutrition in greenhouse tomatoes, to provide a reference for practical production.

Description of the experimental site
The eld experiment was conducted in Shouguang City, Shandong Province, China (36°55 N, 118°45 E) from February 2010 to January 2011. A typical vegetable greenhouse (ground area, 73.0 × 11.5 m 2 ), which has been used to grow tomatoes for 2 years, was selected for the experiment. The soil samples were collected and analyzed at the beginning of the experiment, and the soil characteristics at the study site are listed in Table 1.

Sample collection and analysis
The air temperature at 1 m above the ground in the greenhouse and soil temperature at a depth of 20 cm in each plot were measured from 8:30 am to 10:00 HR. Functional leaves, which are the rst matured leaves below the 2nd-5th fruit cluster, were collected at different vegetative growth stages before topping, and then carefully washed with tap water for further analysis. Chlorophyll concentrations of a 95% (v/v) ethanol extract of fresh tomato leaves were determined using the method described by Arnon [14].
During the nal harvesting stages, plant and fruit samples were collected, oven-dried at 75 °C for at least 48 h, and then ground to ne powder; then, the N, P, K, and Mg contents were analyzed using an elemental analyzer (Costech ECH 4024, Picarro, Italy). For each plot, the tomatoes were picked and weighed at each harvest, and the total yield was calculated as the cumulative weight of tomatoes from all harvest days.

Statistical analysis
All data were analyzed using the SPSS software (SPSS Inc., Chicago, IL, USA). The data were analyzed using analysis of variance, and the least signi cant difference test was employed to determine signi cant differences between treatments at P < 0.05.

Results
Changes in soil temperature and greenhouse temperature The air temperature in the tomato greenhouse during the WS season increased gradually and nally reached a balance at approximately 24°C, while the temperature during the AW season decreased gradually and reached 15°C at the end of harvest in January (Fig. 1a). A similar trend in soil temperature was also found in the unheated tillage layer in the greenhouse (Fig. 1b). The rootzone soil temperature during the WS season increased from 18.2°C in mid-March to 22.8°C, while that during the AW season decreased from 23°C in mid-September to 14.5°C at the end of the harvest. The average temperature of the rootzone without heating was 21.5°C during the WS season, which was higher than that during the AW season (19.2 ℃) at the fruit enlargement stage (Fig. 2).
For the soil warming treatments, the average daily soil temperatures of the rootzone at the fruit expansion stage were 23.1 ℃ and 22.1 ℃ during the WS and AW seasons, respectively, which were 3-5 ℃ higher than that in the unheated soil Effects of Mg fertilizer application on Mg and potassium contents in the functional leaves of tomatoes The changes in K and Mg content of the tomato functional leaves at different periods are shown in Table 2. The MgF, MgSF, and MgSFT treatments signi cantly increased the Mg content in functional leaves compared with the CK and MgS treatments during both the WS and AW seasons, but no signi cant differences were found between the CK and MgS treatments. Compared with the CK treatment, the average Mg content of the functional leaves in the MgF, MgSF, and MgSFT treatments increased by 10.6%, 10.0%, and 10.5% during the WS season, and by 6.6%, 7.6%, and 14.5% during the AW season, respectively. Table 2 The Mg and K contents (g kg −1 ) and K/Mg ratio in functional leaves in two tomato seasons at different growth periods respectively, in the fourth and fth functional leaves during the AW season. The above results indicate that soil warming could improve the Mg absorption by tomatoes. Generally, the Mg content of functional leaves during the WS season was 2-3 fold higher than that during the AW season, which was probably due to the higher soil temperature during the WS season.
Application of Mg fertilizer without soil warming had no signi cant effects on the K content in the functional leaves of tomatoes during the WS season; however, the MgSFT treatment signi cantly decreased the K content in the functional leaves by 29.74% and 32.4% in the third and fourth functional leaves, respectively, compared with the MgSF treatment. The K contents of the functional leaves in the second and third fruit clusters under Mg fertilizer application treatments with or without soil warming were all lower than those under CK treatment.
In general, the application of Mg fertilizers as well as warming tended to increase the Mg content and decrease the K content in functional leaves, resulting in a decrease in the K/Mg ratio in the functional leaves. The most signi cant decrease in the K/Mg ratio appeared under the MgSFT treatment, which decreased the ratio by 30.6% and 29.4%, on average, during the WS and AW seasons, respectively.

Effects of Mg fertilizer application on chlorophyll content of the functional leaves of tomatoes
The chlorophyll content in the functional leaves of tomatoes from different functional leaves is shown in Table 3. During the WS season, there were no signi cant differences in the chlorophyll contents of the functional leaves between the different treatments, except for the fourth fruit cluster. During the AW season, the chlorophyll contents of functional leaves under the MgF, MgSF, MgSFT, and T treatments were signi cantly higher than those under the MgS and CK treatments; the T treatment signi cantly increased the chlorophyll content by 29.2% in the third functional leaves compared with the CK treatment, while the MgSFT treatment signi cantly increased the chlorophyll content of functional leaves by 5.2% and 13.7%, respectively, in the second and third functional leaves, compared with the MgSF treatment. During the AW season, it was found that the chlorophyll contents were higher under the Mg spray treatments (MgF, MgSF, and MgSFT) than those under other treatments, and those of the soil warming treatments were higher than those under the non-warming treatments. Table 3 The chlorophyll contents of functional leaves in different treatments of Mg fertilizers application (mg g −1 ) Treatments

Effects of Mg fertilizer application on nutrient absorption by tomatoes
It can be seen that Mg application and soil warming had large effects on nutrient absorption (Table 4). Compared with the CK treatment, the N and P uptake by tomatoes was improved by Mg application and soil warming treatments during both the WS and AW seasons, and the spraying of Mg fertilizer on leaves was more bene cial for the uptake of N and P compared with soil application.
The K uptake by tomatoes in the soil warming treatments (T and MgSFT) was lower than that under the CK treatment during the WS season, while the opposite results were observed during the AW season. Effects of different treatments on tomato yield As shown in Table 5, compared with the CK treatment, the Mg fertilizer application treatments increased the tomato yields, of which the highest yield was found in the MgSFT treatment with 15.7% and 19.4% increases, respectively, during the WS and AW seasons. Soil warming treatments (T and MgSFT) also signi cantly increased tomato yields compared to no-warming treatments, except for those between T and CK treatments during the WS season. The average yield in the soil warming treatments (T and MgSFT) was 8.2% and 5.7% higher than those in the CK and MgSF treatments, respectively. In addition, foliar spraying of Mg fertilizers had more bene cial effects on tomato yields than did soil application. Different letters in the same column means signi cant differences between different treatments at P < 0.05.

Discussion
Mg is an essential nutrient element for plants, and Mg de ciency can change the metabolism of active oxygen, photosynthesis, and distribution of assimilates in crops, which will ultimately affect the yield and quality of agricultural products [4,15,16]. According to a meta-analysis of 70 years of research, the dry matter formation of species is inhibited when leaf Mg concentrations are lower than 0.35% [17]. In this study, the leaf Mg concentration under the CK treatment ranged from 5.5 to 6.3 g kg −1 and 2.0 to 2.8 g kg −1 during the WS and AW seasons, respectively (  (Tables 2 and 3). The average yield of greenhouse tomatoes varied from 88 t ha −1 to 108 t ha −1 according to the study by Lv et al. [21], which was similar to the results of the present study. These results indicate that the yields obtained in our study were typical grower yields in the mentioned area.
However, under the soil application of Mg treatment, the tomato yield was close to that under CK, and the yield increase was lower than that under the Mg spray treatment (Table 5). In this experiment, 150 kg hm −1 of MgSO 4 was applied to soil, which was the same amount used in the experiment conducted by Li et al. in Shijiazhuang [22]; however, no similar yield increase was observed.
Comparing the two experiments, it can be found that the available soil K content in this experiment was 232 mg kg −1 , whereas it was 126 mg kg −1 in the experiment by Li et al. [22], indicating that Mg uptake was seriously inhibited by high soil K levels, which were caused by a high K fertilizer usage in the present study. Soil investigation suggested that planting tomatoes in soil with high available K content and high K/Mg ratio could reduce Mg content in leaves and tomato yield. The antagonistic levels of K and Mg may be the reason behind the Mg de ciency in crops [8, 13,19]. According to a study by Li et al. [19], when the K/Mg ratio was increased from 4:1 to 8:1, the total biomass and Mg uptake of tomatoes decreased signi cantly, con rming that high K levels inhibited Mg uptake and plant growth. Therefore, in this experiment, the observed results may be due to the relatively high content of K in soil, and the amount  [27,28]. The results of this experiment showed that the Mg content in leaves and Mg amount consumed by tomatoes during the WS season (variety: Hongluoman) were 2 to 3 times higher than those during the AW season (variety: Labi), which were probably caused by the difference in varieties. Increasing the application of Mg fertilizer can increase tomato yield and farmer income [19,29,30]. In addition, soil warming can also increase yield (Table 5), and thus is a recommended treatment for this purpose.

Conclusions
Mg fertilizer application can improve Mg absorption by greenhouse tomatoes. The combination of foliar spray and soil warming showed the highest increase in Mg content and uptake of functional leaves during both the WS and AW seasons. In addition, Mg fertilizer application and soil warming also improved tomato yields, and the combination of foliar spray and soil warming treatment showed the best effect compared with the control. Therefore, foliar spraying of Mg fertilizer and soil warming are two reasonable ways to reduce Mg de ciency in tomato production. Air temperature at 1 m above the ground measured at 8:30-10:00 am in tomato greenhouse (a). Soil temperature at a depth of 20 cm measured at 8:30-10:00 am in tomato greenhouse (b).

Figure 2
Soil temperature at a depth of 20 cm measured at 8:30-10:00 am after soil warming in winter-spring season (a) and autumn-winter season (b).