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Abstract Calcium is an important element that affects vegetable quality and human health. In this paper， the basic nutrition and functions of calcium， the effects of calcium deficiency on vegetable growth and human health， the effect of calcium supply on vegetable quality and the outlook for calcium rich vegetables were overviewed， aiming at providing guidance for the cultivation of high quality calcium rich vegetables.
Key words Calcium; Vegetable quality; Human health
Basic Nutrition and Function of Calcium
Calcium is a medium element required for normal growth and development of plants. Calcium ion can act as a bridge for the collection of the phosphate in phospholipids and the carboxyl group of proteins. It has the function of stabilizing cell membrane structure and enhances the ability of biofilm to selectively absorb ionic nutrients. Calcium is the component and activator of some enzymes in plants， which has a certain influence on the metabolism of nitrogen and carbohydrates.
Calcium is also an important element affecting human health. Calcium is the most important and most abundant mineral element in human body. It not only participates in the composition of bones and teeth， but also participates in metabolism， and plays an important role in regulating the physiological and biochemical reactions in the body. Calcium can help and reduce the excitability of nerve cells and is thus a natural sedative. It also has the effects of strengthening the conduction function of the nervous system， maintaining the normal excitation of muscle nerves， reducing the permeability of cells and capillaries， promoting the activities of various enzymes in the body， maintaining acid base balance in the body and participating in the blood coagulation process. In short， calcium is an indispensable trace element in human body. It is both a constructor of the body and a regulator of the body. It is the source of life for our body.
Effect of Calcium Deficiency on Vegetable Growth and Human Health
Calcium and vegetable diseases
Vegetables require more calcium than other crops. Even in calcareous soils， the soils are not deficient in calcium， but physiological calcium deficiency occurs in vegetable production， especially in facility cultivation with high intensification[1-2]. Jiang et al. [3] compared the exchangeable calcium and magnesium contents and Ca/Mg values in 43 pairs of vegetable protected areas and adjacent open soils in the suburbs of Shenyang. The results showed that compared with the terrestrial soil， the average content of soil exchangeable calcium in the protected areas decreased by 6.38%， the Ca/Mg value decreased by 10.34%， and the average content of exchangeable magnesium increased by 4.74%. The main reason for the decrease in exchangeable calcium content is that the amount of calcium that is exported to the outside of the system with vegetables is relatively high， the application amount of calcium containing fertilizers is reduced and the pH of the soil is lowered. Calcium deficiency caused the decrease of leaf chlorophyll content， the increase in the incidence of soft rot， and the decrease of both fresh and dry weights of leaves; and at the middle and late growth stages， especially the new leaves are prone to calcium nutrition disorder. Calcium and human health
Calcium is an important component of the body. When the calcium content in the blood is lower than 70 mg/L， the neuromuscular excitability increases， accompanied with convulsions， irritable bowel syndrome， etc. The calcium deficiency in human body reduces the elasticity and toughness of the soft tissue and the excitability of nerve cells in the environment. Calcium intake in the diet is difficult to meet physiological needs， and it is easy to cause calcium deficiency， which leads to skeletal dysplasia and osteoporosis. Calcium is mainly distributed in bones and teeth in human body， and it is also distributed in the blood and various body fluids， thereby playing an important role in maintaining the normal metabolism of human body. Normal adults contain 1 000-1 200 g of calcium. Calcium is not only an indispensable component to ensure the integrity of the body， but also plays an extremely important role in maintaining life in various physiological and biochemical processes.
Effect of Calcium Supply on Vegetable Quality
Vegetable nutrition quality
Calcium has a close relationship with the growth and quality of vegetable crops. Zhu et al.[4] studied the effects of calcium application on the yield and quality of five lettuce varieties in acid soil by pot experiment. The results showed that the application of calcium fertilizers increased the yield of lettuce in each variety with a significant difference; and it can also reduce nitrate content and increase vitamin C， soluble sugar and amino acid contents. Sun et al.[5] studied the effects of the liquid calcium fertilizer on the yield and quality of Chinese cabbage. It was pointed out that spraying the liquid calcium fertilizer with calcium nitrate as a main component could increase the yield of Chinese cabbage by 10.5% ， reduce nitrate content by 15.1% and increase vitamin C content by 23.8%. However， Fan and Yi[6] reported that a too high calcium concentration in the nutrient solution would reduce the absorption of potassium and magnesium by leaf lettuce， but has a little effect on the absorption of chlorine and phosphorus; and improving the proportion of NH+ 4 N in the nutrient solution would inhibit the absorption of calcium by crops and reduce its effectiveness. Feng et al.[7] conducted a pot experiment in Holland greenhouse with the cucumber nutrition solution formula of the Horticultural Research Institute as a control， to investigate the effect of calcium on the quality of cucumber by changing the content of calcium in the nutrient solution and the concentration of the nutrient solution. The results showed that with the concentration of the nutrient solution increasing， the content of soluble sugar also increased. Li et al.[8] studied the effects of spraying different calcium fertilizers on the yield and quality of lettuce. The results showed that the CaSO 4 treatment can significantly promote the vitamin C and soluble sugar contents of leaves， and the CaCl 2 treatment can significantly increase the nitrate content in leaves and the vitamin C content in stems. Different calcium fertilizers are applied to vegetable crops， and their effects are different. Xiao et al.[9] pointed out that calcium nitrate and calcium hydroxide had obvious effects on increasing vegetable yield and improving quality components， and were capable of increasing vitamin C and sugar content， reducing acid content and increasing mineral nutrient content， followed by magnesium sulfate. However， the application of calcium chloride had the effect of reducing the quality of vegetables. Among the several vegetables tested， the effect of calcium fertilizers was better for fruits and vegetables than for root vegetables.
Vegetable nitrate content
When the calcium concentration is consistent， there is a significant positive correlation between nitrate content and nitrogen supply concentration. This may be because increasing the amount of nitrogen fertilizer can improve the nitrate reductase activity of vegetables， but the reduction of nitrate by vegetables is still less than absorption， and the absorbed nitrate cannot be reduced in time and is then accumulated in vegetables. At the same time， calcium nutrition also has a significant effect on nitrate content. Under the same nitrogen supply concentration， increasing the calcium supply concentration can also significantly promote the accumulation of nitrate， which may be due to that the excessive calcium ions inhibit the activity of nitrate reductase to a certain degree， which results in the absorption of NO 3- N greater than assimilation， causing accumulation of nitrate in a large quantity in plants.
Oxalic acid content
Calcium nutrition has a significant effect on the oxalic acid content of vegetables. Oxalic acid and nitrate contents are two important indicators that affect the safety of vegetables. It is well known that in plant cells， calcium has important physiological and biochemical effects as a second messenger participating in plant growth and aging， photosynthetic electron transport and photosynthetic phosphorylation， cell tropism and hormone regulation. Calcium is also an important component of calcium oxalate， and adjusting the calcium concentration will inevitably affect the content of oxalic acid in spinach. Zhang[10] studied the effects of different nitrogen and calcium nutrition on the nutritional quality of spinach. It was found that when the supplied calcium concentration was 2 mmol/L， the soluble oxalic acid and total amount of oxalic acid decreased first， then increased and decreased again with the increase of nitrogen concentration. Zindler Frank et al.[11] pointed out that the oxalic acid content of Phaseolus vulgaris increased with the increase of calcium supply level. De et al.[12] also reported that the amount of calcium oxalate crystals in tomato pericarp also increased with the supplied calcium concentration increasing， and under the same nitrogen concentration， the soluble oxalic acid content decreased significantly with the increase of calcium concentration. This means that the calcium ions absorbed by the plant body combine with soluble oxalic acid to form insoluble calcium oxalate， which causes the reduction of soluble sodium oxalate and potassium oxalate. When the concentration of calcium was 5 and 8 mmol/L， the content of oxalic acid in spinach decreased significantly with the increase of nitrogen concentration from 4 to 8 mmol/L， but was basically stable with the further increase of nitrogen concentration， which may be related to the absorption and reduction of NO- 3 N in spinach. When the calcium concentration was 5 mmol/L， the total amount of oxalic acid was the lowest， and it was relatively higher under the conditions of low calcium and high calcium concentrations. Prospect of Calcium rich Vegetables
For most adults， the recommended daily calcium absorption is 1 000 mg/d， and the daily intake per capita in urban and rural areas in China is only 405.2 mg， which is only 40.5% of the recommended supply for adults. For the elderly， the absorption of calcium is 1 300-1 700 mg/d[13-14]. The differences between the calcium intake of children， adolescents and pregnant women and the recommended dietary supply are greater. More than 95% of children have a severe inadequate intake of calcium in their diet， which is about half of the recommended calcium intake for healthy development. The national per capita calcium intake is only about 40% of the recommended dietary supply of the Chinese Nutrition Society. The main source of calcium in humans is food. In recent years， calcium supplementation through the food chain has been considered to be the safest and most effective calcium supplementation pathway. Vegetables are one of the essential foods in peoples daily diet. Calcium can increase the vitamin C， soluble sugar and mineral nutrient contents of vegetables. It is of great practical significance to improve the bodys absorbable calcium content in vegetables through the regulation of nutrient regulation and planting of calcium rich vegetables.
Calcium rich vegetables could supplement calcium instead of medicines. This calcium supplement method is not only safe and convenient， but also has an obvious calcium supplementation effect. At the same time， vegetables are an important part of the human diet and an indispensable daily necessity. However， at present， the use of vegetables to convert inorganic calcium into organic calcium is still in the experimental stage， and it will still take a long term process to finally realize its commercialization. Therefore， solving the enrichment technology of calcium in vegetables and providing theoretical basis and practical reference for the development of plant foods that are easily absorbed by the human body and rich in calcium will be the main direction of future research.
Weiling YUAN et al. Advances in Research on the Effects of Calcium on Vegetable Quality and Human Health
References
[1] ZHANG ZX， YU XC. Principle and technology of vegetable fertilization[M]. Beijing： China Agriculture Press， 1996. （in Chinese）
[2] BAI GY， HUANG DM. New technology of vegetable formula fertilization[M]. Beijing： China Agriculture Press， 1999. （in Chinese） [3] JIANG Y， ZHANG YG， LIANG WW. Soil exchangeable Ca and Mg contents and Ca/Mg ratio in greenhouse vegetable fields in Shenyang suburbs[J]. Rural Eco Environment， 2004， 20（3）： 24-27. （in Chinese）
[4] ZHU HX， DI CX， WANG ZY， et al. Effects of calcium on the yield and quality of lettuce grown in acid soil[J]. Journal of Southwest Agricultural University： NaturalScience， 2005， （4）： 456-463. （in Chinese）
[5] SUN MD， ZHANG L， CAO B. Effects of paste foliage fertilizer and liquid calcium on the yield and quality of chinese cabbage[J]. Chinese Agricultural Science Bulletin， 2006， 22（6）： 255-257. （in Chinese）
[6] FAN SX， YI DZ. Effects of calcium on nutrient absorption and growth and development of Lactuca sativa var. longifolia Lam. in nutrient film technique culture[J]. Acta Horticulturae Sinica， 2002， 29（2）： 149-152. （in Chinese）
[7] FENG J， LIU RP， LUO HY. Effect of calcium level on growth， yield and quality of cucumber in substrate culture[J]. Soil and Fertilizer Sciences in China， 2016（2）： 104-108. （in Chinese）
[8] LI B， WANG CQ， FENG CC， et al. Effects on the Yield and Quality of Lettuce by the Different Calcium Fertilizers Spraying[J]. Hans Journal of Soil Science， 2004， 2（20）： 129-131. （in Chinese）
[9] XIAO CP， YANG ZQ. Influence of different calcium fertilizer and magnesium fertilizer on yield and quality of vegetables[J]. Journal of Changjiang Vegetables， 2000， （4）： 27-29. （in Chinese）
[10] ZHANG YP. Effects of nitrogen and calcium nutrition on nitrogen metabolism and nutritional quality of spinach[D]. Hangzhou： Zhejiang University， 2005. （in Chinese）
[11] ZINDLER FRANK E， HONOW R， HESSE A. Calcium and oxalate content of the leaves of Phaseolus vulgaris at different calcium supply in relation to calcium oxalate crystal formation[J]. J Plant Physiol， 2001， 158： 139-144.
[12] DE KC， JANSE J， VAN GBJ， et al. The incidence of calcium oxalate crystals in fruit walls of tomato （Lycopersicon esculentum Mill.） as affected by humidity， phosphate and calcium supply[J]. Journal of Horticultural Science， 1992， 67： 45-50.
[13] LIU JJ. Study on the migration and transformation of calcium in soil and the calcium enriching characteristic in vegetables and its influencing factors[D]. Taian： Shandong Agricultural University， 2006. （in Chinese）
[14] MEAGY MJ， EATON TE， BARKER AV. Nutrient density in lettuce cultivars grown with organic or conventional fertilization with elevated calcium concentrations[J]. Hortscience， 2013， 48（12）： 1502-1507.
Key words Calcium; Vegetable quality; Human health
Basic Nutrition and Function of Calcium
Calcium is a medium element required for normal growth and development of plants. Calcium ion can act as a bridge for the collection of the phosphate in phospholipids and the carboxyl group of proteins. It has the function of stabilizing cell membrane structure and enhances the ability of biofilm to selectively absorb ionic nutrients. Calcium is the component and activator of some enzymes in plants， which has a certain influence on the metabolism of nitrogen and carbohydrates.
Calcium is also an important element affecting human health. Calcium is the most important and most abundant mineral element in human body. It not only participates in the composition of bones and teeth， but also participates in metabolism， and plays an important role in regulating the physiological and biochemical reactions in the body. Calcium can help and reduce the excitability of nerve cells and is thus a natural sedative. It also has the effects of strengthening the conduction function of the nervous system， maintaining the normal excitation of muscle nerves， reducing the permeability of cells and capillaries， promoting the activities of various enzymes in the body， maintaining acid base balance in the body and participating in the blood coagulation process. In short， calcium is an indispensable trace element in human body. It is both a constructor of the body and a regulator of the body. It is the source of life for our body.
Effect of Calcium Deficiency on Vegetable Growth and Human Health
Calcium and vegetable diseases
Vegetables require more calcium than other crops. Even in calcareous soils， the soils are not deficient in calcium， but physiological calcium deficiency occurs in vegetable production， especially in facility cultivation with high intensification[1-2]. Jiang et al. [3] compared the exchangeable calcium and magnesium contents and Ca/Mg values in 43 pairs of vegetable protected areas and adjacent open soils in the suburbs of Shenyang. The results showed that compared with the terrestrial soil， the average content of soil exchangeable calcium in the protected areas decreased by 6.38%， the Ca/Mg value decreased by 10.34%， and the average content of exchangeable magnesium increased by 4.74%. The main reason for the decrease in exchangeable calcium content is that the amount of calcium that is exported to the outside of the system with vegetables is relatively high， the application amount of calcium containing fertilizers is reduced and the pH of the soil is lowered. Calcium deficiency caused the decrease of leaf chlorophyll content， the increase in the incidence of soft rot， and the decrease of both fresh and dry weights of leaves; and at the middle and late growth stages， especially the new leaves are prone to calcium nutrition disorder. Calcium and human health
Calcium is an important component of the body. When the calcium content in the blood is lower than 70 mg/L， the neuromuscular excitability increases， accompanied with convulsions， irritable bowel syndrome， etc. The calcium deficiency in human body reduces the elasticity and toughness of the soft tissue and the excitability of nerve cells in the environment. Calcium intake in the diet is difficult to meet physiological needs， and it is easy to cause calcium deficiency， which leads to skeletal dysplasia and osteoporosis. Calcium is mainly distributed in bones and teeth in human body， and it is also distributed in the blood and various body fluids， thereby playing an important role in maintaining the normal metabolism of human body. Normal adults contain 1 000-1 200 g of calcium. Calcium is not only an indispensable component to ensure the integrity of the body， but also plays an extremely important role in maintaining life in various physiological and biochemical processes.
Effect of Calcium Supply on Vegetable Quality
Vegetable nutrition quality
Calcium has a close relationship with the growth and quality of vegetable crops. Zhu et al.[4] studied the effects of calcium application on the yield and quality of five lettuce varieties in acid soil by pot experiment. The results showed that the application of calcium fertilizers increased the yield of lettuce in each variety with a significant difference; and it can also reduce nitrate content and increase vitamin C， soluble sugar and amino acid contents. Sun et al.[5] studied the effects of the liquid calcium fertilizer on the yield and quality of Chinese cabbage. It was pointed out that spraying the liquid calcium fertilizer with calcium nitrate as a main component could increase the yield of Chinese cabbage by 10.5% ， reduce nitrate content by 15.1% and increase vitamin C content by 23.8%. However， Fan and Yi[6] reported that a too high calcium concentration in the nutrient solution would reduce the absorption of potassium and magnesium by leaf lettuce， but has a little effect on the absorption of chlorine and phosphorus; and improving the proportion of NH+ 4 N in the nutrient solution would inhibit the absorption of calcium by crops and reduce its effectiveness. Feng et al.[7] conducted a pot experiment in Holland greenhouse with the cucumber nutrition solution formula of the Horticultural Research Institute as a control， to investigate the effect of calcium on the quality of cucumber by changing the content of calcium in the nutrient solution and the concentration of the nutrient solution. The results showed that with the concentration of the nutrient solution increasing， the content of soluble sugar also increased. Li et al.[8] studied the effects of spraying different calcium fertilizers on the yield and quality of lettuce. The results showed that the CaSO 4 treatment can significantly promote the vitamin C and soluble sugar contents of leaves， and the CaCl 2 treatment can significantly increase the nitrate content in leaves and the vitamin C content in stems. Different calcium fertilizers are applied to vegetable crops， and their effects are different. Xiao et al.[9] pointed out that calcium nitrate and calcium hydroxide had obvious effects on increasing vegetable yield and improving quality components， and were capable of increasing vitamin C and sugar content， reducing acid content and increasing mineral nutrient content， followed by magnesium sulfate. However， the application of calcium chloride had the effect of reducing the quality of vegetables. Among the several vegetables tested， the effect of calcium fertilizers was better for fruits and vegetables than for root vegetables.
Vegetable nitrate content
When the calcium concentration is consistent， there is a significant positive correlation between nitrate content and nitrogen supply concentration. This may be because increasing the amount of nitrogen fertilizer can improve the nitrate reductase activity of vegetables， but the reduction of nitrate by vegetables is still less than absorption， and the absorbed nitrate cannot be reduced in time and is then accumulated in vegetables. At the same time， calcium nutrition also has a significant effect on nitrate content. Under the same nitrogen supply concentration， increasing the calcium supply concentration can also significantly promote the accumulation of nitrate， which may be due to that the excessive calcium ions inhibit the activity of nitrate reductase to a certain degree， which results in the absorption of NO 3- N greater than assimilation， causing accumulation of nitrate in a large quantity in plants.
Oxalic acid content
Calcium nutrition has a significant effect on the oxalic acid content of vegetables. Oxalic acid and nitrate contents are two important indicators that affect the safety of vegetables. It is well known that in plant cells， calcium has important physiological and biochemical effects as a second messenger participating in plant growth and aging， photosynthetic electron transport and photosynthetic phosphorylation， cell tropism and hormone regulation. Calcium is also an important component of calcium oxalate， and adjusting the calcium concentration will inevitably affect the content of oxalic acid in spinach. Zhang[10] studied the effects of different nitrogen and calcium nutrition on the nutritional quality of spinach. It was found that when the supplied calcium concentration was 2 mmol/L， the soluble oxalic acid and total amount of oxalic acid decreased first， then increased and decreased again with the increase of nitrogen concentration. Zindler Frank et al.[11] pointed out that the oxalic acid content of Phaseolus vulgaris increased with the increase of calcium supply level. De et al.[12] also reported that the amount of calcium oxalate crystals in tomato pericarp also increased with the supplied calcium concentration increasing， and under the same nitrogen concentration， the soluble oxalic acid content decreased significantly with the increase of calcium concentration. This means that the calcium ions absorbed by the plant body combine with soluble oxalic acid to form insoluble calcium oxalate， which causes the reduction of soluble sodium oxalate and potassium oxalate. When the concentration of calcium was 5 and 8 mmol/L， the content of oxalic acid in spinach decreased significantly with the increase of nitrogen concentration from 4 to 8 mmol/L， but was basically stable with the further increase of nitrogen concentration， which may be related to the absorption and reduction of NO- 3 N in spinach. When the calcium concentration was 5 mmol/L， the total amount of oxalic acid was the lowest， and it was relatively higher under the conditions of low calcium and high calcium concentrations. Prospect of Calcium rich Vegetables
For most adults， the recommended daily calcium absorption is 1 000 mg/d， and the daily intake per capita in urban and rural areas in China is only 405.2 mg， which is only 40.5% of the recommended supply for adults. For the elderly， the absorption of calcium is 1 300-1 700 mg/d[13-14]. The differences between the calcium intake of children， adolescents and pregnant women and the recommended dietary supply are greater. More than 95% of children have a severe inadequate intake of calcium in their diet， which is about half of the recommended calcium intake for healthy development. The national per capita calcium intake is only about 40% of the recommended dietary supply of the Chinese Nutrition Society. The main source of calcium in humans is food. In recent years， calcium supplementation through the food chain has been considered to be the safest and most effective calcium supplementation pathway. Vegetables are one of the essential foods in peoples daily diet. Calcium can increase the vitamin C， soluble sugar and mineral nutrient contents of vegetables. It is of great practical significance to improve the bodys absorbable calcium content in vegetables through the regulation of nutrient regulation and planting of calcium rich vegetables.
Calcium rich vegetables could supplement calcium instead of medicines. This calcium supplement method is not only safe and convenient， but also has an obvious calcium supplementation effect. At the same time， vegetables are an important part of the human diet and an indispensable daily necessity. However， at present， the use of vegetables to convert inorganic calcium into organic calcium is still in the experimental stage， and it will still take a long term process to finally realize its commercialization. Therefore， solving the enrichment technology of calcium in vegetables and providing theoretical basis and practical reference for the development of plant foods that are easily absorbed by the human body and rich in calcium will be the main direction of future research.
Weiling YUAN et al. Advances in Research on the Effects of Calcium on Vegetable Quality and Human Health
References
[1] ZHANG ZX， YU XC. Principle and technology of vegetable fertilization[M]. Beijing： China Agriculture Press， 1996. （in Chinese）
[2] BAI GY， HUANG DM. New technology of vegetable formula fertilization[M]. Beijing： China Agriculture Press， 1999. （in Chinese） [3] JIANG Y， ZHANG YG， LIANG WW. Soil exchangeable Ca and Mg contents and Ca/Mg ratio in greenhouse vegetable fields in Shenyang suburbs[J]. Rural Eco Environment， 2004， 20（3）： 24-27. （in Chinese）
[4] ZHU HX， DI CX， WANG ZY， et al. Effects of calcium on the yield and quality of lettuce grown in acid soil[J]. Journal of Southwest Agricultural University： NaturalScience， 2005， （4）： 456-463. （in Chinese）
[5] SUN MD， ZHANG L， CAO B. Effects of paste foliage fertilizer and liquid calcium on the yield and quality of chinese cabbage[J]. Chinese Agricultural Science Bulletin， 2006， 22（6）： 255-257. （in Chinese）
[6] FAN SX， YI DZ. Effects of calcium on nutrient absorption and growth and development of Lactuca sativa var. longifolia Lam. in nutrient film technique culture[J]. Acta Horticulturae Sinica， 2002， 29（2）： 149-152. （in Chinese）
[7] FENG J， LIU RP， LUO HY. Effect of calcium level on growth， yield and quality of cucumber in substrate culture[J]. Soil and Fertilizer Sciences in China， 2016（2）： 104-108. （in Chinese）
[8] LI B， WANG CQ， FENG CC， et al. Effects on the Yield and Quality of Lettuce by the Different Calcium Fertilizers Spraying[J]. Hans Journal of Soil Science， 2004， 2（20）： 129-131. （in Chinese）
[9] XIAO CP， YANG ZQ. Influence of different calcium fertilizer and magnesium fertilizer on yield and quality of vegetables[J]. Journal of Changjiang Vegetables， 2000， （4）： 27-29. （in Chinese）
[10] ZHANG YP. Effects of nitrogen and calcium nutrition on nitrogen metabolism and nutritional quality of spinach[D]. Hangzhou： Zhejiang University， 2005. （in Chinese）
[11] ZINDLER FRANK E， HONOW R， HESSE A. Calcium and oxalate content of the leaves of Phaseolus vulgaris at different calcium supply in relation to calcium oxalate crystal formation[J]. J Plant Physiol， 2001， 158： 139-144.
[12] DE KC， JANSE J， VAN GBJ， et al. The incidence of calcium oxalate crystals in fruit walls of tomato （Lycopersicon esculentum Mill.） as affected by humidity， phosphate and calcium supply[J]. Journal of Horticultural Science， 1992， 67： 45-50.
[13] LIU JJ. Study on the migration and transformation of calcium in soil and the calcium enriching characteristic in vegetables and its influencing factors[D]. Taian： Shandong Agricultural University， 2006. （in Chinese）
[14] MEAGY MJ， EATON TE， BARKER AV. Nutrient density in lettuce cultivars grown with organic or conventional fertilization with elevated calcium concentrations[J]. Hortscience， 2013， 48（12）： 1502-1507.