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Abstract [Objective] The aim was to explore the nutrient status of tobacco??planting soil， and find out the fertilization strategy for flue??cured tobacco. [Method] The nutrient contents in 142 soil samples from 7 tobacco planting counties in Fu Zhou city were determined and evaluated. [Result] The organic matter content was on the high side， and most soil was acid soil. The soil had abundant total nitrogen， total phosphorus and total potassium， moderate available nitrogen and available phosphorus， low available potassium， high water soluble chlorine， low soil cation exchange capacity and rich effective zinc， but effective boron was famine. [Conclusion] The meta??acid soil should be conducted with pH adjustment， and rational nitrogen， phosphorus and potassium application.
Key words Tobacco soil ； pH ； Water soluble chlorine； Fuzhou
Soil nutrient content is an important factor in the ecological suitability of tobacco??producing areas， and it is also one of the main indicators of soil environmental quality. Soil abundance and supply intensity can directly affect the growth and development， and yield and quality of tobacco leaves[1-2]. In order to further improve the quality of flue??cured tobacco and produce more high??quality tobacco leaves that are suitable for the needs of cigarette industry， efforts must be made to the fertilization in tobacco cultivation in the case that ecological conditions cannot be simulated and changed. Therefore， it is necessary to have a clear understanding of soil， and take scientific and reasonable measures to regulate the soil nutrients in tobacco??planting soil， improve soil microenvironment， and improve soil fertility and production performance， thereby providing good soil conditions for the production of high quality tobacco. The application and promotion of balanced fertilization technology is also an important way to improve the utilization rate of fertilizer， reduce the cost of fertilization and the environmental pollution of fertilizers. Studying the nutrient status of tobacco??planting soil is of great significance for further exploration of the quality characteristics and scientific fertilization of tobacco in Fuzhou. In this paper， the soil nutrient contents such as pH， total nitrogen and available nitrogen in Fuzhou tobacco??planting area were studied.
Materials and Methods
Sample source
The soil samples were collected before fertilization in representative tobacco producing areas from 7 tobacco??producing counties of Guangchang， Lichuan， Le??an， Yihuang， Zixi， Nanfeng and Chongren. The samples involved 125 villages of 65 representative tobacco planting towns （townships） of 7 counties， and there were a total of 142 mixed soil samples. Sampling method
The soil samples were collected within 1 week before tillage， and the samples were collected at the soil depth of 20 cm at multi??points in the ?°S?± route from the selected plots. After the rough samples were fully mixed， 1 kg of the sample was taken using quatering method for later use.
Test items and methods
The test items included 14 nutrient indicators such as soil pH， organic matter， total nitrogen， total phosphorus， total potassium， available nitrogen， available phosphorus and available potassium. The soil samples were sent to the Jiangxi Academy of Agricultural Sciences for testing. The soil pH was determined according to NY/T1121.2??2006； the soil organic matter was determined according to NY/T1121.6??2006； the soil total nitrogen was determined according to NY/T53??1987； the soil total phosphorus was determined according to NY/T88??1988； soil total potassium was determined according to NY/T87??1988； soil available nitrogen was determined by LY/T 1229??1999； soil available phosphorus was determined according to NY/T 1121.7??2006； soil available potassium was determined according to NY/T 889??2004； soil available zinc was determined by LY/T 1261??99； soil available boron was determined according to NY/T1121.8??2006； soil water??soluble chlorine was determined according to NY/T 1121.17??2006； soil exchangeable calcium and magnesium were determined according to NY/T1121.13??2006； soil cation exchange capacity was determined according to LY/T1243??1999.
Results and Analysis
Soil pH
Soil pH is directly related to soil parent material， cultivation system and fertilization structure. The pH value of soil is directly related to various nutrient availability， microbial population composition and activity intensity， root growth and development， tobacco quality and fertilization effect. The suitable soil pH range for flue??cured tobacco is 5.5-6.5 （slightly acidic）[3]. Studies have shown that unsuitable soil pH can be detrimental to the normal absorption and utilization of soil nutrients by tobacco leaves[4-5]， which further affects the quality of tobacco leaves. As shown in Table 1， the pH values of the soil in the sampling areas range from 4.40 to 8.44， and the average value is 5.34， indicating that the soil is at the overall acidity level with the coefficient of variation of 12.76%. Among them， the extremely acidic soil samples with pH less than 4.5 account for 0.7%， the acid soil samples with pH of 4.5-5.5 account for 74.65%， and the total of both is 75.35%. The soil samples with pH of 5.5-7.0 account for 18.52%， and the soil samples with pH higher than 7.0 account for 6.13%. Overall， a considerable majority of tobacco??growing soils in Fuzhou are in a state of acidity. There are a certain differences in soil pH from different tobacco??planting counties. The average pH of the soils of Guangchang and Zixi is slightly acidic， and the average pH of the other counties is within the acid level. There is a sampling point in Lichuan where the soil pH is in the range of strong acidity reaching up to 4.4， while the tobacco soil in the other sampling areas is acidic. And 77.28% of the soil samples collected from Guangchang are at the acidic or slightly acidic levels， and 22.72% at the alkaline levels. There is 1 sampling point in Chongren with the soil in the alkaline range.
Organic matter， total nitrogen， total phosphorus and total potassium in tobacco??planting soil in Fuzhou
Soil organic matter Organic matter is an important indicator to measure soil comprehensive fertility. The content of organic matter is directly related to soil texture， buffering， fertilizer supply and fertilizer retention capacity. Organic matter is also a source of various nutrients. Soil organic matter content is the main source of nutrients for crops[6]. For tobacco， only the soil with moderate organic matter content can produce tobacco with better quality and higher yield[7]. It is generally believed that the organic matter content of tobacco??planting soil is suitable at 15-25 g/kg[3]. The organic matter content of tobacco??planting soil collected from the sampling areas is between 9.09 g/kg and 61.10 g/kg， with an average of 34.94 g/kg， and the coefficient of variation is 31.39%. Therefore， the organic matter content is abundant. The soil samples with organic matter content of less than 10 g/kg account for 1.41% of the total， soil samples of 10-15 g/kg account for 3.52%， soil samples of 15-25 g/kg account for 15.49%， and the total of the 3 is 20.42%. The soil samples with organic matter content of 25-35 g/kg and greater than 35g/kg account for 25.35% and 54.23%， respectively， indicating that nearly 80% of soil samples have high organic matter content.
The soil organic matter contents in different tobacco??producing counties are as follows： soil samples collected from Lichuan， Yihuang and Zixi with the organic matter content of greater than 35 g/kg account for 80.77%， 71.86% and 78.63%， respectively， indicating that the organic matter content of these counties is high. Organic matters are tested in the soil collected from each part of Guangchuang， Le??an. In Le??an， the organic matter content of 15-25 g/kg accounts for 38.89% of the total. In both Nanfeng and Chongren， the organic matter contents of 15-25 g/kg and greater than 35 g/kg account for 40% and 42.86%， suggesting that the organic matter content in Le??an， Nanfeng and Chongren are suitable for tobacco planting. Soil total nitrogen， total phosphorus and total potassium The total nitrogen content of tobacco??planting soil in the whole city is 0.8-3.77 g/kg， with an average of 2.43 g/kg， and the content is generally high. There are 9.86% of soil samples having a total nitrogen content of 1.0-1.5 g/kg， which is moderate level； 1.41% of soil samples less than 1.0 g/kg， which is low； 88.73% of soil samples greater than 1.5 g/kg， which is high. The soil total phosphorus content is 0.17-2.37 g/kg， with an average of 0.77 g/kg， and the content is overall high. There are 11.27% of soil samples having a total phosphorus content of less than 0.4 g/kg， which is low； 19.72% of soil samples of 0.4-0.6 g/kg， which is the moderate level； 69.01% of soil samples of higher than 0.6 g/kg， which is relatively high. The soil total potassium content is 4.24-21.10 g/kg， with an average of 9.68 g/kg， which is rich in content. There are 59.15% of soil samples having a total potassium content of less than 10 g/kg， which is low； 33.8% of 10-15 g/kg， which is moderate； only 7.05% greater than 15 g/kg， which is high. Therefore， the soil is in significant potassium deficiency.
Soil available nitrogen
The content of soil available nitrogen is one of the important basis for determining the amount of nitrogen applied per unit area. Nitrogen has a great influence on the yield and quality of tobacco plants， and the application of nitrogen fertilizer is more complicated and has many influencing factors. Rational application of nitrogen fertilizer can make tobacco plants grow normally， have strong photosynthesis and more dry matter accumulation. As for available nitrogen， the soil with content of less than 65 mg/kg is more favorable for nitrogen regulation[3]. There is a large area with high available nitrogen content， which needs to pay attention to the application amounts in flue0cured tobacco production. As shown in Table 3， the available nitrogen content of tobacco??planting soil in the whole city is 25.1-297 mg/kg， with an average of 156.72 mg/kg. The overall content is moderately high， and the coefficient of variation is 34.59%， indicating moderate variation. The soil samples with available nitrogen content of less than 65 mg/kg account for 18.18% of the total， soil samples of 65-100 mg/kg account for 13.64%， 31.82% for 100-150 mg/kg， and 36.36% of samples with the available nitrogen content of greater than 150 mg/kg. The distribution of soil available nitrogen is generally better than that of organic matter. The soil samples with the content of moderate and below levels account for 63.64%， especially soils with available nitrogen content of less than 65 mg/kg， which is more conducive to nitrogen regulation[3]. For soils with an available nitrogen content of greater than 150 mg/kg， attention should be paid to controlling the application amount of nitrogen fertilizer. The available nitrogen content of tobacco planting soil in different tobacco??producing counties varies greatly. Among them， the average available nitrogen content of soils in Lichuan， Yihuang， Zixi and Chongren counties iss above 150 mg/kg， which is relatively high. The available nitrogen content of Guangchang， Le??an and Nanfeng is moderate， and the average available nitrogen content is 100-150 mg/kg. In some counties， the soil samples with available nitrogen content of 100-150 mg/kg account for a high proportion （85.71 in Zixi and 74.65% in Chongren）， indicating that more than half of the soil available nitrogen in these counties is moderate. In addition， in some counties， the proportion of sil samples with the available nitrogen content of higher than 150 mg/kg is high， such as Guangchang， Lichuan and Le??an， accounting for 80.77%， 62.51% and 71.42%， respectively， suggesting that the area with high soil available nitrogen content is large. Therefore， attention should be paid to the application amount of nitrogen fertilizer in flue??cured tobacco production.
Soil available phosphorus
The available phosphorus content of tobacco??planting soil is relatively rich， which is conducive to the production of high quality flue??cured tobacco. Soil available phosphorus content is an important indicator to determine the ability of soil phosphorus supply， and is also the basic basis for the application of phosphate fertilizer[7]. The content of phosphorus in tobacco leaves is between 0.15% and 0.25%， and the variation range is small. The application amount of phosphorus fertilizer is not quite correlated with the phosphorus content of tobacco leaves. The phosphorus content of tobacco leaves is positively correlated with soil available phosphorus content， and negatively correlated with the taste of tobacco leaves. The migration and leaching of phosphorus in the soil is small， and excessive phosphorus is likely to accumulate.
As shown in Table 4， the available phosphorus content of tobacco??planting soil in the whole city is 2.96-119.00 mg/kg， with an average of 43.26 mg/kg and a coefficient of variation of 60.07%. Among them， soil samples with available phosphorus content of less than 10mg/kg account for 11.97%， 13.38% for 10-20 mg/kg， 20.42% for 20-40 mg/kg ， and 48.59% for 40-80 mg/kg. It is generally believed that soil available phosphorus content of 20-40 mg/kg is more suitable for flue??cured tobacco， and soil available phosphorus content of less than 10 mg/kg is in the state of phosphorus deficiency[3]. Therefore， except a small amount of soil with low phosphorus or phosphorus deficiency， the supply of available phosphorus in most tobacco soils in Fuzhou is abundant， which is very beneficial to the cultivation of high quality flue??cured tobacco. There is also a certain differences in the available phosphorus content of tobacco??planting soil in different tobacco??producing counties. The average available phosphorus content of all 7 counties is above 30 mg/kg， which is at a high level， especially in Yihuang， Nanfeng， Zixi and Chongren counties， where the average contents are 61.02， 57.12， 41.49 and 44.94 mg/kg， respectively. The contents are obviously high， indicating that quite a large area in these counties has relatively abundant content of available phosphorus in the tobacco??planting soils， which is conducive to the production of high??quality flue??cured tobacco. The proportion of phosphorus??deficient soil samples with the content of less than 10 mg/kg is significantly higher in Le??an among different content levels， reaching 30.56%.
Soil available potassium
The potassium content of tobacco leaves is an important indicator to measure the quality of tobacco leaves. Rational application of potassium fertilizer can promote the absorption and assimilation of nitrate nitrogen by flue??cured tobacco， reduce denitrification in soil， enhance enzyme activity， enhance photosynthesis， and improve the resistances of the tobacco plants like drought resistance， disease resistance and pest resistance， thereby achieving the overall improvement of tobacco quality. Flue??cured tobacco is a crop fond of potassium. Potassium content is very important for the normal growth and quality formation of flue??cured tobacco. Soil available potassium content is also one of the main indicators for evaluating soil potassium supply in flue??cured tobacco[8]. It is generally considered that the potassium content of 150 mg/kg is the critical level. The potassium content of 150-220 mg/kg is the moderate level， and the potassium content of 100-150 mg/kg is potassium deficiency， while below 80 mg/kg is extreme potassium deficiency[9].
The test results （Table 5） show that the available potassium content of tobacco??planting soil in Fuzhou ranges from 20.6 to 1 263 mg/kg， with an average of 152.5 mg/kg. The coefficient of variation is 83.49%， indicating large variation. Among them， soil samples with a content of less than 80 mg/kg account for 21.83%， and those with a content of 80-150 mg/kg account for 42.96%， which together account for 64.79%. In addition， soil samples with the content of 150-220 mg/kg account for 18.31%， and 11.27% for the content of 220-350 mg/kg. It indicates that the available potassium content of most tobacco??growing soils in Fuzhou is insufficient. There is also a certain difference in the amount of soil available potassium in each producing county. Among them， the average available potassium content of Guangchang， Lichuan and Zixi is above 150 mg/kg， which is relatively high. Quite a large area of soil in these counties has good potassium supply conditions. The proportion of soil samples with the content levels of less than 80 mg/kg is 40.91% for Lichuan， and 30.56% for Le??an， which suggests the possibility of potassium deficiency in large areas. Therefore， it is recommended that the production of flue??cured tobacco be applied potassium according to soil testing to the potassium??deficient soil， so as to improve the potassium supply of the soil.
Soil water??soluble chlorine
Chlorine is an essential nutrient for flue??cured tobacco， and proper amount of chlorine plays an important role in the yield and quality of tobacco. The test results （Table 6） show that the water??soluble chlorine content of tobacco??planting soil in Fuzhou ranges from 10.7 to 291 cmol/kg， with an average of 88.95 cmol/kg， and the coefficient of variation is 68.31%， indicating high variation range. Among them， there are no soil samples with content of less than 2 cmol/kg， and the soil samples with the content of 2-25 cmol/kg account for 6.34%， 19.72% for 25-40 cmol/kg， 5.63% for 40-45 cmol/kg， and up to 68.31% for the samples with content of greater than 45 cmol/kg. The results suggest that the tobacco??planting soil in most tobacco??producing areas in Fuzhou has high chloride ion content， which is unfavorable for tobacco planting.
The water??soluble chlorine content of soil in each producing county is consistent. The average water??soluble chlorine content of the 7 tobacco??producing counties is higher than 45 cmol/kg， especially Chongren， reaching up to 171.6 cmol/kg. The proportion of soil samples with different content levels of greater than 80 mg/kg reaches 100%， indicating that the soil in large areas of these counties has high water??soluble chlorine content. In production， attention should be paid to the control of chlorine. In areas where the chlorine content is unsuitable， in addition to the reduction of chloride ions in the season of fertilization in tobacco， it is necessary to control the use of chlorine??containing fertilizers before the control.
Soil available zinc
The test results （Table 7） show that the available zinc content of tobacco??planting soil in Fuzhou ranges from 0.2 to 23.2 cmol/kg， with an average of 5.22 cmol/kg， and the coefficient of variation is 68.81%， suggesting great variation range. Among them， soil samples with a content of less than 0.3 cmol/kg account for 2.1%， 2.82% for 0.5-1 cmol/kg， 22.54% for soil samples with the content of 1-3 cmol/kg， and 72.54% for greater than 3 cmol/kg. The data show that most of the tobacco??planting soils in the tobacco??growing areas of Fuzhou are rich in available zinc. The soil available zinc content in each tobacco??producing county is consistent. Except for Guangchang， the average value of available zinc in the other 6 tobacco??producing counties is higher than 3 cmol/kg， especially in Zixi， which is 11.6 cmol/kg. In different levels， the proportions of soil samples with the content of greater than 3 cmol/kg reach 100% in Lichuan and Zixi， and the proportions of other counties also reach above 50%， indicating that the available zinc content of large areas of these counties is relatively abundant.
Soil available boron
Boron is a trace element essential for tobacco growth and is involved in protein metabolism， alkaloid synthesis and organic matter transport[1]. Nicotiana is a medium boron??demand crop. The test results （Table 8） show that the available boron content of tobacco??planting soil in Fuzhou ranges from 0.01 to 0.6 cmol/kg， with an average of 0.14 cmol/kg， and the coefficient of variation is 70.65%， indicating great variation range. Among them， soil samples with content of less than 0.2 cmol/kg account for 81.69%， soil samples with content of 0.2-0.5 cmol/kg account for 17.61%， and soil samples with content of 0.5-1.0 cmol/kg account for 0.7%. Overall， most of the tobacco??planting soils in Fuzhou are in the significant state of boron deficiency.
The soil available boron content in each producing county is consistent. The average available boron content of the 7 tobacco??producing counties is less than 0.2 cmol/kg， which is at a very low level. As far as the sampling area is concerned， soil boron nutrition is one of the soil barrier factors for high??quality tobacco cultivation. The soil available boron content in each sample township （town） is in the state of boron deficiency or potential deficiency， so it is recommended that the tobacco??planting soil in the sampling areas be generally supplemented with boron fertilizers.
Soil cation exchange capacity， exchangeable magnesium and exchangeable calcium
Soil cation exchange capacity Soil cation exchange capacity is closely related to soil fertility and nutrient availability[1]. Under normal circumstances， it is considered that the cation exchange capacity of the cultivated layer is 10-20 cmol/kg， and if it is less than 10 cmol/kg， the soil has low fertility. The cation exchange capacity of tobacco??planting soil in the city ranges from 4.53 to 19.3 cmol / kg， with an average of 9.67 cmol/kg， and the coefficient of variation is 30.26%， of medium variation. Among them， soil samples with exchange capacity of less than 10 cmol/kg account for 63.38%， 36.62% for 10-20 cmol/kg， and no soil samples of greater than 20 cmol/kg. Therefore， the cation exchange capacity of tobacco??planting soil in the whole city is generally low， and the fertility is slightly insufficient. There is a small difference in the average cation exchange capacity of soil in different tobacco??producing counties. Zixi and Guangchang have slightly higher cation exchange capacity， with an average of higher than 10 cmol/kg， followed by Lichuan， Yihuang， Lean and Nanfeng， with an average value of higher than 8 cmol/kg. In addition， Chongren is slightly lower， and the average value is higher than 7cmol/kg. The distribution of the quantity and quantity of exchange is relatively concentrated. Except Zixi， the cation exchange capacities of the other counties are less than 10 cmol/kg， which reflects the fact that the cation exchange capacity is generally low in the tobacco??planting soils of the whole city.
Soil magnesium and calcium Magnesium is an important medium nutrient element that affects the yield and quality of flue??cured tobacco. Calcium is the total regulator of cellular metabolism and maintains the pH required for normal growth of tobacco plants[11]. Magnesium is an important component of chlorophyll. Appropriate amount of magnesium can promote the growth and development of flue??cured tobacco， which is conducive to the improvement of the intrinsic quality of tobacco leaves[12]. For most crops， the soil exchangeable calcium and magnesium are lower than 400 and 50 mg/kg， respectively， and the application of calcium and magnesium fertilizers can have obvious effects[13].
The exchangeable magnesium content of tobacco??planting soil in the city is 0.07-18.8 cmol/kg， with an average of 2.66 cmol/kg， and the coefficient of variation is 114.02%， suggesting the largest variation range. The exchangeable calcium content of tobacco planting soil in the city is 0.18-3.16 cmol/kg， with an average of 0.88 cmol/kg， and the coefficient of variation is 51.42%， indicating that the variation is great.
Conclusion and Discussion
The test results show that a considerable majority of tobacco??planting soils in Fuzhou are in a state of acidity， and alkaline fertilizers such as calcium magnesium phosphate， dolomite powder， and quick lime can be reasonably applied to adjust the pH of the soil. The tobacco??planting soil of the whole city has rich nitrogen， total phosphorus and total potassium contents， which is suitable for high??quality flue??cured tobacco production. For pH， nearly 80% of the soil has high organic matter content. For the sampling areas with high organic matter content （greater than 35）， the ratio of carbon to nitrogen in the organic fertilizer product should be controlled in organic fertilizer application， and the active organic fertilizer product should be selected but avoid the application of inactive organic fertilizer products with grass char as the main raw material. The soil available nitrogen content is generally moderately high； except for a small proportion of soil with low phosphorus or phosphorus deficiency， most of the tobacco??planting soil has rich available phosphorus， which is very beneficial to the cultivation of high quality flue??cured tobacco. Most of the tobacco??growing soil has insufficient available potassium， and the water??soluble chloride ion content is too high， both of which are unfavorable for tobacco planting. In the future production， attention should be paid to the control of chlorine. In areas where the chlorine content is unsuitable， in addition to the reduction of chloride ions in the season of tobacco， it is necessary to control the use of chlorine??containing fertilizers. For trace elements， most of the tobacco??growing soil in the tobacco??producing areas of Fuzhou has rich available zinc content， but the phenomenon of boron deficiency is significant. It is recommended that the tobacco??planting soil in the sampling area should be generally supplemented with boron fertilizers. The cation exchange capacity of tobacco??planting soil in the whole city is generally low， and the fertility is slightly low. From the view of low soil cation exchange capacity and the high temperature and rain during the production period in Fuzhou， it is speculated that as long as the potassium fertilizer is properly added， the utilization of soil and applied potassium by flue??cured tobacco can be better， and the potassium content of the tobacco leaves would not be low. References
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[2] ZHOU JM. Farmland nutrient balance and management[M]. Nanjing： Hehai University Press， 2000.
[3] CHEN JH， LI ZH， LIU JL， et al. Evaluation of soil nutrients condition in major tobacco production region of China[J]. Acta Tabacaria Sinica， 2004， 10（3）： 14-18.
[4] GUO PG， CHEN JJ， LI RH. Effects of pH values on the activity of roots and chemical compositions of the cured leaves in flue??cured tobacco[J]. Acta Agriculutra Sinica， 2005， 33（1）： 39-41.
[5] CUI XY， CHEN ZY， ZHANG MS， et al. Effects of soil pH on physiological??biochemical characteristics of flue??cured tobacco （Nicotiana tabacum L.） leaves[J]. Plant Physiology Communications， 2005， 41（6）： 737-740.
[6] CARTER MR. Soil quality for sustainable land management： organic matter and aggregation interactions that maintain soil functions[J] . Agronomy Journal， 2002， 94： 38-47.
[7] HAO C， TIAN XH. Analysis and research on soil physical properties in high quality tobacco areas[J] . Tobacco Science & Technology， 1996（5）： 34-35.
[8] LI TF， CHEN P， RAN BD. Relationship between potassium content in tobacco leaves and soil physical and chemical properties[J] . Tobacco Science Research， 2000（3）： 17-19.
[9] CAO WZ. Soil conditions of flue??cured tobacco and fertilization of high??quality tobacco in Guizhou[M]. Guiyang： Guizhou Science and Technology Press， 1995： 1-25.
[10] Mengel K. Principles of plant nutrition[M]. ZHANG YC， translated. Beijing： China Agricultural Publishing， 1987.
[11] ZUO TJ. Tobacco production， physiology and biochemistry[M]. Shanghai： Shanghai Far East Publishing House， 1993： 209-213.
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[13] LI YZ， JIANG ZH， YANG ZX， et al. Effects of magnesium supply on main economic characters in flue??cured tobacco[J]. Journal of Southwest Agricultural University， 2002， 24（3）： 200-203.
Key words Tobacco soil ； pH ； Water soluble chlorine； Fuzhou
Soil nutrient content is an important factor in the ecological suitability of tobacco??producing areas， and it is also one of the main indicators of soil environmental quality. Soil abundance and supply intensity can directly affect the growth and development， and yield and quality of tobacco leaves[1-2]. In order to further improve the quality of flue??cured tobacco and produce more high??quality tobacco leaves that are suitable for the needs of cigarette industry， efforts must be made to the fertilization in tobacco cultivation in the case that ecological conditions cannot be simulated and changed. Therefore， it is necessary to have a clear understanding of soil， and take scientific and reasonable measures to regulate the soil nutrients in tobacco??planting soil， improve soil microenvironment， and improve soil fertility and production performance， thereby providing good soil conditions for the production of high quality tobacco. The application and promotion of balanced fertilization technology is also an important way to improve the utilization rate of fertilizer， reduce the cost of fertilization and the environmental pollution of fertilizers. Studying the nutrient status of tobacco??planting soil is of great significance for further exploration of the quality characteristics and scientific fertilization of tobacco in Fuzhou. In this paper， the soil nutrient contents such as pH， total nitrogen and available nitrogen in Fuzhou tobacco??planting area were studied.
Materials and Methods
Sample source
The soil samples were collected before fertilization in representative tobacco producing areas from 7 tobacco??producing counties of Guangchang， Lichuan， Le??an， Yihuang， Zixi， Nanfeng and Chongren. The samples involved 125 villages of 65 representative tobacco planting towns （townships） of 7 counties， and there were a total of 142 mixed soil samples. Sampling method
The soil samples were collected within 1 week before tillage， and the samples were collected at the soil depth of 20 cm at multi??points in the ?°S?± route from the selected plots. After the rough samples were fully mixed， 1 kg of the sample was taken using quatering method for later use.
Test items and methods
The test items included 14 nutrient indicators such as soil pH， organic matter， total nitrogen， total phosphorus， total potassium， available nitrogen， available phosphorus and available potassium. The soil samples were sent to the Jiangxi Academy of Agricultural Sciences for testing. The soil pH was determined according to NY/T1121.2??2006； the soil organic matter was determined according to NY/T1121.6??2006； the soil total nitrogen was determined according to NY/T53??1987； the soil total phosphorus was determined according to NY/T88??1988； soil total potassium was determined according to NY/T87??1988； soil available nitrogen was determined by LY/T 1229??1999； soil available phosphorus was determined according to NY/T 1121.7??2006； soil available potassium was determined according to NY/T 889??2004； soil available zinc was determined by LY/T 1261??99； soil available boron was determined according to NY/T1121.8??2006； soil water??soluble chlorine was determined according to NY/T 1121.17??2006； soil exchangeable calcium and magnesium were determined according to NY/T1121.13??2006； soil cation exchange capacity was determined according to LY/T1243??1999.
Results and Analysis
Soil pH
Soil pH is directly related to soil parent material， cultivation system and fertilization structure. The pH value of soil is directly related to various nutrient availability， microbial population composition and activity intensity， root growth and development， tobacco quality and fertilization effect. The suitable soil pH range for flue??cured tobacco is 5.5-6.5 （slightly acidic）[3]. Studies have shown that unsuitable soil pH can be detrimental to the normal absorption and utilization of soil nutrients by tobacco leaves[4-5]， which further affects the quality of tobacco leaves. As shown in Table 1， the pH values of the soil in the sampling areas range from 4.40 to 8.44， and the average value is 5.34， indicating that the soil is at the overall acidity level with the coefficient of variation of 12.76%. Among them， the extremely acidic soil samples with pH less than 4.5 account for 0.7%， the acid soil samples with pH of 4.5-5.5 account for 74.65%， and the total of both is 75.35%. The soil samples with pH of 5.5-7.0 account for 18.52%， and the soil samples with pH higher than 7.0 account for 6.13%. Overall， a considerable majority of tobacco??growing soils in Fuzhou are in a state of acidity. There are a certain differences in soil pH from different tobacco??planting counties. The average pH of the soils of Guangchang and Zixi is slightly acidic， and the average pH of the other counties is within the acid level. There is a sampling point in Lichuan where the soil pH is in the range of strong acidity reaching up to 4.4， while the tobacco soil in the other sampling areas is acidic. And 77.28% of the soil samples collected from Guangchang are at the acidic or slightly acidic levels， and 22.72% at the alkaline levels. There is 1 sampling point in Chongren with the soil in the alkaline range.
Organic matter， total nitrogen， total phosphorus and total potassium in tobacco??planting soil in Fuzhou
Soil organic matter Organic matter is an important indicator to measure soil comprehensive fertility. The content of organic matter is directly related to soil texture， buffering， fertilizer supply and fertilizer retention capacity. Organic matter is also a source of various nutrients. Soil organic matter content is the main source of nutrients for crops[6]. For tobacco， only the soil with moderate organic matter content can produce tobacco with better quality and higher yield[7]. It is generally believed that the organic matter content of tobacco??planting soil is suitable at 15-25 g/kg[3]. The organic matter content of tobacco??planting soil collected from the sampling areas is between 9.09 g/kg and 61.10 g/kg， with an average of 34.94 g/kg， and the coefficient of variation is 31.39%. Therefore， the organic matter content is abundant. The soil samples with organic matter content of less than 10 g/kg account for 1.41% of the total， soil samples of 10-15 g/kg account for 3.52%， soil samples of 15-25 g/kg account for 15.49%， and the total of the 3 is 20.42%. The soil samples with organic matter content of 25-35 g/kg and greater than 35g/kg account for 25.35% and 54.23%， respectively， indicating that nearly 80% of soil samples have high organic matter content.
The soil organic matter contents in different tobacco??producing counties are as follows： soil samples collected from Lichuan， Yihuang and Zixi with the organic matter content of greater than 35 g/kg account for 80.77%， 71.86% and 78.63%， respectively， indicating that the organic matter content of these counties is high. Organic matters are tested in the soil collected from each part of Guangchuang， Le??an. In Le??an， the organic matter content of 15-25 g/kg accounts for 38.89% of the total. In both Nanfeng and Chongren， the organic matter contents of 15-25 g/kg and greater than 35 g/kg account for 40% and 42.86%， suggesting that the organic matter content in Le??an， Nanfeng and Chongren are suitable for tobacco planting. Soil total nitrogen， total phosphorus and total potassium The total nitrogen content of tobacco??planting soil in the whole city is 0.8-3.77 g/kg， with an average of 2.43 g/kg， and the content is generally high. There are 9.86% of soil samples having a total nitrogen content of 1.0-1.5 g/kg， which is moderate level； 1.41% of soil samples less than 1.0 g/kg， which is low； 88.73% of soil samples greater than 1.5 g/kg， which is high. The soil total phosphorus content is 0.17-2.37 g/kg， with an average of 0.77 g/kg， and the content is overall high. There are 11.27% of soil samples having a total phosphorus content of less than 0.4 g/kg， which is low； 19.72% of soil samples of 0.4-0.6 g/kg， which is the moderate level； 69.01% of soil samples of higher than 0.6 g/kg， which is relatively high. The soil total potassium content is 4.24-21.10 g/kg， with an average of 9.68 g/kg， which is rich in content. There are 59.15% of soil samples having a total potassium content of less than 10 g/kg， which is low； 33.8% of 10-15 g/kg， which is moderate； only 7.05% greater than 15 g/kg， which is high. Therefore， the soil is in significant potassium deficiency.
Soil available nitrogen
The content of soil available nitrogen is one of the important basis for determining the amount of nitrogen applied per unit area. Nitrogen has a great influence on the yield and quality of tobacco plants， and the application of nitrogen fertilizer is more complicated and has many influencing factors. Rational application of nitrogen fertilizer can make tobacco plants grow normally， have strong photosynthesis and more dry matter accumulation. As for available nitrogen， the soil with content of less than 65 mg/kg is more favorable for nitrogen regulation[3]. There is a large area with high available nitrogen content， which needs to pay attention to the application amounts in flue0cured tobacco production. As shown in Table 3， the available nitrogen content of tobacco??planting soil in the whole city is 25.1-297 mg/kg， with an average of 156.72 mg/kg. The overall content is moderately high， and the coefficient of variation is 34.59%， indicating moderate variation. The soil samples with available nitrogen content of less than 65 mg/kg account for 18.18% of the total， soil samples of 65-100 mg/kg account for 13.64%， 31.82% for 100-150 mg/kg， and 36.36% of samples with the available nitrogen content of greater than 150 mg/kg. The distribution of soil available nitrogen is generally better than that of organic matter. The soil samples with the content of moderate and below levels account for 63.64%， especially soils with available nitrogen content of less than 65 mg/kg， which is more conducive to nitrogen regulation[3]. For soils with an available nitrogen content of greater than 150 mg/kg， attention should be paid to controlling the application amount of nitrogen fertilizer. The available nitrogen content of tobacco planting soil in different tobacco??producing counties varies greatly. Among them， the average available nitrogen content of soils in Lichuan， Yihuang， Zixi and Chongren counties iss above 150 mg/kg， which is relatively high. The available nitrogen content of Guangchang， Le??an and Nanfeng is moderate， and the average available nitrogen content is 100-150 mg/kg. In some counties， the soil samples with available nitrogen content of 100-150 mg/kg account for a high proportion （85.71 in Zixi and 74.65% in Chongren）， indicating that more than half of the soil available nitrogen in these counties is moderate. In addition， in some counties， the proportion of sil samples with the available nitrogen content of higher than 150 mg/kg is high， such as Guangchang， Lichuan and Le??an， accounting for 80.77%， 62.51% and 71.42%， respectively， suggesting that the area with high soil available nitrogen content is large. Therefore， attention should be paid to the application amount of nitrogen fertilizer in flue??cured tobacco production.
Soil available phosphorus
The available phosphorus content of tobacco??planting soil is relatively rich， which is conducive to the production of high quality flue??cured tobacco. Soil available phosphorus content is an important indicator to determine the ability of soil phosphorus supply， and is also the basic basis for the application of phosphate fertilizer[7]. The content of phosphorus in tobacco leaves is between 0.15% and 0.25%， and the variation range is small. The application amount of phosphorus fertilizer is not quite correlated with the phosphorus content of tobacco leaves. The phosphorus content of tobacco leaves is positively correlated with soil available phosphorus content， and negatively correlated with the taste of tobacco leaves. The migration and leaching of phosphorus in the soil is small， and excessive phosphorus is likely to accumulate.
As shown in Table 4， the available phosphorus content of tobacco??planting soil in the whole city is 2.96-119.00 mg/kg， with an average of 43.26 mg/kg and a coefficient of variation of 60.07%. Among them， soil samples with available phosphorus content of less than 10mg/kg account for 11.97%， 13.38% for 10-20 mg/kg， 20.42% for 20-40 mg/kg ， and 48.59% for 40-80 mg/kg. It is generally believed that soil available phosphorus content of 20-40 mg/kg is more suitable for flue??cured tobacco， and soil available phosphorus content of less than 10 mg/kg is in the state of phosphorus deficiency[3]. Therefore， except a small amount of soil with low phosphorus or phosphorus deficiency， the supply of available phosphorus in most tobacco soils in Fuzhou is abundant， which is very beneficial to the cultivation of high quality flue??cured tobacco. There is also a certain differences in the available phosphorus content of tobacco??planting soil in different tobacco??producing counties. The average available phosphorus content of all 7 counties is above 30 mg/kg， which is at a high level， especially in Yihuang， Nanfeng， Zixi and Chongren counties， where the average contents are 61.02， 57.12， 41.49 and 44.94 mg/kg， respectively. The contents are obviously high， indicating that quite a large area in these counties has relatively abundant content of available phosphorus in the tobacco??planting soils， which is conducive to the production of high??quality flue??cured tobacco. The proportion of phosphorus??deficient soil samples with the content of less than 10 mg/kg is significantly higher in Le??an among different content levels， reaching 30.56%.
Soil available potassium
The potassium content of tobacco leaves is an important indicator to measure the quality of tobacco leaves. Rational application of potassium fertilizer can promote the absorption and assimilation of nitrate nitrogen by flue??cured tobacco， reduce denitrification in soil， enhance enzyme activity， enhance photosynthesis， and improve the resistances of the tobacco plants like drought resistance， disease resistance and pest resistance， thereby achieving the overall improvement of tobacco quality. Flue??cured tobacco is a crop fond of potassium. Potassium content is very important for the normal growth and quality formation of flue??cured tobacco. Soil available potassium content is also one of the main indicators for evaluating soil potassium supply in flue??cured tobacco[8]. It is generally considered that the potassium content of 150 mg/kg is the critical level. The potassium content of 150-220 mg/kg is the moderate level， and the potassium content of 100-150 mg/kg is potassium deficiency， while below 80 mg/kg is extreme potassium deficiency[9].
The test results （Table 5） show that the available potassium content of tobacco??planting soil in Fuzhou ranges from 20.6 to 1 263 mg/kg， with an average of 152.5 mg/kg. The coefficient of variation is 83.49%， indicating large variation. Among them， soil samples with a content of less than 80 mg/kg account for 21.83%， and those with a content of 80-150 mg/kg account for 42.96%， which together account for 64.79%. In addition， soil samples with the content of 150-220 mg/kg account for 18.31%， and 11.27% for the content of 220-350 mg/kg. It indicates that the available potassium content of most tobacco??growing soils in Fuzhou is insufficient. There is also a certain difference in the amount of soil available potassium in each producing county. Among them， the average available potassium content of Guangchang， Lichuan and Zixi is above 150 mg/kg， which is relatively high. Quite a large area of soil in these counties has good potassium supply conditions. The proportion of soil samples with the content levels of less than 80 mg/kg is 40.91% for Lichuan， and 30.56% for Le??an， which suggests the possibility of potassium deficiency in large areas. Therefore， it is recommended that the production of flue??cured tobacco be applied potassium according to soil testing to the potassium??deficient soil， so as to improve the potassium supply of the soil.
Soil water??soluble chlorine
Chlorine is an essential nutrient for flue??cured tobacco， and proper amount of chlorine plays an important role in the yield and quality of tobacco. The test results （Table 6） show that the water??soluble chlorine content of tobacco??planting soil in Fuzhou ranges from 10.7 to 291 cmol/kg， with an average of 88.95 cmol/kg， and the coefficient of variation is 68.31%， indicating high variation range. Among them， there are no soil samples with content of less than 2 cmol/kg， and the soil samples with the content of 2-25 cmol/kg account for 6.34%， 19.72% for 25-40 cmol/kg， 5.63% for 40-45 cmol/kg， and up to 68.31% for the samples with content of greater than 45 cmol/kg. The results suggest that the tobacco??planting soil in most tobacco??producing areas in Fuzhou has high chloride ion content， which is unfavorable for tobacco planting.
The water??soluble chlorine content of soil in each producing county is consistent. The average water??soluble chlorine content of the 7 tobacco??producing counties is higher than 45 cmol/kg， especially Chongren， reaching up to 171.6 cmol/kg. The proportion of soil samples with different content levels of greater than 80 mg/kg reaches 100%， indicating that the soil in large areas of these counties has high water??soluble chlorine content. In production， attention should be paid to the control of chlorine. In areas where the chlorine content is unsuitable， in addition to the reduction of chloride ions in the season of fertilization in tobacco， it is necessary to control the use of chlorine??containing fertilizers before the control.
Soil available zinc
The test results （Table 7） show that the available zinc content of tobacco??planting soil in Fuzhou ranges from 0.2 to 23.2 cmol/kg， with an average of 5.22 cmol/kg， and the coefficient of variation is 68.81%， suggesting great variation range. Among them， soil samples with a content of less than 0.3 cmol/kg account for 2.1%， 2.82% for 0.5-1 cmol/kg， 22.54% for soil samples with the content of 1-3 cmol/kg， and 72.54% for greater than 3 cmol/kg. The data show that most of the tobacco??planting soils in the tobacco??growing areas of Fuzhou are rich in available zinc. The soil available zinc content in each tobacco??producing county is consistent. Except for Guangchang， the average value of available zinc in the other 6 tobacco??producing counties is higher than 3 cmol/kg， especially in Zixi， which is 11.6 cmol/kg. In different levels， the proportions of soil samples with the content of greater than 3 cmol/kg reach 100% in Lichuan and Zixi， and the proportions of other counties also reach above 50%， indicating that the available zinc content of large areas of these counties is relatively abundant.
Soil available boron
Boron is a trace element essential for tobacco growth and is involved in protein metabolism， alkaloid synthesis and organic matter transport[1]. Nicotiana is a medium boron??demand crop. The test results （Table 8） show that the available boron content of tobacco??planting soil in Fuzhou ranges from 0.01 to 0.6 cmol/kg， with an average of 0.14 cmol/kg， and the coefficient of variation is 70.65%， indicating great variation range. Among them， soil samples with content of less than 0.2 cmol/kg account for 81.69%， soil samples with content of 0.2-0.5 cmol/kg account for 17.61%， and soil samples with content of 0.5-1.0 cmol/kg account for 0.7%. Overall， most of the tobacco??planting soils in Fuzhou are in the significant state of boron deficiency.
The soil available boron content in each producing county is consistent. The average available boron content of the 7 tobacco??producing counties is less than 0.2 cmol/kg， which is at a very low level. As far as the sampling area is concerned， soil boron nutrition is one of the soil barrier factors for high??quality tobacco cultivation. The soil available boron content in each sample township （town） is in the state of boron deficiency or potential deficiency， so it is recommended that the tobacco??planting soil in the sampling areas be generally supplemented with boron fertilizers.
Soil cation exchange capacity， exchangeable magnesium and exchangeable calcium
Soil cation exchange capacity Soil cation exchange capacity is closely related to soil fertility and nutrient availability[1]. Under normal circumstances， it is considered that the cation exchange capacity of the cultivated layer is 10-20 cmol/kg， and if it is less than 10 cmol/kg， the soil has low fertility. The cation exchange capacity of tobacco??planting soil in the city ranges from 4.53 to 19.3 cmol / kg， with an average of 9.67 cmol/kg， and the coefficient of variation is 30.26%， of medium variation. Among them， soil samples with exchange capacity of less than 10 cmol/kg account for 63.38%， 36.62% for 10-20 cmol/kg， and no soil samples of greater than 20 cmol/kg. Therefore， the cation exchange capacity of tobacco??planting soil in the whole city is generally low， and the fertility is slightly insufficient. There is a small difference in the average cation exchange capacity of soil in different tobacco??producing counties. Zixi and Guangchang have slightly higher cation exchange capacity， with an average of higher than 10 cmol/kg， followed by Lichuan， Yihuang， Lean and Nanfeng， with an average value of higher than 8 cmol/kg. In addition， Chongren is slightly lower， and the average value is higher than 7cmol/kg. The distribution of the quantity and quantity of exchange is relatively concentrated. Except Zixi， the cation exchange capacities of the other counties are less than 10 cmol/kg， which reflects the fact that the cation exchange capacity is generally low in the tobacco??planting soils of the whole city.
Soil magnesium and calcium Magnesium is an important medium nutrient element that affects the yield and quality of flue??cured tobacco. Calcium is the total regulator of cellular metabolism and maintains the pH required for normal growth of tobacco plants[11]. Magnesium is an important component of chlorophyll. Appropriate amount of magnesium can promote the growth and development of flue??cured tobacco， which is conducive to the improvement of the intrinsic quality of tobacco leaves[12]. For most crops， the soil exchangeable calcium and magnesium are lower than 400 and 50 mg/kg， respectively， and the application of calcium and magnesium fertilizers can have obvious effects[13].
The exchangeable magnesium content of tobacco??planting soil in the city is 0.07-18.8 cmol/kg， with an average of 2.66 cmol/kg， and the coefficient of variation is 114.02%， suggesting the largest variation range. The exchangeable calcium content of tobacco planting soil in the city is 0.18-3.16 cmol/kg， with an average of 0.88 cmol/kg， and the coefficient of variation is 51.42%， indicating that the variation is great.
Conclusion and Discussion
The test results show that a considerable majority of tobacco??planting soils in Fuzhou are in a state of acidity， and alkaline fertilizers such as calcium magnesium phosphate， dolomite powder， and quick lime can be reasonably applied to adjust the pH of the soil. The tobacco??planting soil of the whole city has rich nitrogen， total phosphorus and total potassium contents， which is suitable for high??quality flue??cured tobacco production. For pH， nearly 80% of the soil has high organic matter content. For the sampling areas with high organic matter content （greater than 35）， the ratio of carbon to nitrogen in the organic fertilizer product should be controlled in organic fertilizer application， and the active organic fertilizer product should be selected but avoid the application of inactive organic fertilizer products with grass char as the main raw material. The soil available nitrogen content is generally moderately high； except for a small proportion of soil with low phosphorus or phosphorus deficiency， most of the tobacco??planting soil has rich available phosphorus， which is very beneficial to the cultivation of high quality flue??cured tobacco. Most of the tobacco??growing soil has insufficient available potassium， and the water??soluble chloride ion content is too high， both of which are unfavorable for tobacco planting. In the future production， attention should be paid to the control of chlorine. In areas where the chlorine content is unsuitable， in addition to the reduction of chloride ions in the season of tobacco， it is necessary to control the use of chlorine??containing fertilizers. For trace elements， most of the tobacco??growing soil in the tobacco??producing areas of Fuzhou has rich available zinc content， but the phenomenon of boron deficiency is significant. It is recommended that the tobacco??planting soil in the sampling area should be generally supplemented with boron fertilizers. The cation exchange capacity of tobacco??planting soil in the whole city is generally low， and the fertility is slightly low. From the view of low soil cation exchange capacity and the high temperature and rain during the production period in Fuzhou， it is speculated that as long as the potassium fertilizer is properly added， the utilization of soil and applied potassium by flue??cured tobacco can be better， and the potassium content of the tobacco leaves would not be low. References
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