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Abstract The effects of amount of green manure returned to field on yield and quality of flue??cured tobacco were studied by field experiment. The results showed that significant positive correlation existed between tobacco leaf yield and small or moderate amount of green manure returned to field. Path analysis showed that moderate amount of green manure returned to field affected yield and quality of tobacco leaves， small green manure returned to field mainly affected leaf yield， while large amount of green manure returned to field mainly affected leaf quality. Therefore， the effect of moderate amount of green manure returned to field on yield and quality of tobacco leaves was best， and moderate amount of green manure returned to field was recommended in production of flue??cured tobacco.
Key words Flue??cured tobacco； Returning green manure to field； Yield and quality
The amount of green manure returned to field decides soil nutrient content， organic content and effect of tobacco??planting soil improvement， and affects quality and yield of tobacco leaf. With rapid development of flue??cure tobacco production in China， chemical fertilizer has been used in quantity， and unreasonable crop rotation has been adopted for a long time. As a result， more and more soil is very hardened and acidified， and contains unbalanced nutrients as well[1]. Applying green manure is one of the important measures to improve soil[2]. A lot of studies has shown that decomposed matter produced from green manure returned to field improves soil and enhanced edaphon activity[3-4]， and is beneficial to the improvement of tobacco yield and quality[5-6]. Therefore， the study on the effects of green manure returned to field on improvement of yield and quality of tobacco is of great significance and will provide a theoretical basis for large??area popularization of green manure， improvement of tobacco yield and quality and sustainable development of tobacco production.
Materials and Methods
Materials
The experimental variety of flue??cured tobacco was Yunyan87， and the green manure was developed from Vicia sativa.
Experimental design of green manure returned to field
The amounts of green manure returned to field suitable for promotion were designed in 2013 （fresh weight）： small （3 750， 7 500， 11 250 and 15 000 kg/hm2） （treatment I）， moderate （18 750， 22 500， 26 250 and 30 000 kg/hm2） （treatment II）， and large （33 750， 37 500， 41 250 and 45 000 kg/hm2） （treatment III）， i.e.， there were 3 massive treatments and 12 micro treatments in total. The experiment adopted randomized block arrangement. Each plot had an area of 50.4 m2， where 84 tobacco plants were planted. Experimental method
The soil for growing green manure plant last year was selected. On April 9， the green manure plants were first cut off and weighed according to different treatments. The green manure was returned to field by turning over and burying in soil with a depth of 10-20 cm. next， ridging and flue??cured tobacco planting were performed. The N application rate for flue??cured tobacco was 105 kg/hm2， and as conventional production of sound tobacco， N， P and K ratio was 1?? 1.3?? 3. The flue??cured tobacco was transplanted on April 28. The farming operations were consistent， and the same management measure was required to be finished in same date. Other cultivation and management agreed with the production of sound tobacco.
Results and Analysis
Effects of amount of green manure returned to field on agronomic characters of flue??cured tobacco
As shown in table 1， among all the treatments of amount of green manure returned to field， the treatment I was no good， and the maximum leaf length， maximum leaf width， plant height， pitch， stem girth and valid leaves increased in the medium treatment II and treatment III， which were significantly different from treatment I， while there were no significant differences between the moderate treatment II and large treatment III.
Stepwise regression analysis between amount of green manure returned to field and tobacco quality and yield
The yield and quality results of different treatments are shown Table 2. It could be seen from Table 3 that tobacco yield was in significant positive correlation with small and moderate amounts of green manure returned to field， but had no significant correlation with large amount of green manure； inferior tobacco proportion was in significant negative correlation with small and moderate amounts of green manure returned to field， but in significant positive correlation with large amount of green manure returned to field； and superior and medium tobacco proportions had significant positive correlation with small and moderate amounts of green manure returned to field， but no significant correlation with large amount of green manure. Above analysis showed that small amount of green manure returned to field mainly affected tobacco yield and quality； moderate amount of green manure returned to field mainly played a leading role in improving tobacco yield and quality； and large amount of green manure returned to field mainly affected inferior tobacco proportion. The path analysis on amount of green manure returned to field indicated that the effect of moderate amount of green manure returned to field on yield and quality was best， followed by the effect of small amount of green manure returned to field on yield and quality， while large amount of green manure returned to field was no good on yield and quality （Table 4）. Path analysis between amount of green manure returned to field and tobacco quality
Inferior tobacco proportion It could be seen from table 5 that all the treatments directly or indirectly affected inferior tobacco proportion. The effect of moderate amount of green manure returned to field on inferior tobacco proportion was best， followed by the effect of small amount of green manure returned to field on inferior tobacco proportion， while large amount of green manure returned to field was hardly good on inferior tobacco proportion. All the treatments less indirectly affected inferior tobacco proportion. Only the absolute value of large amount of green manure returned to field was higher than indirectly effect.
Medium tobacco proportion As shown in table 6， the path analysis showed the indirect coefficients of all the treatments on medium tobacco proportion were lower， while their direct coefficients were higher， so all the treatments mainly directly affected medium tobacco proportion. The effects of the various amounts of green manure returned to field on medium tobacco proportion ranked as medium large amount of green manure>moderate amount of green manure>small amounts of green manure amount returned to field.
Superior tobacco proportion As shown in table7， large amount of green manure returned to field had the absolute value of direct path coefficient higher on superior tobacco proportion， suggesting that the effect of large amount of green manure returned to field on superior tobacco proportion was best， and small and moderate amounts of green manure returned to field less directly affected superior tobacco proportion.
Discussion and Conclusions
After the harvest of tobacco leaves， green manure plants were cultivated in winter， and tobacco was transplanted after ploughing back and returning the green manure to field. Under the effect of microbe， the green manure is constantly decomposed and generates rotten matter， which not only could form granular structures with calcium， but also could enhance the preservation capacity of water and fertility and reduced soil bulk density[7-8]. Green manure offers fresh organic energy material for rapid microorganism multiplication[4-9]， promotes nutrient availability， and reinforces soil??ripening. The secreta of green manure crop in growing process and organic acids generated by it also promote conversion of indissoluble nutrients in soil[6-7]. Kong et al.[10] showed that among anterior five weeks after Vicia villosa was ploughed back， nitrogen was released 86.90%-92.27%， and potassium was released 90.10%-90.16%. With the combined application of green manure and inorganic fertilizers into soil， Chemical components in baked tobacco were more coordinated， and tobacco leaves had better appearance quality[11] as well as the higher smoking score， which accord with the results of this study. The results of this study showed that the effect of moderate amount of green manure returned to field on tobacco yield and quality leaves was best， and moderate amount of green manure returned to field was recommended in production of flue??cured tobacco. References
[1] YANG YG， WANG SL， LIU G， et al. Effect of Bio??logical organic fertilizer on yield and quality of flue??cured tobacco and soil improvement[J]. Chinese Tobacco Science， 2012， 33（4）： 70-74.
[2] WANG RB， LIAN FF， XIA KB， et al. Effect of different pattern of green manure returning to field on yield and quality of flue??cured tobacco[J]. Anhui Agricultural Science， 2010， 38（2）： 6183-6188.
[3] WU WC， MENG QJ. First expiration on stable production development of green manure[J]. Journal of Guangxi Agriculture， 2008， 23（3）： 39-41.
[4] CHENG S， WU JS， WANG P， et al. Effect of application of green manure??chicken manure??and calcium fertilizer on fertility of under??crop red soil tobacco growing[J]. Acta Tobacaria Sinica， 2008， 10（14）： 39-44.
[5] WANG XW， MENG QH， WANG SK， et al. Utilization of green manure in production of flue??cured tobacco[J]. Chinese Tobacco Science， 2001（4）： 44.
[6] ZHANG HF， LIU XP， CUI WY， et al. Application of green manure in production of flue??cured tobacco and its effect on quality of tobacco leaf[J]. Anhui Agricultural Science， 2007， 35（29）： 9291-9292.
[7] LIN KH. Potassium improves quality of flue??cured tobacco[J]. Better Crops International，1993， 12： 14-15.
[8] LUO HY， CHENG CX， WANG SK， et al. Effect of amount of green manure retuning to field on enzyme activity of tobacco leaf during the field period （from transplanting to budding） and quality of tobacco leaf after it was baked[J]. Journal of Mountain Agriculture and Biology， 2010， 29（6）： 495-501.
[9] SHI Q， JI Y， JIANG PC， et al. Effect of grain amaranth of rich potassium on quality of summer tobacco leaf[J]. Chinese Tobacco Science， 2002， （3）： 5-7.
[10] KONG W， GENG MJ， CHU LZ， et al. Study of dynamic on decomposition of Vicia villo Rothvar in tobacco field and nutrient??release[J]. Soil and Fertilizer in China. 2011（1）： 64-68.
[11] YANG BJ， LIU H. Effect of combined application of organic fertilizer and chemical fertilizer on nutrient and quality of potassium[J]. Journal of Southwest Agricultural University， 1993， 15（6）： 578-583.
Key words Flue??cured tobacco； Returning green manure to field； Yield and quality
The amount of green manure returned to field decides soil nutrient content， organic content and effect of tobacco??planting soil improvement， and affects quality and yield of tobacco leaf. With rapid development of flue??cure tobacco production in China， chemical fertilizer has been used in quantity， and unreasonable crop rotation has been adopted for a long time. As a result， more and more soil is very hardened and acidified， and contains unbalanced nutrients as well[1]. Applying green manure is one of the important measures to improve soil[2]. A lot of studies has shown that decomposed matter produced from green manure returned to field improves soil and enhanced edaphon activity[3-4]， and is beneficial to the improvement of tobacco yield and quality[5-6]. Therefore， the study on the effects of green manure returned to field on improvement of yield and quality of tobacco is of great significance and will provide a theoretical basis for large??area popularization of green manure， improvement of tobacco yield and quality and sustainable development of tobacco production.
Materials and Methods
Materials
The experimental variety of flue??cured tobacco was Yunyan87， and the green manure was developed from Vicia sativa.
Experimental design of green manure returned to field
The amounts of green manure returned to field suitable for promotion were designed in 2013 （fresh weight）： small （3 750， 7 500， 11 250 and 15 000 kg/hm2） （treatment I）， moderate （18 750， 22 500， 26 250 and 30 000 kg/hm2） （treatment II）， and large （33 750， 37 500， 41 250 and 45 000 kg/hm2） （treatment III）， i.e.， there were 3 massive treatments and 12 micro treatments in total. The experiment adopted randomized block arrangement. Each plot had an area of 50.4 m2， where 84 tobacco plants were planted. Experimental method
The soil for growing green manure plant last year was selected. On April 9， the green manure plants were first cut off and weighed according to different treatments. The green manure was returned to field by turning over and burying in soil with a depth of 10-20 cm. next， ridging and flue??cured tobacco planting were performed. The N application rate for flue??cured tobacco was 105 kg/hm2， and as conventional production of sound tobacco， N， P and K ratio was 1?? 1.3?? 3. The flue??cured tobacco was transplanted on April 28. The farming operations were consistent， and the same management measure was required to be finished in same date. Other cultivation and management agreed with the production of sound tobacco.
Results and Analysis
Effects of amount of green manure returned to field on agronomic characters of flue??cured tobacco
As shown in table 1， among all the treatments of amount of green manure returned to field， the treatment I was no good， and the maximum leaf length， maximum leaf width， plant height， pitch， stem girth and valid leaves increased in the medium treatment II and treatment III， which were significantly different from treatment I， while there were no significant differences between the moderate treatment II and large treatment III.
Stepwise regression analysis between amount of green manure returned to field and tobacco quality and yield
The yield and quality results of different treatments are shown Table 2. It could be seen from Table 3 that tobacco yield was in significant positive correlation with small and moderate amounts of green manure returned to field， but had no significant correlation with large amount of green manure； inferior tobacco proportion was in significant negative correlation with small and moderate amounts of green manure returned to field， but in significant positive correlation with large amount of green manure returned to field； and superior and medium tobacco proportions had significant positive correlation with small and moderate amounts of green manure returned to field， but no significant correlation with large amount of green manure. Above analysis showed that small amount of green manure returned to field mainly affected tobacco yield and quality； moderate amount of green manure returned to field mainly played a leading role in improving tobacco yield and quality； and large amount of green manure returned to field mainly affected inferior tobacco proportion. The path analysis on amount of green manure returned to field indicated that the effect of moderate amount of green manure returned to field on yield and quality was best， followed by the effect of small amount of green manure returned to field on yield and quality， while large amount of green manure returned to field was no good on yield and quality （Table 4）. Path analysis between amount of green manure returned to field and tobacco quality
Inferior tobacco proportion It could be seen from table 5 that all the treatments directly or indirectly affected inferior tobacco proportion. The effect of moderate amount of green manure returned to field on inferior tobacco proportion was best， followed by the effect of small amount of green manure returned to field on inferior tobacco proportion， while large amount of green manure returned to field was hardly good on inferior tobacco proportion. All the treatments less indirectly affected inferior tobacco proportion. Only the absolute value of large amount of green manure returned to field was higher than indirectly effect.
Medium tobacco proportion As shown in table 6， the path analysis showed the indirect coefficients of all the treatments on medium tobacco proportion were lower， while their direct coefficients were higher， so all the treatments mainly directly affected medium tobacco proportion. The effects of the various amounts of green manure returned to field on medium tobacco proportion ranked as medium large amount of green manure>moderate amount of green manure>small amounts of green manure amount returned to field.
Superior tobacco proportion As shown in table7， large amount of green manure returned to field had the absolute value of direct path coefficient higher on superior tobacco proportion， suggesting that the effect of large amount of green manure returned to field on superior tobacco proportion was best， and small and moderate amounts of green manure returned to field less directly affected superior tobacco proportion.
Discussion and Conclusions
After the harvest of tobacco leaves， green manure plants were cultivated in winter， and tobacco was transplanted after ploughing back and returning the green manure to field. Under the effect of microbe， the green manure is constantly decomposed and generates rotten matter， which not only could form granular structures with calcium， but also could enhance the preservation capacity of water and fertility and reduced soil bulk density[7-8]. Green manure offers fresh organic energy material for rapid microorganism multiplication[4-9]， promotes nutrient availability， and reinforces soil??ripening. The secreta of green manure crop in growing process and organic acids generated by it also promote conversion of indissoluble nutrients in soil[6-7]. Kong et al.[10] showed that among anterior five weeks after Vicia villosa was ploughed back， nitrogen was released 86.90%-92.27%， and potassium was released 90.10%-90.16%. With the combined application of green manure and inorganic fertilizers into soil， Chemical components in baked tobacco were more coordinated， and tobacco leaves had better appearance quality[11] as well as the higher smoking score， which accord with the results of this study. The results of this study showed that the effect of moderate amount of green manure returned to field on tobacco yield and quality leaves was best， and moderate amount of green manure returned to field was recommended in production of flue??cured tobacco. References
[1] YANG YG， WANG SL， LIU G， et al. Effect of Bio??logical organic fertilizer on yield and quality of flue??cured tobacco and soil improvement[J]. Chinese Tobacco Science， 2012， 33（4）： 70-74.
[2] WANG RB， LIAN FF， XIA KB， et al. Effect of different pattern of green manure returning to field on yield and quality of flue??cured tobacco[J]. Anhui Agricultural Science， 2010， 38（2）： 6183-6188.
[3] WU WC， MENG QJ. First expiration on stable production development of green manure[J]. Journal of Guangxi Agriculture， 2008， 23（3）： 39-41.
[4] CHENG S， WU JS， WANG P， et al. Effect of application of green manure??chicken manure??and calcium fertilizer on fertility of under??crop red soil tobacco growing[J]. Acta Tobacaria Sinica， 2008， 10（14）： 39-44.
[5] WANG XW， MENG QH， WANG SK， et al. Utilization of green manure in production of flue??cured tobacco[J]. Chinese Tobacco Science， 2001（4）： 44.
[6] ZHANG HF， LIU XP， CUI WY， et al. Application of green manure in production of flue??cured tobacco and its effect on quality of tobacco leaf[J]. Anhui Agricultural Science， 2007， 35（29）： 9291-9292.
[7] LIN KH. Potassium improves quality of flue??cured tobacco[J]. Better Crops International，1993， 12： 14-15.
[8] LUO HY， CHENG CX， WANG SK， et al. Effect of amount of green manure retuning to field on enzyme activity of tobacco leaf during the field period （from transplanting to budding） and quality of tobacco leaf after it was baked[J]. Journal of Mountain Agriculture and Biology， 2010， 29（6）： 495-501.
[9] SHI Q， JI Y， JIANG PC， et al. Effect of grain amaranth of rich potassium on quality of summer tobacco leaf[J]. Chinese Tobacco Science， 2002， （3）： 5-7.
[10] KONG W， GENG MJ， CHU LZ， et al. Study of dynamic on decomposition of Vicia villo Rothvar in tobacco field and nutrient??release[J]. Soil and Fertilizer in China. 2011（1）： 64-68.
[11] YANG BJ， LIU H. Effect of combined application of organic fertilizer and chemical fertilizer on nutrient and quality of potassium[J]. Journal of Southwest Agricultural University， 1993， 15（6）： 578-583.