Contents and Changes of Potassium in Plough Layers of Xuancheng , South Anhui Province

Xuancheng City is the dominant tobacco-planting area of Anhui province due to the high quality of the tobacco leaves. However, the potassium content in the tobacco leaves shows a gradual decreasing trend in recent years, which may be attributed partly to the possible low potassium content in topsoil of tobacco-planting farmland. Therefore, the content data in the year of 20052007 of rapidly available potassium (RA-K) of 7730 topsoil samples mainly under double rice rotation or wheat/rice-rice rotation at that time and the content data of slowly available potassium (SA-K) and RA-K of 124 typical topsoil samples in the year of 2015 under tobacco-rice rotation were used and compared in order to disclose the status of topsoil potassium and to provide a guidance for reasonable potassium fertilization in Xuancheng. The results showed that in 2005-2007 RA-K content ranged from 1 mg·kg to 844 mg·kg with an average of 68 mg·kg, and 82.7% of topsoil samples were insufficient in RA-K (<100 mg·kg). Comparatively in 2015 SA-K content ranged from 230 mg·kg to 1340 mg·kg with an average of 595 mg·kg, and 13.7% of soil samples were insufficient in SA-K (<400 mg·kg); RA-K content ranged from 46 mg·kg to 352 mg·kg with an average of 134 mg·kg, and 25.8% of soil samples were insufficient in RA-K (<100 mg·kg). The above data show that RA-K content has increased gradually in farmland topsoil mainly due to fertilization since 1980s, particularly to farmland under tobacco-rice rotation, but insufficient RA-K in topsoil is still widely and serious for farmland under double rice rotation or wheat/rice-rice rotation. Under tobacco-rice rotation, RA-K insufficiency usually occurred in the farmlands which plant tobacco less than 3 years, indicating more potassium fertilizer should be applied. How to cite this paper: Lin, K., Li, D.C., Song, X.D., Zu, C.L., Jiang, C.Q., Shen, J., Ma, C.X., Gao, Q., Zhu, Q.F., Ji, X.J., Zhang, G. and Xue, L. (2017)


Introduction
The potassium content is an important index to evaluate the quality of fluecured tobacco [1] [2].Generally speaking, the potassium content in tobacco leaves with high quality should not be less than 2% [3].The potassium content in tobacco leaves of the United States, Zimbabwe and Brazil and other countries ranges from 4% to 6% while it is only ranges from 1.5% to 2.0% in China, except for the Yunnan-Guizhou Plateau of southwest China and Sanming of Fujian province of southeast China [4] [5].Thus, promoting the potassium content in tobacco leaves has been one of the key studies in China.
Xuancheng City (29˚57'N -31˚19'N，117˚58'E -119˚40'E), located in south Anhui Province, has been the dominant tobacco-planting region of this province since tobacco was initially planted in 2007 due to its well-known "burnt-sweet smelling" style [6] [7].The tobacco-planting area is nearly 10,000 hm 2 with 10,000 tons of tobacco leaf yield [6]- [11].However, according to the annual monitoring data, the average potassium content in tobacco leaves of Xuancheng City shows a decreasing trend, it alarmingly decreased from 2% in 2007 [6] to 1.5% in 2014.
The potassium in tobacco leaves mainly comes from soil and applied potassium fertilizer [12], and the rational potassium fertilization depends upon the background of soil potassium content [13].Therefore, in this paper, we analyzed the potassium contents in the topsoil in 2005-2007 and in 2015 of Xuancheng City, disclosed the changes and mechanism of potassium status in order to instruct the reasonable potassium fertilization for producing tobacco leaves with high content of potassium.

The Sources of Soil Potassium Data
The contents of potassium in topsoil were from the following sources in this paper, one is the contents of rapidly available potassium (RA-K) of 7730 topsoil samples (0 -20 cm) in 2005-2007 obtained from the programme of Soil Testing and Formula Fertilization (STFF), and the other is the contents of slowly available potassium (SA-K) and RA-K of 124 topsoil samples collected in 2015 from the farmlands of model tobacco-farmers.
The spatial distribution of topsoil samples in the two periods are shown in Figure 1 and Table 1.The contents of SA-K and RA-K were determined by the method of flame photometer after topsoil samples were digested by HNO 3 and extracted by acetamide for the content measurements of SA-K and RA-K, re  spectively [14].

The Grading Standards of Soil Potassium Contents
Table 2 is the grading standards of SA-K and RA-K used in this study, which is simplified on the basis of grades of potassium contents adopted in the 2 nd National Soil Survey (http://wenku.baidu.com/view/3f7f297e27284b73f242505c.html).

Methods of Data Statistics and Mapping
Data were analyzed with Microsoft Excel 2013 and IBM Statistics SPSS 20.0, the spatial distribution of topsoil samples was mapped on the platform of Esri Arc-GIS 10.3.

Rapidly Available Potassium Contents of Topsoil in 2005-2007
Table 3 shows the statistic information of RA-K contents of topsoil samples in 2005-2007.To the whole Xuancheng City, RA-K content ranged from 1 mg•kg −1   to 844 mg•kg −1 with an average of 68 mg•kg −1 , while the average RA-K contents in different administrative regions ranged from 47 mg•kg −1 to 94 mg•kg −1 , generally in the deficient grade.
Table 4 shows the sample number distribution in different grades of RA-K contents in 2005-2007.To the whole Xuancheng City, 82.7% (6392 samples) of the total samples were in the deficient grade while 5.8% (9 samples) of the total samples were in sufficient grade, which indicate generally RA-K content of topsoil samples are insufficient.To various administrative regions, the deficiency of RA-K contents were most serious in Jingde and Xuanzhou, more than 90% of the corresponding total samples were in the deficient grade, followed by GuangDe, LangXi and Jixi, about 80% -90% of the corresponding total samples were in the deficient grade, while 60% -80% of the corresponding total samples in Jingxian and Ningguo were in the deficient grade.

Potassium Contents of Topsoil Samples in 2015
Table 5 shows the statistic information of the contents of SA-K and RA-K in topsoil samples from the model tobacco-farmers' farmlands of Xuancheng City in 2015.
To the whole region, SA-K content ranged from 230 mg•kg −1 to 1340 mg•kg −1   with an average of 595 mg•kg −1 , in the sufficient grade; RA-K content ranged from 46 mg•kg −1 to 352 mg•kg −1 with an average of 134 mg•kg −1 , in the medium grade.
To various administrative regions, 1) average SA-K content in Jingxian was  the total samples were in the deficient grade while 62.9% (78 samples) of the total samples were in sufficient grade.To various administrative regions, SA-K content in Jingde was in the sufficient grade, while 8% -30% of the corresponding total samples in other regions were in the deficient grade.

K. Lin et al.
Table 7 shows the sample number distribution in various grades of RA-K contents of topsoil samples in 2015.To the whole region, 25.8% (32 samples) of the total samples were in the deficient grade, while 29.8% (37 samples) of the total samples were in the sufficient grade.To various administrative regions, RA-K content in Langxi and Ningguo were in the sufficient grade, while 15-36% of the corresponding total samples in other regions were insufficient in RA-K.

Discussion
According to the data of RA-K contents in 1987 obtained during the 2 nd National Soil Survey in Xuancheng City, 89.4% of topsoils were lack of RA-K (<100 mg•kg −1 ), however, in 2005-2007 the percentage of RA-K deficient was decreased down to 82.7%, indicating RA-K status of farmlands were slightly improved, which is attributed to potassium fertilization: before 2005-2007, most of the farmlands in Xuancheng were given priority to double rice rotation or ricewheat rotation.Historical field survey data showed that during this period the annual average crop yield (double rice or rice + wheat) was 1000 kg/667 m 2 in total, the grains usually contain 0.4% of K 2 O, almost all the crop straws were returned to the fields or burnt directly in the fields [15] [16], thus about 4 kg /667 m 2 of K 2 O was removed out from the topsoil by the harvested grains.Meanwhile, average 80 kg/667 m 2 of the compound fertilizer (containing 8% of K 2 O) was applied annually to the fields, about 6.4 kg/667m 2 of K 2 O brought into the topsoils, so annually the "net" input of K 2 O to the topsoils is about 2.4 kg/667m 2 .
For the surveyed model farmers' farmlands with tobacco-rice rotation, only 25.8% of the total 124 topsoil samples are insufficient in RA-K content, while the proportion of RA-K content in medium or above grades was 74.2% in total, indicating that obvious improved in RA-K content in topsoil under the tobacco-rice rotation compared with the data under double rice cropping or rice-wheat rotation in 2005-2007, which is resulted from surplus potassium fertilizers applied into the topsoils during the tobacco-planting period.The field survey data surveyed in 2015 showed that during the tobacco-planting period, 55 Some tobacco-farmers thought that no potassium fertilizers should be applied into fields during the rice planting period, but the field survey data surveyed in 2015 showed further that for the 87.5% of 'new tobacco-planting farmlands' (under tobacco-rice rotation less than 3 years, 23 topsoil samples in total in 2015), the potassium accumulative effect in topsoils is not obvious, RA-K content in topsoil is still in the deficient grade, so the fields need continuous potassium fertilization during the rice planting period.

Conclusion
Generally, the content of rapidly available potassium content has increased gradually in topsoil farmlands in Xuancheng City, which is mainly due to potassium fertilization since 1980s, particularly due to farmlands under tobacco-rice rotation which initially started since 2007.However, more potassium fertilizers should be applied into the farmlands under double rice rotation or wheat-rice rotation, while more potassium fertilizers should be used during the rice planting period to the farmlands under tobacco-rice rotation less than 3 years.
485 mg•kg −1 , in the medium grade, average SA-K contents ranged from 545 mg•kg −1 to 810 mg•kg −1 in other region, in the sufficient grade.2) average RA-K contents were 151 mg•kg −1 and 185 mg•kg −1 in Guangde and Ningguo, respectively, in the sufficient grade; average RA-K content ranged from 105 mg•kg −1 to 141 mg•kg −1 in other region, in the medium grade.

Table 1 .
Regional distribution of topsoil samples in Xuancheng City.

Table 2 .
Content grades of slowly available potassium and rapidly available potassium used in this study.

Table 3 .
Statistics of rapidly available potassium contents in topsoil samples in 2005-2007, Xuancheng City.

Table 6
shows the sample number distribution in various grades of SA-K contents of topsoil samples in 2015.To the whole region, 13.7% (17 samples) of

Table 4 .
Statistics of rapidly available potassium contents in different grades in topsoil samples in 2005-2007, Xuancheng City.

Table 5 .
Statistics of slowly available potassium and rapidly available potassium contents in topsoil samples in 2015, Xuancheng City.

Table 6 .
Statistics of slowly available potassium contents at various grades in topsoil samples in 2015, Xuancheng City.

Table 7 .
Statistics of rapidly available potassium contents at various grades in topsoil samples in 2015, Xuancheng City., also almost all the rice straws were returned to the fields or burnt directly in the fields, thus, about 5.0 kg of K 2 O was removed out from the soil by the leaves and stems of tobacco and the grains of rice, so the "net" input of K 2 O into the topsoil is about 15.85 kg/667m 2 annually.It is the reason for mean RA-K content (134 mg•kg −1 ) in the model tobacco-farmers' farmlands in 2015 is 49.3% higher than that under double rice rotation or rice-wheat rotation in 2005-2007 (68 mg•kg −1 ), also higher than that of the 20 long-term monitoring farmlands (under double rice rotation or rice-wheat rotation) in Xuancheng City in the same year (ranged from 58 mg•kg −1 to 188 mg•kg −1 with an average of 104 mg•kg −1 ).