Analysis on the Change of Soil pH in Tobacco Fields During the Cultivation of Flue-cured Tobacco

Yunfei CHEN Ailian OUYANG Jiye QIANG

Abstract ［Objectives］ This study was conducted to understand soil pH changes during tobacco planting and its reasons.

［Methods］ Soil samples were collected from Jingdong, Jinggu and Zhenyuan tobacco-planting areas of Pu餰r City before tobacco planting and during different growth stages of tobacco.

［Results］ Planting flue-cured tobacco changed the soil pH value. First, it was affected by the nicotine produced by the root system of tobacco plants. Second, it was affected by the type of soil.

［Conclusions］ This study has certain theoretical guiding significance for the cultivation management of flue-cured tobacco and the arrangement of ensuing crops.

Key words Tobacco cultivation; pH value; Change

Received: February 23, 2020Accepted: April 20, 2020

Supported by University-level Project of Pu餰r University (No.k2017019).

Yunfei CHEN (1981-), male, P. R. China, teaching assistant, master, mainly devoted to research about tobacco physiology and biochemistry and cultivation physiology.

*Corresponding author. E-mail: 1249891704@qq.com.

Flue-cured tobacco, as a pillar industry in Yunnan Province, has made tremendous contributions to the local economic construction. The added value created by the tobacco industry in Yunnan during the "12th Five-Year Plan" period is about 888.8 billion yuan, accounting for 13.8% of the province餾 GDP, with an average of 117.6 billion per year, 320 million per day. Tobacco cultivation is arguably one of the most important crops in Yunnan Province. The sustainable development of tobacco, especially the flue-cured tobacco industry, directly affects the economic construction and people餾 living standards in Yunnan Province.

The development of flue-cured tobacco in Yunnan in these years can be described as vigorous and well developed. However, because flue-cured tobacco planting has strict requirements on the soil［1-2］, whether the flue-cured tobacco planting process affects the soil and whether it will have an effect on the cultivation of the crops planted before and after and the change of the farming system are problems very worth studying. Studying the changes in soil pH during the process of flue-cured tobacco plantation is of great practical significance for scientifically guiding flue-cured tobacco production, protecting soil resources and environment, providing a theoretical basis for establishing a reasonable planting system and achieving sustainable agricultural production.

Materials and Methods

Basic situation of the experimental field

Pu餰r City is located in the southwest of Yunnan Province and has sufficient light and heat resources. It is one of the main production bases for fragrant flue-cured tobacco raw materials in Yunnan Province. In Jinggu County, the most suitable area accounts for 4.3%, and the suitable area accounts for 6.6%; in Jingdong County, the most suitable area accounts for 3.4%, and the suitable area accounts for 1.8%; and in Zhenyuan County, the most suitable area accounts for 1.2% , and the suitable area accounts for 2.4% ［3］. This experiment was selected in Jinggu County, Jingdong County and Zhenyuan County. In this study, fields with flat terrain, convenient irrigation and drainage, sufficient sunshine and convenient transportation, planted with the previous crop wheat, moderate in soil fertility, were selected.

Experimental design and methods

The flue-cured tobacco variety, Yunyan 87, was selected for all the three counties. As to the fertilization, 70% of the fertilizer was used as the base fertilizer, and the remaining 30% was used for top dressing during the resettling stage. The cultivation management methods and time were guaranteed to be completely consistent. The soil samples were collected randomly during such four periods as before planting, the resettling stage, the vigorous growth stage and after topping to determine the indices.

Determination indices and methods

In the three key flue-cured tobacco planting counties of Jinggu, Jingdong and Zhenyuan, the tobacco fields of three key townships were selected as the experimental fields for soil sample collection, respectively. In each tobacco field, the soil samples were collected by five-point sampling method, and a hoe was used to dig the soil at 20 cm of the cultivated layer. The soil collected should be brought back to the laboratory in time to make a mixed soil sample of about 1 kg for analysis.

The pH value of soil was measured by the pH meter method (the soil-water ratio was 1∶2.5). Specifically, large particles were removed from the mixed soil sample (it does not need to be air-dried), and then a 20 g of sample was weighed into a small beaker, added with 50 ml of distilled water, and stirred to fully disperse the soil particles. The mixture was stood for half an hour for determination. A pH electrode was turned on, and the probe was inserted into the liquid.The displayed value was recorded as the soil pH value after the data was stable.

Data processing and analysis

SPSS25.0 and excel were used for statistical analysis of the data.

Results and Analysis

General characteristics of pH changes during tobacco planting

It can be seen from Fig. 1 that the soil before tobacco planting in all three tobacco planting sites was acidic.

The initial pH in the Jingdong experimental area was between 5 and 5.5. After the flue-cured tobacco planting was started, the pH was on the increase and reached a maximum value of about 8.5 in the vigorous growth stage, and then fell to 5.5-6.0 after topping.

The change trends in the Jinggu and Zhenyuan experimental areas were about the same. The initial pH was between 4 and 4.5, and the peak value was between 5.5 and 6 in the vigorous growth stage. After topping, it dropped to 5.0-5.5 and 4.5-5.0, respectively.

The pH of Jingdong was generally higher than that of other two experimental sites, which was caused by regional differences. The soil in Jingdong experimental area is brown soil, which is gray-brown at the surface and may have a low free calcium carbonate content in the whole profile, and the soil pH is 5.0-6.5. However, since the same flue-cured tobacco was grown, the change trends were similar.

In the process of tobacco planting, the pH changes in three places were similar. The pH gradually increased during the period from the beginning of planting tobacco to the vigorous growth period, then reached the highest value during the vigorous growth period, and finally decreased slowly. It could be speculated that the plant absorbed a large amount of anions (NO-3) from the soil during the period from the resettling stage to the vigorous growth stage, so the soil pH tended to increase. During the period from the vigorous growth stage to the topping stage, the absorption of NO-3 ions by flue-cured tobacco decreased, which made the anions in the soil increase, and the soil pH showed a downward trend. However, on the whole, during the cultivation process, the pH increased to a certain extent and gradually decreased after topping, and there were fluctuations during the process, but there was not much difference between after planting tobacco and before planting tobacco.

Analysis of the causes of pH changes during tobacco planting

The effect of nicotine on soil pH during tobacco planting

Most of the total alkaloids in flue-cured tobacco are nicotine［4］, which is one of the important chemical components in flue-cured tobacco. Sun et al. studied the change laws of chemical substances and dry matter during the maturation of flue-cured tobacco using Longjiang 911, JY-2-03 and Yun 87 as experimental materials. It was found that the total plant alkaloid content basically increased first and then decreased. The nicotine content of the three varieties reached a peak at 60 and 70 d after transplanting, but dropped sharply at 80 d after transplanting when the tobacco leaves began to mature［5］, which is roughly similar to the change law of pH value in the soil after flue-cured tobacco planting. According to this inference, the change in nicotine content may have a correlation with the change in soil pH, and nicotine may have a certain effect on the change in soil pH.

Soil parent materials and soil type also affect soil pH

Among the three experimental counties, the soil pH value of Jingdong County was significantly higher than those of Jinggu County and Zhenyuan County, and the pH range of the whole tobacco planting process also changed greatly, which might be due to the differences in soil parent materials.

The soil of Jingdong is brown soil, and the main parent materials of which are the weathered products of granite, gneiss, limestone, sandstone and shale［6］. It has a thin skeleton soil layer, and the soil layer is thin, acidic or slightly acidic.

The soil of Jinggu is red soil. The weathering and leaching effect of red soil is slightly weaker than that of laterite. It has a red color, thick soil layer, thick texture and poor fertility and is acidic.

The soil of Zhenyuan is yellow-brown soil. The yellow-brown soil has both the characteristics of aluminization in yellow and red soils and the clayification of brown soil［7］.

Conclusions

During the process of tobacco planting, the pH changes in the three counties were basically similar. In the 60-70 d from planting tobacco to the vigorous growth stage, the pH gradually increased, reached the highest value during the vigorous growth period, and then slowly decreased. It could be speculated that as the mature nicotine content of flue-cured tobacco reached the highest value in the vigorous growth stage, the nicotine in the root system of flue-cured tobacco was lost to the rhizosphere soil with secretions, which increased the soil pH to a certain extent. After the vigorous growth period, the nicotine content dropped and the soil pH recovered accordingly.

Different soil types can also affect soil pH. The red soil and yellow-brown soil have stronger leaching effects than brown soil, and it is easier for them to lose more soluble ions during the rainy season. Therefore, the soil acidity of Jinggu and Zhenyuan areas is stronger than that of Jingdong.

References

［1］ LIU GS, FU YP, DING SS, et al. Tobacco cultivation science［M］. Beijing: China Agriculture Press, 2017. (in Chinese)

［2］ CHENG BY, MA JM, CHENG L, et al. Analysis of present soil nutritional situation in major flue-cured tobacco production regions［J］. Journal of Agricultural Science and Technology, 2009, 11(3): 131-136. (in Chinese)

［3］ LI M, YANG M, WANG W, et al. Fine division of climate suitability for flue-cured tobacco cultivation in Pu餰r City, Yunnan［J］. Crops, 2010(6): 75-78. (in Chinese)

［4］ ZHANG YY, ZHOU JH. A study on the spatio temporal variability of nicotine content in flue-cured tobacco ( Nicotiana tabacum )［J］. Acta Agriculturae Universitis Jiangxiensis, 2008, 30(3): 449-454. (in Chinese)

［5］ SUN LJ, AN CR, WU GH, et al. Chemical compositions and dry matter change of tobacco leaves during the ripening of flue-cured tobacco［J］. Hunan Agricultural Sciences, 2017, (6): 77-80. (in Chinese)

［6］ HU XD, DENG XH, WANG F, et al. Soil pH characteristics and its relation to soil nutrients in Qianxi餹an tobacco-growing region［J］. Journal of Anhui Agricultural University, 2014(6): 1070-1074. (in Chinese)

［7］ ZHANG W, LI QQ, WANG CQ, et al. Spatial variability of soil pH and its influence factors at a county scale in hilly area of mid-Sichuan basin——A case study from Renshou in Sichuan［J］. Resources and Environment in The Yangtze Basin, 2015, 24(7): 1192-1198. (in Chinese

Editor: Yingzhi GUANGProofreader: Xinxiu ZHU