Relationship Between pH Value and Physical and Chemical Properties of Tobacco-growing Soil

Yunfei CHEN Jiye QIANG Ailian OUYANG

Abstract ［Objectives］ This study was conducted to understand the relationship between soil pH and soil physical and chemical properties during tobacco planting.

［Methods］Through a field experiment, the pH value of tobacco-growing soil, the contents of mineral nutrients (ammonium nitrogen, alkali-hydrolyzable nitrogen, available potassium, available phosphorus), soil bulk density and porosity were investigated.

［Results］ The pH value of the soil after tobacco planting increased, and the contents of ammonium nitrogen, alkali-hydrolyzable nitrogen, and available potassium were closely related to the change of pH value. The bulk density of the tobacco-planting soil decreased and the porosity increased. Planting flue-cured tobacco had certain impacts on soil pH, soil bulk density and other physical and chemical properties.

［Conclusions］This study provides a theoretical basis for the investigation of the fertilization laws in the process of tobacco planting and the selection of subsequent crops.

Key words Tobacco growing soil; pH value; Mineral nutrients; Bulk density; Porosity

Received: December 30, 2020  Accepted: February 22, 2021

Supported by School-level Project of Puer University (K2017019).

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

*Corresponding author. E-mail: 330887433@qq.com.

The flue-cured tobacco industry has made a great contribution to the construction of the national economy, especially in Yunnan, where flue-cured tobacco is a pillar industry of the economy. The sustainable development of flue-cured tobacco is affected by climate, soil, moisture, cultivation techniques and measures, etc., and the guarantee of the quality of flue-cured tobacco and the establishment of the planting system of flue-cured tobacco and other crops are mainly affected by natural conditions such as soil, temperature, and moisture, of which soil is one of the most important factors［1-2］. Soil is the foundation of crop growth, and good soil quality is the prerequisite for the production of high-quality tobacco. Flue-cured tobacco must obtain sufficient nutrients from the soil to ensure its normal growth and development and the formation of yield quality, thereby obtaining higher economic benefits.

In recent years, the excessive use of land, the unreasonable application of chemical fertilizers, the erosion of industrial pollution, man-made destruction and other factors have caused the soil to become increasingly acidified and heavy［3-4］, which seriously affects the quality of flue-cured tobacco and affects the economic income of tobacco farmers, and the green and sustainable development of the flue-cured tobacco industry has been severely hindered. To study the relationship between the pH of tobacco-planting soil and the changes in the physical and chemical properties of tobacco fields is very important to slowing down or hindering soil acidification, scientifically guiding fertilization of flue-cured tobacco and its scientific production, improving the yield, quality and economic benefits of flue-cured tobacco, and ensuring the continuous, efficient and green development of flue-cured tobacco. Meanwhile, it provides a certain theoretical basis for the establishment of a planting system with flue-cured tobacco as the main crop, which has very important economic and social significance.

Materials and Methods

Basic information of the test site

Jinggu County, Puer City was selected as the representative for the test site. The test plot was flat, convenient for irrigation and drainage, and had sufficient sunshine, convenient transportation. The previous crop was wheat, and the soil fertility was moderate.

Experimental design and methods

The flue-cured tobacco variety Yunyan 87 was selected. 70% of the fertilizer was used as base fertilizer, and the remaining 30% was top-dressed in the rosette stage. The cultivation and management methods and time were all guided in accordance with the local high-quality tobacco production techniques. The soil samples were collected by random sampling at such four periods as before tobacco planting, the rosette stage, the vigorous growing stage, and after harvesting for index determination and analysis.

Determination indexes

pH value of tobacco-growing soil

A portable pH meter was used to measure the pH value of the soil in different periods.

Mineral nutrient contents

The contents of ammonium nitrogen, alkali-hydrolyzale nitrogen, available phosphorus and available potassium in tobacco-growing soil at different periods were determined, and the determination in the rosette stage was carried out before topdressing. Ammonium nitrogen was determined by the Nesslers reagent colorimetric method; alkali-hydrolyzale nitrogen was determined by the alkaline hydrolysis diffusion method; the rapidly available phosphorus was determined by molybdenum blue colorimetry; and available potassium was determined by the sodium tetraphenylborate turbidimetric method.

Soil bulk density and porosity

The soil bulk density was determined by the cutting-ring method, and the soil porosity was calculated. Each soil sample was determined for 3 times to obtain an average.

Data processing and analysis

SPSS 25.0 was used to statistically analyze the data.

Results and Analysis

Changes in various indexes in different growth and development periods

The pH value, alkali-hydrolyzale nitrogen, ammonium nitrogen, available potassium, and available phosphorus were determined at different stages of the entire period of flue-cured tobacco cultivation. The results are shown in Table 1.

Soil pH is related to the activities of soil microorganisms, the synthesis and decomposition of organic matter, the transformation, release and effectiveness of various nutrient elements, the ability of the soil to retain nutrients, base saturation, and the existence［5-7］ and possible poisoning［8-9］ of iron, manganese, aluminum, calcium and other elements. It can be seen from Table 1 that before planting tobacco, the soil pH value was 4.7, which was acidic, and the soil pH value had been rising from the rosette stage to the vigorous growing stage. In the vigorous growing stage, the soil pH had become neutral. After the tobacco was harvested, the pH value dropped to 5.3. It can be seen that during the tobacco planting process, due to the influence from the growth of flue-cured tobacco plants, the soil pH value will have a tendency to increase, which is rather obvious.

Soil alkal-hydrolyzable nitrogen, also called available nitrogen, can reflect the recent supply of nitrogen in the soil, including inorganic nitrogen (mainly ammonium nitrogen and nitrate nitrogen) and easily hydrolyzed organic nitrogen (amino acid amide and easily hydrolyzed protein). The amount of soil available nitrogen affects the growth and development of tobacco plants［1］. Ammonium nitrogen is one of the main nitrogen forms of flue-cured tobacco, and the effects of ammonium nitrogen on the growth and development, economic characteristics and internal quality of flue-cured tobacco are slightly better than that of nitrate nitrogen［10］. From before planting to after harvesting, the content of alkali-hydrolyzale nitrogen showed a gradual decrease in the whole period, which was especially obvious in the rosette and vigorous growing stages, but it did not change much from the vigorous growing stage to after harvesting. For ammonium nitrogen, its decrease was more obvious in the vigorous growing stage, but after harvesting, its content rose to basically the same as that in the rosette stage.

The soil potassium content did not change much before the rosette stage of flue-cured tobacco. It was the lowest in the vigorous growing period, and began to increase after the tobacco was harvested, indicating that the flue-cured tobacco absorbed more potassium in the later period of its growth, which is in line with the growth of flue-cured tobacco. The available phosphorus in the soil declined after the vigorous growing stage, but it was not obvious.

Correlation between pH value and mineral nutrient contents

After determining the soil pH value, alkali-hydrolyzable nitrogen, ammonium nitrogen, available potassium and available phosphorus in different periods of flue-cured tobacco cultivation, correlation analysis was carried out to explore the relationship between soil pH and these several soil chemical properties. The results are shown in Table 2.

The above table shows that soil pH had an extremely significant negative correlation with soil alkal-hydrolyzable nitrogen, ammonium nitrogen, and available potassium, but no correlation with available phosphorus; alkaline hydrolyzable nitrogen had an extremely significant positive correlation with ammonium nitrogen; and there was no significant correlation between other indexes. It can be concluded that in the process of tobacco planting, the soil pH value had extremely obvious effects on the contents of alkali-hydrolyzable nitrogen, ammonium nitrogen, and available potassium. The higher the pH value, the lower the contents of these three indexes in the soil, which had a very important impact on the supply capacity of soil fertility, thereby affecting the yield and quality of flue-cured tobacco.

Ammonium nitrogen is one of the important members of alkal-hydrolyzable nitrogen, which exists in the form of ammonium ion (NH+4), but because ammonium ion is acidic, it will neutralize with alkaline soil and cause nitrogen loss［10,13］. The significant positive correlation between the content of ammonium nitrogen and the content of alkali-hydrolyzed nitrogen revealed that ammonium nitrogen changed the pH value and caused the decrease of the content of alkali-hydrolyzed nitrogen, which might be the main reason why the content of alkali-hydrolyzed nitrogen was affected by soil pH.

Changes in soil bulk density and porosity

Comparison of differences in soil bulk density

The soil in such two typical periods as the rosette and vigorous growing stages during the planting period of flue-cured tobacco was selected for the determination of bulk density and porosity. The results are shown in Table 3.

The soil bulk density and porosity were measured at different time periods, and it was found that the soil bulk density and porosity were different between the rosette and vigorous growing stages. Specifically, the soil bulk density in the vigorous growing stage was lower than that in the rosette stage, while the soil porosity was the opposite. The variance analysis was performed on the soil in the rosette stage and the soil in the vigorous growing period, and the results are shown in Table 4.

The analysis of variance showed that comparing soil bulk density and porosity in the vigorous growing period with those in the rosette stage, the sig values were 0.015 and 0.017, respectively, both less than 0.05, which meant that there were a significant difference, indicating that the flue-cured tobacco planting made the soil looser, the soil bulk density decrease, and the porosity increase.

Relationship of soil bulk density and porosity with pH

According to SPSS statistical analysis, soil bulk density and porosity were extremely significantly correlated with pH, with the correlation coefficients of 0.647 and -0.652, respectively. The change in soil pH was closely related to the change in bulk density and porosity. The bulk density increased and the porosity decreased, which was consistent with the previous analysis conclusion that the pH value in the vigorous growing stage was large and the soil bulk density decreased significantly. It can be seen that the change of soil bulk density and porosity can be determined to some extent by measuring the change of soil pH value.

Conclusions

During the planting process of flue-cured tobacco, the pH value of the soil first increased, then decreased, and its largest value reached at the vigorous growing stage. It dropped after harvesting, but was still greater than before tobacco planting. The overall trend of pH value was on the increase, which might mean that flue-cured tobacco planting can improve soil acidification to a certain extent. Whether it is specifically related to cultivation and management, or to flue-cured tobacco root exudates, or to other related factors［14-15］, requires further research and demonstration.

With the change of pH value, the nutrient elements in the soil changed greatly. Among them, alkali-hydrolyzable nitrogen, ammonium nitrogen, and available potassium were obviously related to the pH value. With the increase of pH value, the contents of alkali-hydrolyzed nitrogen, ammonium nitrogen and available potassium were obviously reduced, which can be used as one of the important indicators of fertilization in production. Meanwhile, the positive correlation between alkali-hydrolyzable nitrogen and ammonium nitrogen also revealed the change of alkali-hydrolyzable nitrogen content to some extent, which might be mainly caused by the rapid change of ammonium nitrogen with the rapid change of pH value. The rapid change of ammonium nitrogen might be related to the absorption of different nitrogen forms (NO-3 and NH+4) by flue-cured tobacco［13,16］.

The pH of tobacco-growing soil is closely related to soil bulk density and porosity. According to this study, it was found that when the soil pH was small, the soil bulk density was larger and the porosity was small, and when the soil pH was high, the results were the opposite. The bulk density of the soil after tobacco planting was smaller than that before tobacco planting, and the porosity was greater than that before tobacco planting. The soil of the

tobacco field became loose, the aeration condition was improved, and the nutrient release was more favorable, which was beneficial to the development of the root system of the crops in the later seasons, and could effectively promote the growth and development of the crops in the later seasons, which was beneficial to production.

The conclusions of this study were drawn under the conditions of specific years, specific locations and specific flue-cured tobacco variety, so it is inevitable that there were certain limitations, and whether the universal law is like this requires further verification and improvement through in-depth and extensive experiments and obtained experimental data.

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