Study on Soil Nutrient Fertility of Cultivated Land in Laos

Yuyi HUANG Guifen CHEN Yanfei HUANG Shuyi LIU Yancheng MENG Bin LIU Liping XIE Pheng SENGXUA Moryang CHUAYENG Qizhan TANG

Abstract ［Objectives］ This study was conducted to explore the differences in soil nutrient contents of cultivated land in different regions of Laos, and to provide a basis for soil nutrient management and scientific fertilization in Laos.

［Methods］ By collecting 166 samples of the 0-20 cm cultivated soil layer from farmland in the farming areas of Luang Namtha Province in the northern part of Laos and Vientiane Province in the middle of the country, the contents of soil organic matter, total nitrogen, available phosphorus, available potassium, available calcium and available magnesium were analyzed, so as to evaluate the current status of soil nutrient fertility and explore the differences in soil nutrient contents of regional cultivated land.

［Results］ The soil organic matter was relatively abundant, the total nitrogen and available potassium contents were at the lower-middle level, and the available phosphorus, available calcium and available magnesium were insufficient or extremely insufficient. The soil nutrient contents of cultivated land were different in different regions.

［Conclusions］ It is suggested that lime or other alkaline fertilizers should be applied on the farmland with acid soil, and nitrogen, phosphorus, and potassium fertilizers should be added at the same time to improve soil nutrient fertility.

Key words Cultivated land; Soil available nutrients; Nutrient content; Laos

Received: February 9, 2020Accepted: April 8, 2020

Supported by Guangxi R & D Project (GuiKe-AD17195026); Fund of Guangxi Academy of Agricultural Sciences (2019ZX121).

Yuyi HUANG (1962-), male, P. R. China, researcher, master, mainly devoted to research about soil environmental quality and plant nutrition.

*Corresponding author. E-mail: tangqizhan@gxaas.net.

Laos is located in the northern part of the Indochina Peninsula. It is the only landlocked country in Southeast Asia, with a land area of 236 800 km2. It belongs to the tropical and subtropical monsoon climate zone and has abundant agricultural resources. The country餾 arable land area is about 8 million hm2, and the agricultural land is about 4.7 million hm2. Agriculture is the pillar industry of Laos economy, accounting for 51% of Laos GDP, and the agricultural population accounts for about 90% of the country餾 total population［1］. The main soil types in Laos are alisols, acrisols and luviols. Among them, the distribution areas of alisols and acrisols are the largest, which together account for 67.64% of the land area of Laos［2］. The results of studies on the soil nutrient fertility of different soil types in local areas show that the soil nutrient content varies greatly and the overall soil fertility is low［3-4］, but there are few reports on the soil fertility of large-scale cultivated land in Laos. In order to learn more about the current status of cultivated land fertility in Laos and scientifically guide the rational fertilization of farmland to achieve the purpose of increasing crop yields and increasing economic benefits, we began to investigate the nutrient fertility of the soil in the farming areas of Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country from 2015 to 2018. This study evaluated the current status of soil nutrient fertility in the cultivated areas of Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country, and explored the differences in soil nutrient contents of cultivated land in different regions, providing a basis for soil nutrient management and scientific fertilization in Lao farmland.

Material and Methods

Soil sample collection

Soil samples were collected in the farming areas of Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country. Each sampling unit was 10 to 20 hm2 in area, and the sampling field area was 0.1 to 1.0 hm2. For the collection of each sample, soil was randomly selected from 5 to 10 sample points and mixed into one sample, which was divided by the quartile method, giving a 1 kg of soil sample, which was air-dried indoor and ground for later use. A total of 166 soil samples were collected. The sampling time is from 2015 to 2018 in the dry season from December to April of the following year.

Determination methods

Soil pH value was determined by water leaching-potentiometry (soil to water ratio 1∶2.5). The organic matter, soil total nitrogen, rapidly available phosphorus and rapidly available potassium contents were determined by potassium bichromate titrimetric method, Kjeldahl method, Olsen method, and 1N neutral ammonium acetate extraction-flame photometry, respectively. The available calcium and magnesium were determined by 1N neutral ammonium acetate extraction-atomic absorption method.

Evaluation criteria of soil nutrient fertility

The evaluation was performed according to the soil nutrient grading index of the second national soil survey of China.

Data processing

The average values, coefficients of variation and distribution frequencies of soil organic matter and nutrient contents in the cultivated land were calculated, and the abundance of soil nutrients was discussed. Data processing and statistics were performed using excel.

Results and Analysis

Soil nutrient fertility status of cultivated land

The organic matter and the contents of macroelements in the cultivated soil of Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country are shown in Table 1. The organic matter content was in the range of 8.50-59.49 g/kg, with an average content of 23.23 g/kg. The total nitrogen content was in the range of 0.48-1.54 g/kg, with an average value of 0.083 g/kg . The available phosphorus content was in the range of 0.0-19.0mg/kg, with an average value of 3.8 mg/kg. The available potassium content ranged from 9.0 to 310.3 mg/kg, with an average of 89.0 mg/kg. The soil organic matter and total nitrogen showed small differences, while the contents of available phosphorus and available potassium were quite different. The coefficients of variation of the latter two were 96.26% and 81.81% respectively.

The contents of available calcium and magnesium in the soil of Laos cultivated land were analyzed. The results showed that the available calcium content ranged from 16.0 to 1 016.0 mg/kg with an average of 232.0 mg/kg. The available magnesium content was between 7.2 and 234.0 mg/kg with an average 70.8 mg/kg. The coefficients of variation of available calcium and magnesium were 78.02% and 83.05%, respectively, indicating that there were large differences in the contents of available calcium and magnesium in the cultivated soil (Table 2).

Grading and evaluation of soil nutrient contents in cultivated land

According to the soil nutrient grading standards for the second national soil survey in China［5］, the soil nutrient contents of cultivated land in Luang Namtha Province in the northern part of Laos and Vientiane Province in the central part of the country were graded, as shown in Table 3. The contents of soil organic matter of the third and fourth grades accounted for 27.11% and 46.38% , respectively, and those of the first and second grades accounted for 8.44% and 15.06%, respectively. The overall evaluation was the middle-upper level. The total soil nitrogen contents of the fourth and fifth grades accounted for 35.54% and 38.56% , respectively, and those of the second and third grades accounted for 8.43% and 15.06%, respectively. The content was at the lower-middle level overall. The available phosphorus contents were all lower than the first and second levels, and those of the fifth and sixth grades accounted for 83.74%, so the overall evaluation was that the soil of the cultivated land lacked available phosphorus. The available potassium contents of the first and second grades accounted for 31.32%, and those of the fifth and sixth grades accounted for 47.59%. The overall evaluation was the lower-middle level.

The grading criteria of soil middle elements in cultivated land in Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country are shown in Table 4. Only 2.41% of the available calcium contents in the soil were at the first and second levels, and 92.17% at the fourth and fifth levels. The overall evaluation was extremely insufficient. As to the total available magnesium content, 5.42% of the values were at the first and second levels, and those at the fourth and fifth levels accounted for 72.29%. The average content of available magnesium in the soil was extremely insufficient.

On the whole, the organic matter content of the cultivated soil in Laos was at the middle-upper level, the total nitrogen and available potassium content were at the lower-middle level, and the available phosphorus, available calcium and available magnesium contents were insufficient or extremely insufficient. Therefore, the soil nutrient fertility level of the cultivated land in Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country was low and the abundance of different nutrients varied greatly.

Evaluation of current status of soil nutrient fertility in cultivated land in different regions

The soil nutrient contents of cultivated land in Luang Namtha Province in the northern part of Laos and Vientiane Province in the middle of the country were analyzed and the results are shown in Table 5. The average contents of organic matter, total nitrogen, available phosphorus and available potassium in the cultivated land of Luang Namtha Province in northern Laos were 21.49 g/kg, 0.81 g/kg , 5.2 mg/kg and 66.1 mg/kg, respectively, and the average contents of calcium and magnesium in soil were 294.0 and 96.0 mg/kg, respectively. The soil organic matter content was overall at the middle-upper level, and the total nitrogen, available phosphorus and available potassium contents were at the lower-middle level, while the available calcium and magnesium contents were insufficient or extremely insufficient. The average contents of soil organic matter, total nitrogen, available phosphorus and available potassium in the cultivated land of Vientiane Province in the middle of the country were 25.09 g/kg, 0.86 g/kg, 2.8 mg/kg and 112.0 mg/kg, respectively, and the average contents of calcium and magnesium in the soil were 188.0 and 50.4 mg/kg, respectively. The contents of soil organic matter and available potassium were at the middle-upper level, the total nitrogen content was at the lower-middle level, and the contents of available phosphorus, available calcium and available magnesium were insufficient or extremely insufficient. Compared with the soil nutrient contents of Vientiane Province in the middle of the country, the soil organic matter and total nitrogen contents of the cultivated land in Luang Namtha Province in the north were equal, and the average contents of available phosphorus, available calcium and available magnesium were 85.71%, 56.38% and 90.48% higher, but the soil rapidly available potassium content was 69.44% lower.

Conclusions and Discussion

The soil organic matter content of the cultivated land in Luang Namtha Province in northern Laos and Vientiane Province in the middle of the country was relatively abundant, which was at the middle-upper level, the contents of total nitrogen and available potassium were at the lower-middle level, while the contents of available phosphorus, available calcium and available magnesium were very low, at the insufficient or extreme insufficient level. There are two main reasons for the lack of available phosphorus, available calcium and available magnesium in the soil. One is that the soil pH in the study area was low, with an average pH value of 5.00 , and the number of soil samples with a soil pH value lower than 5.50 accounted for 84.34%. The acidity of the soil leads to easy loss of calcium and magnesium in the soil. Second, the main soils in Luang Namtha Province in northern Laos and Vientiane Province in the middle of Laos are alisols and acrisols［2］, plus no or less application of phosphorus fertilizer, which causes serious phosphorus insufficiency in the cultivated soil. Therefore, it is recommended to apply lime or other alkaline fertilizers on the farmland with acid soil in order to increase the soil pH and increase the supply of calcium and magnesium in the soil. Meanwhile, the application of nitrogen, phosphorus and potassium fertilizers can improve soil nutrient fertility.

The soil nutrient contents of the cultivated land were different in different regions. The average contents of available phosphorus, available calcium and available magnesium in the cultivated soil of Luang Namtha Province in the northern part were higher than the corresponding soil nutrient contents in Vientiane Province in the middle of the country, but the average soil rapidly available potassium content was lower than the average soil rapidly available potassium content in Vientiane Province in the middle of the country.

Agricultural Biotechnology2020

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