Characterization of Physicochemical Properties and and Heavy Metals Content of Soils under Kratom (Mitragyna speciosa) Cultivation, Kapuas Hulu District, Indonesia

Mitragyna speciosa or purik is a traditional medicinal plant native to Kapuas Hulu District, West Kalimantan Province, Indonesia (Kementan, 2020) and classified as a non-narcotic plant (Kemenkes, 2019 and 2020). The global market name of M. speciosa is kratom and it was exported to the USA and India until the end of 2020.       

Kratom’s cultivation is still traditionally managed and carried out on alluvium land and river banks. Alluvial plain soil is formed from several materials and deposited at flat to nearly flat slope due to the process of fluvial/colluvial through gravity force and water flow (Hikmatullah, 2007), which cause variations in chemical, physical and mineral properties and nutrient content (Brubaker et al., 1993). This causes the productivity of alluvial soil to sometimes be higher than the productivity of upland soil. On the other hand, the soil characteristics of kratom plants in this area have not been widely studied. Therefore, studying this aspect is important because soil properties affect the yield and quality of kratom production. Apart from that, based on the results of our pre-survey, several kratom production locations are former gold mining sites without permits (PETI). On the other hand, heavy metals and metalloids such as Pb, As, Hg and Cd are requirements for the quality of traditional medicine raw materials, where the presence of these heavy metals must be absent (BPOM, 2014). Considering that there is still very little scientific data regarding the characteristics of the soil where kratom grows, it is necessary to carry out this research to provide information regarding the physicochemical properties and heavy metal content of soil planted with kratom.
    
The physical properties of the soil play an important role in the ability of plants to utilize soil nutrients and water. In evaluating agricultural practices, soil physical properties are an important indicator (Yang et al., 2012). Poor land management can lower soil quality, which will make kratom farming less sustainable. The texture of the soil has an impact on soil water circulation, plant roots, plant absorption of nutrients and soil cultivation. (Lal and Shukla, 2004). High soil density can inhibit the movement of soil water and plant roots; On the other hand, low soil density makes it very easy for groundwater to pass through so that plants have sufficient water availability. Soil porosity characteristics are closely related to soil physical properties, water movement and root penetration (Pagliai and Vignozzi, 2002; Sasal et al., 2006). Very high and/or very low total soil porosity will inhibit the development and growth of cultivated plants. Field capacity water content is the initial limit for soil water storage for plants (Shukla, 2013). In addition, the content of heavy metals in soil and plants depends on the physicochemical properties of the soil, cultivation method, type of plant, as well as the availability, solubility and type of metal in the soil (Sinha et al., 2006).

Study areas
 
The research was conducted in Kapuas Hulu Regency which is located at Located between 0°04’20”N-1°37’20” N latitude and between 111°34’20”E-114°09’20”E longitude which is the easternmost tip of West Kalimantan Province. The research area is in the lowlands and hills, with an average annual rainfall of 404.7 mm. For the mineral soil, the sample was taken in Nanga Mentebah Village; for the peat soil, it was taken in Kedamin Darat Village, South Putussibau District. For the mineral soil, Nanga Mentebah Village served as the sampling location, while Kedamin Darat Village, South Putussibau Sub-District, served as the place for the peat soil. The coordinates of sampling locations and their codes are shown in Table 1 and codes and description of the sampling locations in Table 2, while the study locations on a map is as in Fig 1.
 

 

 

 
Sampling and preparation
 
Soil samples with three replications were collected during the rainy season (February-March, 2020) from six sampling locations at a depth of 0-30 cm. Whole soil samples were used to analyze for the weight, total porosity and water content at field capacity. Meanwhile, non-intact soil samples were used for soil texture analysis, particle density, pH, concentration of N, P and K and heavy metals. All those samples were stored in sterile boxes and then brought to the laboratory for further analysis.
 
Soil physicochemical analysis
 
Soil-specific gravity was analyzed using the ring sampler (core) method, particle density using the pycnometer method, total porosity using mathematical equations and texture using the hydrometer method. Soil acidity (pH) was analyzed using a pH meter, total N was measured using the Kjeldahl method, P₂O₅ using the Bray I method and K using the 1N NH₄OAC at pH: 7 extraction method.

Metals analysis
 
Analysis of the metal elements lead (Pb), cadmium (Cd) and chromium (Cr) contained in each soil sample was tested by using Atomic Absorption Spectrophotometry ((AAS-6300, Shimadzu), with the samples previously prepared using wet digestion of nitric acid (HNO₃).

Soil physical properties
 
The soil’s physical properties are displayed in Table 3. The textures of mineral soils were classified as sandy loam and silty clay loam. The peat soil was classified as hemic to sapric maturity. These textures can be considered suitable for plant growth. The clay texture, including a slightly fine texture, may increase corn and wheat production (Jalota et al., 2010) and is suitable for plant growth, root development, seed germination and Senna obtusifolia nurseries (Abubakar, 2017). The bulk density of the minerals soil was categorized as moderate. Rieley et al., (1996) found that the bulk density of tropical peat is generally low, ranging from 0.1-0.32 g/cm³. The bulk density character of mineral soil was consistent with the analysis of soil texture obtained. Gama-Castro et al., (2000) stated that the bulk density range of alluvial soils might be extensive; smaller values were observed in the Ap horizons and increased with depth. A very high bulk density of 1.7 g/cm³ will inhibit root penetration and further reduce the roots’ ability to reach a usable amount of water at specific soil depths (Reeve, 1986). The suitable total porosity for agricultural land is 50-60% (Arsyad, 2010). For peat soil at the research location, the total porosity is around 76.8%, which has a low bulk density and peat maturity. As emphasized by Buol et al., (1973) which states that the total porosity of sapric peat is less than 85%; where the presence of water and air is balanced and dynamic. Becomes an optimal medium for plant growth and development. It becomes the optimum media for plant growth and development.
 

   
At the mineral soil research location, the water content and field capacity of mineral soil is related to total porosity indicated almost all soil pores are filled with water (92.1% by vol). Soil bulk density, field capacity water content and total porosity in this study are similar to profile four at a depth of 0-30 cm in Tworków (Borek and Bogdał, 2018). Hong et al., (2013) explained that the most influential mineral soil agricultural factor is field capacity water content (at pF 2.5), which ranges between 0.2690 and 0.6784 cm³.
     
For peatlands, it is approximately ±3 km away from the river with a depth of groundwater level that is also quite deep (50-70 cm), so the field capacity water content is lower than the results of research by Schwärzel et al., (2002) which states that the water content of peat soil is pF 2.54 ranges from 69 -76% vol, at pF 4.2 ranges from 23-38% vol and available water content ranges from 31-53% vol. However, the peat water content at the research location is in line with research by Suryadi, (2013). Thus, the physical characteristics of the soil at the research location are suitable for kratom cultivation.
 
Soil chemical properties
 
The chemical properties of mineral soil and peat soil in Table 3 show acidic properties (pH 4.6-5.7) as stated by Gunawan et al., (2019) which states that the soil pH in Eucalyptus sp-based vegetable agroforestry forests is acidic to very acidic (pH 2.65-4.85), as well as in peat soils, it has a very acidic pH (pH 2.8), supported by Utami and Indrawati, (2024) which states that peat is soil that is poor in macro and micro nutrients and has an acidic pH. The chemical characteristics of peat soil in Indonesia are very diverse and are determined by the mineral content, thickness, types of plants that make up the peat, types of minerals in the substratum (at the bottom of the peat) and the level of peat decomposition. The N nutrient content (%) in mineral soil is classified as low to medium (0.14-0.37%) as stated by Cyio (2017) and Gunawan et al., (2019), but is classified as very high in peat soil (1, 86%). The P content (ppm) in mineral soil is classified as high to very high with a value of 25.85-39.47 and is very different from peat soil which is classified as very high with a value of 194.94. The K content [cmol (+) Kg^-1] in mineral soil is low (0.13-0.27) and in peat soil it is also moderate (0.39). Based on the data, the chemical properties of the soil at this research location are considered fertile for kratom cultivation. The Physical and Chemical Properties of Soil in Kratom Land Processing can be seen in Table 3.

Heavy metals content
 
Heavy metal content is showed in Fig 2 and Table 4. The metal content was found below the critical limit as stated by Haris and Aris (2013) for Fe and Cd metals (Syachroni, 2018), Cu, Zn, Pb and Cd metals (Sun et al., 2020). The presence of heavy metals that are still below the critical threshold is possible in the upstream part of the research, there is gold mining without permits, so the metals are carried away by the current downstream. According to Gaur et al., (2023), Iron is one of the most deficient nutrients in Indian soils. It is a structural component of cytochrome, hematin and leghaemoglobin. It is also important in activation of several enzymes including fumaric hydrogenase, catalase, dehydrogenase, oxidase and peroxidase. It helps in the absorption of other plant nutrients. Iron is also associated with chloroplast and protein synthesis. Molybdenum is an essential plant nutrient found in soil. It is also known as ultramicronutrient as this is required in very less amount. As its less amount is required, the deficiency and sufficiency range are narrow. Meanwhile, according to Patel and Parikh (2012), many metals are essential, all metals are toxic at higher concentrations because they cause oxidative stress by the formation of free radicals.
 

 

     
 According to Karalić et al., (2013) and Kashem and Singh (2001), in acidic soil, the availability of heavy metals is higher than in neutral or alkaline soil. Koèevar Glavaè et al., (2017) found that the availability of this metal will depend on several factors other than pH. Rieuwerts et al., (1998) stated that in mineral soils the factors that cause the presence of heavy metals are mineral composition, soil microbiota and redox potential, while in peat soils the concentration of soil organic matter, fulvic acid and humic acid are potential ligands for the complexation of these metals. The Hg element is stable in the soil environment. It has a strong tendency to form complexes with other anions (such as Cl^-, OH^- and S^2-) and humic matter. Yin et al., (1996) observed in an experimental study that soil adsorption decreased significantly above pH 5 as a result of increased amounts of dissolved organic matter and the tendency of mercury to complexly strong against organic carbon. However, heavy metals’ relative phytotoxicity may depend on plant species, soil properties, age of the plant and heavy metals types and concentration (Maleki et al., 2017). It is further stated that these metals may influence secondary metabolite production, which is substantially signiûcance in medicinal plants.

The research results show:

On mineral soil:

Soil texture is classified as sandy loam and silty clay, Soil specific gravity is low to medium, Total porosity is suitable for plant growth, Field capacity water content ranges from 50-59.2% vol, Soil reaction: acid to very acid, Total N is low to medium, P availability is high to very high, Potassium exchangeability is low, Fe, Mn, Cu, Zn, Pb, Hg and Cd metals in the soil are below the critical limit.

On peat soil:

The specific gravity of the soil is very low, the total porosity is classified as porous so that groundwater can easily pass through it, the field capacity water content is around 48.7% vol., the total N content is very high, the availability of P is very high, the exchangeability of potassium is moderate and. The metals Fe, Mn, Cu, Zn, Pb, Hg and Cd in the soil are below the critical limit.
     
The research results show that both lands for kratom cultivation are fertile. Further research is expected to increase understanding regarding the transport of heavy metals from soil to plants, as well as from plants to extracts.

The authors and co-author would like to thank Institution of Research and Community Services, Tanjungpura University and Development Planning Agency at Sub-National Level, Kapuas Hulu District by funding this research in 2020.

There is no conflict of interest.