Introduction

The Dominican Republic is a country in the Caribbean Sea of ​​Central America, located on the eastern part of the island of Hispaniola, the second largest of the Great Antilles Islands. It has a mainly tropical climate, with an average annual temperature of 25 - 30°C and an average annual rainfall of 1,500 - 1,800 mm. It is divided into a dry season (November - April) and a rainy season (May - October). During the dry season, it is vulnerable to drought, and during the rainy season, it is vulnerable to storms, floods, and landslides. Agriculture in the Dominican Republic employs 9.5% of the national labor force and contributes 5.5% of the GDP. Major agricultural exports include processed tobacco products, raw sugar, cocoa, fruits, and juices, while imports include tobacco raw materials, beans, alcohol, and malt extracts (OEC, 2022).

According to data from the Ministry of Agriculture and Forestry of the Dominican Republic, rice is one of the main crops that is the staple food of the country, and in 2023, it was cultivated on 200,000 ha, harvested on 186,000 ha, and the average production was 3,309.09 kg ha^-1. Approximately 30,000 farmers and approximately 250,000 citizens depend on it directly and indirectly, and it is very important for the diet of the Dominican people. According to data from the Ministry of Agriculture and Forestry of the Dominican Republic, domestic production of rice accounts for more than 95% of the consumption of the Dominican Republic diet. However, farm income is low at less than US$560 ha^-1. In addition, the opening of the rice market from 2025 due to the conclusion of the FTA with the United States requires efforts to increase international competitiveness and farm income through the dissemination of high-quality rice production technology, development of post-harvest management technology, and development of branded rice (MDA, 2023).

The first soil survey of agricultural land in the Dominican Republic was conducted by the Organization of American States (OAS), which produced a 1:250,000 soil map that took into account topography, soil texture, nutrients, and drainage, and combined it with forest and vegetation data to provide soil use information (OAS, 1967). Afterwards, areas with similar soil characteristics were separated based on LANDSAT satellite images and spatially expressed as planned resource units (URPs) on a 1:250,000 scale map, which was used for future agricultural planning and detailed soil research. (SEA, 1985). Chemical survey on agricultural land are aimed at maintaining sustainable soil productivity, preventing environmental leakage due to over-input, and increasing crop productivity (Kim et al., 2010; Núñez et al., 2020). Chemical survey of agricultural lands in the Dominican Republic have been actively conducted for crops such as rice (Jiménez and Núñez, 2015), coffee (Núñez et al., 2011), the Santo Domingo livestock production area (Núñez et al., 2020), and pastures and citrus (Núñez and Almonte, 2022). However, the reality in the Dominican Republic is that there is almost no survey of the fertility of farmland for proper farmland nutrient management, so the soil is deteriorating due to poor management, and the productivity of some crops is low. Therefore, this study was conducted to improve the fertility of paddy soil to increase rice productivity and farm income, prevent excessive input of nutrients to prevent nutrient accumulation and leakage into the environment, and use it as basic data to identify long-term changes in paddy soil through periodic monitoring.

Materials and Methods

In order to determine the chemical properties of paddy soils in the Dominican Republic, soil samples were collected from 28 rice-producing farms from January to March 2024. The samples were collected in a zigzag pattern at a depth of 0 to 15 cm from the soil surface, dried in the shade, and prepared according to the National Institute of Agricultural Sciences and Technology (NIAST, 2000). The chemical properties of the soil were analyzed according to the method of Page et al. (1982). Soil pH and EC were measured at a ratio of 1:2 by the potentiometric method, and organic matter was quantified by the Walkley and Black method after oxidation with potassium dichromate. Available phosphorus was measured by Mehlich III, and exchangeable cations and available silica were quantified after extraction with NH₄OAc and NaOAc, respectively. Trace elements such as iron, copper, manganese, and zinc were quantified by atomic absorption spectrophotometry after decomposition with perchloric acid. The total heavy metal content in the soil was analyzed by soaking the soil in aqua regia at 30°C for 2 h, extracting it using a Kjeldatherm block digestion system (Gerhardt GmbH, Northants, UK) at 90°C for 150 min, and analyzing it using hydride generation inductively coupled plasma atomic emission spectrometry (HG-ICP-AES; Integra XL Dual, GBC, Melbourne, Australia). Standard materials (SRM; BAM-U112a) were used for quantification and verification (MOE, 2010b).

Results and Discussion

The average content and range of chemical components of the paddy soil of the Dominican Republic are neutral at pH 6.9 (5.4 - 7.9), but compared to the level recommended by the agricultural guidance agency of the Dominican Republic, the soil acidity is found to be considerably weakly alkaline. It is presumed that this is due to the high calcium content in the soil, which is closely related to the report that the average acidity of irrigation water in the Dominican Republic is 7.7 ± 0.02, and 94.4% of the water has a pH greater than 7.0. (Cepeda, 2019). EC was measured to be 0.55 (0 - 1.32) dS m^-1, which means that most soils showed an appropriate level. Organic matter content was 56.4 (35.5 - 86.5) g kg^-1 and available phosphorus was 25 (4 - 118) mg kg^-1, indicating that some soils were lacking in phosphorus. Considering the high price of phosphate fertilizer, it is presumed that sufficient fertilization has not been performed so far, and considering that many types of phosphate fertilizers contain Ca, and the content of exchangeable Ca in the soil is high, the use of low-Ca phosphate fertilizer is required. The content of available silica was 405 (141 - 539) mg kg^-1, so it is thought that additional application of silica fertilizer is not necessary (Table 1).

The average content and range of exchangeable Ca in the surveyed paddy soil were 20.5 (6.7 - 32.5) cmol_c kg^-1, which was higher than the appropriate level of the guidance agency, while the exchangeable Mg content was 0.64 (0.16 - 0.92) cmol_c kg^-1, and the exchangeable K content was 0.36 (0.08 - 0.85) cmol_c kg^-1, which were lower than the appropriate level (Table 1, Fig. 1). This was similar to the results obtained in coffee farm soils, where the content ranges of exchangeable Ca and Mg were 2.26 - 43.4 cmol_c kg^-1 and 0.55 - 3.56 cmol_c kg^-1, respectively (Núñez et al. 2011). In addition, the Dominican Republic uses the relative ratio of cations as an evaluation index, expressing the ratio of each cation in the total content of Ca, Mg, K, and Na cations as saturation (%) and setting the appropriate levels as 60 - 85%, 10 - 20%, 3 - 7%, and <5%, respectively. The results of this survey showed that the saturation of Ca was high at 93.5%, while the saturation of Mg was low at 3.2%, requiring appropriate management of Ca and Mg. This promotes the insolubilization of phosphorus by calcium, raising concerns about phosphorus deficiency in crops (Table 2).

Table 3 shows the contents and ranges of available trace elements such as Fe²⁺, Mn²⁺, Cu²⁺, and Zn²⁺ in paddy soils of the Dominican Republic. In general, the contents are higher than the guidance agency’s appropriate level, so no additional fertilization of trace elements is required. These results are slightly different from those of other agricultural fields that showed high Fe²⁺ contents, appropriate Mn²⁺, Cu²⁺ contents, and low Zn²⁺ contents (Núñez et al., 2011; Núñez et al., 2020; Núñez and Almonte, 2022).

Fig. 1.

Relative distribution percentage (%) of paddy soils with range of chemical properties in Dominican Republic.

The average total contents of As, Cd, Cr, Cu, Ni, Pb, Zn, and Hg in the paddy soils of the Dominican Republic were 1.5, 0.21, 223, 74.9, 149.5, 2.4, 66.7, and 0.04 mg kg^-1, respectively (Table 4). Although the Cr and Ni contents were relatively high, the Dominican Republic government has not yet established management standards. This is consistent with the report by Delanoy et al. (2022) that Cr and Ni exceeded the standard in rice cultivation soils in the San Francisco de Macoris and La Vega areas. Similar results were obtained in the rice cultivation area of ​​the Sánchez Ramírez area reported in 2024 (Alberto Then et al., 2024). In addition, compared to Korean paddy soil, As and Pb are at low levels, Cd, Zn, and Hg are at similar levels, and Cr, Cu, and Ni are at high levels, and Cr and Ni in particular require management (Kunhikrishnan et al., 2015). There are several theories as to the cause of the high concentrations of Cr, Cu, and Ni in agricultural soils in the Dominican Republic. Delanoy et al. (2022) suggested that the northeastern region contains high amounts of Cr, Fe, Cu, and Ni from the parent rock, such as harzburgite, lherzolite, tholeiite and gabbro, suggesting that they may have originated from nature. Other theory suggests that the increase may have been caused by human activities such as mining, pesticide and fertilizer use, and transportation (Alberto Then et al., 2024).

In summary with the above results, it is believed that the management of paddy soil in the Dominican Republic should be promoted extensively. Currently, chemical analysis of agricultural land in the Dominican Republic is extremely limited. Although some crop fields and farms have been surveyed, the benefits have not yet reached general farmers for calculating the appropriate amount of fertilizer. In addition, the rice industry in the Dominican Republic generates more than US$570 million for the economy. However, since the DR-CAFTA agreement between the United States and the governments of the Caribbean and Central America, which will come into effect in 2025, rice producers in the Dominican Republic are threatened by low competitiveness and high production costs. Currently, rice is experiencing one of the largest economic imbalances due to the continuous increase in fertilizer and pesticide prices, the increase of productivity cost, and shortage of local labor (MDA, 2023). Therefore, proper nutrient management is essential as part of improving rice productivity, and the introduction of agricultural machinery, proper pest control and solving the labor shortages is also necessary.

Conclusion

Dominican Republic paddy soils generally have neutral to slightly alkaline soil acidity and very low phosphorus content, requiring the use of nitrogenous acid fertilizers, phosphate fertilizers that do not contain lime, and potassium fertilizers at higher than the existing recommended levels. In addition, the high silica content of Dominican Republic paddy soils was investigated, so application of silicate fertilizers to paddy soils is not necessary. The contents of trace elements in the soils are high, but the contents of heavy metals are relatively low, although Cr and Ni are high. These results show that the levels of As, Cd, and Pb, for which the Codex standards for agricultural products are set, are generally low, so it can be estimated that they are below the level harmful to the human body.