Teknologi Budidaya Kedelai yang Adaptif pada Gawangan Tanaman Karet Belum Menghasilkan Adaptive Soybean Cultivation Technology on Space Between Immature Rubber Plants

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Introduction
Soybean is one of the strategic foodcrops in Indonesia after rice and corn. The high amino acid quality of its protein makes soybean used for various products in the food and animal feed industries (Zakaria, 2016). People's consumption patterns prioritizing low-carbohydrate foods with higher protein make soybeans superior (Anjani, 2019). The demand for various processed soybean products such as tempe, tofu and soy sauce is predicted to continue to increase along with the increase in population (Mahdoh & Risyanto, 2018).
The increasing demand for soybeans is not followed by an increase in domestic soybean production. The Ministry of Agriculture stated that domestic soybean production only covered less than 10 percent of the national soybean demand in 2021 (Pusdatin, 2022). Indonesia's soybean imports from 2014 to 2023 are forecasted to increase by 6,81 percent per year (Permadi, 2015). Nevertheless, Indonesia has the opportunity to become self-sufficient in soybeans as long as it can maintain production growth above consumption growth (Aldillah, 2015).
Soybean production growth can be increased by utilizing sources of production growth. One source of production growth is the expansion of the planting area by increasing the Planting Index (Adnyana & Kariyasa, 1999). The expansion of planting areas can utilize irrigated paddy fields, rain-fed paddy fields and dry land, including immature plantation land (oil palm and rubber) as intercrops. Rubber plantation areas that can be utilized for intercrops are around 50-60% of the land area (Rodrigo et al., 2001;Xianhai et al., 2012;Sahuri et al., 2016). Short-term intercropping in rubber plantations is usually done when the rubber is 1-2 years old after planting. This condition will provide additional income when there is no latex produced and can protect farmers from fluctuating rubber prices if done sustainably (Huang et al., 2020;Sahuri, 2019).
Soybean cultivation on gawangan of immature rubber plant will not only provide benefits for farmers but also the main crop. Tistama et al. (2016) reported that the intercropping of sorghum and soybean on gawangan of immature rubber plant could increase the content of P, N, pH and CEC and can inhibit the development of white root fungus (Rigidoporus microporus). Rhizobium sp symbiosis with soybean can help improve soil biology in marginal drylands (Watkins et al., 2012).
Rubber is the primary commodity after oil palm, which plays a crucial role in the people's economy in Bengkulu Province. Based on data from the Central Bureau of Statistics in 2016, the area of rubber plants in Bengkulu Province reached 117,064 ha consisting of Mature Plants 72,320 ha, immature Plants 37,945 ha and old/dead plants/damaged plants 6,799 ha. The amount of immature plant area is still quite large, so it has the potential to cultivate soybean in the area. However, there is a need for soybean production technology that can increase productivity and be profitable for farmers. This study aims to obtain a package of adaptive soybean cultivation technology in the gawangan of immature rubber plant.
The technology applied consisted of complete tillage, Rhizhoplus seed treatment 40 g/8 kg of seed, planting method by direct sowing, with a spacing of 40 cm x 20 cm, fertilizer according to the treatment, weed control two times before fertilization, hilling together with weeding, pest management by applying integrated pest management, harvest time was marked by fallen leaves and yellow pods. Anjasmoro variety, fertilizer recommendation PUTK analysis Data collected consisted of growth components (plant height), yields and yield components (number of branches, number of pods, number of filled pods, number of empty/damaged pods, weight of filled pods, number of filled seeds, number of empty/damaged seeds, number of seeds per plant), input costs and output costs of each technology package. Data on growth and yield components were analyzed using analysis of variance with the model = + + + and if there were differences, it was followed by Duncan Multiple Range Test (DMRT).
The technical feasibility of soybean cultivation farming was measured using the analysis of revenue return on cost (R/C ratio) based on the formula,

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Where: R/C = ratio of revenue and cost TR = total revenue (Rp/ha) TC = total cost (Rp/ha) with decision: R/C > 1, the farming business is feasible to develop R/C = 1, farm business is at break-even point (BEP) R/C < 1, the farming business is not worth developing Description: = Soybean planting = Rubber planting

Result
Gardu Village is one of the villages in the administrative area of Arma Jaya Sub-district, North Bengkulu Regency. The research area land is located at an altitude of 106 m above sea level (ASL) with undulating flat land conditions. Soil testing using the Dry Soil Test Device (PUTK) shows that the research soil is in zones I and II which have low available P nutrient status, medium available K, slightly acidic pH, and medium C-Organic (Table 2). The research area has a marginally suitable P nutrient status (S3), a moderately suitable K status (S2), and a suitable C-Organic nutrient status (S1), according to the parameters of the nutrient status. The C-Organic content falls within the medium category, which is 2 to 3%. In comparison, the slightly acidic pH status shows that the soil at the research site has a pH value range between 5,6 -6,5 (Ritung et al.,2011). The nutrient status of this soil can be improved using the recommendations of the PUTK (Table 3). The components of growth and yield of soybean plants grown on immature rubber spacings showed significant and insignificant results. The components of number of branches, number of empty pods, weight of filled pods per plant (g), number of filled seeds, and seed weight per plant showed significant results. In contrast, plant height, number of filled pods, number of pods, number of empty or damaged seeds, number of seeds, and percent of empty or damaged seeds showed insignificant results (Table 4). DMRT further analysis test at the 5% level on components that showed different results showed P3 and P4 gave the best results (Table 5).  The calculation of the farm business also shows that P3 and P4 have higher R/C ratio values, although all technology packages are technically feasible to develop because they have R/C ratio values > 1 ( Table 6). The similarity between P3 and P4 is that both use the Anjasmoro variety. This shows that the Anjasmoro variety has better performance compared to the Dena 1 variety. Even with different fertiliser recommendations, the Anjasmoro variety is able to produce higher yields than the Dena 1 variety.

Discussion
The rubber planted at the research area is approximately two years old and comes from the superior clone PB260 with a planting distance of 5 x 4 m. Farmers generally do not utilise immature rubber plantations, leaving most of them unused. Some farmers utilise the gawangan for the cultivation of kencur plants but only for personal consumption. There are no obstructions from the rubber plant canopy around the research area. This condition causes unobstructed airflow, and sunlight can shine on the soybean plants because they are not yet covered by the plant canopy. Rubber plant crowns will cover each other at the age of 2 years (Sahuri, 2017). If the light intensity is reduced, it will reduce the leaf area even though the leaves play an important role in photosynthesis (Susanto & Sundari, 2010).
The application of fertilisers in accordance with PUTK recommendations can enhance soil nutrient status and potentially elevate the land suitability class from S3 to S2 or even S1, and from S2 to S1 . This circumstance demonstrates that if the land is processed in a way that and applying ameliorants to soils that react to acid before fertilization can overcome the deficiencies of each limiting component (Nurmegawati et al., 2021), it has the ability to grow and produce optimal results. The application of N, P and K fertilizer has its technical recommendations. According to Husnain et al. (2016), the application of N fertilizer can be done by mixing the fertilizer with the soil before spreading, buried in the soil so that it does not evaporate and giving it in stages 2-3 times in granule form for optimal absorption. P fertilizer is recommended to be applied according to the needs of plants, added organic fertilizer, at a soil pH of 6 -7 and increased interaction with mycorrhiza. Meanwhile, K fertilizer can be applied at a distance of 2,5 cm beside the plant by burying it.
The number of branches in the P3 and P4 treatments showed significantly different results. This result shows there is no difference between the two treatments. The number of branches in the Anjasmoro variety was higher when compared to the Dena 1 variety. Regarding the component of empty or damaged pods, the Anjasmoro variety also showed the best results with fewer empty or damaged pods in the P4 treatment. The weight of filled pods, filled seeds and seed weight per plant also showed the highest results in the P4 treatment and were not significantly different from P3.
The significant difference in the number of branches in P3 and P4 compared to P1 and P2 is presumed due to genetic factors possessed by Anjasmoro varieties. Based on the description, the number of branches of Anjasmoro ranged from 2,9-5,6 higher when compared to Dena 1 which only ranged from 1-3 branches. So based on the description, the number of branches of the Dena 1 is less than that of the Anjasmoro. Branches on soybean plants are one of the yield components because the position of pods is on branches or leaf axils (Tamba et al., 2017). The pods produced depend on the number of branches produced. The more the number of branches produced, the more potential the pods will appear (Herawati et al., 2017). Fertilizer treatment also influenced the number of empty or damaged pods in each treatment. The number of empty or damaged pods per plant showed the lowest results in P3 and P4 treatments compared to P1 and P2. This shows that the P3 and P4 treatments produce more number of pithy seeds compared to P1 and P2. In the P1 and P2 treatments, in addition to empty seeds, there were also seeds that were damaged by pests. The low number of empty pods in the P4 treatment was probably due to a lower dose of N fertilizer.
Nitrogen is one of the essential nutrients for plants whose contents have an influence on insect development. Using nitrogen in large quantities can increase plant growth rapidly, especially in the stems and leaves that become dark green and the plants become succulent, making it easier for pests and diseases to attack (Trisnawati et al., 2017). In addition, the application of high amounts of N and rhizobium can suppress nodule growth and reduce nodule activity in tethering N from the air, thus affecting the number of pods formed (Meitasari dan Wicaksono, 2018).
The yield components that showed the highest yields in the P3 and P4 treatments showed that the Anjasmoro variety is a variety that is widely adapted to various environments. The adaptability of the Anjasmoro variety is better compared to the Dena 1 variety. Variations that arise in plant populations grown under the same environmental conditions are caused by variations in the genotypes of individual members of the population. In addition, the application of urea fertilizer could support appropriate growth so that the growth and development of plant vegetative organs becomes optimal, that also help generative development growth. The provision of nitrogen in small amounts with inoculation from rhizobium bacteria and sufficient water availability can increase the production of seed weight of soybean plants (Meitasari dan Wicaksono, 2018).
The use of Anjasmoro varieties in P3 and P4 significantly affected the production and R/C value. Despite applying different fertilizer recommendations, P3 and P4 still produced higher production and R/C value than P1 and P2. These results are in line with the results of research by Kuncahyo et al. (2019) who planted nine varieties of soybean on paddy fields in the dry season; Novianto et al. (2022) on palm-soybean intercropping pattern and Somantri et al. (2019) in monoculture pattern. The farming R/C ratio of Anjasmoro variety in this study was also higher compared to several studies with different varieties. Nuswantara et al. (2019) reported an R/C value of 1,47 for the Grobogan variety with a selling price of IDR 7.916 kg -1 . In line with the research of Farikin et al. (2016) who also used the Grobogan variety with a selling price of Rp 6.940/kg obtained an R/C ratio of 1,73. Arifin dan Sahrawi (2014) who conducted research in Pakong District, Pamekasan Regency on the Wilis soybean variety obtained an R/C ratio of 1,56. The technology packages that apply fertilizer based on KATAM recommendations (P1 and P3) have higher costs. This is because the amount of urea fertilizer used in this recommendation is higher than the fertilizer based on the PUTK analysis. The higher amount of fertilizer also results in higher fertilizer purchase costs. However, using more fertilizer is not directly proportional to the production. Not all technology packages that applied fertilizer based on the Katam recommendations resulted in higher production compared to the fertilizer technology package with the PUTK recommendations.
The difference in fertilizer recommendations based on Katam recommendations and PUTK analysis lies only in the amount of urea fertilizer dosage. Urea fertilizer is closely related to the availability of N elements that plants will utilize. Prakoso et al. (2018) reported that urea fertilization had no effect on the growth and yield of Anjasmoro cultivar soybean. According to Purnamasari dan Tarbiyatul (2016), root nodules formed optimally in soybean can reduce the need for N elements when symbiotic with rhizobium. Root nodules can function as N fixers from the air through N2 fixation, so these plants only require a small amount of additional N fertilizer.

Conclusion
Technology package 3 (P3), which uses the Anjasmoro variety with fertiliser doses based on KATAM recommendations, produces the highest production compared to the other technology packages. Although all technology packages are technically feasible to develop because they have an R/C ratio value >1, this technology package is not only the most feasible to develop because of its better yields, but also because it has the highest R/C ratio value.
The Anjasmoro variety also showed a better performance in comparison to the Dena 1 variety in this study. The Anjasmoro variety still produced higher yields than the Dena 1 variety, despite being grown with different recommended fertiliser doses (KATAM and PUTK). This suggests that compared to Dena 1, the Anjasmoro variety has a better adaptability.

Acknowledgement
We would like to thank the Agricultural Research and Development Agency of the Ministry of Agriculture for funding this research, Darkam Musaddad as head of the Assessment Institute for Agriculture Technology Bengkulu and also Mr. Ir. Syahrial A., M.S for his guidance when writing the manuscript.

Declaration of Conflicting Interests
The authors have declared no potential conflicts of interest concerning the study, authorship, and/or publication of this article.