Soil Outperforms Rice Husk Ash as a Seedling Medium for LUSI Glutinous Rice in Suboptimal Swampy Lands of South Sulawesi, Indonesia

As the population continues to grow, the demand for food also increases. Indonesia has vast potential for swamp land, including both tidal swamps and lowland swamps [1, 2]. Current agricultural development faces various obstacles, including limited productive land due to conversion to non-agricultural land. Agricultural development will be directed more towards suboptimal non-agricultural land, which is generally found outside Java. The total area of swamp land in Indonesia reaches 34.1 million hectares, of which 25.2 million hectares consists of lowland swamp land and 8.9 million hectares of swamp land. Swamp land is spread across the islands of Sumatra, Papua, Sulawesi, and Kalimantan, with potential for food crops, horticulture, and annual crops [3, 4]. The area of swamp land in Sulawesi is South Sulawesi (218,523 ha), Southeast Sulawesi (87,969 ha), Central Sulawesi (69,412 ha), West Sulawesi (88,937 ha) [4-6].

LUSI is a popular glutinous rice variety released in 1989 with a life span of 130-140 days and a typical yield of 4.5 tons per hectare, with a potential yield of 6.0 tons per hectare, higher than local varieties, with a plant height of 120-130 cm [7]. Suboptimal land, such as that found on the shores of Lake Tempe, South Sulawesi, is land that often faces various challenges for the agricultural sector [8, 9]. These soils generally have low fertility [10], salinity problems due to the influence of seawater [11], and fluctuations in soil moisture due to tidal fluctuations [12] and climatic conditions [13]. Another limiting factor is acidity. Swampy land generally has a pH < 4.5. This high acidity comes from the oxidation of pyrite (FeS₂), which produces sulfuric acid when the soil experiences excessive drying. Slowly decomposed organic matter, which produces organic acids (humic acid, fulvic acid). High rainfall leaching (dissolving and washing away) base cations such as Ca²⁺, Mg²⁺, K⁺, and Na⁺. Decreased soil microbial activity, plant root damage due to overly acidic conditions, decreased electrical conductivity, and cation exchange capacity [3, 8, 9]. Under highly acidic conditions, aluminum and iron ions become highly soluble. Aluminum (Al³⁺) is toxic to roots, causing them to become short, branch less, and inhibit nutrient absorption [8]. Phosphorus (P) availability is low in swampy soils because, at low pH, Al³⁺ and Fe³⁺ bind with P to form insoluble Al–P and Fe–P compounds. In sulfate-acidic soils, pyrite oxidation reactions further enhance P binding. High kaolinite clay mineral content also binds P. As a result, plants often show symptoms of purple leaves and slow growth. P fertilizer use efficiency is low without amelioration (lime, dolomite, rice husk ash, compost) [3, 8, 10]. Poor drainage, swampy land tends to experience waterlogging that is difficult to drain due to concave topography and low-lying terrain. Low soil permeability, often associated with a high clay fraction and elevated organic matter content, impedes water movement and gas exchange in the soil profile. Moreover, repeated cycles of oxidation and reduction degrade soil structure over time. Poor drainage conditions lead to anaerobic (oxygen-deprived) environments, which significantly inhibit root growth and function [3, 8-10]. These factors can inhibit plant growth and reduce rice productivity [14, 15]. Therefore, a specific strategy is needed to improve rice growth success in this area, one of which is through the use of appropriate seedling media. Seedling media play an important role in the early stages of growth [16, 17]. The right medium will provide optimal support for root development [18, 19], aid nutrient absorption [20, 21], and maintain soil moisture. In suboptimal land, choosing the right seedling medium [22] becomes even more important as it can reduce the negative impact of limited soil conditions.

Ash from rice husk combustion is known as rice husk ash [23]. Because it contains silica, the ash produced from rice husk combustion has pozzolanic properties [24]. Rice husk ash has the function of binding metals. In addition, rice husks serve to loosen the soil [25, 26] so that plant roots can more easily absorb nutrients in the soil. The advantages of using burnt rice husks are that they are sterile, porous, rich in nutrients, and not too light for mobilization [27]. Rice husk ash is an organic material and a compost for soil [28], where the material will serve to improve soil properties and help bind nitrogen, phosphorus, and potassium in the soil [29]. Soil structure can be improved by adding rice husk ash [30]. By reducing soil compaction, the use of rice husks will also improve the physical characteristics of the soil [31]. The disadvantages of rice husk ash are that it is poor in nutrients, dries quickly, and if it is excessive, the roots will not receive enough nutrition [32, 33]. Research on the effect of various growing media on LUSI sticky rice in suboptimal land on the shores of Lake Tempe is highly relevant. This study aims to find the most effective growing medium to support the early growth of LUSI sticky rice in difficult land conditions. By finding the optimal growing medium, farmers in coastal areas can increase crop productivity despite facing environmental limitations. Additionally, it is hoped that this can be a solution to agricultural challenges in other suboptimal lands in Indonesia.

3.1 Sprout formation (days)

The results of the experimental evaluation confirm that the growing medium greatly affects the growth of glutinous rice seedlings, in terms of sprout formation parameters in the Tempe Lake swamp (TLS), Walanae River swamp (WRS), and Talotenreng pumped land (TPL) (Table 2). Soil medium was the best among the different media, and the average sprouting time was 1.17 days around TLS, followed by approximately 2.00 days around TPL, followed by 2.17 days in the WRS area. Treatments involving rice husk ash (RHA) or its combination with soil generally led to longer sprouting durations. These differences were statistically significant based on the Least Significant Difference (LSD_0.05) test.

Table 2. Formation of LUSI glutinous rice shoots at planting locations

3.2 Leaf formation (days)

The results of the experimental evaluation confirm that the growing medium significantly affects the growth of glutinous rice seedlings, in terms of leaf formation parameters in the coastal swamp areas of Lake Tempe (TLS), Walanae River (WRS), and Talotenreng pump area (TPL) (Table 3). Soil medium was the best among the other media, and the average leaf formation was 3.00 days at the TLS location, followed by the TPL and WRS locations, each with 3.50 days. Treatments involving rice husk ash (RHA) or its combination with soil generally led to longer sprouting durations. These differences were statistically significant based on the Least Significant Difference (LSD_0.05) test.

Table 3. Leaf formation of LUSI glutinous rice at planting sites

3.3 Shoot formation 50% (days)

The results of experimental evaluations in Table 4 prove that the planting media significantly increased the growth of sticky rice seedlings, with a parameter of 50% shoot formation in the Tempe Lake coastal swamp area (TLS), the Walanae River coastal swamp area (WRS), and the Talotenreng pump area (TPL). Treatments involving rice husk ash (RHA) or its combination with soil generally led to longer sprouting durations. These differences were statistically significant based on the Least Significant Difference (LSD_0.05) test.

Table 4. Shoot formation of LUSI glutinous rice at 50% at planting sites

3.4 Seedling height (cm)

The results of the experimental evaluation in Table 5 and Figure 2 show that the growing medium significantly increased the growth of glutinous rice seedlings in terms of peak plant growth parameters (at 8, 15, and 22 days after planting) in the coastal swamp areas of Lake Tempe (TLS), Walanae River (WRS), and Talotenreng pump area (TPL). Soil media was of high quality among the different media, and the general peak of 8-day-old flowers was 12.85 cm in the TLS area, observed through the WRS area at 11.91 cm, and the next TPL area at 11.13 cm, while the 15-day-old age was 23.29 cm in the WRS area, observed through the TLS area at 22.35 cm, followed by 22.18 cm in the TPL area. At 22 days old, the peak height was 33.36 cm in the WRS area, 32.03 cm in the TLS area, and 30.78 cm in the TPL area (LSD test at 95%). This indicates that flower peaks in soil medium tend to be higher than in rice husk ash. Further comparison of the morphology of LUSI glutinous rice seedlings at 22 days after treatment (Figure 3), and the condition of LUSI glutinous rice seedlings before harvest and ready for planting (Figure 4).

Table 5. Seedling height of LUSI glutinous rice at planting sites

Figure 2. Seedling height of LUSI glutinous rice at planting sites

Figure 3. Comparison of the morphology of LUSI glutinous rice seedlings at 22 days after treatment. (A) Soil media (M₀), (B) Rice husk ash media (M₁), (C) Soil + rice husk ash 1:1 media (M₂), (D) Soil + rice husk ash 2:1 media (M₃), and (E) Soil + rice husk ash 1:2 media (M₄)

Figure 4. Condition of LUSI glutinous rice seedlings. (A) Before harvest and (B) Ready for planting

3.5 Number of plants per tray (seedlings)

The results of the experimental evaluation in Table 6 show that soil medium is the best variable with the highest value for the parameter of number of plants per tray (seedlings) in the TLS, WRS, and TPL. The soil medium produced the highest number of seedlings compared to other media. The highest among the three soil samples was 1,385.83 seedlings in the TLS area, followed by the TPL area with 1,219.17 seedlings, and the WRS area with 1,196.67 seedlings (LSD Test at 95%).

Table 6. Number of LUSI glutinous rice seedlings per tray at planting sites

The results of the soil analysis (Table 1) indicate that the three planting locations are categorized as moderately suitable to very suitable to support increased rice production [34]. The soil media at the three locations—TLS, WRS, and TPL—inherently have good fertility, due to their edaphic characteristics that support basic soil fertility [35-39]. Several supporting factors include: Swamp environment and organic matter enrichment. Swamp ecosystems naturally receive high biomass inputs from aquatic vegetation, litter, and plant residues. The slow anaerobic decomposition process causes the long-term accumulation of organic matter. This organic matter increases cation exchange capacity (CEC), and provides macronutrients (N, P, K) and micronutrients, and improves soil structure and water retention [35-39]. The Lake Tempe area is a dynamic hydrological system. Annual sedimentation carries minerals from the catchment areas in the surrounding hills. This mineral-rich sedimentation makes the soil relatively fertile and less nutrient-poor than peat swamps [35, 36]. The mineral swamps in Lake Tempe do not have the extreme acidity levels of peat swamps. The relatively near-neutral pH supports phosphate availability, stable ammonium nitrogen availability, and optimal nutrient uptake. These conditions create an ideal environment for early root development of rice seedlings. All of these factors explain why M₀ (pure soil) provided the best and most consistent response in seedling growth parameters. M₀ excelled because the soils in all three swamp locations have inherently high fertility [34-46].

Pure rice husk ash (M₁) has significant physical and chemical limitations. The limitations of physical properties are: Rice husk ash has very high Porosity causing water to absorb quickly so that the seeds experience physiological drought, and Low bulk density resulting in difficulty supporting the roots, causing shallow and unstable roots [44], and very low water retention capacity so that the media dries quickly and interferes with the initial hydration of the sprouts [32, 44]. As a result, the root environment is unable to maintain the moisture necessary for germination and shoot formation [32, 35, 36, 41, 47, 48]. The limitations of chemical properties are: Despite its high silica (Si) content, rice husk ash has very low N, P, and K content, and provides almost no essential micronutrients, and does not have adequate cation exchange capacity to bind and store nutrients. As a result, seedlings grown in M₁ experience nutritional deficiencies from the start, characterized by slow growth, pale leaves, and a small number of shoots (Figure 3) [32, 35, 36, 41, 45, 47, 48].

This combination of deficiencies causes the seedlings' energy to be spent on survival, rather than forming new tissue, and photosynthesis rates are low due to low N and Mg supplies, as well as delayed shoot and leaf formation. This is why M₁ is consistently the worst-performing treatment, because it is poor in nutrients and unable to retain water [32, 35, 36, 41, 45, 47, 48].

The 2:1 soil + rice husk ash treatment (M₃) performed similarly to pure soil (M₀). The 2:1 ratio (two parts soil: one part rice husk ash) represents the optimum balance between the benefits and limitations of rice husk ash [35, 36, 42, 46]. The mechanisms that enable M₀-equivalent performance are aeration properties, and the improvement mechanism in rice husk ash improves soil pore space, leading to healthier roots. The resulting soil, which is moist yet not excessively wet, arises from the soil's ability to retain water, the improvement in the soil mixture's water storage capabilities, and the role of rice husk ash in regulating soil aeration [42]. The nature of lower bulk density, the repair mechanism in the roots more easily penetrates the rice husk ash media, thus accelerating the growth of primary roots [47]. The rooting environment is balanced, with a combination of good physical structure and repair mechanisms, without significant nutrient loss from the soil. With a 2:1 ratio, the soil component remains dominant, preventing loss of its chemical fertility, while the rice husk ash provides physical benefits to the medium. Therefore, M₃ is an ideal compromise, enough soil for nutrients, enough rice husk ash for physical repair, resulting in performance similar to M0 [33, 35, 36, 38, 41-45, 49, 50].

The soil + rice husk ash 2:1 treatment (M₃) was not significantly different from the soil treatment (M₀) in various early plant growth indicators, such as: shoot formation (days), leaf formation (days), 50% shoot formation (days), seedling height at 22 days after planting, and the number of plants per tray. This is greatly influenced by the initial nutrient status and root condition. The reason: Soil as the main nutrient provider is still dominant in M₃, so that the seedlings receive almost the same nutrient supply as M₀ [35, 36]. Rice husk ash only acts as a physical modifier, not a source of nutrients, so it does not significantly change the nutritional function [45, 49, 51]. In the early growth phase, the plant's nutrient requirements are still relatively low, so the additional water retention and aeration from rice husk ash do not result in a drastic growth spurt [50]. There was no significant physical or chemical stress in M₃, in contrast to the physically extreme and nutrient-poor M₁. Therefore, the plant response in M₃ was close to that of M₀, resulting in a 95% LSD analysis showing no significant differences. Therefore, the insignificant differences between M₃ and M₀ in all initial growth indicators indicate that mild physical modification without soil reduction was not strong enough to produce statistically different agronomic responses [35, 36, 38, 43-45, 49-51].