Acid sulfate soil at the beginning of the Dry-Season crop: why should fused phosphate be applied as a base fertilizer before sowing?
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Acid sulfate soil at the start of the Dry-Wet season: why should fused phosphate be applied as a base fertilizer before sowing?
Early November marks the beginning of the main sowing period for the 2026–2027 Winter-Spring rice crop. According to the seasonal calendar from the Department of Crop Production and Plant Protection, the main sowing period from November 1 to November 30, 2026, accounts for approximately 53 percent of the planned area across the region, while coastal areas such as Bến Tre and Trà Vinh have already sown early from October 10–30 to avoid salinity intrusion (Dân Việt, 2026). Concurrently, the El Niño phenomenon is forecast to extend into early 2027, with dry season salinity intrusion potentially comparable to the 2015–2016 dry season and beginning to affect areas from late December 2026 (thitruonghanghoa.com, 2026).
For farmers in acid sulfate soil areas — a type of soil covering approximately 1.5 million hectares, equivalent to 40 percent of the natural area of the entire Mekong Delta, concentrated in Đồng Tháp Mười, Tứ Giác Long Xuyên, and the Cà Mau Peninsula (doc.edu.vn, n.d.) — this is a critical moment: thoroughly treat the soil before sowing, or let the acidity "surface" and then try to mitigate it.
Why does acid sulfate soil damage young rice roots?
When potential acid sulfate soil is tilled and exposed to air, pyrite minerals are oxidized; every 1 mole of completely oxidized pyrite produces up to 4 moles of H+ ions, lowering the soil pH, often to only about 3.5 (doc.edu.vn, n.d.). At pH below 3.5, Fe3+ and Al3+ ions dissolve strongly, causing direct toxicity to plants; when the pH is raised to about 4, iron is fixed, but dissolved aluminum remains the most concerning toxic substance — which is why farmers often worry more about "cold acidity" (due to aluminum) than "hot acidity" (due to iron) (doc.edu.vn, n.d.).
Aluminum toxicity causes roots to stunt, preventing water and phosphorus uptake; rice plants become dwarfed, leaves stand upright, and turn dark green or purple due to phosphorus deficiency. Scientifically, acidic soil causes phosphorus to be fixed by aluminum and iron oxides, making it harder for plants to absorb. A study on acid sulfate saline soils in Bến Tre and Kiên Giang showed that only about 12 percent of the total phosphorus in the soil is in an available form, with the rest "held tightly" by aluminum and iron (Linh et al., 2023). A recent research overview also noted that single amelioration measures are unlikely to completely resolve all limitations of waterlogged acidic soils, requiring an integrated strategy—soil diagnosis, long-term monitoring, and the coordinated use of complementary amelioration materials (Anusha et al., 2026).
How does fused phosphate differ from superphosphate?
According to technical analysis, superphosphate is slightly acidic, readily available, and quickly absorbed by plants — but applying too much on acidic (phèn) soil will make the soil even more acidic, so it is usually only used for sweet alluvial soil. In contrast, fused magnesium phosphate is alkaline (pH around 8.0), and in addition to phosphorus, it also contains calcium, magnesium, and silicon. Its alkalinity helps neutralize acidic (phèn) soil, and the micronutrients contribute to stronger plants (ECOMCO, 2026).
Manufacturer-published data indicates that every 100 kg of fused magnesium phosphate contains approximately 16 kg of phosphorus (P2O5), 30 kg of lime (CaO), 24 kg of silicon (SiO2), 15 kg of magnesium (MgO), and various micronutrients (Văn Điển, n.d.). Studies by the National Institute of Soils and Fertilizers and Vietnam National University of Agriculture also confirm that fused magnesium phosphate effectively neutralizes acidity and detoxifies soils in the Mekong Delta (Văn Điển, n.d.).
A notable characteristic: fused magnesium phosphate is insoluble in water and thus not leached away, but only dissolves in the acidic exudates secreted by plant roots — thereby providing nutrients gradually throughout the entire crop cycle (ECOMCO, 2026). When applied to acid sulfate soils, phosphorus also reacts with aluminum to form a solid precipitate, helping to "lock up" toxic aluminum — colloquially known as "sacrificing a portion of phosphorus to ameliorate acidity" (ECOMCO, 2026).
This approach is also consistent with international research: alkaline materials (lime, alkaline inorganic materials containing CaO, MgO, SiO2) help increase soil pH by 0.33–1.57 units, reduce exchangeable aluminum, and improve the health of rice-growing soil (Liu et al., 2026). At the same time, scientists also note the need to balance inputs: recycling organic fertilizers helps combat soil acidification but may come with the risk of cadmium accumulation if quality is not controlled (Xu et al., 2026) — therefore, basal fertilization should combine multiple measures, not rely on a single type of material.
Proper basal application of fused phosphate
The general principle consistently stated in technical documents is to apply fertilizer early as a basal dressing, incorporating it into the soil, rather than spreading it on the surface or waiting until the rice plants have emerged:
- Timing: Apply basal fertilizer during the final soil preparation, before sowing. Early application helps phosphorus take effect in neutralizing acidity right from seed germination (Mai Thành Phụng, n.d.; ECOMCO, 2026).
- Application Method: Spread evenly, then harrow to incorporate the fertilizer into the soil layer about 10–15 cm deep, so that young rice roots immediately encounter the phosphorus source (ECOMCO, 2026).
- Tip to avoid "soil locking phosphorus": Phosphorus applied to the soil is easily bound by iron and aluminum, allowing plants to absorb only a fraction. The recommended solution is to mix fused magnesium phosphate with organic fertilizer or well-decomposed farmyard manure before application — organic matter coats the phosphorus particles, limiting direct contact with iron and aluminum; decomposing organic humus also helps dissolve phosphorus for easier plant absorption (ECOMCO, 2026). Research in Bến Tre and Kiên Giang also showed that organic materials help convert phosphorus into a more available form in acid sulfate saline soils (Linh et al., 2023).
- Reference Dosage: Documents by Assoc. Prof. Dr. Mai Thành Phụng suggest approximately 200 to 400 kg per hectare depending on the soil's acidity (Mai Thành Phụng, n.d.); another recommendation for the Winter-Spring crop is about 40 to 50 kg per công (1,000 m²) of fused magnesium phosphate in the basal fertilization formula (ECOMCO, 2026). Farmers should adjust based on the actual acidity of their fields and refer to the manufacturer's packaging.
Technical example: Ninh Bình fused phosphate
Ninh Bình fused phosphate is one of the fused phosphate types commonly used by farmers in the Mekong Delta for pre-planting application to suppress acidity at the beginning of the season. It belongs to the same group as other fused phosphates with similar alkalinity and micronutrient content (ECOMCO, 2026). When purchasing, farmers should compare the ingredients listed on the packaging (phosphate, lime, magnesium, silicon content) and ask the dealer about the correct dosage for their specific soil type — this article only outlines general principles and does not replace manufacturer instructions.
Conclusion
The beginning of the Winter-Spring crop is the "golden time" to treat acid sulfate soil: early application of fused phosphate as a base fertilizer, thoroughly incorporated, combined with well-decomposed organic fertilizer will help suppress acidity, raise soil pH, and create momentum for healthy rice root development right from germination — especially as the 2026–2027 dry season is forecast to have severe drought and salinity intrusion. Thorough land preparation at the start of the season is always cheaper than firefighting mid-season.
Dosage varies depending on soil, crop variety, and plant condition. For further consultation, please call Út Nữ: 02943 882069.
References
Anusha, B., Joseph, B., Rani, B., George, T., Leno, N. and Gopinath, P.P. (2026) 'Managing Soil Acidity and Fe–Al Toxicity in Submerged Paddy Soils: Biogeochemical Processes, Environmental Implications and Integrated Amelioration Strategies', *International Journal of Environment and Climate Change*, 16(10), pp. 184–197. doi: 10.9734/ijecc/2026/v16i105705.
Dân Việt (2026) 'Latest seasonal calendar to "avoid" saltwater intrusion due to El Nino for the 2026-2027 Winter-Spring crop, what does the Department of Crop Production and Plant Protection recommend?', *Dân Việt Newspaper*, 19 June. Available at: https://danviet.vn/khung-thoi-vu-moi-nhat-ne-han-man-do-el-nio-cho-vu-dong-xuan-2026-2027-cuc-trong-trot-va-bvtv-khuyen-cao-gi-d1436554.html (Accessed: 19 October 2026).
doc.edu.vn (n.d.) 'Thesis on Acid Sulfate Soil Improvement for Socio-Economic Development in the Mekong Delta' [thesis, publication year not verified]. Available at: https://doc.edu.vn/tai-lieu/tieu-luan-cai-tao-dat-phen-phuc-vu-phat-trien-kinh-te-xa-hoi-vung-dong-bang-song-cuu-long-9644/ (Accessed: 19 October 2026).
ECOMCO (2026) 'Rice basal fertilization techniques: The secret to using Lime & Phosphorus to reduce alum, effectively detoxify salinity'. Available at: https://ecomco.vn/ky-thuat-bon-lot-cho-lua-bi-quyet-dung-voi-lan-ha-phen/ (Accessed: 19 October 2026).
Linh, D.T.T., Khoi, C.M., Ritz, K., Van Sinh, N., Phuong, N.T.K., My, H.M.T., Linh, T.B., Minh, D.D., Linh, T.T. and Toyota, K. (2023) 'Effects of Rice Husk Biochar and Compost Amendments on Soil Phosphorus Fractions, Enzyme Activities and Rice Yields in Salt-Affected Acid Soils in the Mekong Delta, Viet Nam', *Agronomy*, 13(6), 1593. doi: 10.3390/agronomy13061593.
Liu, J., Wang, T., Lan, L., Meng, J., Hu, M., Sajid, T. and Meng, J. (2026) 'Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils', *Agronomy*, 16(1), 9. doi: 10.3390/agronomy16010009.
Mai Thanh Phung (n.d.) 'Fertilization techniques for summer-autumn rice in the Mekong Delta' [PDF document, publication year unverified]. Available at: https://lib.vn/upload/library/1003188/preview/ky-thuat-bon-phan-cho-lua-vu-he-thu-o-dbscl-1003188.pdf (Accessed: 19 October 2026).
thitruonghanghoa.com (2026) 'Salinity intrusion 'threatens' the Mekong Delta', 10 October. Available at: https://www.thitruonghanghoa.com/tin-tuc/han-man-de-doa-dbscl-355462.html (Accessed: 19 October 2026).
Văn Điển (n.d.) '"Văn Điển fertilizer is the optimal solution for improving acidic, waterlogged, and marshy soils"' [reposted on vandienfmp.vn from Nông thôn mới Magazine; publication year unverified]. Available at: https://vandienfmp.vn/phan-bon-van-dien-la-giai-phap-toi-uu-de-cai-tao-dat-chua-trung-lay-thut/ (Accessed: 19 October 2026).
Xu, D., Ros, G.H., Liu, P., Zhu, Q., Zhang, F. and de Vries, W. (2026) 'Nutrient management modulates acidification-induced risks to yield and cadmium contents in paddy rice', *Nature Food*, 7(3), pp. 272–282. doi: 10.1038/s43016-026-01315-2. [Information cited from a scientific bulletin; original article not directly accessed.]
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