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414 PROCEEDINGS OF INTERNATIONAL SCIENTIFIC CONFERENCE ON APPLIED BIOTECHNOLOGY20% ratio, the density of microbial strains consistently exceeded 106 CFU/g, fulfilling the Vietnamese standards for the quality of organic microbial fertilizer. With a control formula and three experimental formulas with a mixture of microbial fertilizer and sea sand, the results showed that vegetable yield increased in all experimental formulas with an increase in biomass efficiency of over 2.67 times in formula number 2, mixing 20 kg of sea sand + 10 kg of organic microbial fertilizer (2:1); over 3.40 times in formula number 3, mixing 15 kg of sea sand + 15 kg of organic microbial fertilizer (1:1) and formula number 4, mixing 10 kg of sea sand + 20 kg of organic microbial fertilizer (1:2 The experiment on tomato plants produced results that were even better than those on vegetable plants. However, the yield of vegetables and tomatoes between formula 3 and formula 4 did not show a statistically significant difference, so formula 3 would be the most optimal, ensuring both yield and saving the amount of fertilizer used.Keywords: organic microbial fertilizer, sandy soil, vegetable yield.1. INTRODUCTIONSea sandy soil, generally and specifically in the Truong Sa archipelago, is characterized by low fertility, poor nutrition, low organic matter content, acidic soil, poor permeability, low water retention, and nutrient absorption capacity [1], this makes it difficult to grow vegetables[2], so they should regularly receive vegetables from the mainland. To increase local crop production, improving the quality of coastal sand is essential. There have been many solutions to improve the quality of coastal sand, such as using microorganisms that can synthesize polysaccharides as a bio-adhesive to bind soil particles, limiting water loss, and enhancing the water retention capacity of sandy soil [3]. Some studies [4,5,6] have shown that residents in some parts of the