. Humic acid is a natural organic material relatively abundant and potentially be used to improve plant growth. The role of humic acid in improving nutrient content of cocoa seedlings and in improving the growth of soil bacteria and fungi, as well as improving chemical properties of sandy Humic Dystrudept were the main foci of this research. This research was carried out at Indonesian Research Institute for Biotechnology and Bioindustry in 2014. Population of Azotobacter beijerinckii and Aspergillus Niger in the soil were evaluated in the laboratory. Treatments were the addition of 0; 2.5; 5.0; 7.5; 10.0; and 12.5 mL humic acid into 10 kg of sterile soil. The greenhouse experiment to determine the effects of humic acid on the growth of cocoa (Theobroma cacao) seedlings was arranged in a completely randomized design with three replications and twelve treatments: 25; 50; 75; 100 % dosages of NPK fertilizer; 3.75; 7.5; 11.25; and 15 mL humic acid; 100% NPK + 7.5 mL humic acid; 50% NPK + 7.5 mL humic acid; 25% NPK + 7.5 mL humic acid into 10 kg of soil; and blank (without fertilizer and humic acid). The results of these research showed that humic acid have a positive correlation in increasing soil microbial population and nutrient uptake. The most abundant population of A. beijerinckii and A. Niger were under the addition of the 7.5-12.5 mL of humic acid. The addition of humic acid and its combination with NPK fertilizer increased N content 5.7%, P content 21.4%, and K contents 17.2% in cocoa seedling leaves and seedling dry weight 5.4 - 41.7%.