Objective This study aimed to evaluate the effects of different fertilizer types and application rates on the growth of middle-aged teak (Tectona grandis) forests.

Method A 33-year-old teak plantation was selected for the study, and a mixed fertilizer application experiment was conducted using the U12(1210) uniform experimental design. Fertilizers tested included organic fertilizer, compound fertilizer, borax, zinc sulfate, magnesium sulfate, and lime. Based on the results after two years of fertilization, organic fertilizer, compound fertilizer, magnesium sulfate, and slow-release fertilizer were applied again to three replicates for a second round of fertilization. Growth data were observed continuously for four years.

Result 1. After four years, the growth rates of diameter at breast height (DBH) and volume in the treatment groups ranged from 3.4% to 6.7% and 6.9% to 16.0%, respectively, compared to 3.8% and 7.7% in the control group. The highest treatment growth rates for DBH and volume were 89.6% and 88.0% greater than those of the control, but differences among treatments were not statistically significant (P>0.05). 2. In the first year after the initial fertilization, magnesium sulfate and compound fertilizer significantly promoted DBH growth (P<0.01). Increasing organic fertilizer application also enhanced DBH growth, while variations in borax and zinc sulfate application rates had minimal effects. Lime application inhibited growth. In the second year, only magnesium sulfate significantly improved DBH growth (P<0.05). During the second fertilization round, none of the treatments showed significant effects on DBH growth (P>0.05). 3. Fertilization in the first year generally resulted in greater DBH growth compared to subsequent years, with the effect diminishing over time. Single applications outperformed double applications in the first year, but over the entire study period, double applications produced slightly better results. 4. Cost-benefit analysis indicated that appropriate fertilization treatments achieved a benefit ratio of 1∶3.5. However, doubling the fertilization frequency reduced economic benefits.

Conclusion Fertilization of middle-aged teak forests effectively promotes tree growth. The optimal treatment combination per 10 cm DBH is 2 kg organic fertilizer, 0.25 kg compound fertilizer, 5 g borax, 10 g zinc sulfate, and 5 g magnesium sulfate.