Objectives The application effect of humic acid is uncertain due to ecological environment and management conditions. We assessed the comprehensive application effects and key influencing factors of humic acid on wheat and maize across China, so as to increase the application efficiency of humic acid.

Methods The data of this study were derived from databases such as “China National Knowledge Infrastructure (CNKI)”, “Wanfang Data”, and “VIP Information”, and the relevant literature was searched with “humic acid”, “maize”, “wheat” and “yield” as the main keywords. This study collected 117 articles with 319 sets of valid data published from 2000 to 2023 to quantitatively analyze the impact of humic acid on wheat and maize yields as well as yield components under different climatic, soil, and management conditions used the Meta-analysis method. The control group was taken without the addition of humic acid, and the addition of humic acid was used as the experimental group. Furthermore, the random forest and structural equation model was used to quantitatively assess the importance and paths of various factors influencing the effectiveness of humic acid.

Results Humic acid application increased average yields of wheat and maize by 8.0%, spikes per unit area by 17.4%, grains per spike by 5.2%, and 1000-grain weight by 4.3%. Humic acid-nitrogen fertilizer had a yield increase of 11.9%, higher than other humic acid-based fertilizers. Combination of basal dressing and topdressing significantly increases yield by 12.0%. The yield effects of humic acid was decreased with increasing application rate. The yield effect of humic acid was positively (P<0.05) correlated with soil pH, but negatively correlated with soil organic matter, mean annual temperature, and mean annual precipitation. The random forest model indicated that soil available nitrogen, phosphorus and potassium, organic matter, pH were the key variables affecting the responses of yield and yield component to humic acid. Structural equation model showed that mean annual temperature, and mean annual precipitation had negative impact on the yield effect of humic acid application, through directly or indirectly affecting soil properties.

Conclusions Humic acid application exhibits significant effects on increasing yield and yield components of wheat and maize. Comparing the influencing factors, humic acid nitrogen fertilizer, combination of basal dressing and topdressing, application rate <15 kg/hm², and infertile soil and stress climate will enhance effects of humic acid.