The Impact of Humic Acid Fertilizers on Crop Yield and Nitrogen Use Efficiency: A Meta-Analysis

Humic acid, as a novel organic fertilizer, has the potential to improve soil fertility and the agronomic performance of crops. However, the efficacy of humic acid varied in previous studies and the mechanisms by which climatic conditions, soil properties, and fertilization regimes regulate it remain unclear. To test this, a comprehensive meta-analysis was conducted to evaluate the effects of humic acid across various environmental and agricultural conditions. The results showed that the humic acid amendment significantly increased the crop yield by 12%, nitrogen use efficiency by 27%, and nitrogen uptake by 17%, on average. Optimal effects were observed in regions with an annual precipitation > 300 mm and mean annual temperatures > 10 °C, and in soils with a moderate pH (6 < pH ≤ 8) or low total nitrogen. Its efficacy was attenuated in alkaline soils and high total nitrogen (≤1.5 g kg⁻1) or low soil organic carbon (<10 g kg⁻1) conditions. Cash crops and upland cereals showed a superior response compared to paddy rice, with optimal nitrogen application rates of 100–200 kg ha⁻1. This study quantifies humic acid’s positive impacts on crop productivity and nitrogen utilization across various conditions, providing empirical evidence for optimizing its application in sustainable agricultural systems, potentially reducing excessive fertilizer use and its associated environmental pollution.