Soil salinization is a slow-motion crisis, affecting over 10% of global agricultural soils and threatening food security. In arid regions like Xinjiang, China—a major global cotton producer—this problem is especially severe. Decades of irrigation, often with mildly saline water, have led to high salt stress in farm fields, which is the primary limiting factor for cotton growth. This high salinity not only harms plants but also damages soil structure and disrupts the vital microbial communities that keep soil healthy. A new study in the journal Agronomy by Xiao Wang, Jianli Ding, Jinjie Wang, and their colleagues investigates a powerful two-part solution: combining biochar with humic acid (HA) to see if the mixture is greater than the sum of its parts.

The research team conducted a field experiment in a saline-alkali cotton field to test this “co-application” strategy. They compared the effects of no treatment (control), humic acid alone (HUM), and two different types of biochar—one made from walnut shells (SBC) and one from livestock manure (MBC). Most importantly, they also tested these two biochars when mixed with humic acid (H-SBC and H-MBC). After a full cotton growing season, the team sampled the soil to see what had changed, both chemically and biologically.

The results showed that while applying biochar or humic acid alone helped, the combined application “yielded better results than their individual applications”. The specific benefits depended on the biochar’s feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More. The combination of manure biochar and humic acid (H-MBC) was the most effective treatment for tackling salinity. It slashed the soil’s electrical conductivity (EC) by 44.1% compared to the untreated control. This same mixture was also the best for feeding the soil, boosting available phosphorus by 45.8%. Meanwhile, the walnut shell biochar and humic acid (H-SBC) combination excelled in a different area: it increased the soil’s water content by an impressive 80.3%, while also being the most effective at increasing nitrate nitrogen.

This chemical and physical transformation created a new habitat for soil microbes. The co-application treatments significantly increased the bacterial community’s richness, meaning more different types of bacteria were able to thrive. The researchers also analyzed the “co-occurrence network,” a way of mapping the “social network” of the microbes. They found the biochar-HA combination created a “more complex and stable microbial network” compared to biochar alone. This new network had more members (nodes) and more connections (edges). Crucially, the network shifted to be dominated by positive, cooperative interactions (68.4% of connections) rather than negative, competitive ones. This suggests the microbes were no longer just fighting for survival in a high-salt environment but were beginning to work together in a healthier, more resource-rich one.

The study identified the key factors driving this microbial shift: electrical conductivity (salinity), nitrate nitrogen, and available phosphorus. The co-application strategy works by first fixing the biggest problem—high salinity. Once that primary stressor was reduced, nutrient availability became the main factor shaping the new, healthier microbial community. By alleviating salt stress and improving nutrient levels, this biochar-HA mixture effectively steers the soil’s ecosystem toward a state “more favorable for ecological restoration”. For farmers in arid regions, this study provides a practical, powerful strategy to reclaim damaged land and support crop growth.

Source: Wang, X., Ding, J., Wang, J., Han, L., Tan, J., Liu, J., & Ge, X. (2025). Humic Acid Enhances the Soil Amelioration Effect of Biochar on Saline-Alkali Soils in Cotton Fields. Agronomy, 15, 2412.