Changes of soil properties caused by acidification of yellow-cinnamon soil and the acid damage threshold for soybean production in southern Henan

Objectives In recent years, the acidification of yellow-cinnamon soil has posed a significant threat to the profitability and sustainability of soybean production in eastern Henan Province. This study investigated the variations in soil chemical and biological indicators following acidification and assessed the impact of varying degrees of acidification on soybean growth and yield.

Methods A pot experiment was conducted in Xinxiang, Henan Province. Seven initial soil pH levels were established by adding different amounts of ferrous sulfate: 3.5, 3.7, 4.0, 4.4, 4.8, and 5.3 (control), along with an additional treatment using wood ash (pH 5.7). Soybean plant height was measured at 16, 25, 35, 45, 55, 71, and 93 days after emergence (DAE). Aboveground biomass and grain yield were determined at harvest. Soil samples were collected at 25, 46, 68, and 89 DAE, as well as at harvest, to analyze exchangeable nutrients, available nutrients, and related enzyme activities. The Gompertz Equation was employed to model the relationship between soybean grain yield, biomass, and soil pH. Acidification thresholds corresponding to 95% (pH₉₅), 50% (pH₅₀), and 5% (pH₅) of maximum yield, as well as the sensitive pH value, were calculated.

Results With increasing acidification, soil exchangeable acidity and ammonium nitrogen content increased, whereas base cations, available phosphorus, available potassium, and nitrate nitrogen decreased. Concurrently, soil polyphenol oxidase activity increased, while acid phosphatase and catalase activities declined. Within the pH range of approximately 6.0, soybean biomass and grain yield increased significantly with rising soil pH, with maximum increases of 60.60% and 59.85%, respectively. Gompertz modeling revealed that the soil pH values corresponding to pH₉₅, pH₅₀, and pH₅ for grain yield were 5.23, 3.64, and 2.74, respectively, with a sensitive pH of 3.41. For biomass, the corresponding pH values were 5.38, 3.60, and 2.61, with a sensitive pH of 3.35.

Conclusions Acidification of yellow-cinnamon soil in southern Henan leads to elevated exchangeable acidity and ammonium nitrogen, reduced base cations, available phosphorus, available potassium, and nitrate nitrogen, decreased soil organic matter stability, and deterioration of the microbial environment, ultimately impairing soil fertility. A soil pH of 5.23 serves as the critical threshold for maintaining normal soybean growth and yield, regulated jointly by exchangeable calcium, soil nutrients, and enzyme activities.