In-situ and laboratory determined Ultisol-derived paddy soil pH varied with flooding condition in subtropical China

Soil pH measured by laboratory standard method does not accurately reflect its actual acidity. In this study, we compared the pH values of anaerobic and aerobic farmland soils in Langxi, Yujiang, and Jinggangshan Counties in subtropical China, measured using in-situ and laboratory standard methods. The results showed that the in-situ pH values of anaerobic Ultisol-derived paddy soils were significantly higher than the laboratory values. Although there was an extremely significant correlation between the pH values obtained by the two methods (R_adj²=0.15, P = 1.77 ×10⁻⁴, n = 81), extrapolating these results across different soil types proved challenging due to the specificity of paddy soils. The in-situ pH values of aerobic paddy soils were 0.52 units lower than the laboratory values, with an extremely significant correlation (R_adj²=0.67, P = 5.95 ×10⁻²¹, n = 81). In-situ pH mapping of farmland soils under both anaerobic and aerobic conditions revealed that the paddy soils in northwest Jinggangshan County, along with the majority of aerobic paddy soils in the study areas, exhibited acidification and aluminum-induced phytotoxicity. These findings provide valuable insights into estimating the in-situ soil pH of paddy soils under anaerobic and aerobic rotations, providing a more accurate and efficient representation of actual soil pH conditions during different cultivation stages in the acidic paddy soil regions of southern China.