Soil pH is a critical parameter influencing numerous soil properties including nutrient cycling, microbial activity, inorganic carbon and metal speciation and criteria for classifying acid sulfate soils and soils with reactive aluminium (Podzols and Andosols). Accurate measurement of soil pH is essential for effective soil assessment, management and crop production. This review of soil pH focuses on techniques and challenges for measurement and utilises insights from a comprehensive global soil dataset (n = 655,336). Soil pH has a global average of 6.36 with considerable regional variability. A Random Forest model of the global dataset identified total carbonate content, rainfall, evaporation, clay and organic carbon content as key drivers of soil pH (R2 = 0.77, mean absolute error = 0.46 pH units). The advantages and limitations of various soil pH measurement methods (e.g., glass and solid‐state electrodes, colorimetric and spectrophotometric methods, infrared spectroscopy, remote sensing and specialised field assessment methods) are discussed. Care should be taken in choosing appropriate pH measurement techniques depending on the purpose of the measurement and nature of the soils. The use of electrolytes (e.g., CaCl2 and KCl) introduces variable changes in measured pH and, on average globally, pH measured in a 1:5 soil:0.01 M CaCl2 extract is ~0.7 pH units lower than that measured in a 1:5 soil suspension. Although the use of electrolytes can help stabilise pH measurements in low ionic strength soils, their use requires careful consideration, as they do not eliminate all seasonal variability (which may also be important to assess) and they can introduce large pH perturbations in sodic‐alkaline and acid sulfate soils. Varying soil‐to‐solution ratios have less influence on resultant pH values than electrolytes however the measurements at low soil‐to‐solution ratios (e.g., 1:1 soil:water) are preferable for accuracy. Perturbations from field pH conditions due to CO2 outgassing and oxidation are likely when wet soils, acid sulfate soils and/or sub‐soils are measured in the laboratory. These findings underscore the need for improved field measurement technology and further research on the measurement protocols to ensure accurate and reliable soil pH data, which are crucial for optimising agricultural practices and environmental management.

中文翻译：

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土壤 pH 值：来自全球数据集的技术、挑战和见解


土壤 pH 值是影响许多土壤特性的关键参数，包括养分循环、微生物活性、无机碳和金属形态以及酸性硫酸盐土壤和活性铝土壤（灰化土和安道醇）分类的标准。准确测量土壤 pH 值对于有效的土壤评估、管理和作物生产至关重要。本综述侧重于测量技术和挑战，并利用来自全球土壤综合数据集 （n = 655,336） 的见解。土壤 pH 值的全球平均值为 6.36，具有相当大的区域差异。全球数据集的随机森林模型确定总碳酸盐含量、降雨量、蒸发量、粘土和有机碳含量是土壤 pH 值的关键驱动因素（R2 = 0.77，平均绝对误差 = 0.46 pH 单位）。讨论了各种土壤 pH 值测量方法（例如玻璃和固态电极、比色和分光光度法、红外光谱、遥感和专门的现场评估方法）的优点和局限性。根据测量目的和土壤的性质，在选择合适的 pH 测量技术时应小心。电解质（例如 CaCl2 和 KCl）的使用会带来测得的 pH 值变化，平均而言，在全球范围内，在 1：5 土壤：0.01 M CaCl2 提取物中测得的 pH 值比在 1：5 土壤悬浮液中测得的 pH 值低 ~0.7 pH 单位。尽管使用电解质有助于稳定低离子强度土壤中的 pH 值测量，但使用它们需要仔细考虑，因为它们并不能消除所有季节性变化（评估这一点也很重要），并且它们会在钠碱性和酸性硫酸盐土壤中引入较大的 pH 值扰动。 与电解质相比，不同的土壤与溶液比对所得 pH 值的影响较小，但是在低土壤与溶液比（例如，1：1 土壤：水）下的测量更可取。在实验室中测量潮湿土壤、酸性硫酸盐土壤和/或底土时，可能会因 CO2 脱气和氧化而对现场 pH 条件造成干扰。这些发现强调了改进田间测量技术和进一步研究测量协议的必要性，以确保准确可靠的土壤 pH 数据，这对于优化农业实践和环境管理至关重要。