Abstract. In situ soil pH measurements with ion-selective electrodes (ISEs) are receiving increasing attention in soil mapping for precision agriculture as they can avoid time-consuming sampling and off-site laboratory work. However, unlike the standard laboratory protocol, in situ pH measurements are carried out at lower and varying soil moisture contents (SMCs), which can have a pronounced effect on the sensor readings. In addition, as the contact with the soil during in situ measurements should be relatively short, effects of soil texture could be expected because texture controls the migration of protons to the electrode interface. This may be exacerbated by the fact that the electrodes used for in situ measurements are made of less sensitive but more robust materials as compared to the standard glass electrode. Therefore, the aim of the present study was to investigate the effect of soil moisture and soil texture on pH measurements using robust antimony and epoxy-body ISEs pressed directly into the soil for 30 s. The SMC was gradually increased from dry conditions to field capacity. A wide range of soil texture classes were included, with sand, silt, and clay contents ranging from 16 % to 91 %, 5 % to 44 %, and 4 % to 65 %, respectively. An exponential model was fitted to the data to quantify the relationship between SMC and pH. The results show that an increase in SMC causes a maximum increase in pH of approximately 1.5 pH units, regardless of the type of pH ISE used. Furthermore, for sandy soil textures, a rather linear relationship between pH and SMC was observed, whereas, with decreasing mean particle diameter (MPD), the model had a pronounced exponential shape, i.e., a greater pH increase at low SMC and a plateau effect at high SMC. With increasing SMC, the pH values asymptotically approached the standard pH measured with a glass electrode in 0.01 M CaCl2 (soil : solution ratio of 1:2.5). Thus, at high SMC, subsequent calibration of the sensor pH values to the standard pH value is negligible, which may be relevant for using the sensor pH data for lime requirement estimates. The pH measurement error decreases exponentially with increasing soil moisture and increases with decreasing MPD. Using a knee point detection, reliable pH values were obtained for SMC > 11 %, irrespective of the pH ISE used. An analysis of the regression coefficients of the fitted exponential model showed that the maximum pH increase also depends on soil texture; i.e., the influence of soil moisture variation on the pH value increases with decreasing MPD. Moreover, the concavity of the exponential curve increases with decreasing MPD.

中文翻译：

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土壤水分含量和土壤质地对两种类型的稳健离子选择电极快速原位 pH 测量的影响

摘要。使用离子选择电极 (ISE) 进行的原位土壤 pH 值测量在精准农业土壤测绘中受到越来越多的关注，因为它们可以避免耗时的采样和场外实验室工作。然而，与标准实验室协议不同的是，原位 pH 测量是在较低且变化的土壤水分含量 (SMC) 下进行的，这会对传感器读数产生显着影响。此外，由于原位测量期间与土壤的接触应该相对较短，因此可以预期土壤质地的影响，因为质地控制着质子向电极界面的迁移。与标准玻璃电极相比，用于原位测量的电极由灵敏度较低但更坚固的材料制成，这一事实可能会加剧这种情况。因此，本研究的目的是使用将坚固的锑和环氧体 ISE 直接压入土壤 30 秒，研究土壤湿度和土壤质地对 pH 测量的影响。 SMC从干燥条件逐渐增加到田间持水量。包括多种土壤质地类别，沙子、淤泥和粘土含量分别为 16% 至 91%、5% 至 44% 和 4% 至 65%。对数据拟合指数模型以量化 SMC 和 pH 之间的关系。结果表明，无论使用何种类型的 pH ISE，SMC 的增加都会导致 pH 值最大增加约 1.5 个 pH 单位。此外，对于沙质土壤质地，观察到 pH 值和 SMC 之间呈相当线性的关系，而随着平均粒径 (MPD) 的减小，该模型具有明显的指数形状，即在低 SMC 和平台效应下 pH 值增加更大在高 SMC 下。随着 SMC 的增加，pH 值逐渐接近用玻璃电极在 0.01 M CaCl2 中测量的标准 pH（土壤：溶液比例为 1:2.5）。因此，在高 SMC 下，随后将传感器 pH 值校准到标准 pH 值可以忽略不计，这可能与使用传感器 pH 数据进行石灰需求估计相关。 pH 测量误差随着土壤湿度的增加而呈指数减小，并随着 MPD 的减小而增大。使用拐点检测，无论使用何种 pH ISE，都能获得 SMC > 11% 的可靠 pH 值。对拟合指数模型回归系数的分析表明，最大 pH 值增加还取决于土壤质地；即土壤湿度变化对pH值的影响随着MPD的减小而增大。此外，指数曲线的凹度随着 MPD 的减小而增大。