The effect of reaction between soil and phosphate on pH has not previously been explored or explained. We incubated samples of a soil with calcium carbonate to give a range of ${\mathrm{pH}}_{{\text{CaCl}}_2}$ from 4.2 to 7.2, and then measured the change in pH when these samples were reacted with phosphate solutions. These solutions were at the same pH as the soil to which they were added. We also measured exchangeable cations and cation exchange capacity. At low pH, reaction with phosphate increased pH; at high pH it decreased it. These effects are a consequence of the way that the charge on the phosphate molecules is distributed. Reaction with phosphate conveys negative charge to the soil. This charge may be located partly on the surface side of the adsorbed phosphorus molecule and partly on the solution side; that is either ‘inside’ or ‘outside’. At low pH the electric potential before reaction is less negative than at higher pH and it is easy to increase the negative charge inside the adsorbed phosphate molecules. Changes in charge inside the phosphorus atom have a large effect on potential; a small amount of charge is required to achieve the necessary electric potential. The charge conveyed to the surface is less than the mean charge of the ions in solution and the pH therefore increases. At high pH, because of the more-negative potential, it is more difficult to increase the negative charge inside the adsorbed phosphorus atom. A larger fraction of the charge is therefore located outside it. A large amount of charge is required to achieve the necessary electric potential. This charge is greater than the mean charge on the ions in solution and the pH therefore decreases. The cation exchange capacity and the exchangeable cations reflect the charge inside the adsorbed phosphate molecule. At low to medium initial pH, reaction with phosphate did not decrease pH. The decreases in pH that are sometimes observed following application of phosphate are due to ancillary effects.

中文翻译：

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磷吸附对土壤pH值的影响

土壤和磷酸盐之间的反应对 pH 值的影响以前没有被探索或解释过。我们用碳酸钙培养土壤样品，得到一系列${\mathrm{酸碱度}}_{{\text{氯化钙}}_2}$从 4.2 到 7.2，然后测量这些样品与磷酸盐溶液反应时的 pH 值变化。这些溶液的 pH 值与添加它们的土壤相同。我们还测量了可交换阳离子和阳离子交换容量。在低 pH 值下，与磷酸盐的反应会增加 pH 值；在高 pH 值下，它会降低它。这些影响是磷酸盐分子上的电荷分布方式的结果。与磷酸盐的反应将负电荷传递给土壤。该电荷可能部分位于吸附的磷分子的表面侧，部分位于溶液侧；那是“内部”或“外部”。在低 pH 条件下，反应前的电势比在高 pH 条件下的负电势更小，并且很容易增加吸附的磷酸盐分子内的负电荷。磷原子内部电荷的变化对电位有很大的影响；需要少量电荷才能达到必要的电势。传递到表面的电荷小于溶液中离子的平均电荷，因此 pH 值增加。在高 pH 值下，由于电势更负，吸附磷原子内部的负电荷更难增加。因此，大部分电荷位于其外部。需要大量电荷才能达到必要的电势。该电荷大于溶液中离子的平均电荷，因此 pH 值降低。阳离子交换容量和可交换阳离子反映了吸附的磷酸盐分子内部的电荷。在低至中等初始 pH 值下，与磷酸盐的反应不会降低 pH 值。在施用磷酸盐后有时观察到的 pH 值降低是由于辅助效应。