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Soils - Part 4: Soil pH

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How is Soil pH Determined?

The most common technique for determining pH is with a pH meter, using glass and reference electrodes. Before electronic devices were available, color indicators were used. These were dyes which changed color at certain pH values. Kits containing mixed indicator dyes are still available. The degree of accuracy varies but most are not as accurate as pH meters, especially for soil.

The pH in a soil sample will vary, depending on handling and sample preparation. It would be best to determine soil pH within the range of moisture content found in the field. This is not possible with current pH meters. With laboratory pH meters, the standard ratio of soil:water used is 1:1. The soil pH tends to increase as the ratio of soil:water goes to 1:5. Two probable reasons for this are: 1) H+ ions in solution are diluted as the amount of water increases; and 2) basic ions, such as calcium and magnesium, increase in the solution, which tends to increase the amount of OH- ion according to Equation 7:

   CaCO3 +  ~2HOH <--> H2CO3 + Ca++ + 2OH          (7)


As shown in Equation 7, calcium hydroxide Ca(OH)2 is a stronger base than H2CO3 (carbonic acid) is acid; therefore, there will be more OH- so the pH will increase.

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