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Soils - Part 4: Soil pH

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Why Does Soil pH Change?

Several factors influence soil pH. Some of these are:

1. Parent material. Rocks from which parent material originally formed vary from acidic to alkaline in reaction. Soils formed from sandstone or shale are more acidic than soils formed from limestone.

2. Precipitation. As annual precipitation increases, leaching of Ca and Mg increases, allowing the pH to decrease. For example, most Nebraska soils were formed from a calcareous parent material; however, increased precipitation in the eastern part has moved the lime layer deeper in the soil than in western Nebraska, where the lime layer is still at the surface. When moving from east to west across the state the pH goes from acid to alkaline.

3. Native vegetation. Soils formed under prairie (grasses) tend to be less acid than soils formed under forests.  Residue from coniferous(evergreen) trees is more acidic than from deciduous (broadleaf) ones. Further, roots of growing plants produce CO2 and small amounts of organic acids which increase soils acidity.

4. Crop grown. Legumes remove more calcium and magnesium (replaced with hydrogen) than some grasses, such as corn or sorghum. Removing the basic cations decreases OH- ions and increases H+ thus lowering the pH.

5. Nitrogen and other acid forming fertilizers and organic material (manure, crop residue and soil organic matter). These are all potential sources of H+ which lowers the pH. Nitrogen sources such as anhydrous ammonia, ammonium nitrate and urea require about 1.8 pounds of CaCO3 per pound of actual nitrogen to neutralize the H+ when the nitrogen is converted to NO3-.  All nitrogen fertilizers have an acidifying effect except those containing base carriers which offset the effect, like a calcium nitrate fertilizer. High analysis phosphate fertilizers are also acid formers. CO2 formed from mineralization of organic material has an acidifying effect (Equation 2), i.e., adds H+ to the soil.

HOH + CO2 <--> H+ + HCO3(Carbonic acid)          (2)

6. Burning fossil fuels. Acid rain? This is not a serious problem for most agricultural production areas of Nebraska. However, the acidifying effect of nitric and sulfuric acids from burning fossil fuels has and can seriously influence the vegetation in industrial areas where the acid-forming compounds are not removed from gases released from burned fossil fuels.

7. Irrigation water. Irrigation water pumped from groundwater sources often has quite a bit of calcium and magnesium dissolved in it which makes it “hard water.”  This is a significant source of cations which offset the acidifying effects of fertilizer or organic matter.

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