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The Soil Environment Soil Acidity Part 2
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Last week’s column defined what acidity is and how it is described. This week, what determines soil pH, what causes the pH in soils, and what it means for plant growth. Please remember this is a very condensed overview. First, what determines the pH of undisturbed soil?
• Parent material – What the soil forms from helps determine pH. If the soil forms from material like limestone it will be higher in base (nonacid) cations and can have a pH well above neutral. The reverse is true also. When forming under materials like sandstone and shale it is more likely to be nearer neutral or even acidic depending on the factors listed below.
• Climate – As rainfall increases, especially with moderate to warm climates, soils tend to be naturally more acid (pH less than 7).  This occurs for two reasons. First, many basic cations such as sodium and potassium are leached down into the soil profile as the soil forms and even below it in some cases. This creates an acid surface soil environment over time with a higher subsoil pH. In drier climates, these cations remain and create high pH, nonacid, and in extreme cases alkali soils such as in the desert Southwest. Second, rainfall determines vegetation and the type of vegetation helps determine the native pH.
• Native vegetation – Climate (primarily temperature and rainfall along with periodic fires) also have an effect on native vegetation. Higher rainfall areas not experiencing periodic fires developed into forested areas and the soil, especially the upper soil horizons tend towards slightly too very acid. With the introduction of periodic fires and as rainfall decreases, native vegetation moves progressively from tall to mixed to short grass prairie. Eventually as precipitation decreases further xerophytic (desert) vegetation predominates. In this situation soils are slightly to well-above neutral (basic soils) as you moved to less and less precipitation. This is caused partially by the climate and reinforced by the shift from tree to grass species.  
• Soil moisture – This may seem part of climate but it’s a bit more complicated. Soil water content determines the oxygen status of the soil. Too wet or too dry inhibit normal microbial activity. This determines the rate of breakdown of organic materials added to the soil (plant materials, etc.). Poorly drained soils inhibit the downward movement of ions and can result in higher pH levels.
There is more but to wrap this up for this area the soils here formed under grassland conditions with moderate rainfall. There are some differences depending on whether the soil is north or south of the river. When the area was first settled the soil pH was generally neutral to slightly above neutral (7 to a bit above 8) with excellent structure and organic matter content. Next week how agriculture has changed soil pH.