How parent material influences soil pH

Although the pH scale ranges from 0 to 14, most soils have pH values between 4 and 9.  Many soils of the world are acidic for a variety of reasons including parent material, weathering and pollution.  Overtime soils have a general tendency to become more acidic.  The pH of a soil is often a limiting factor in plant nutrient availability. Soil pH may affect the water solubility of a nutrient and therefore plant uptake of that nutrient.  A common agronomic practice associated with highly acidic soils is liming.  Lime is a common amendment added to raise soil pH.  However, as was discussed in class, a soil may have high reserves of H+ ions which means that large amounts of lime would have to be added to have an impact on the soil pH.  The reserve acidity is dependent on soil texture and cation exchange capacity.  An example of this would be a high clay soil with high organic matter content which results in a high buffering capacity.  In a soil like this, it would be very difficult and expensive to change the soil pH.

Parent material can exert a strong influence on soil pH.  Materials such as shale, sandstone and granite generally produce more acidic soils.  In Western Canada however, soils are generally neutral to basic and resist acidification.  This is in large part due to the limestone parent material.  The soils in Western Canada therefore tend to resist acidification hence are buffered in the basic pH direction.  Saskatchewan has about 45 million acres of cultivated soil, of that 17.5 million has a pH > 7.0 and 1.5 million has a  pH value of > 5.5.  This is a result of parent material and the relatively recent glaciation of the soils.  The general bedrock geology (parent material) can be observed in Figure 1.  The yellow and orange areas on the map indicate areas of limestone parent material.  This band of limestone strongly influenced the distribution of pH values seen in Figure 2.

Figure 1. Bedrock geology of Saskathcewan.  Yellow and orange areas indicate limestone parent material.
Image from Atlas of Saskatchewan, 1969

Figure 2. pH values of Saskatchewan soils.
Image prepared by H.P.W. Rostad, J.J. Kiss, and A.J. Anderson, 1983, University of Saskatchewan

The soils on the east half of the province are dominated by basic and neutral soils while the soils on the western half of the province are neutral to acidic (Figure 2).  When the last glaciation occurred the thickest part of the glacier was on the east side of the province and scraped over the widest band of limestone parent material (Figure 1 - orange area).  Figure 3 illustrates the retreat of the last glaciation in Saskatchewan.  It was the advance and retreat of the glacier that incorporated the limestone parent material into the soil.  The glacier was thinner on the western half of the province.  The limestone band is also thinner in the northwestern part of Saskatchewan (Figure 1 - yellow area), so less limestone was incorporated into those soils.  The glaciation and parent material are the reasons that our agricultural soils are predominantly calcareous and have neutral to basic pH.  Once the lime has been exhausted though, Saskatchewan soils will lose the capacity to buffer against acidification.

Figure 3. Glacial retreat in Saskatchewan.