Which factor promotes high pH levels in soils in Southern Ontario?

The promotion of high pH levels in soils is closely associated with parent material that is rich in base cations. Base cations, which include calcium, magnesium, potassium, and sodium, neutralize acidity in the soil. The presence of these cations can increase the alkalinity of the soil, leading to higher pH values. In Southern Ontario, much of the soil is derived from limestone and dolostone parent materials, which naturally contain high levels of these base cations. As these minerals weather, they release these cations into the soil, effectively raising the pH.

While organic matter can influence soil structure and fertility, it generally contributes to soil acidity rather than alkalinity due to the production of organic acids during decomposition. High rainfall amounts can lead to leaching of base cations, which may reduce soil pH rather than increase it. Similarly, humidity primarily affects the moisture in the air and does not directly alter soil pH levels the way base cation presence does. Thus, the parent material rich in base cations is the most significant factor in promoting high pH levels in the soils of Southern Ontario.