How Does Parent Material Determine Soil Texture and Mineralogy?

Parent material is the geologic "raw ingredient" from which a soil is formed. It has a direct and powerful influence on the soil's inherent properties, especially texture and mineralogy, particularly in young soils.

Influence on Soil Texture:

Soil texture refers to the relative proportions of sand, silt, and clay particles.

• Coarse Parent Material: If a soil forms from sandstone or sandy glacial outwash, the resulting soil will be dominated by large sand particles. It will be a sandy soil, which is gritty, drains quickly, and has low fertility.
• Fine Parent Material: If a soil forms from shale (which is compressed clay) or fine lake-bed sediments (lacustrine), the resulting soil will be dominated by tiny clay particles. It will be a clayey soil, which is sticky, drains poorly, and can be nutrient-rich.
• Mixed Parent Material: Parent materials like glacial till (a mix of boulders, sand, silt, and clay) will produce a mixed-texture soil, often a loam, which is ideal for agriculture.

Influence on Soil Mineralogy and Chemistry:

Soil mineralogy refers to the types of minerals present in the soil, which dictates the soil's natural fertility and chemical properties.

• Felsic vs. Mafic Rock:
  • Soils from felsic rocks (like granite) are rich in quartz (silica) and feldspar. They weather to form acidic, sandy, light-colored soils that are often low in nutrients.
  • Soils from mafic rocks (like basalt) are low in quartz but rich in iron and magnesium. They weather to form dark-colored, clay-rich soils that are typically more fertile.
• Calcareous Rock: Soils that form from limestone or chalk (which are rich in calcium carbonate) will be alkaline (high pH). They will be rich in calcium and may have issues with iron and phosphorus availability.

Over very long periods of time, the effects of climate (weathering and leaching) can eventually override the original parent material's influence, but in most soils, the parent material sets the initial foundation.