Soil Science

Parent Material

Quartz and Chemistry

Soil parent material is simply the material from which the soil is formed. Mineral soils are derived from rocks or unconsolidated deposits such as alluvium. A glance at any geology map will show that there is a wide variety of soil parent materials. However, much of their influence on soils can be understood by looking at the following two properties:

Quartz content

When a rock weathers, most primary minerals are converted to clay minerals. The main exception is quartz, which is inherited by the soil relatively unaltered and comprises the bulk of the sand fraction in soils. Consequently, the texture of the soil is related to the size and abundance of the quartz grains in the parent material.

 

Chemical composition

Many chemical elements found in soil are inherited from the parent material. The chemical composition of the parent material influences the concentrations of plant nutrients in the soil.


Examples

 

 GraniteBasaltShaleSandstone
Sand-sized quartz contentHighNilLow -ModerateVery high
Nutrient concentration (% by mass)
P0.060.160.070.02
Ca1.306.972.223.91
Mg0.453.801.470.70
K3.341.032.691.07

Look at the properties of the rocks in the table above and answer the questions about the properties of soils derived from them.

 

 
 

Which parent rock will tend to produce soils with the highest clay content??

Correct! Basalt and other basic igneous rocks are composed almost entirely of weatherable minerals, which mostly produce soil clay minerals as they decompose. There is no quartz of any size, and so no resistant sand grains occur in the soil.

Try again.

 

Which parent rock will produce soils of the lowest chemical fertility??

Correct! This quartz sandstone has several features which will cause it to produce poor soils. The dominance of quartz sand is associated with low content of clays or clay forming minerals. The low clay content of its soils implies low cation and anion exchange capacity, low available water capacity, excessive drainage and a potential for leaching. It is low in K and Mg and, most significantly, very low in P.

Try again.

 

 

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