soil

Soil

Soil - one of the best by-products of weathering!

A typical soil will consist of the following:

Mineral Matter 45%
- Sand - a byproduct of mechanical weathering. It is pretty much what is left behind after a chunk of granite has been weathered. Sand is nothing more than quartz - pure silica. Quartz is very resistant to chemical weathering but is constantly being broken down to smaller and smaller pieces by mechanical processes.
- Silt
- Clay - Both silt and clay are the byproducts of chemical weathering of granite. Many types of feldspar release silt and clay when attacked by acid rain.
Organic Matter 5%
Water 25%
Air 25%

Permission from U of Nebraska

There are many different types of soils. Soil scientists classify soils by there relative percents of sand, silt, and clay (see image below).

Permission from U of Nebraska

We (in Southeast Wisconsin) have a rather high silt content. Click here to see a soil map of Wisconsin.

The different types of soil result from differences in:

Parent minerals (local bedrock or transported material)
Climate
- temperature
- humidity
- rainfall
- vegetation (which can hold the soil as well as provide organic matter for future soil)
- animals (which eat the vegetation as well as provide organic matter for future soil)
Slope (good soil is usually not found on steep slopes but is usually washed to lower, flatter areas)

The vertical profile of soil indicates the composition changes with depth. These "layers" are called horizons. Typically the upper layers are richer in organics (which decreases with depth). However, mineral composition usually increases with depth. Burrowing animals and root action help "mix" the composition between layers.

The Midwest is considered the "nations bread basket" because of its rich soil and moderate climate. One contributing factor is the supply of a wide variety of minerals provided by glacier weathering and erosion (which formed over Wisconsin several thousand years ago).

Soil should be valued! When it is gone, it takes several thousand years to re-generate. The famous "Dust Bowl" in the 1930's (and clear cutting of forests...which leads to soil run-off) is a reminder that we must treat soil as a valuable natural resource.

You can bet that soil scientists play an integral part in helping farmers manage their soil to insure that crops have the proper nutrients as well as strategies to maintain the soil for future generations.