Five Factors Of Soil Formation - University Of Minnesota Extension

Minnesota is a land of geologically young soils with many different parent materials (Figure 1). The common factor among Minnesota soils is that they were formed by the last glacier in the northern United States, 11,000 to 14,000 years ago.

This may seem like a long time but is considered recent in the context of soil formation and geology. Figure 1 lists five major parent materials: Till, loess, lacustrine, outwash and till over bedrock.

Till

Till is predominant in the south-central, west-central and southwestern parts of the state. As the last glacier was melting, these materials were deposited.

Soils formed in this material generally have silty clay loam to silty clay textures, many different rock sizes and poor internal drainage. The poor drainage has a large influence on nitrogen management and cultural practices.

Loess

Loess is windblown, silt-sized material deposited after the glacier melted. These silt deposits can range in depth from a few inches to many feet. Soils formed in loess generally have silt loam textures and no rocks.

Most soils formed in loess occur in southeastern Minnesota where the loess deposits are on top of limestone or sandstone. Because of the porous state of the underlying materials in southeastern Minnesota, the soils are generally well-drained.

Loess in southwestern Minnesota is deposited over glacial till. Soils formed in this material are generally poorly drained and behave similarly to soils formed in glacial till. Erosion is a major concern for these soils because of the silt loam texture. Residue management becomes an important factor in maintaining high productivity.

Lacustrine

Lacustrine parent materials result from sediment deposited in lakes formed by glacial meltwater. The lakes existed long enough that the large particles, such as rocks and sand, were deposited immediately after the lake was formed, while the smaller clay-sized particles were deposited later.

An example is the soil formed under Glacial Lake Agassiz in northwestern Minnesota and eastern North Dakota (Red River Valley of the North). Soils formed in lacustrine deposits have clay, clay loam and silty clay loam textures, poor internal drainage and no rocks. Many soils in northwestern Minnesota were formed in lacustrine material.

Outwash

Outwash is material deposited on the edges of fast-running rivers from the melting ice of receding glaciers. This includes rocks, gravel, sand and other materials large enough to drop out of the water flow, as the river current continued transporting smaller particles.

Soils formed in outwash are excessively well-drained and have sand and sandy loam textures. Examples of Minnesota areas with soils formed in outwash include the Anoka Sand Plain, North Central Sands and Bonanza Valley regions in east-central, north-central and central Minnesota, respectively.

Till over bedrock

Till bedrock deposits occur in northeastern Minnesota. Materials from the glacier were deposited over bedrock, similar to south-central Minnesota but with material from different glacial ice.

There are also significant areas of soils formed directly from bedrock. These soils tend to be shallow and aren’t extensively used for crop production.