Soil Parent Material

Amber Anderson; Lee Burras; and Gerald Miller

Soil Parent Material

The loose or disintegrated rock material from which the mineral part of the soil originated is called the soil parent material for mineral soils. The parent material for organic soils is peat, but organic soils are relatively rare in Iowa.

Parent material influences soil texture and all of the properties related to soil texture. Also, the gradual transfer of minerals from the parent material releases nutrient elements that are essential to plant growth. Once released, these nutrients may be cycled many times from growing plants to soil organic matter and back to another generation of growing plants.

Soil parent materials will be divided into six classes for contest purposes. These six classes are:


This is made of ground-up material left by glaciers. Glacial sediments may be divided into till or outwash.

Glacial till can be recognized by mixed textures (loam, silt loam, clay loam) and the likely presence of occasional rocks in the soil pit.

Glacial outwash is generally sorted coarser materials that were deposited from flowing meltwater from glaciers.


Dominantly silt-sized material transported and deposited by wind. Loess is a rather uniform material with little or no apparent layering. The origin of most loess is related to glacial action. The particles were first transported by the ice, then carried by glacial melt water to broad floodplains,

and finally picked up by the wind and deposited as a loess parent material. Loess deposits range in thickness from many feet down to a layer of dust. Loess deposits less than 20 inches thick are difficult to identify because shallow loess becomes mixed with the material it covers and loses its characteristic high silt content. In certain landscapes, loess-covered stream terraces exist. These landscapes are broader and flatter to still have this layer intact.

Soil pit in alluvium, where alluvial deposits are distinct different-colored layers


Sediments deposited by running water. The water usually sorts the material by particle size, and deposits coarser materials where the current is faster and the finer material where the flow is slower. Variable flow rates and shifting currents cause most alluvial material to be stratified into layers and lenses of differing textures.



Disturbance uphill led to instability and erosion. The dark layer at 25 cm is the original A horizon, with colluvium above. This profile has two parent materials (colluvium over glacial till).


Material that has moved down a steep slope with gravity as the pulling force (water usually acts as a lubricant but not as the transporting agent) and accumulated on a lesser slope at the bottom. The movement may be either very fast (a landslide) or very slow (soil creep), but in either event, the colluvium is unsorted in contrast to the sorted nature of alluvium.



Material formed in place by the weathering of bedrock into a disintegrated mass. Weathered bedrock occurring within judging depth will be considered residuum for the purpose of this contest.



This area drained for peat harvest in Minnesota has organic material (peat) over lakebed materials (light layer).

Organic materials that accumulate in bogs, marshes, and wetlands because the wet conditions
slow decomposition.



There are circumstances in which multiple depositing factors contributed to the soil profile presented. When this is the case, select both of the contributing parent materials.


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Soil Judging in Iowa Copyright © 2023 by Amber Anderson; Lee Burras; and Gerald Miller is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.