Soil Development

Soil develop, or weathering, is the amount of change that has occurred since the material was originally deposited. In order to get change, energy is put in, materials are added or taken away. Rainfall moving through the soil may carry away calcium; plant growth may add organic material to the soil; biological as well as physical interactions change the soil structure; clays may accumulate or break down.

Factors

A variety of factors influence the amount of change that has occurred since deposition. Generally referred to as soil forming factors, the following five aspects of a soil’s history significantly influence what it looks like today.

Material

The type of material makes a significant difference in the rate of development. One example is a loose material, like glacial till, compared to a shale bedrock. As the material receives rainfall, water can move into the loose material, whereas the shale will take first breaking up the material before it can start the same process. Water can move most easily through (and therefore change) sands most easily, then other loose materials, then a loosely cemented material (like sedimentary sandstone), and very hard materials (like slate) will be slowest to change, given identical other factors.

Topography

While material may be consistent across an area, the same hillslope will not develop or change at the same rate. Stable upper parts of the landscape will have water moving through them, changing the profile (moving or transforming clays, carbonates, etc), and the organic-rich surface or residues are likely to stay in place. In the steeper slopes, water may run off or erosion may remove the top layer of soil. Additionally, low parts of the landscape may receive deposits from above (erosion) or below (flooding) and then need to start developing or changing those materials. Therefore, holding other factors equal, we see the most development on the stable upper portion of the landscape.

Organisms (including humans)

Organisms can have a significant impact on changes. In former prairie regions, like across Iowa, areas where prairie dominated have or had A horizons of significant thickness with less developed B horizons, whereas areas where trees dominated are more likely to have an O-A-E-Bt or A-E-Bt horizon sequence. Humans drastically alter the landscape and soil processes as well, moving A material, removing cover that significantly increases the rate of erosion, compacting soil (on purpose for structural support or unintentionally), to name a few.

Climate

Since energy to change a soil and reactions require both moisture and warmth, warmer and wetter conditions lead to more development. Changes are slow or non-existent if the soil is in a frozen state-as water can’t move through, biological activity is minimal, and chemical reactivity is generally decreased.

Time

As one might expect, more time since deposition means more time for changes to occur. Therefore, time is a significant factor, with more time leading to more development if other factors are the same.