Soil Depth
Amber Anderson and Lee Burras
The total thickness of material available for plant root development is important. Plants obtain not only water and nutrients from the root zone but gain mechanical support as well. The thickness of soil above a layer that stops root development is therefore considered to be the soil depth. The limiting layer for root penetration and development may be a hard layer such as bedrock or a strongly cemented layer, or it may be a very coarse material greater than 2 mm. in size such as gravel, cobbles, or boulders that store very little water and nutrients for plant growth. A porous layer such as coarse sand and fine gravel does not stop root development and should be considered as part of the soil depth. For purposes of the contest, a porous layer is defined as a layer consisting of boulders, cobbles, gravel, and very coarse sand that contains voids 1 mm. or larger in size. These voids are not filled with finer sand, silt, or clay material. About 30-70 inches of soil depth is enough for most crops to grow quite satisfactorily. Any soil less than 20 inches deep is likely to be restrictive for most plants. The depth classes to be used for the contest are:
DEEP*: more than 40 inches of usable soil.
MODERATELY DEEP: 30-40 inches of usable soil.
MODERATELY SHALLOW: 20-30 inches of usable soil.
SHALLOW: less than 20 inches of usable soil.
In the case that the soil pit is not clearly visible to 40”, assume the material in the base of the pit continues to 40 inches unless a clear boundary such as bedrock underlies the pit.
Key Takeaways
- The thickness of soil above a layer that stops root development is therefore considered to be the soil depth.
- About 30-70 inches of soil depth is common in Iowa and is enough for most crops to grow quite satisfactorily
- To be considered deep, there needs to be more than 40 inches of usable soil.
- To be considered moderately deep, there must be 30-40 inches of usable soil.
- To be considered moderately shallow, there must be 20-30 inches of usable soil.
- To be considered shallow, there will be less than 20 inches of usable soil.
