Over the past few weeks we have discussed soil properties and how they can be managed or not, what sorts of clues soil colors give us, and how water is held and moves through the soil to meet plant transpiration needs. The controlling factors of soil water supply potential, soil texture, and rooting zone depth are closely associated with soil development, particularly the processes that give the soils their "features". Let's talk about restrictive layers defined as a layer that restricts or reduces the movement of water and penetration of roots. Examples would include bedrock (impenetrable to roots and water), fragipans, claypans, tillage pans, caliche (hardened calcium carbonate) and
Fragipans are soil horizons with a high bulk density, that are reversibly cemented and restrict root penetration. They typically develop over thousands of years in soils that contain silt. The main distribution of soils with fragipans are located in the southern Mississippi River valley and parts of the Appalachian area. Research at the University of Kentucky has demonstrated some promise of annual ryegrass' ability to proliferate roots that penetrate the fragipan and leave pathways as the roots decay. They also saw a commensurate increase in crop yields following the use of an annual rye cover crop.
Distribution of soils containing fragipans (A) and (B)annual ryegrass used as remediation of fragipans as roots have some ability to penetrate the brittle soil layer
Claypans develop over thousands of years due to specific soil forming processes. Older soils, typically formed in ancient glacial till plains, experience a movement of clay particles that are deposited in the upper subsoil. These layers often become dense. They often include shrink-swell clays that when wet prevent movement of water downward. Claypan soils are slow to dry in wet springs as internal drainage is very slow to non-existent. There are few management options to improve the claypan soils. Nitrogen management is key for corn and wheat, as there is risk for denitrification when the soils are wet. Water management (surface drainage) and tillage operations that avoid surface compaction are key to getting good production on these soils.
Claypan soils showing upper-enriched organic matter Ap horizon overlaying the zone of eluviation (clay removal) and then the underlying zone of clay deposition (Bt horizon). The white arrows indicate the width of the clay removal above them is the Ap horizon, and below them the Bt horizon
Tillage pans are best addressed by avoidance rather than mitigation. Soils most vulnerable to the formation of tillage pans are the loamy sands that dominate the Atlantic Coastal plain soils. The tillage pans can reform and re-cement annually. The soils they form in are prone to drought and low native nutrient levels which exacerbate the problem of limited rooting zone that tillage pans create. Subsoiling, ripping, and other forms of deep tillage can create access points for roots to penetrate. A particularly promising tillage program is a subsoil-bedding approach at planting. Roots can get to the clay subsoil before the tillage pan reforms.
Caliche is a dense calcium-based layer that occurs in semi-arid and arid regions. There is no real way to manage these soils.
Plinthite is primarily found in the tropics and is not of great concern to our readership.
Bedrock soils have limitations that can't be fixed. Most soils with bedrock close to the soil surface are left in natural vegetation or cropped with trees or pasture.
Restrictive soil layers can confine rooting volume and alter water infiltration. There is some hope for managing fragipans and tillage pans. Claypan soil crop production takes special attention to how soil-water changes affect nutrient status and rooting.
Coming next week: Life at the C HOPKNS CaFe