Maintaining good soil structure is a fundamental aspect of managing productive and fertile soils. In soil, organic matter, mineral soil particles and roots group together to form larger physical units called aggregates. Aggregates start small and eventually form into larger clusters, but both small and large aggregates play a major role in how friable, or easily crumbled. Aggregates are the site where organic matter is stored (sequestered) in soil, so building organic matter is largely related to building good soil structure. Good structure is ideal for promoting good seed-soil contact and germination, development of extensive root systems that allow for optimum water and nutrient utilization, and free water and air movement through the root zone. Several conditions can develop when soil structure is degraded. These include surface crusting, surface and subsurface compaction, and creation of large, unmanageable clods. None of these is desirable for crop production.
Surface soils with relatively low organic matter and high silt concentrations often form hard, impermeable crusts that interfere with seedling emergence and restrict the intake of midseason rainfall. Important agricultural soils, including Blount, Canfield, Cardington, Crosby, and Fincastle, fall into this category. Hard crusts form when raindrop impact destroys weak aggregates at the soil surface, causing the surface to disintegrate and dry into a solid, impermeable mass. Young seedlings may not be able to generate force sufficient to break through the crust and may be trapped and die underground. Maintaining good soil structure helps facilitate emergence and increase stand significantly.
Crops growing on crusted soils may suffer moisture stress if the soil has not stored enough water prior to crusting to support the crop until maturity. On such soils, preventing or eliminating crusting can increase water infiltration and yield, often significantly.
Using a tillage system that leaves crop residue on the surface reduces crusting. Residues reduce direct raindrop impact and promote biological activity at the soil surface, which maintains permeability and promotes greater infiltration (Table 2.1). Increasing midseason infiltration is a major reason why no-till practices increase yields to the extent they do on well-drained soils subject to crusting.
Driving on soils when they are too wet can cause compacted layers to develop in the soil profile. Such layers can restrict crop rooting depths and reduce percolation of excess water following rain. This situation can cause the upper layer of the soil to become saturated more quickly than normal and may prevent roots from obtaining nutrients and water from deeper in the soil during dry periods. Both effects are detrimental to yield. Compaction problems seem to be increasing in Ohio, likely a result of heavier equipment and loads, coupled with warmer winters that reduce opportunities for deep freezing and thawing.
Surprisingly, much compaction occurs when soils are moist, not saturated. Unfortunately, this condition is common in midspring and fall, when timely fieldwork is critical. Because it is almost impossible to avoid working in fields when they are sensitive to compaction, producers need to take as many steps as possible to minimize damage to the soil. These include inflating tires to proper pressure, using float tires, reducing axle loads (particularly during harvest), and limiting the area driven upon (i.e., controlling traffic).
Most Ohio soils contain significant quantities of clay and are subject to smearing and clod formation if they are worked when wet. Working wet soils often can create large clods that may persist and interfere with production throughout the growing season. Poor seed-soil contact (and impaired germination) is a major problem that results from planting into a cloddy soil. Planting into wet soils also may create other unfavorable conditions, including smeared seed furrows that are impenetrable by young roots (sidewall compaction), or furrows that reopen upon drying, exposing the seed and severely limiting germination. Planting always should be delayed until soils are crumbly and good seed-soil contact can be obtained.