Discing a field is a practice of secondary tillage, often performed after primary tillage like plowing, or sometimes used alone for lighter soil preparation. The implement is a disc harrow, consisting of concave metal discs arranged in gangs. These discs cut into the soil, lift a portion, and invert it as the implement is pulled across the field.
The physical interaction involves cutting, crushing, and mixing, which breaks up larger soil clods. This process generally achieves a shallow to medium depth of mixing, often penetrating 10 to 15 centimeters (4 to 6 inches). This action creates a more uniform soil surface by reducing the size of large aggregates left by previous operations.
Key Outcomes for Field Preparation
The most immediate outcome of discing is the refinement of the soil surface into a suitable seedbed for planting. The chopping and mixing action creates a smoother, finer soil tilth in the uppermost layer. This finer texture ensures optimal seed-to-soil contact, which is necessary for uniform germination and consistent emergence of the crop seedlings.
Discing also plays a significant role in managing previous crop residue, such as cornstalks or wheat stubble. The process cuts up this surface residue and mixes it into the top layer of the soil. Incorporating this organic matter accelerates its decomposition by soil microbes, clearing the surface for efficient planting equipment while simultaneously distributing nutrients back into the soil profile. A single pass typically leaves 40 to 70% of residue on the surface, depending on the crop, which helps mitigate some erosion risk.
Another primary reason for discing is mechanical weed control before the new crop is sown. The implement shears off and uproots small, emerging weeds, effectively destroying them. The disturbance of the soil can also bring dormant weed seeds closer to the surface, exposing them to light and moisture and encouraging them to germinate. These newly sprouted weeds can then be killed by a subsequent, lighter pass of the disc harrow or another tillage tool before the main crop is planted, reducing the overall weed population.
Impact on Soil Structure and Water Management
The turning and mixing of the soil significantly alters its physical environment, leading to both benefits and drawbacks for the soil structure. Discing increases soil aeration by introducing more air pockets, which enhances microbial activity and promotes better root growth. This disturbance also helps the soil warm up faster in the spring, which can be advantageous for farmers aiming for early planting dates.
In terms of water management, discing presents a trade-off: it can break up surface crusts, allowing rainwater to penetrate the soil more easily. However, by exposing moist soil to the atmosphere, the practice substantially increases the rate of evaporation, leading to greater moisture loss from the field. Furthermore, repeated passes, or discing when the soil is too wet, can create a dense, compacted layer just below the working depth of the disc, known as a diskpan.
This diskpan-compaction layer restricts the downward growth of crop roots and impedes the infiltration of water, which can lead to localized flooding or reduced yields, especially during dry years. Most notably, turning and disturbing the soil leaves the surface unprotected from environmental forces. Compared to no-till practices, the bare, loosened soil after discing is highly susceptible to wind and water erosion, which can accelerate surface runoff and lead to the loss of fertile topsoil.