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No-till systems affect soil structure and the potential for soil compaction in many ways. Some changes with reduced tillage are rapid, such as increases in residue cover, while others occur slowly, such as improved aggregation and increased biological activity. Many changes are positive, but others may have somewhat adverse effects, especially in the short-term. The extent of changes are affected by soil type, drainage, climate, and cropping system. Differences between no-till and other tillage practices also depend upon the type of tillage system no-till is compared to.

A soil with good tilth or structure is one where individual soil particles are bound together in stable aggregates. Aggregation increases pore space, reduces soil density, and helps maintain a healthy balance between air and water in the plant root zone. Tillage can destroy soil aggregates and result in degradation of soil structure. No-till increases surface residue and decreases its decomposition rate. This results in organic matter accumulation in the upper layer of soil and improved soil aggregation over time. Organic matter promotes aggregation by supplying breakdown products such as gums, gels, and other chemical cementing agents as it is decomposed. Binding agents are also produced by the metabolism of soil organisms that use organic matter as a food source. It may take several years for the benefits of no-till on aggregation and soil structure to become apparent.

Increases in surface residue and soil organic matter with no-till stimulate microbial activity and earthworm populations. Bacterial exudates, hyphal strands of fungi, earthworm casts, organic matter decomposition products, and other forms of biological activity are extremely important in soil aggregation and the development of well-structured soil. Earthworms, soil inhabiting insects, other burrowing animals, and channels left by decayed plant roots also can be important agents in improving water movement into and through the soil. Tillage disrupts burrows and old root channels, while no-till permits the development and retention of a semi-permanent network of interconnected pores that improve water infiltration and drainage. Tillage also speeds soil drying, which reduces biological activity, buries the organic matter food sources important for surface-feeding earthworms and other organisms, and destroys networks of fungal filaments and fine roots that help create stable soil aggregates.

Maintenance of residue cover through reduced tillage protects the soil surface from raindrop impact and dispersion of soil aggregates. Aggregate breakdown can lead to sealing of the soil surface and formation of a dense, hard crust as the soil dries. Crusting may prevent seedling emergence and reduce stands, reduce water infiltration, and increase runoff. Rotary hoeing, cultivation, and other tillage operations break up surface crusts, but crusts may readily reform following rainfall on soils subject to sealing. Excessive tillage promotes sealing by breaking up soil aggregates. No-till management, by stabilizing aggregates and increasing residue cover, is one of the best methods for reducing surface crusting.

Soil compaction reduces water infiltration and drainage, limits aeration, restricts root growth, and reduces water and nutrient uptake. Compaction contributes to excessive runoff and erosion, in addition to reducing crop yields. Wheel traffic is the major cause of compaction, but tillage of wet soil and animal grazing are other important contributors. Soil compaction increases soil strength and density, while total porosity and average pore size are reduced. Although events causing extreme compaction leave visual evidence such as wheel tracks and ruts, less severe or cumulative impacts result in compaction frequently being a "hidden" culprit in crop yield losses.

In the long term, no-till results in a soil more resistant to compaction by creating a firmer soil surface and a more stable, well aggregated structure. In the short term, before structure is stabilized, increases in residue cover and wetter soil conditions can create conditions more prone to compaction. In some situations, such as when a wet fall necessitates harvest under wet conditions, some tillage may be necessary to relieve compaction and smooth the soil surface.

Prevention of compaction is much easier than alleviating compaction after it occurs. Compaction increases with soil wetness, and wet soils also transmit pressures to greater depths, so the most important practice to minimize soil compaction is to keep equipment out of fields when they are wet. Short-term losses, e.g. from delayed planting, must be balanced against the long-term losses that can result from compaction.

Improvements in surface and subsurface drainage can reduce compaction by increasing the period of time when a field is at suitable moisture content for the operation of heavy equipment. Providing adequate drainage is especially important for no-till systems, where increases in residue cover reduce the loss of water from the soil surface.

Controlled traffic reduces compaction by restricting it to permanent lanes between rows where tires from all equipment are run. This requires that all machinery has the same tire spacing and all field equipment covers multiples of a specific width. Implementation of controlled traffic patterns may have to be implemented over a period of years as equipment is replaced. Reducing tire inflation pressure to the minimum necessary to support a load, and the use of wider tires, duals, or tracks can all reduce compaction.

Reducing tillage can prevent compaction by reducing trips across the field, in addition to its effects on aggregation and structure. Tillage can reduce compaction and create looser, softer soil conditions, but these same conditions result in a soil that is easier to recompact. If regular tillage is used, altering the depth of tillage can be helpful in breaking up, or preventing the development of, a tillage pan.

Crop rotations that include deep-rooted perennials can alleviate compaction. Deep rooting physically breaks up compacted layers and removes large amounts of water that promotes drying and cracking of the subsoil. Forage cropping also helps prevent compaction by reducing field operations after establishment, generally limiting them to hay harvesting during periods when the soil is dry.

Subsoiling can be used to break up compacted layers in subsurface soil that restrict root growth and reduce water movement. If a plowpan or other dense layer has developed under a previous tillage system, it is most effective to subsoil before making the change to no-till. Subsoiling is only useful when a discrete compacted layer is present above soil that is less dense and more permeable. It is not effective in improving conditions where the subsoil is naturally dense and impermeable to great depth.

Successful subsoiling requires that the compacted layer of soil is dry enough to fracture. If it is too wet, compaction problems may increase due to smearing. Use a soil probe or dig a hole to determine the moisture content and exact depth of the compacted layer. It will stay wet much longer than the surface soil. The subsoiler should be operated at a depth that penetrates completely through the compacted zone, but there is no benefit to subsoiling deeper than 1 or 2 inches beyond the bottom of the compacted layer. The depth and speed of operation determine the amount of residue covered by a subsoiling operation.

The benefits of subsoiling can be quickly lost by as few as 1 or 2 equipment trips across a field. If compaction occurs, it is important to determine what caused it and institute management changes that will prevent further compaction in the future.

For further information contact: Peter Bierman

Research & Extension Associate

Soil & Water Resources