In its simplest definition, soil compaction is a form of soil degradation in which soil bulk density increases as the porosity, or space between solid soil particles, decreases when particles are pushed together. Pore space is essential for the movement of air, water, nutrients, soil organisms and plant roots, which is why losing porosity to compaction is such a threat to crop health and yields.
Compaction comes in many forms with many causes, some natural due to interactions of soil type and weather, but most caused by choices in timing, cropping methods, traffic patterns, equipment types and settings used in fieldwork. Unfortunately, compaction is not always easy to spot, literally hiding below the surface as a factor in nutrient deficiency, stress from drought or too much water, and other yield robbers.
The illustration below show the five main types of soil compaction in farm fields.
Surface crusting can trap germinating plants and cause corkscrewed mesocotyls in corn (left). The other two plants made it through the crust, but the purple one shows signs of phosphorus deficiency due to root growth stunted by surface and shallow compaction.
Moderate to intense rain after planting causes fine surface particles to bind together and fill in surface pores. Thick, impermeable crust forms as soil dries. Fast-drying soil and excessive secondary tillage increase the risk of crusting.
Impact: Reduced seedling emergence and crop stand establishment; poor water infiltration; more runoff and erosion.
Prevention/remedy: Increase organic matter. Improve soil structure and increase surface crop residue with reduced tillage or no-till. Break up crusts with rotary hoe to improve emergence and stand.
Tightly packed crumbs and large blocks of compacted soil in the tillage zone restrict root growth and deprive crops of the oxygen needed for nutrient and water uptake.
Wheel traffic compresses soil particles from surface down through normal tillage zone, especially in moist conditions. Settling of tilled soil can also cause shallow compaction.
Impact: Varies, but can harm crop health and yield, especially in wet growing seasons. Compaction usually temporary and can be remediated.
Prevention/remedy: Stay off field in poor conditions and use lower tire pressures. Can partially remediate with normal tillage. Over time, natural processes of freeze-thaw and wet-dry cycles, microbial activity and plant root growth can rebuild structure.
When corn roots encounter tillage and plow pan layers, they’re forced to grow where they can find weaknesses, and end up stubby with few root hairs.
Repeated tilling at the same depth (especially with discs, moldboard plows and sweep-type implements) and wheel traffic can result in a hard layer of subsoil compaction a few inches thick, directly below normal tillage zone (about 6-12 inches below surface).
Impact: Reduced yields in extreme weather years. In wet years: drainage problems, waterlogging and denitrification. In dry years: roots confined by compaction and have less access to deep available soil water.
Prevention/remedy: Complicated, but consider less-intensive tillage while adding organic matter; strip tilling; breaking up the pan with deep tillage/subsoiling (but only when entire tillage depth is dry and the depth of the tillage layer is known); reduce heavy traffic.
Deep subsoil compaction can cause long-lasting areas of dense, unhealthy soil that drains poorly, lacks oxygen and inhibits root growth, plant health and yields.
High axle-weight loads from grain carts and combines can compact soil more than 2 feet down into soil profile, especially on wet soils.
Impact: Poor crop growth and yield for years after it occurs.
Prevention/remedy: Stay off wet fields. Evaluate all traffic practices. Use better weight distribution with larger tire footprints, more axles, tracks. Use controlled traffic lanes, fill carts only on headlands, and don’t fill carts or combine grain tanks to capacity, when possible. Hard to eliminate.
Sidewall compaction, especially as furrow walls dry and harden, can leave nodal roots with nowhere to go but up and down the seed slot. It also can cause furrows to reopen, letting light trigger the plant to start to leaf out before it reaches the soil surface.
Planting in too-wet soils and/or with too much row-unit downforce is a main cause. Disc openers shear into wet soils, making furrow sidewalls hard. Can get worse with shallow planting depth and poor soil structure in tilled fields.
Impact: Poor crop emergence and stunted root development. Crop more susceptible to drought and nutrient stress. Severe sidewall compaction can cut corn yield 50%.
Prevention/remedy: Don’t plant into wet soil. Reduce (or automate) downforce on gauge and closing wheels. Try spiked closing wheel or wheels and seed firmer to reduce sidewall effects. For corn, plant seed 1.5 inches or deeper.
Agronomic Impacts of Compaction
Compaction, especially in fine- and medium-textured soils, takes a toll on growing plants. As a physical barrier, it can delay or prevent seedling emergence (crusting), or cause root tips to be club-like or stunted, with few of the fine lateral roots and root hairs needed to take in water and nutrients. Roots may shoot to the sides, looking for a way through the soil, unable to grow past subsoil compaction and left more vulnerable to different stress factors.
And there’s a more direct impact on plant growth, beyond the physical barriers to healthy roots. In what is thought to be a natural survival mechanism, “a root system that’s up against mechanical barriers sends a hormonal signal to the plant shoot, which then slows down respiration and growth.”*
The loss of porosity also cuts down on the soil’s ability to store ample water and allow the gaseous exchange needed to supply oxygen to plant roots and beneficial soil organisms. That contributes to moisture stress and nutrient deficiencies. If crops have trouble emerging, establishing a robust root system, reaching and taking in nutrients, and even tell themselves to slow down to survive, plant health and yields suffer.
Signs of Compaction
While compaction can reduce crop growth and yield without revealing itself as the cause, there are several visual signs that the condition may be affecting your fields and crops:
Ways To Avoid Compaction
Soil scientists and agronomists agree on the best way to avoid topsoil compaction at planting or harvest time: Stay out of the field if the soil is too wet to plant without smearing or creating sidewall compaction, or too wet to support the weight of the combine.
When it is time to be in the field, a major way to limit compaction damage is to choose tires with a large footprint, inflated only to the minimum allowable pressures for the weight. Central tire inflation systems, such as the VarioGrip™ tire pressure regulation system on Fendt® row-crop tractors and the Load Logic™ weight management system on the Fendt Momentum™ planter, automate the process of inflating and deflating tires to correct pressures for field and transport. Operators save time while making the right choice for their soil, instead of hurting yields by creating new compaction.
Growers trying to get all their acres planted in windows shortened by weather and soil conditions are drawn to the productivity of high-speed, central-fill planters, but until now may have lost significant yield in the center rows due to wheel compaction from large, heavy commodity tanks.
With Load Logic on the high-capacity Fendt Momentum planter, growers can benefit from fewer fill stops for seed and fertilizer without paying a penalty from creating more compaction. The weight management system not only manages tire pressures, but also gives you the choice of spreading the weight equally across all the planter wheels, or concentrating the weight on the in-line tandem central transport wheels to limit compaction to a controlled traffic path.
Having tire pressure regulation on both the Fendt planter tractor and the Momentum planter reduces that first-pass compaction and offers growers a complete compaction management systems approach.
Additional choices soil experts recommend to avoid surface and sidewall compaction at planting time include:
At harvest time, they give similar advice: don’t drive on overly wet fields; use wide tires or tracks; keep trucks out of the field; establish and stay on grain cart paths, ideally same as the combine; don’t till wet soils; use cover crops to help build soil structure while protecting soils from erosion. AGCO’s Fendt IDEAL™ combines offer hydraulic suspended rubber tracks with the largest footprint on every belt size, for maneuverability on any terrain while mitigating compaction.