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Soil compaction occurs when mechanical loads are applied to susceptible soils. Water in the soil acts as a lubricant between soil aggregates, allowing them to become tightly packed together when pressure is applied. Coarse textured soils and those with high levels of organic matter are less prone to compaction. Medium and fine textured soils typically have a greater moisture holding capacity, are slower to dry, and are more vulnerable to compaction.
Sidewall compaction refers to all soil compaction and soil smearing in and around the seed furrow and occurs when planting operations take place on wet soils.1,6 Furrow openers can smear the soil on the sidewall of the furrow and effectively seal it, creating a barrier to seedling root growth. When the seed slot is properly closed, the sidewalls should be fractured around the seed, providing good seed-to-soil contact. Press wheels set with too much down-pressure to close the seed slot tend to over-pack the soil. If the seed placement is too shallow relative to the press wheel positioning, this packing occurs below the seed, again causing difficulty for root penetration.
Sidewall compaction can cause poor seed-to-soil contact. Often, shallow placement of the seed is associated with sidewall compaction. Consequences can include: reduced germination and poor stands, uneven emergence and growth, restricted root growth, and stunted seedlings (Figures 1 and 3). Plants with restricted root growth often show symptoms of nutrient deficiencies, even in soils with adequate soil test values, as the roots are not able to intercept enough nutrients. Nutrient deficiency symptoms can become more pronounced as seedlings reach the V3 growth stage. At this stage, the kernel reserves are depleted and the plant begins to rely on its nodal root system for nutrients to sustain development. Often, the seed furrow is not completely closed when sidewall compaction occurs. If dry conditions develop after planting, the germinating seedling and its early roots may suffer from inadequate amounts of moisture. Longer-term effects on yield potential may be possible in corn plants subjected to sidewall compaction. Floppy corn syndrome is often associated with sidewall compaction and shallow planting because the roots are unable to grow deep enough to anchor the plant.3
To verify possible sidewall compaction, use a spade to carefully remove one side of the seed slot to a depth of about 3 to 4 inches. Corn roots will be found following the seed furrow down the row (Figure 2).
Signs of sidewall compaction are flattened roots with a proliferation of secondary roots growing horizontally along the planter trench. Evidence of the furrow opener smearing the soil on the sidewall and sealing it may also be found.
Planting when the soil is too wet is the cause most commonly given for sidewall compaction. Soils are generally considered fit for field operations when soil from a 3- to 4-inch depth is formed into a ball that will fracture easily when dropped, or will not form a ball at all. Another technique is to press soil between your thumb and fingers in an attempt to form a soil ribbon. Fit soil will crumble and will not form a ribbon of any significant length. If the ribbon reaches 3 inches or more before breaking, the soil is probably too wet to plant.6 While planting, make sure that soil is not building up on the rubber closing wheels, which would mean it is too wet, and check the sides of the seed furrow for a smooth, shiny appearance, which is a sign of soil smearing.
Planting at a depth that is too shallow can also contribute to sidewall compaction.4 Depth gauge wheels set with too much down-pressure may compact the soil beyond what the closing wheels can fracture. Applying too much down-pressure on seed furrow closers to get the seed slot to close may also contribute to unnecessary compaction. Down-pressure on both the row unit’s depth gauge wheels and the press wheels should be reduced if planting into wet soil conditions. If there is good seed-to-soil contact and the seed slot remains open, there are other devices that can be used aside from standard closing devices or press wheels. Some examples are coulters, intermeshing row cleaners, and numerous types of closing wheels. In heavy, wet soils, seed slot “pinch65533;? from angled closing wheels can also be a problem. This happens when the soil in the seed slot dries, shrinks, and opens up the slot to expose the seed. A suggestion is to remount the wheels one ahead of the other, which causes the second wheel to move soil over the first wheel’s path.