Soil crusts form a barrier at the soil surface – against oxygen and water from above, and emerging seedlings from beneath. A surface crust is more hard, compact, and brittle than the soil beneath it, which can be loose and friable. Certain soils are more prone than others, and some management factors worsen the problem. Soils that are weak in structure for a variety of reasons or lack protection from cover crops and residue often crust more readily.
Soil texture, organic matter content, and sodium content are all soil characteristics that influence crusting potential. Fine textured soil, such as high silt or clay soils, are especially vulnerable to becoming cement-like when dry. Surface crusts tend to be thin and weak – not as much of a problem – on coarse-textured, sandy soil.
Soil with low organic matter tends to have poor structure and weak aggregates. The more soil aggregates break apart, the more individual soil particles can bind to one another and plug up pore spaces, sealing off the surface and creating a crust as it dries.
Soils with high sodium and low calcium levels are also at greater risk. High-sodium soil particles are more readily dispersed by rainfall or irrigation.
Dynamic factors are more easily shifted by management choices from one year to the next. Management practices that deplete soil and leave it bare are risk factors, including excessive tillage and no cover cropping. Disturbance of soil with tillage destroys organic matter and soil structure, creating risk factors for crusting – despite the fact that they may create a smooth seedbed first. Removal, harvest, or burning of crop residue leaves the bare soil surface to heat up and dry rapidly in direct sunlight, making crusting more likely.
In the short term, field operations in wet conditions create high crusting risk. The water within pore spaces lubricates particles, and they pack together more easily. Then when hot and dry conditions arrive, the soil dries out rapidly, and the particles are left compacted against one another. But the extent of crusting depends on how well the particles can be compacted. Soil characteristics like low organic matter, high silt content, low surface residue, and small aggregate size make for more easily compacted particles.
High intensity rain on exposed soil can also play a role in breaking down soil integrity and creating crusts – especially if soil health is poorer to begin with. Dr. Sjoerd Duiker at Penn State explains, “When the soils are weakly aggregated, the impact of drops causes the fine particles to fill in the pores between the large particles.” When the fine particles block pores and seal surface, the soil crusts over as it dries. The thin surface layer cements into a crust with low permeability and high tensile strength.
Soil crusting causes a cascade of negative consequences. Poor water infiltration and restricted seedling emergence are the two most obvious. Soybean hypocotyls can be broken while pushing up against a solid crust. Oxygen diffusion into soil can be reduced by as much as 50 percent if the crust is wet, and there is less overall gas exchange between soil and atmosphere. The increased runoff and water erosion brought about by the sealed surface can mean reduced infiltration of water for plants. The surface reflects more solar energy, keeping the subsurface soil cooler. This can be a problem for late spring and early summer crops like corn, which tend to be more successful the faster they can germinate. If there is a silver lining, it would be that the crust is less prone to wind erosion and is a barrier for evaporation.
Shallow rotary hoeing or row cultivating is considered the quick fix. Scout fields and make sure the crust is actually sealing the soil before going in to remedy it. Take a break in hoeing to check for stand loss; slow the field speed if needed to be less aggressive. The decision to use the rotary hoe involves weighing the lesser of two evils – whether more damage would result from the rotary hoe or from leaving the soil crust as is. A penetrometer can be used to measure the resistance and thickness of the crust.
Since an ounce of prevention is worth a pound of cure, think about how to protect the soil surface long term, for as many days of the year as possible. Continuous no-till, even “most of the time no-till” keeps depositing organic matter at the top. Work to build organic matter in as many ways as possible, with manure, compost, cover crops, and crop rotation. Cover cropping boosts biodiversity in the rotation, which builds the biological health, resilience, and structure of soil.
Try to leave the soil surface a little rough after any tillage. A smooth, firm seedbed is great for small seeds, but it’s best avoided when planting larger seeds into soils prone to crusting. Also, reduce packing pressure in wetter conditions; the compaction can increase crusting risk.
In a field with known crusting problems, seeds planted closer together (increasing row spacing to maintain the seeding rate) can push through crusting with greater joint force. Or, simply increase seeding rates to capture some of this force and make up for losses. Try to avoid deeper-than-necessary planting, so the seed can emerge before soil has a chance to crust.
Because there are many factors that contribute to soil crusting, the risk of crusting varies largely by situation. Sometimes, the best solution is to just wait for rain to loosen the soil, but before making this call, make sure to evaluate the factors contributing to the crusting and the potential effectiveness of each solution.