Rill and gully erosion is the detachment and transport of soil by concentrated flow of water. Rills are small enough to be removed by normal tillage operations. Once they become deeper than about a foot, and too big to drive across with a tractor, they become gullies.
Reports are coming in of rill erosion on fields with little crop residue — especially in soybean stubble or bare corn silage ground. The problem is increased if the subsoil is frozen but the surface starts to thaw because the water cannot infiltrate. The surface soil turns into a liquid, and soon after water starts to puddle, runoff is the inevitable result.
If the soil is bare and unprotected rill erosion can become a serious problem, especially if slopes are long. This is especially worrisome in long-term no-till fields where these concentrated flow paths will grow over time and turn into gullies. This is why it is so important to manage your soil so that the chance of rill erosion in the winter is reduced.
Key points are:
- Leave soil completely covered. This means no residue should be removed in the fall, or a cover crop should be planted early enough to provide full cover going into the winter to protect soil against rill erosion throughout the cold winter months and early spring.
- Employ living root systems to hold soil in place. Cover crop or economical small grain crop roots are especially effective in keeping soil in place. It is highly preferred to use no-till methods to establish these crops in the fall to avoid loosening the soil and reducing its residue cover over the winter. Make sure the cover crops are established early enough to provide 80% soil cover and a root system that will hold soil in place.
- Leave soil very rough and covered with residue until shortly before planting if you do use tillage in your system. Chisel plowing with straight shanks and leaving the soil very rough helps create many pockets that hold water, giving it more time to infiltrate before running off. This practice may have limited applicability on slopes steeper than 8% due to increased risk of break-through that now creates deep incisions in the field.
- Plant on the contour. Runoff is slowed down by the closely spaced plants in the row. On the other hand, runoff often flows between the rows if they are planted up-and-down the slope.
- Drain perennial wet spots. Drainage helps soil stay aerated so that cover crops can grow there. It is not uncommon to see cover crops die in wet spots which then become sediment sources later on. Check with USDA-NRCS that the area is not designated as a wetland or you may lose farm payments.
- Plant grassed waterways in areas of concentrated flow. This may be the solution where concentrated flow customarily occurs, for example where seeps exit the soil on hillsides, or near a road which dumps runoff into a field. A grassed waterway is a natural or constructed vegetated channel that is shaped and graded to carry surface water at a sub-erosive velocity to a stable outlet that spreads the flow of water before it enters a vegetated filter. The disadvantage of this practice is that it takes cropland out of production.
Soil erosion is still our No. 1 enemy in agriculture — it causes soil productivity to decline and pollutes our surface waters. Keeping soil in place should be first on every producer’s mind but it involves planning and preparation because once runoff starts it is too late and one is left with only remediation as an option.