By Sjoerd Willem Duiker and Jen Weld
The important soil erosion types in our environment are:
- Sheet and rill erosion
- Ephemeral gully erosion
- Tillage erosion
Management practices such as cover crops, contour farming, and no till can be used to control erosion. However, understanding the different types of erosion can help to determine the best approaches for controlling erosion.
Sheet and rill erosion
The best way to protect soil from erosion by water is to keep it covered. The industry standard is to maintain at least 30% crop residue cover after planting. The only way this can be done is by using no-till or reduced tillage. Reduced tillage is only effective if the previous crop left large amounts of crop residue. For example, when using reduced tillage, it is difficult to maintain 30% crop residue after planting into soybean or wheat stubble. No-till has become the most widely used conservation practice in Pennsylvania because 60% of Pennsylvania cropland is HEL (Highly Erodible Land). No-till is also a cost-effective conservation practice because it does not take any land out of production and is compatible with use of large-scale equipment common in today’s farming landscape.
After crops that leave little residue, it is recommended to plant cover crops for erosion control. The cover crops should be planted immediately after harvest to take advantage of the growing season. Alternatively, cover crops could be established in the standing crop, with methods such as frost seeding or interseeding. Cover crops should be left to grow sufficiently to at least achieve the 30% crop residue cover in the following crop.
Another important soil conservation practice is to alternate summer crops with winter crops and perennial crops. While continuous corn for grain leaves the soil completely protected with no-till practices, continuous corn has many other disadvantages over more diverse crop rotations such as greater risk, weed and pest control problems and lower crop yields. In a central PA trial, the average yield advantage to growing corn after soybeans instead of continuous corn was 7%- and first-year corn after alfalfa or red clover hay had a 15% yield advantage to continuous corn. Further, from the soil health point of view, diverse crop rotations are recommended.
Contour farming is planting crops on or near the contour. It is most effective on moderate slopes (2-6%) when crops are planted in tilled soil where ridge height is 2-3 inches. Even in no-till contour planting can be useful if residue cover is marginal and ridge height is 2 inches or more. The ridges block runoff and allow water longer to infiltrate.
Contour strip-cropping involves alternating strips of perennial grass or close-growing crops with strips with low residue cover. The strips should be laid out close to the contour, which is not always possible in rolling landscapes. Strip width is typically between 75-120 feet. Soil that erodes from the bare strips is deposited in the vegetated strips and runoff velocity is reduced there. The practice is most useful in tilled fields or in those left with little residue in no-till systems.
It is uncommon to change slope grade in Pennsylvania farmlands, but slope length is commonly changed by installing terraces and diversions. A terrace is a cross-slope channel constructed along the contour that is cropped. Storage terraces allow water to infiltrate in front of the terrace or be released through a stable outlet, while gradient terraces allow the water to run across the terrace to a stable outlet like a grass waterway. A diversion is similar to a terrace but wider and higher and usually planted to permanent grass and not used for crop production. Diversions are built typically on steep slopes where terraces are impractical. They can also be used to protect barnyards or farmsteads from runoff.
Ephemeral gully erosion
As explained in You Have Met T Values. What Now? (Penn State) , ephemeral gully erosion is not estimated in RUSLE simulations and not part of the calculation of A (annual soil loss), but its magnitude may be as much as or more than that of sheet and rill erosion. It is also a form of erosion that farmers need to address to be in compliance with their conservation plan. Ephemeral gullies are between 4" and 18" deep, can be erased with tillage, form in the same landscape position, and can become classic gullies if left unattended. Gully erosion can sometimes be addressed by improving soil health so that water infiltration increases to the point that runoff is negligible. If runoff still occurs and the ephemeral gully forms again, other practices may be needed. The most common conservation practice to address ephemeral gully erosion in fields is a grassed waterway to stabilize the area of concentrated flow. Other practices include methods to shorten the slope such as strip cropping, terraces, or diversions. Further, contour planting and contour buffer strips are ways to improve infiltration in-field. Other practices can be used to limit soil from sheet, rill, and ephemeral gully erosion to leave the field and possibly lead to stream water degradation. These methods include field borders (bands or strips of permanent vegetation at the edge of a field), filter strips (permanent vegetation to filter sediment from runoff), and vegetative barriers (narrow strips of stiff-stemmed, tall, dense vegetation established across the area of concentrated flow to keep the area stable).
Tillage erosion
Tillage erosion is the mere displacement of soil in undulating landscaped due to gravity. As discussed in the article Tillage Erosion – a Matter of Great Concern (Penn State) , tillage erosion is probably the major cause of topsoil loss and subsoil exposure in our environment causing significant crop yield reduction. Unfortunately, many practices promoted to control soil erosion by water do nothing to control tillage erosion. For example, research suggests chisel plows cause as much tillage erosion as moldboard plows. Field cultivators run at high speed can also move substantial amounts of soil. Crop residue does not control tillage erosion. Narrow contour strips favor subsoil exposure at the top of the strip and soil accumulation at the bottom of the strip. The best way to control tillage erosion is to stop tilling! If tillage is still part of crop production, eliminate all unnecessary tillage trips, reduce speed, avoid down-hill tillage, turn soil uphill (easier said than done), and run tillage tools at a uniform depth and speed.
Soil erosion by water and tillage erosion are serious threats to soil productivity and water quality in undulating landscapes in the U.S. As we have seen in this article, diverse no-till systems with high residue cover are the best way to guarantee preservation of soil productivity and avoidance of sediment loss to water bodies.
Source : psu.edu