The weeks between harvest and when snow flies can be a good time to evaluate and perform maintenance on terraces. In Kansas, over 9 million acres of land is protected by more than 290,000 miles of terraces, making Kansas #2 in the U.S. for this soil and water conservation practice. To accomplish the goal of erosion control and water savings, terraces must have adequate capacity, ridge height, and channel width.
Without adequate capacity to carry water, terraces will be overtopped by runoff in a heavy storm. Overtopping causes erosion of the terrace ridge, terrace back slope, and lower terraces and may result in severe gullies. Terraces are typically designed to handle runoff from a 1-in-10-year storm. The rainfall amounts for such a storm are approximately five inches for eastern Kansas, four inches for central, and three inches for western Kansas during a 24-hour period.
Terraces need regular maintenance to function for a long life. Erosion by water, wind, and tillage wears the ridge down and deposits sediment in the channel, decreasing the effective ridge height, and channel capacity. The amount of capacity loss depends on the type and number of tillage operations, topography, soil properties, crop residue, and precipitation. Terrace maintenance restores capacity by removing sediment from the channel and rebuilding ridge height.
Typically, more frequent maintenance is required for steep slopes and/or highly erodible soils. Annual maintenance is necessary for intense tillage operations and heavy rainfall runoff. Less frequent maintenance is often adequate with high residue levels or where lower rainfall occurs and runoff intensity is low.
Check for needed repairs
Terraces degrade naturally by erosion and sediment, and can be damaged by machinery, animals, settling, and erosion. Check terraces and terrace outlets regularly (at least annually) for needed repairs. The best time to check is after rains, when erosion, sedimentation, and unevenness in elevation are easiest to spot. Specific items to note are overtopping, low or narrow terrace ridges, water ponding in the channel, terrace outlets, erosion, and sediment clogging near waterway or pipe outlets.
Reshaping the terrace
Terrace maintenance can be done with virtually any equipment that efficiently moves soil. Common tools include those that turn soil laterally, such as a moldboard plow, disk plow, one-way, terracing blade (pull-type grader), or 3-point ridging disk (terracing disk, etc.); those that convey or throw soil (belt terracer, scraper, whirlwind terracer, etc.); and those that push or drag soil (dozer blade, straight-wheeled blade, 3-point blade, etc.).
This article discusses procedures for the common plow. For other equipment, get advice from manufacturers, other users (contractor), or experiment to find what works best.
The primary objective in reshaping the terrace is to move soil from the channel to the ridge. Work done on the terrace back slope or cut slope above the channel may help maintain or improve shape but does little to add significant ridge height or channel capacity. Because of improved efficiency, a two-way (rollover) plow is ideal for terrace maintenance. It can usually achieve the desired shape with fewer passes than the conventional plow. Turn the soil in one direction to counteract erosion or turn it in either direction to clear the channel or raise and widen the terrace ridge.
The number of passes required for maintenance depends on the size of the tool, the depth of operation, travel speed (which controls distance of throw), and the amount of soil moved. The plow throws soil further at higher speeds, so a minimum ground speed of 5 mph in loose soil is suggested, but 6 mph or more is better.
Maintenance controls terrace shape. Assess what needs to be done before beginning maintenance. Compare the existing cross-section shape with the desired shape and size, and determine where soil should be removed and where it should be placed for the desired result. Back furrows are placed where more soil is needed, while dead furrows are located where soil needs to be removed. In this way, passes or sets of passes with the equipment are located to achieve the desired results.
Terrace dimensions can be changed by carefully planned placement of back furrows and dead furrows. Large changes in dimension and shape require several sets of passes with the tools or earthmoving equipment. Plan the terrace cross-section shape and size and terrace slope segment length to fit current and future tillage, planting, and harvesting equipment size.
The number of rounds or passes with maintenance equipment depends on the beginning shape of the terrace, size of equipment, and the desired size and shape. If in doubt, make more passes rather than stop too soon. Remember, the loose soil will settle a lot.
Plowing the ridge. The terrace ridge is raised and widened by plowing up from both sides as shown in Figure 1. When a 2-way plow is used, plow just the front slope from the channel to the ridge. Plowing the backslope makes it steeper.
Figure 1. Double back furrow. Arrow indicates the back furrows meeting on the top of the ridge. Image from K-State Research and Extension.
The back furrows are placed on top of the ridge, and the dead furrows are placed at the desired center of the channel and at the toe or beyond on the backslope. Avoid making a depression on the backslope by varying where the dead furrow is placed. Plowing the ridge is recommended for maintaining or adding ridge height. To make the ridge wider and not so sharply peaked, the back furrows should come together, but not overlap and make additional rounds. Correct a narrow-peaked ridge resulting from too few passes by moving the plow over only one or two bottom widths with each pass. This process requires many more rounds.
To make the terraces slopes long enough to fit equipment, always leave dead furrow the desired distance from the ridge. For the three-segment shape, locate the back and dead furrows in the same place each year, keeping the cross-section uniform in size and shape. Vary the back furrow and dead furrow locations each time to maintain the rounded shape of the channel and ridge for the large smooth section.
Plowing the channel. Sometimes even when the ridge is large enough, the channel can have inadequate capacity. To enlarge and widen the terrace channel, plow out to both sides as shown in Figure 2.
Figure 2. Enlarging and widening the terrace channel. Arrow indicates the two dead furrows meeting at the center of the channel. Image from K-State Research and Extension.
Back furrows are placed on the ridge and on the uphill cut-slope side the same distance from the desired center of the channel. Begin at a distance equal to that from ridge to desired channel center. A double side-by-side dead furrow should result at the desired channel center. Locate the plow back furrow on the ridge and the dead furrows in the desired channel bottom to achieve and maintain the desired shape. Vary the back furrow location to avoid leaving a large ridge on the cut slope.
Plowing out the channel periodically is recommended for steeper slopes to help maintain adequate channel capacity. Alternating between plowing the channel out and plowing the up from one time to the next is a good practice.
Consider conservation agriculture practices to increase terrace life
When silt bars and sediment deposits accumulate frequently in a terrace channel, excessive erosion is the cause. A change in tillage and cropping practices is needed to correct this cause. Adding cover crops to a system, switching to no-till or conservation tillage, and using crop rotations that retain crop residue will reduce erosion substantially. This will reduce the frequency of terrace maintenance needs. Many no-till producers find terrace systems require little maintenance. Although runoff still occurs, there is very little soil movement in a no-till system. Remember, terraces are there to help in extreme weather events, and terraces prevent gullies and are only a part of an overall erosion control plan. Conservation farming methods, especially retaining crop residue or using cover crops, compliments erosion control structures and has been shown to be both economically and environmentally sound.
For more information, refer to publication Terrace Maintenance, C-709 available online at: http://www.ksre.ksu.edu/bookstore/pubs/C709.pdf
Another great resource is this KSRE YouTube video: Basics of Terrace Maintenance: http://youtu.be/CcoITeP9QRA
Additional sources for technical information include your local USDA-Natural Resources Conservation Service and County Conservation District offices.
DeAnn Presley, Soil Management Specialist