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Deep Tilling

General

Deep tilling (or subsoiling) is a practice that rips the soil perpendicular to the direction of flow. This creates a cut in the soil that results in a series of horizontal ridges and depressions, which slows runoff and encourages infiltration and deposition, decreasing soil loss from a site. In addition, deep tilling relieves soil compaction and increases pore space, fostering plant growth by increasing nutrient availability and water retention.

Deep tilling may be performed on a wide variety of sites to mitigate erosion. However, safety may become an issue on steeper slopes, as ordinary construction equipment may be prone to rollover. In these instances, alternative practices, such as diversions, slope drains, or construction sequencing may be necessary. Because deep tilling does not protect the soil from raindrop impact, it is best used in conjunction with other management practices, such as mulch, erosion matting, and seeding.

Advantages

  • Cost-effective
  • Prevents sheet and rill erosion
  • Fosters vegetative growth
  • Effective way to mitigate soil compaction
  • Low maintenance

Disadvantage

  • May be difficult on sites with slopes steeper than 3:1
  • Low practice efficiency

Equipment

The effectiveness of deep tilling depends on the type of equipment used. Heavy, tracked machinery is used to pull heavy steel shanks through the soil, perpendicular to the slope. Tracked machinery is used because it provides the stability, horsepower, and weight required to achieve a depth of 2-3 feet. Other types of equipment, such as large tractors or other large wheeled vehicles may experience difficulties when attempting to till to these depths.

Generally, the vehicle will pull 2 shanks, which are usually parabolic in shape, positioned behind the tracks of the vehicle, and should be spaced 4-5 feet apart. Parabolic shanks, in addition to creating larger ridges than straight shanks, generally require less power to pull and are better suited for this purpose.

Soils

The length and size of the shank used will also depend upon the type of soil and the depth of compaction, as soils should be ripped at least 1 to 2 inches below the hardpan layer or compacted zone. Soils should be tested with a soil probe or soil penetrometer to determine the extent of compaction and the required depth.

Deep tilling should be practiced in dry soils, as it more effectively breaks up the soil and leaves larger ridges on the surface. Deep tilling in wet soils, although it generally is easier and requires less power, often results in minimal soil fracturization between the shanks.

Construction and Maintenance

  • Shanks should be spaces 4-5 feet apart
  • Must be performed on the contour
  • Areas that have been deep tilled should be inspected after each storm event for signs of erosion, with necessary repairs made immediately

Method to Determine Practice Efficiency

Deep tilling prevents soil loss by reducing the flow velocity of runoff. The efficiency of this practice is dependent upon the depth of tillage and the height of the ridge that is produced by the shank. Efficiency is also dependent upon site disturbance. Any disturbance that may cause compaction, such as vehicular traffic, greatly reduces the efficiency and requires that the practice be repeated. However, when properly performed, deep tilling yields an efficiency of up to 20%, which may be taken for the time period between the completion of the practice and the application of seed.