Tillage Erosion and Its Interaction with Water Erosion

While we are all familiar with wind and water erosion, tillage erosion has been something that has been ignored for a very long time. Tillage erosion is the downslope movement of soil due to gravity. It does not occur in fields that are completely flat, but is the most important cause of soil movement in sloping tilled fields. Tillage erosion redistributes soil from high spots in a field to lower areas. The most important effect of tillage erosion is to ‘truncate’ soil profiles at high spots in the landscapes and deposit the topsoil in the lower spots. The clearest evidence of tillage erosion in our state are 'clay knobs'.

Soil profiles in Pennsylvania are often characterized by a silt loam surface horizon underlain by a subsoil that is heavier – clay loam or clay. The topsoil contains more organic matter and is therefore darker colored than the subsoil which often has a bright (reddish) color. Although under natural forest the organic-rich layer is shallow, this organic matter was plowed into the soil and became the plow layer when farmers first took the soil into cultivation. When tillage continues, however, the topsoil slowly moves downslope until it has completely disappeared and the heavier subsoil appears. We are now farming the subsoil.

This soil is typically heavier, contains more stones or rock fragments, has weaker soil structure, high bulk density, lower infiltration rate, lower percolation and water holding capacity, and contains fewer plant nutrients. It becomes more challenging to get a good stand and the crop suffers from greater stress. Therefore, these outcroppings become the lower-yielding areas on the farm. Tillage erosion has been observed on soils of all textures (sandy, silty, and clayey soils). It is worst in rolling landscapes with complex topography containing lots of high and low spots. Of course, the steeper the soil, the more severe tillage erosion is.

Tillage erosion has become a greater focus of soil conservation research because continuous annual crop production has become more common (less sod in the rotation) and because more powerful tractors can pull tillage equipment faster. The type of tillage is also important – the higher the drag, the greater the amount of soil moved, while greater depth and faster speed also increase tillage erosion. The number of passes also matters – it has been shown, in effect, that the third tillage pass can do the most damage by dragging the loose soil downslope. Finally, the direction of tillage impacts tillage erosion – with downslope tillage doing the most damage. However, any type of soil disturbance causes some tillage erosion in undulating landscapes because of the physics of gravity. Tillage erosion in itself does not move soil from a field, but it loosens soil and moves it downslope where runoff can more easily transport it. Tillage erosion is the worst on shoulders and upper side slopes, while it deposits soil at lower side slopes and foot slopes. This contrasts with water erosion, which is typically greatest at the lower side slopes and foot slopes.

In fact, there is an insidious negative interaction between tillage erosion and water erosion. First, the reduced soil health on the higher spots of the field causes lower infiltration and therefore more runoff. The reduced productivity of the outcroppings also results in less soil cover and because less crop residue is returned to the soil the organic matter content becomes very low – further decreasing infiltration and increasing runoff. Because tillage loosens soil and deposits it where water erosion is greatest, it increases the rates of water erosion and sets us up for losing more soil from the field – this is soil that will be forever lost.

Now the best way to reduce tillage erosion is to stop tillage. You can also manipulate tillage practices like plowing on the contour, using tools that drag less soil, cultivating more shallow and slower, and using fewer tillage passes. Other practices that help reduce tillage erosion are increasing sod in the rotation, and using organic amendments such as manure or compost. Nonetheless, it takes a long time and a lot of effort to bring clay knobs back to productivity. Therefore, we need to do anything we can to avoid them from forming by eliminating tillage erosion. The progress we have made with permanent no-till systems allows us now to do so.