A new look at soil erosion
By Rosalie I. Tennison
For decades, soil erosion has been blamed on wind and water.
|This potato field in New Brunswick exhibits the signs of eroded knolls where soil has been removed by tillage erosion.
For decades, soil erosion has been blamed on wind and water. After the 1930s, cultural practices changed to reduce the effects of wind and cropping techniques were adjusted to minimize the damage from water. However, recent work conducted by researchers in three major cropping areas, Manitoba, Atlantic Canada, and Ontario, proves that tillage is equally culpable when it comes to soil erosion. Surprisingly, one of the main contributors to tillage erosion is the human characteristic to maintain patterns, such as always cultivating or ploughing in a clockwise or counterclockwise direction around the field.
Research conducted in the potato growing areas of Atlantic Canada, where fields are steeply sloped, shows that the tillage practices necessary to grow the crop and the direction of the tillage used on the slopes, is causing as much erosion as wind and water. “Tillage erosion is the loss and gain of soil that occurs within a field due to tillage practices and, typically, happens when more soil is moved down-slope than up-slope,” explains Kevin Tiessen, a research associate at the University of Manitoba. “While the severity of tillage erosion is usually greatest on hilly land, tillage erosion is showing considerable degradation on crop land in all areas of Canada.” Yes, even the “flat” prairie soils can be degraded, according to the research.
“Tillage erosion isn’t spectacular like wind and water erosion can be, but it occurs little by little over time,” comments Tiessen. “However, the evidence of tillage erosion is all of those eroded hilltops and knolls in the field. Always farming in the same pattern can also increase tillage erosion.” He says the common practice of tilling in circles around the field can eventually create a “bowl” effect in the middle of the field and always tilling from top to bottom on hills drags the soil to the lower areas. With the bowl effect, the soil is moved out of the centre of the field to the edges creating an area of low fertility and collection area for water in the middle. By dragging soil off hilltops, the best soil is left at the bottom.
“We can’t predict how long it will be before our potato production land will become unproductive,” Tiessen admits. “In Atlantic Canada, it is estimated that up to 75 percent of the potato land is at risk to unsustainable levels of tillage erosion, but we don’t know when it will cause an insurmountable problem. So, we need to manage our soils better in order to sustain the industry.”
One of the challenges facing potato growers in Atlantic Canada is the minimal number of rotation crops available. Tiessen says growing potatoes in a two year rotation increased the soil losses due to tillage erosion because of the number of tillage operations involved. The problem is reduced when a three year rotation is practised. A four year rotation is even better, simply because the total number of tillage operations is reduced. In other potato growing areas in Canada, the availability of many rotation options and access to markets for them lessens the rotation factor as it pertains to erosion.
The challenge does not end with changing the direction or the style of tillage or in increasing the years in a rotation because, as Tiessen learned, equipment is also contributing to the problem. He sites three forms of tillage that are typically used to grow a crop of potatoes in Atlantic Canada: primary fall tillage with mouldboard or chisel ploughs, secondary spring tillage using offset disc or vibrashank, and tertiary tillage that includes the planting, hilling and harvesting operations. All tillage operations involved in potato production moved large quantities of soil and have the potential to cause erosion. Of all the types of equipment, there was very little difference in the tillage erosion between the chisel plough, the mouldboard plough, or the offset disc. The greatest possibility for tillage erosion actually occurs in the post-plant hilling and harvesting operations. “The surprising part of the implement study was that by changing from a mouldboard plough to a chisel plough, little difference was made in terms of reducing tillage erosion,” Tiessen comments.
The threat of erosion can be reduced somewhat, he continues, if the depth of the tillage and the speed at which it is being done can be reduced. As well, matching the size of the tractor to the size of the plough will make a difference because a constant speed can be maintained. He says pulling a large plough with a smaller tractor means the tractor labours up slopes and returns down them at a greater speed displacing more soil.
In areas with steep slopes, growers often plan their operation to drive down the steepest slopes because it is easier and, often safer, but that results in soil always being dragged down the hill. “Since tillage erosion moves soil to the lower parts of hills, where water erosion is most severe, tillage erosion is actually a major delivery mechanism for water erosion,” Tiessen explains. “When we repeat patterns, we cause erosion over time.” He recommends breaking up patterns of tillage whenever possible.
Another suggestion Tiessen makes is to reduce operations where possible. “You can’t cut out all operations in potato production, but I would suggest not doing any extra,” he says. In the past, growers often did three hilling operations, but many have cut back to two and some have cut back to one. “Cutting back one operation is hard to do, but it will make a difference.”
There have been no comprehensive studies on the effect equipment has on erosion and Tiessen’s examination only showed there were problems. He suggests equipment designers need to get involved to develop equipment that causes less disturbance which would cut down the chance for erosion.
Tiessen says the study of how tillage causes erosion is still in its infancy. “We are still in the early stages of understanding this issue, but we need to start thinking about it and learning ways to manage it,” he adds.
Certainly, the manner in which tillage erodes the soil is not as obvious as watching the wind blow soil from field to field, but Tiessen’s research shows there is need for concern. While he admits the damage is happening slowly over time, he cautions against ignoring the problem. He would like to see growers begin to make small changes, such as reducing tillage speed and depth, changing the direction and pattern of their tillage operations or lengthening the rotation to slow and, possibly, reverse the effects of tillage erosion. He believes by taking small steps now, it might not be necessary to take big steps later.
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