Perennials in rotation reduce erosion

Conventional vs.organic: A U of M study measured erosion in both systems

By Alison Nelson and Jane Froese,
Department of Plant Science, University of Manitoba

The risk of soil erosion can be reduced by choosing a crop rotation that includes perennials or green manures, according to a recent research study completed at the University of Manitoba. The two-year study by the Department of Plant Science compared soil erosion risk on organic and conventional farms across Canada. Organic agriculture is increasing in popularity in Canada, but little is known about the environmental impacts of this farming system. Organic farms have different cropping practices than conventional farms, such as a greater reliance on tillage and green manures to control weeds. Since they use more tillage than most conventional operations, organic farms have been accused of increasing soil erosion risk.

For our study, we were interested in answering the question: do organically managed farms have a higher risk of soil erosion than conventionally managed farms? We hoped to identify farming practices that could be used by both organic and conventional farmers to protect their soil against erosion. This study did not consider zero tillage, a farming practice known to help prevent erosion since this system includes the use of pesticides to control weeds and this is not an option for organic producers.

Our study looked at soil samples from long-term studies that compare organic and conventional farming systems. To make sure that the results reflected actual farm situations, soil was also sampled from organic farms and their conventional farming neighbours in the Prairies, Ontario and the Maritimes. The soil samples were tested for soil stability, measured by dry and wet aggregate size distribution, and organic carbon content, a factor known to help to protect the soil against erosion and measured using the tube digestion method. First, we wanted to know if organic management does in fact lead to increased soil erosion risk. In fact, we found the differences in erosion risk between the organic and conventional farms to be quite small. The organic systems tended to have slightly lower levels of organic carbon in the soil, probably due to the higher levels of tillage on organic farms. Despite these differences in organic carbon levels, however, there was no difference in soil stability between the organic and conventional farms. This means that although the organic systems did not have as much soil organic carbon as the conventional systems, the organic soils were just as resistant to erosion forces.

Perennials key factor

Our next step was to compare farms that had perennials in rotation versus farms that only rotated annual crops to determine if there were any differences in soil erosion risk. We found that crop rotation had a larger effect on soil erosion risk than how the farm was managed (organically or conventionally). Farms that included perennials in their rotation had over 33% higher organic carbon levels than farms that had only annuals in rotation.

In contrast to comparing the organic and conventional farms (where a difference in organic carbon did not lead to a difference in soil erosion risk), farms with perennial rotations were also significantly more resistant to water erosion. The higher levels of organic carbon will help reduce the risk of soil erosion, as well as improve other soil properties, such as soil tilth and soil nutrient cycling. Having perennials in rotation is a good thing!

These results are not surprising as the benefits of rotating crops have been known for a long time. Next to a total reduction in tillage, crop rotation is probably the most effective tool available to farmers to prevent soil erosion. Perennials and green manures in rotation help to build soil structure and increase organic carbon levels. As more organic farms have perennials and green manures in rotation than conventional farms, the lack of differences in soil erosion risk on organic and conventional farms begins to make sense. While organic farms use more tillage than conventional farms, these farms also tend to have more soil-building crops in rotation.

Our conclusions? Organic systems do not necessarily have a higher risk of soil erosion than conventional systems. Additionally, farms that include perennials or green manures in their rotation, whether they are organic or conventional, are helping to protect their soil from wind and water erosion.

This research was funded by the Natural Sciences and Engineering Research Council and the Organic Agriculture Centre of Canada.

This article first appeared in Farmers' Independent Weekly. The OACC gratefully acknowledges permission to re-publish it here.

Contact
Jane Froese
Assistant Professor of Farming Systems
Department of Plant Science
University of Manitoba
Winnipeg, Manitoba
R3T 2N2 Canada
Phone: (204) 474-6504
Fax: (204) 474-7528