Taking a closer look at how cover crops can help your soil and potato yields.
Editor’s note: This interview has been edited for length and clarity.
Spud Smart (SS): Why grow a cover crop?
Scott Gillespie (SG): The first set of reasons is to hold the soil in place and to hold on to the nutrients. To me, this is something that will always pay. Losing soil costs you money. You may be able to compensate for it by adding more fertilizer, but you lose much more than that… if you regularly have water flowing through the profile, you can lose the mobile nutrients such as nitrogen.
The next set of reasons to grow a cover crop is to increase water infiltration, increase nutrient availability and to lower pest pressures. Water infiltration may be increased within weeks of a cover crop being established, but it’s more likely something that will gain over multiple cycles. Water infiltration is not the total amount the soil can hold. This refers to how fast the water can move into the soil. When you have good water infiltration, you can take in rainstorms quickly and have less runoff.
The final reason of better reasons to grow cover crops is to lower pest pressure. Herbicide tolerant weeds are a common reason that most farmers in the United States got into cover crops, and it is where they have the most success. By covering the land with the best species for the job, they crowd out the undesirable species. It’s not 100 per cent control, but it may allow them to use the herbicides that they do have more effectively.
SS: How can cover crops help reduce expenses on the farm?
SG: I want you to think about something that all potato growers use — fungicides. Even if you’re organic, I’m sure you’ve used an organic approved product. The first application will usually give the greatest return. Think about the week before canopy closure. Early blight may or may not be appearing. But as soon as the canopy closes it has the ideal environment, the humidity will be higher, and the spores will be coming from the soil. Good coverage slows or stops this initial infection and makes the whole rest of the season easier.
If you only sprayed once, this is the one that would give you the most money back. Throughout the season the subsequent sprays should continue to give you more money back than spent on the product and the application costs. Scouting spore traps, weather, irrigation and many other factors will help will play into how many sprays, what products and how often they’re used. The best time to stop is just before the cost is greater than the return.
Shifting back to cover crops, if you can solve an immediate problem, you should get a payback within the growing season. Holding onto your soil and the nutrients pays back right away. Solving a short-term problem should still pay back more money than the money that was spent. It can be tough to see the payback immediately on this as it could take until the next year’s potato year to see the difference. This is why it’s so important to use the right species for the job.
SS: Why is important to reduce nitrate leaching when selecting a cover crop?
Judith Nyiraneza (JN): It’s really important when we assess the benefits of cover crop, to look also at how they contribute to reduce nitrate leaching risk because P.E.I. relies mostly on the groundwater for its drinking water. The report of the commission on nitrate in groundwater had many recommendations. Among those recommendations, there was a mandatory three-year crop rotation system. So, it set it this way that it is recommended that a province wide mandatory three crop rotation in fields and integrated crop production, with no exemptions be implemented.
P.E.I. has observed stagnant potato marketable yield over a decade. That was attributed to combined factors such as disease pressure, drought spells, and soil degradation. So, in the cropping systems, we have limited access to manure and compost, cover crops become important to sustain crop production. There are many trials right now in P.E.I. testing a large diversity of cover crops.
SS: What crops have traditionally been used as cover crops in P.E.I. and why?
JN: The traditional potato rotation system in P.E.I. included barley mixed with red clover in year one, a pure stand of clover in year two and potatoes in year three. The benefits associated with this rotation would be to provide a ground cover after barley harvest during fall and winter and would minimize the soil disturbance by reducing the tillage frequency. In addition, red clover residues constitute a good carbon and nitrogen input.
However, red clover has been a host for wireworm and root lesion nematodes. In addition, incorporating red clover in fall contributes to increasing nitrate leaching over the fall and winter. Right now, there are trials testing a large variety of cover crops to find alternatives to red clover.
SS: Are there any cover crops which help increase potato yield?
JN: Red clover improved certain soil health-related parameters health faster than grasses or brown mustard, but it also increased the soil nitrogen supply capacity and the risk of nitrate leaching. Red clover was not associated with increased potato yield. Mixing legumes with grasses so that the grass is dominant would decrease the risk of nitrate leaching without impacting potential yield.
In a recently published article, compared red clover, sorghum Sudan grass and forage pearl millet returned high biomass, enhanced potato yield, reduced the soil nitrate leaching risk and have the potential to decrease root lesion nematodes.
Cover photo — A cover crop of forage pearl millet. Photo: Agriculture and Agri-Food Canada