It might be time to add a little spice to the potato rotation.
Researchers from Washington State University have been planting mustard green manures in the year preceding a potato crop, a strategy some Manitoba agronomists believe might protect the crop and improve soil health.
Many of the arguments for green manures will sound familiar to anyone following the agricultural trend towards soil health.
Biomass, both above and below the surface, is a key element of the practice during the green manure years, Andrew McGuire of Washington State University says. The researcher’s green manure system is practised on 25,000 to 30,000 acres in the Columbia Basin, with wheat followed by a fall green manure, followed by potatoes the following year.
When still green — and, as McGuire notes, high in nitrogen — green manure is plowed into the soil to decompose where it eventually increases organic matter, and therefore the soil structure, water infiltration and microbial activity.
The last, in particular, might be of interest to potato producers, a crop that by its nature involves high soil disturbance. Tillage is an often-cited issue when it comes to growing soil microbiology, although McGuire has noted increased microbial activity in his potato green manure plots, despite the disturbance from growing potatoes.
That microbial activity may play into yet another possible benefit, and one that’s attracted the attention of Manitoba agronomists: verticillium control.
While the pathogen is a noted problem for canola growers, McGuire’s studies have tagged mustard for biofumigation, or the production of compounds unfriendly to soil-borne pests, including verticillium.
The researcher found little yield difference between plots that were chemically treated and those that directly followed a green manure over three separate trials.
Those benefits, however, are less sure a few years down the road. A one-in-four-year green manure showed inconsistent results, something that adds an economic burden in Manitoba, where green manures typically take a field out of production for a full year tempting farmers to consider stretching out the period between green manures.
“You may be OK every four years or you might not,” McGuire said.
Spreading chaff will also be key, he added, not only for nitrogen considerations in the residue leading up to the green manure, but because chaff rows tend to suppress the mustard following it.
Mustard green manures in potatoes are not totally novel in Manitoba, according to Zack Frederick, agronomist with Manitoba Horticulture Productivity Enhancement Centre Inc. (MHPEC), but attempts have been sporadic and there are still bugs to work out to adapt the practice.
Some of those bugs are literal. Flea beetle pressure is higher in Manitoba compared to Washington, where canola is not the powerhouse crop that it is here.
That pressure adds a new wrinkle to bringing in mustard as a green manure, according to Frederick. Producers would have to put more effort into management.
Manitoba’s shorter growing season also sets it apart.
Producers may balk at losing a productive year for the sake of a green manure, unlike trial areas in the U.S., where the longer growing season means the green manure takes up only part of a season, and the field can still produce.
“It’s not a cheap crop to grow, because you fertilize it, you water it, you put a herbicide on it to take out volunteer wheat or whatever,” McGuire said. “So, I think there’s going to have to be some economic calculation of, can we grow potatoes every other year if we’re doing the mustard for that in-between year?”
There must also be time for that green manure to grow. In Washington, mustard is planted in August. Manitoba’s earlier winter, however, means less time for that green manure to gain biomass.
Producers may also run into trouble if they attempt to salvage the year by underseeding mustard into the wheat, and therefore ending the season with a mustard cover crop after wheat is taken off.
“You really want the wheat to dry down and you’re still trying to keep that mustard alive and vegetative,” McGuire said. “I think it tends to stress the mustard and then it tends to bolt or flower and you’d lose the vegetative biomass production.”
Looking to adapt
Frederick is currently working with Manitoba’s Crop Diversification Centres to adapt the practice.
Manitoba’s specific planting and incorporation dates have yet to be ironed out, he said. A crop such as fall rye leading up to the mustard crop might help limit the economic hit, since the cash crop is taken off early with room in the fall for a green manure.
There are other crops than mustard for a potential green manure, he said, although the tough survival structure of verticillium makes mustard a front-runner when it comes to biocontrol.
“The benefit of the mustard is we already know that specific system works,” he said. “It doesn’t mean that there are others that don’t exist, but they would have to produce actively toxic compounds to kill (verticillium). There is the possibility, if you’re incorporating green plant matter and you have a microbial flush, verticillium is a very poor competitor with microbial life, so if you have that microbial flush, it is entirely possible to arrest the growth of verticillium.”
“Arrest,” however, does not mean kill, and the producer would have to carefully maintain that control, since verticillium can survive in the soil for well over a decade.