Timing is important when planting organic potatoes

Potato research continues at the Intermountain Research and Extension Center in Tulelake, with the major objective of the research being to look at the advantages and disadvantages of using a cover crop within an organic potato rotation, and what the impacts are on soil fertility.

Rob Wilson, IREC center director and farm advisor, said they are in the second phase of the project. In the first phase, they grew a variety of cover crops at different times of the year.

"Some were planted in the spring, some were planted midsummer, some were planted in the fall and managed a different way," Wilson said.

This year, Wilson is growing the potatoes where the cover crops were grown last year.

"We're trying to manage them just like an organic grower is. We're not applying herbicides or pesticides to control pests, and so we're trying to see how these interactions actually affect an organic production, not only the potato crop, but weeds and everything else," he said.

Wilson said he will also be looking at the advantages and disadvantages with the different cover crops and amendments that were used.

"There's been a lot of cover crop research that's been done already, but a lot of it's not very specific to our climate and for large-scale potato production," Wilson said.

"Growers have a lot of questions and it's hard to have a treatment for every question, but we're trying to determine what are the true benefits and pitfalls to watch out for," he said.

Some of the questions Wilson is trying to answer with his research include:

•  In a limited growing season, is it better to grow in the spring, summer or fall?
•  What are the impacts other than nutrients—weeds, diseases, nematodes?
•  Are some cover crops more competitive with weeds than others, and will this result in fewer weeds in the potato crop?
•  Is there any reduction in disease or nematode pressure in the cover crops?
•  Do certain cover crops increase disease loads?
•  Is it better to grow a mix, or just grow one species?
•  If growing a mix, what is the best mix?
•  Are there long-term benefits?

Wilson is looking at diseases such as Rhizoctonia and Verticillium wilt and other diseases that are common in potatoes, and not treatable in an organic setting with chemicals.

"That's been one of the problems with organic potatoes, is there are a lot of different diseases that appear on potatoes. Some of them really have an impact on yield, and some of them, like Rhizoctonia and black dot, have a big impact on quality," Wilson said, adding a grower could very easily lose 40 percent of his crop.

"In a conventional setting, there are eight fungicides that work well. In organics, there's nothing, and so if the organic grower gets that disease, there's really no recourse," Wilson said, other than altering the irrigation and other things that don't promote the disease.

"We've had a lot of growers that have had significant losses just because there aren't those fungicide options," Wilson said, and if organic growers used a cover crop that encourages or increases the disease, it could be devastating.

"Those are some of the other things that we're trying to quantify," he said.

To grow a cover crop, a grower would have two options—potentially double-cropping or not growing a crop that year, Wilson said.

This could mean growing a short-season cash crop, then a cover crop, or maybe the opposite. Farmers could grow a green crop, then plant a cover crop in early fall, and grow it through the fall and early spring, he said.

"Growing a cover crop in the fall, in our system, can be a little difficult for it to get adequate growth," Wilson said. "There needs to be enough time to incorporate it into the soil where it can actually break down, provide available nutrients, and reduce the likelihood of disease carryover between the cover crop and the potato crop.

"From an economic standpoint, the best approach would be to grow an organic grain crop, plant something like vetch in the early fall, incorporate it in in early April, and then plant the potatoes in May," he said.

"There are a lot of the different things that we're looking at, and trying to give growers a better picture of what we say, and we've had a lot of growers that have been experimenting on their own," Wilson said.

"What we're finding is that it's important to do the research, but also really do the extensive sampling to know exactly what's going on, because many of the growers have had success and then failures, and can't really explain why," Wilson said.

The problem with the cover crops and amendments is that there are some general principles that organic growers can follow, but it's not as easy as a conventional fertilizer, Wilson said.

The amendments are all in organic form when applied, but then it's a wait-and-see process for them to break down with soil microbial activity into a plant-available form, Wilson said.

"That takes time, and then in our cold soils, that can take more time," he said.

Another issue Wilson has found is that some of the cover crops and amendments that were put on last year looked beautiful in the fall, but then higher than expected precipitation equaled more nitrogen loss over the winter due to denitrification, he said.

"It's just a lot more complex," Wilson said.

"I think for the small-scale grower that's doing these things, growing organics on the small garden setting, the economics don't get that much in play, and they can do a lot of things like just composting, and it's affordable," he said.

But for an organic potato grower farming 160 acres, the cost of failure is a lot higher, Wilson said.

(Kathy Coatney is a reporter in Bend, Ore. She may be contacted at kacoatney@gmail.com.)