Call in a nurse. A “nurse” crop that is

A second crop planted along with potatoes protects the soil and boosts yields

Recent years have seen increased attention on the health of the soil used in potato production, and attempts to bring potatoes into longer rotations with other crops. In the Maritimes, seed companies are promoting soybeans or corn as rotational options and McCain has developed one- and two-year multi-species cover crop blends to help boost organic matter and limit erosion.

Sheldon Hann, a biologist with Agriculture and Agri-Food Canada (AAFC) in Fredericton, says there is a direct correlation between organic matter and crop production — the higher the level, the higher the yield. The challenge across much of Eastern Canada is that some rotational crops such as soybeans and dry beans don’t add much residue to the soil. Aggressive tillage on sandier, porous soils with potato production in the Maritimes also makes it difficult to maintain organic matter.

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That’s prompted an interest in “nurse” crops planted along with potatoes. In 2015, Bernie Zebarth (now retired), one of Hann’s colleagues in Fredericton, began a multi-faceted project to find opportunities to lengthen rotations, improve soil health, improve yields and enhance biodiversity.

“My portion of the project was to run the nurse crop trials. We came up with the idea of running these companion crops planted with the potatoes and when we hill the potatoes, the process kills those crops.”

The project ran for three years with one-year and three-year trials. The three-year study was done with field pea, winter rye and a control. Each was planted just ahead of the potatoes, with two different kill treatments — one physical (hilling the potatoes) — and the other using a desiccant before hilling.

“We’d go in two days prior to planting and plant a nurse crop using a grain drill at two different seeding rates — high and low,” Hann says. “We went in two days later with the two-row potato planter, and planted the potatoes right into the nurse crop, and the nurse crop was allowed to emerge out on to the soil, growing for about 20 days before we’d hill the potatoes. Then we implemented the two kill treatments, where a third of those plots received a herbicide spray which desiccated the nurse crop and then was mechanically hilled with the one-pass hiller.”

Hann found that the field pea nurse crop didn’t need a spray treatment since the mechanical hilling killed the crop. Winter rye, a denser, more aggressive species, required desiccant. Hann says if the winter rye remained it would act as a weed and compete with potatoes for nutrients and water.

“We also ran those two different seeding rates and found that if you use a higher rate of field pea, the potato yield response was better than a low rate. With winter rye, the higher rate didn’t show that increase in potato yield compared to the low seeding rate. That’s telling us that there’s a competitive effect where if you use a higher rate of winter rye, you’re going to get competition with the potato plant but with the lower rate, you’re not going to have that same yield effect on the potato crop.”

Hann also tested a one-year trial with winter rye together with spring barley to measure the difference between two cereals versus one legume and one cereal.

“The winter rye seems to be a little more aggressive a plant and when we put it in with the spring barley in the second trial, we just did one lower seeding rate for both the rye and barley crops,” Hann says. “There’s an increase in yield of the potatoes, but it wasn’t significant in three replications.”

What’s next?

Hann wants to expand the research and grow potatoes to a two- or three-year rotation with a grain or a forage-and-grain option to determine whether there’s an increase in biodiversity, and what happens beyond boosting yields. What happens to the soil or plant-soil interactions?

He would also like to study multi-species blends in rotations. He acknowledges the launch of the one- and two-year blends by McCain Fertilizer and wants to know their effects on the soil’s microbial communities and the plant’s interactions in the soil. It’s one thing to have a single species as a nurse crop, but Hann would like to know the effect of a multi-species blend, particularly on the effects of hilling versus desiccants. He’d also like to test the overall effect on productivity and soil health if the multi-species blend is planted as a cover crop instead of a nurse crop.

Hann says a nurse crop adds costs for seed and running a planter but no additional inputs are required. He drilled-in the nurse crops, which were able to use some of the fertilizer applied to the potatoes.

“In Bernie’s study, one of his main findings was that a relatively small increase in tuber yield would be sufficient to justify the extra costs associated with growing the nurse crops,” Hann says. “They would provide that extra protection of soil and when you plant potatoes, you have nearly a month of bare soil, and the nurse crop is going to provide that cover, provide that interception and minimize soil erosion.”

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