Until the 1970s, Maine potato farmers were some of the most productive in the world, growing thousands of pounds more per acre than the rest of the United States.

Since then, though, Maine’s potato yields have remained pretty much flat, at about 300 hundredweight (or 30,000 pounds) per acre — only a little bit more than when Dwight Eisenhower was president.

Meanwhile, in Idaho, Washington and other major potato growing states, yields have consistently increased from less than 200 hundredweight per acre in the years after World War II to around 400 hundredweight today. Amid agriculture’s “green revolution” for commodity crops, Maine potatoes have been something of an exception to the rule.

“We’ve had the same or better work with genetics, diseases, fertility and so forth, but we really haven’t realized the benefits,” said Tony Jenkins, the U.S. Department of Agriculture’s Maine state resource conservationist. “The soils are maxed out.”

Jenkins was among several researchers and farmers at a recent soil health field day in Presque Isle, organized by the Central Aroostook Soil and Water Conservation District, which is working to share best practices in soil management that can increase yields and reduce fertilizer pollution.

In recent years, Aroostook farmers have noticed harder, compacted soil that struggles to hold water and requires fertilizer. Farmers such as Matt Porter, who runs 700 acres around Presque Isle, have turned to strategies such as nurse crops, where they grow rye or barley at the same time as potatoes, planting winter cover crops, or rotating fields over several years through potatoes, grains, legumes or radish — all to build soil layers with plant matter that feed worms and microorganisms and, in turn, generate soil nutrients and supports.

“We’re hoping we can spend less money and have better soils,” said Porter, who is participating in a nurse crop initiative by McCain Foods called the Drive for 45, aiming to help Maine and New Brunswick farmers increase yields by 45 hundredweight per acre.

It’s a long-term, multigenerational goal, said Porter. “I have young kids and I’m starting to look out in the future, and hoping that I can do a little so it’s better for them.”

Farmers have long known the importance of composting plant and natural waste matter or using “green manure” from cover crops. “Feed the soil, not the plant,” is an adage cited especially by organic farmers.

Recently, though, new research has quantified some of the potential benefits. A 2013 study by USDA researchers in Orono found that green manures, fall cover crops and rotations were associated with increased potato yields of as much as 40 percent and up to 58 percent reductions in soil-borne diseases.

A group led by USDA plant pathologist Bob Larkin concluded that a three-year cover crop rotation can reduce potato diseases, by helping “break the host-pathogen cycle,” and that moisture from added compost limits the need for irrigation in the peak August summer.

Adopting a two- or three-year rotation does mean that farmers aren’t getting money that would have come from growing potatoes in a given acre. But cover crops such as barley, canola and oats can be sold, and those fields that are being devoted to potatoes may end up with more spuds than previously.

In the long run, “you can get a few more bags per acre, less input cost, and the environmental benefits as well,” said Jenkins.

The evidence has inspired USDA scientists to study and promote cropping practices for soil health, and prompted others to try to help farmers better understand their soil.

“Find different ways to measure this, and you’ll start getting different ideas of how you can manage it,” said Will Brinton, the CEO of Woods End Laboratories and a soil scientist at the University of Maine Cooperative Extension, who has developed several soil health tests available to farmers and labs.

“You don’t need to necessarily revolutionize all your methods, but by making some small changes, like a winter cover crop, over time, the system is changing.”

With a cycle of crops, the soil system can contribute to its own management. Earthworms and microbes feeding on decaying roots from cover crops “work to make the soil deeper and spread,” Brinton said. “Earthworm canals are highly enriched with carbon, nitrate and available phosphate. The worms are there because the litter is there from extra cover cropping, and roots can penetrate deeper and deeper because the worm canals are present.”