Editor’s note: This is the fourth story in a series exploring opportunities and issues facing potato growers across the region. Part two of this installment runs tomorrow.

When it comes to crops, potato is king for Flying H Farms of Mountain Home, Idaho.

Though co-owner Jeff Harper talks passionately about raising spuds and other crops—everything from alfalfa to spearmint—he wants to focus this particular discussion on composted manure and what it has done for his farm’s productivity.

BROWN GOLD: Composted manure, which benefits agricultural fields in a variety of ways, is spread on a farm near Mountain Home, Idaho, owned by Jeff Harper and his family as part of a University of Idaho research project.

“Manure has a great sustainability story behind it, one that I believe all agriculture needs to talk about with the public. It’s a story we need to get out to the consumer,” Harper says. “You can draw a cycle between crops growing in the ground, and those crops going into milk cows, and the cows turning those crops into milk and manure.”

The manure is then composted and goes onto the ground, adding valuable organic matter and putting back what the crops took out, including phosphate and potash.

“It’s very much a cycle. It’s very organic. It’s very healthy. You’re giving back what you take, and this is a good message for everyone involved,” Harper says.

That is only the beginning of the story for farmers like Harper who happen to live close to dairies, feedlots and other operations that churn out large concentrations of manure. Some have figured out how to turn that manure into money.

Flying H Farms grows feed for a large southwest Idaho dairy, and the roundtrip haul is 120 miles.

Jeff Harper: "Our yields keep getting better and better, and composted manure is a big part of that."

“Every load of silage we take to them, we bring back a load of composted manure,” Harper says. “The manure is paying freight costs both ways, which is helping both the dairy and our farm.”

Crop yields rise

Since Harper started applying composted manure to his fields, crop yields have gone up while commercial fertilizer costs have dropped substantially.

“On fields that have been in our manure program for several years, we have virtually eliminated commercial phosphorus and potassium applications, and we’re also using less commercial nitrogen.”

Meanwhile, organic matter continues to rise, increasing nutrient- and water-holding capacity, boosting water infiltration into the soil and reducing compaction, which benefits all crops.

“Our yields keep getting better and better, and composted manure is a big part of that,” says Harper, who notes that the desert soils in this area of Idaho are low in organic matter. “Soil tests show that spreading about 7-½ tons per acre works well for us. This adds about 25 units of phosphate and 50 units of potash per ton.”

The amount of manure they apply to fields is dictated by how many tons of silage they sell each year to dairies.

“It typically runs between 10,000 and 15,000 tons,” says Harper, who notes that employees truck about 200 tons of silage per day, six days a week throughout winter. Manure is then hauled back to farming operations owned by the Harpers and other families.

Harper didn’t start adding composted manure to his potato fields until about five years ago and is already sold.

“If you want to build your soil, this is definitely the way to do it,” he says. “One of my neighbors is an outstanding organic farmer; he has been using composted manure and cover crops for a long time, and you can tell they work when he starts combining corn. It’s a nice sustainable story, and we need sustainable stories like this in agriculture.”

Don't get carried away

Adding composted manure to fields rotated into potatoes brings many benefits, but also potential negatives that should be carefully examined before starting such a program, according to ongoing University of Idaho research.

“If you get carried away, you can get in trouble with salt levels,” says one of the participating farmers, Jeff Harper.

The study, led by UI Extension soils specialist Amber Moore and scientific aide Megan Satterwhite, shows that field applications of dairy manure can boost tuber yields and lead to larger size-class tubers.

Improvements in yields and size class are suspected to be related to improved nutrient and water retention, increased organic matter, improved soil structure, the slow release of nitrogen from organic matter to plant roots throughout the season and reduced fluctuations in soil temperature.

But this comes with an important caveat.

Intensive and/or recent manure applications can increase salt levels above the electrical conductivity (soluble salt) tolerance thresholds, the UI studies have revealed. When this happens, tuber yields and quality can go down, and it can trigger environmental issues including nitrate leaching, phosphorous runoff and soil copper accumulations.

The study recommends applying between five and 20 tons of composted dairy manure per acre preceding a potato, dry bean or sugarbeet crop.

Suggested rates preceding corn, alfalfa and wheat are 10 to 40 tons.

However, Moore and Satterwhite emphasize that the interactions between animal manures, soils and crops are complex. Therefore, farmers are encouraged to seek guidance from a crop consultant, company agronomist, Extension specialist or other professional and also test for phosphorus, nitrates and, in the case of fields with a known history of lagoon sludge or water applications, copper.

Manure samples should also be analyzed prior to application to determine nutrient levels.

Harper says that he relies heavily on soil testing, which has helped him avoid problems with salt.

“If there are free salts in the soil, you have to leach them out of the root zone prior to planting. A winter with good snows usually works real well. If you don’t get a moist winter, you crank up a pivot and wash them out the expensive way.”

As a final resort, calcium nitrate can be applied to fields at a predetermined rate.

“The calcium replaces the sodium in the soil, which allows the salt to leach out,” Harper says. “In the tougher soils, it’s making a huge difference in their ability to produce.”

The ongoing UI study is partially funded by the Idaho Potato Commission and Northwest Potato Research Consortium (NPRS).

To learn more, see Potato Progress volume 15, number 13, on the NPRS website at nwpotatoresearch.com.

UI Extension is also planning a bulletin on the research.

Tomorrow we offer a closer look at the soil benefits of using composted manure.