Cull Potato Composting

OBJECTIVES
This project's objective is to compost two local waste streams - cull potatoes and sawdust - for application to farmland with the hope this soil amendment will help stem the spread of European late blight virus.

ABSTRACT
Five seed potato growers in Townsend, Mont., hoping to use composted cull potatoes as a method of controlling the spread of European late blight, used a farm-scale approach to create the compost from two local waste products, cull potatoes and sawdust. Most of the culls are generated in the spring when the oversized, bruised or deformed potatoes are sorted from the high-quality seed potatoes as they come out of storage.

The growers achieved their goal of turning waste products into a useable soil amendment. But determining whether the compost destroys the European late blight will require further laboratory analysis.

METHODS
The cooperating farmers planned to place their cull potatoes atop sawdust beds (prepared by the Broadwater County Solid Waste Board) measuring 600 feet long, 8 feet wide and 12 inches deep from mid March to Mid May. They expected to add 750 tons of culls to the beds. Manure or added sawdust was to be used to cover the potatoes and inoculants applied if needed.

The beds would be windrowed and mixed with either a loader rented from the waste board or a Wildcat compost turner. If sawdust cannot be picked up from the bottom of the windrow for proper mixture, more will be added. The temperature and pH will be monitored during heating to keep the temperature under 130 degrees F. to avoid overheating, which can destroy the compost process, before a gradual cooling. The proper C:N ratio will be maintained, with moisture, materials and aeration adjusted as needed. Finished compost will be tested at Montana State University for ring rot and late blight of potatoes.

The project team will also investigate possible end uses for the finished compost, including applying it to farmland of cooperating producers. In the first year, the compost will be applied to their land, but the land will be rotated for three years before returning to potato production.

SPECIFIC RESULTS
At the local landfill, the growers set up 100-foot-long rows of sawdust 12 inches deep as a base, and then layered about 200 tons of cull potatoes on top. Fewer-than-normal spoiled potatoes came out of storage that year. The county's solid-waste crew used their front-end loader and grader to turn the piles.

The sawdust worked well for absorbing the liquid from the cull potatoes as they began to break down, but the C:N ratio required adjusting given the slow start of the composting process. The project team learned that they needed to monitor the windrows more closely. The heating process occurred faster than anticipated and some piles overheated, destroying the beneficial organisms in the composting mixture. This sterile compost was not sent to the Montana State University lab for nutrient analysis.

Two of five growers participated in a second attempt, hauling 85 tons of culls applied in a layer over an 8-inch sawdust bed, with a layer of sawdust piled atop the spuds. The easier-to-manage smaller rows turned out compost of higher quality.

POTENTIAL BENEFITS
State law, which dictates disposal of cull potatoes, says composting is an acceptable disposal method that helps prevent European late blight from contaminating marketable seed potatoes. It provides one more layer of protection for the seed potato industry in southwest Montana.

What's more, compost added to soils has been shown to increase the soil's biological activity, improve soil tilth and increase the availability of certain plant nutrients. Whether it will inhibit soil-borne pathogens, as some research has shown, has yet to be determined for Montana growers.

FARMER ADOPTION AND DIRECT IMPACT
Because composting is a viable method for disposing of cull potatoes, farmers in the region have expressed interest experimenting with on-farm composting even though it adds to their labor and management. By composting cull potatoes, farmers won't have to disturb their own ground to cover cull piles, which can be sources of late blight. Such cultural measures of reducing the spread of late blight also reduce the need for chemical controls.

One farmer has asked the county to help select a permanent site for the composting project, and another stated that all Montana growers could use the process to dispose of unmarketable potatoes, at the same time yielding a useful product. The project participants have received queries from the Dillon and Kalispell areas.

FUTURE RECOMMENDATIONS OR NEW HYPOTHESES
To expand the composting potential, the project team says it would like to incorporate leaves and grass clippings generated by homeowners and to find the right ratios for such a mixture. In addition, the project report suggests placing the compost project closer to the storage cellars, which would allow farmers to take a more active role in monitoring and turning the piles.

DISSEMINATION OF FINDINGS
The participants discussed the project with other seed growers in the region, and the project was mentioned during the Montana Potato Growers Conference.