Double-cropping trial reduces soil phosphorus

A trial program whereby local farmers harvested two crops per year roughly doubled the reduction of soil test phosphorus (STP) compared with harvesting one crop per year, according to a news release from the Mercer Soil and Water Conservation District. The program was implemented on farmland in the Beaver Creek watershed, one of seven subwatersheds within the much larger Grand Lake watershed.

Phosphorus is the favorite food source of the toxin-producing blue-green algae in Grand Lake, into which water from the Beaver Creek watershed drains. Agriculture is the largest land use in the 58,000-acre, livestock-heavy watershed area that drains to the lake.

Area officials over the years have been testing methods to try to reduce the amount of phosphorus that runs off farmland into the 13,500-acre recreational lake.

In 2018 the Mercer SWCD was awarded an Ohio EPA 319 grant to install multiple best management practices in the Beaver Creek watershed, according to the release.

Six farmers participated and six fields totaling about 120 acres were involved in the trial, said Ag Solutions Coordinator Theresa Dirksen.

The SWCD offered cost-share to farmers to enroll fields in a double-cropping program in the Beaver Creek watershed. During the two-year program, farmers harvested two crops each year from the same acreage. No manure or commercial phosphorus was applied to the fields during the two-year period.

As a general practice, wheat was planted in the fall and it was then harvested as a forage crop the following spring. The forage wheat harvest was followed by planting and harvesting a traditional crop of either corn or soybeans. Other options were available, but this was the most common double cropping method used during this trial, according to the release.

It was determined that this practice dropped the STP value an average of 11 parts per million (ppm) across the fields during the two years. The Bray P1 extraction method was used to determine STP.

This drop in soil test phosphorus is roughly double what is typically seen with harvesting only one crop per year from the same acreage, according to the release. Any field with a STP value above 92 ppm Bray P1 would be a good candidate for the double cropping practice to promote the reduction of STP in that field.

Soil phosphorus levels are strongly related to the amount of phosphorus in runoff water and less phosphorus in the soil should result in reduced phosphorus runoff risk, according to The Ohio State University's College of Food, Agricultural, and Environmental Sciences.

Dirksen said SWCD officials plan to promote the double cropping system to area farmers. Some aready are using th concept.

"It is always beneficial to have a growing crop on the field. Cover crops do this already, but adding the harvest of the crop is removing additional nutrients instead of leaving them in place," Dirksen said. "This practice is being used widely by local dairy farmers (additional forage is beneficial for the farms)."

However, it is hard for other farmers to accomplish a forage harvest if they do not have the proper equipment, Dirksen added.