With runoff, timing is everything

Farmers cannot stop soil and nutrient runoff from fields, but they may have some control over what’s in it.

The Discovery Farms programs in Minnesota and Wisconsin have collected surface runoff samples representing more than 150 site years and covering 3,285 runoff events. A multistate Discovery Farms Summit was convened in January in Bloomington to celebrate 20 years for the Wisconsin program and 10 years in Minnesota.

This extensive dataset provides actual measurements to go along with the years of farmer experience. Many farmers may have seen runoff evidence on their farms, yet few have actual data. We can now characterize runoff in a more objective way. Here are some highlights.

Runoff. Across the 151 site years sampled, runoff occurred in 150. Across the 35 monitored sites, every location experienced surface runoff at some point. Runoff happens most often during two time periods. Snowmelt runoff is concentrated in the period from February to April, while rainfall runoff is most prevalent in the months of May through July.

Soil loss. Most soil loss occurs with surface runoff prior to crop canopy. Soil loss measured at the edge of monitored fields typically ranged from 33 to 375 pounds per acre, with 76% measured in May, June and July.

Phosphorus loss. Phosphorus loss occurs mainly with surface runoff. However, phosphorus can also be found in tile water at very low levels. Phosphorus in surface runoff ranged from about 0.5 to 2 pounds per acre. P in tile flow ranged from about 0.03 to 0.33 pound per acre. Phosphorus loss happens during both the snowmelt period and with early season runoff. However, there is a difference. Phosphorus loss during snowmelt is about 70% in the dissolved form, since it is largely from the breakdown of crop residue at the soil surface. During spring runoff, 70% of the phosphorus is bound to soil particles.

Lessons learned

Second, consider timing and placement of nutrient applications. Incorporation helps reduce the risk of phosphorus loss for both commercial fertilizer and manure, so late fall applications with incorporation can be effective, though excessive tillage that may leave the soil vulnerable to spring runoff should be avoided. Another tradeoff.

Third, for livestock operations that spread manure throughout the winter, runoff risk can be reduced by ranking the runoff likelihood of fields on which you plan to spread throughout the winter. Reserve the lowest risk areas for spreading late in the winter as snow melt becomes more likely.

There are probably few surprises here for farmers. However, this data helps us explain to regulators and others the many efforts farmers make to reduce farm runoff.

Formo is executive director of the Minnesota Agricultural Water Resource Center.

Warren Formo | Jan 16, 2020