Thirty percent of the farms in Ohio’s edge-of-field water quality monitoring study have phosphorus loss levels at or below target levels for protecting water quality. That percentage shows that controlling phosphorus loss from farmland is achievable, but also that most farms will need to do more to keep phosphorus from leaving their farms.

Kevin King, the research engineer with the USDA Agricultural Research Service who is coordinating Ohio’s edge-of-field research, outlined the next-generation production practices farmers will need to adopt during the recent Ohio Grain Farmers Symposium. “We’ve accumulated a lot of phosphorus in our soils,” he said. “The water quality problems in Lake Erie are probably going to persist for some time.”

Kevin King

The edge-of-field water quality monitoring involves 20 paired sites in the state’s northwest quadrant that are representative of crop production practices in Ohio. Water samples are collected from both tile discharge and surface runoff, except on a few flat fields with virtually no surface runoff, King explained. The paired study design allows for comparison of different production practices.

Government leaders in Ohio, Michigan and Ontario agreed to work together to reduce phosphorus entering Lake Erie’s Western Basin as part of a collaborative agreement in 2015. The goal is to achieve a 40% reduction in the amount of total and dissolved reactive phosphorus entering Lake Erie’s western basin by 2025. For producers, that translates into a goal of losing an average of a quarter pound of phosphorus per acre or less, King said. “It’s going to be difficult for us to meet that low of a threshold.”

Based on the farms in the study, King estimates that 30% of the state’s farmers are already doing their share to keep phosphorus out of the lake. He estimates an additional 10% are “way out there” in terms of high nutrient losses. However, just dealing with the farms with particularly high nutrient losses won’t be enough to bring nutrient loading levels down to the reduction goal, he said. “That’s not going to get it.” The 60% of farmers with moderate nutrient losses will also need to do better.

Currently, farmers in Ohio are taking off about as much phosphorus with crop removal as they are applying to fields. However, King said, some fields have high levels of phosphorus, and that phosphorus is likely to continue causing water quality problems. “What is going to be difficult, I think, is this idea of legacy phosphorus.” Even without additional phosphorus application, it could take a generation to draw down some of those high phosphorus levels with corn and soybean crop removal. While cover crops can help reduce erosion, they won’t help address high phosphorus levels in the soil, he added. If a crop isn’t harvested and removed from the field, it won’t reduce phosphorus in the soil.

First, test soil
The first step for farmers is to test the soil if they aren’t already doing so, said King. “We need to be soil-testing, and we need to be adhering to those soil tests.” He recommends using application rates based on the Tri-State Fertilizer Recommendations.

Comparisons of production practices show that putting fertilizer or manure in contact with the soil rather than on the surface helps keep it on the fields. King explained that he’s not advocating increased use of tillage, but he does advocate subsurface placement of nutrients. “We know by doing that we can significantly reduce the losses that we see.”

Another concern is a field’s discharge-to-precipitation ratio, said King. Reducing the amount of water that leaves a field can reduce nutrient losses, he explained.

It’s important to consider both the concentration of nutrients in water and the quantity of water leaving fields, King added. For instance, in fields with systematic tile drainage, the concentration of nutrients in the drainage water might be low. But if the flow is high, the total nutrients leaving the field might be high. He sees a need for more use of drainage water management to reduce nutrient loss.

The water quality research is ongoing with funding from a coalition of agricultural and environmental organizations, as well as businesses, government agencies and universities. The ongoing research will give farmers additional scientifically based recommendations for managing nutrients, King said. Researchers are continuing to compare the effectiveness of a variety of practices, including:
• gypsum as a surface amendment
• fall vs. spring application (organic and inorganic sources)
• rate (full v. half rate)
• drainage water management
• multiple vs. single application
• drawdown rates with alfalfa
• conventional tillage vs. no-till or reduced tillage
• banding vs. broadcast application
• cover crops vs. no cover crops
• bioreactors and steel slag filters

Risky business
While most farms don’t have the monitoring equipment to do their own edge-of-field water quality research, data collected through the state’s collaborative water quality research is offering them a way to estimate the water quality risks or advantages of various production practice. Libby Dayton, a soil scientist at Ohio State University, is working on revising the phosphorus risk index for the state based on research results. The P Risk Index estimates phosphorus runoff risk using various combinations of nutrient levels, application rates and methods, field characteristics, and management practices. Dayton, who described her work during the recent Ohio Grain Farmers Symposium, explained that she is focusing on factors that farmers can control. “Farmer management does matter,” she stressed.

Libby Dayton

Dayton said soil test phosphorus levels are one of the most important factors affecting the amount of phosphorus carried out of fields with water. While crops need phosphorus, many fields already have plenty without adding fertilizer, she said. She recommended maintaining soil test phosphorus within agronomic ranges, but she suggested, “Go as low as you can tolerate it.”

Consider whether your soil test shows enough total phosphorus for the crop rotation. Farmers sometimes apply additional P even when levels in the field are sufficient, Dayton notes. “It’s going to become available to the crop.”

Reducing soil disturbance has proven to be another important factor for reducing loss of particulate-bound phosphorus from fields, Dayton added. Avoiding surface application of fertilizer or manure also reduces the risk of phosphorus loss. “There are huge opportunities for reducing runoff phosphorus based on fertilizer placement,” she explained.