By Dusty Sonnenberg, CCA, Ohio Field Leader, a project of the Ohio Soybean Council and soybean checkoff
Soil test phosphorus (P) numbers in the State of Ohio have continued to decline in recent years. “Much of the land in the state has soil phosphorus levels within the agronomic range. Even so, there are still some high phosphorus areas and issues with legacy phosphorus from past practices that we continue to work through,” said Greg LaBarge, OSU Extension Field Specialist, Agronomic Systems. “We are learning how slowly phosphorus changes in the soil when it is at high levels. There are opportunities to make changes, but simply employing the single practice of reducing phosphorus application is probably not going to be enough, and we need to make sure we are putting the right practice in the right place.”
Looking at the big picture, when we talk about agricultural impacts from the standpoint of water quality, there are three main components. “The first is soil erosion and soil loss. Agricultural practices, conservation tillage and cover crops all work to help reduce erosion. When the eroded soil washes into the ditches and rivers it ends up in Maumee Bay and the sediment settles out and needs to be dredged annually,” said LaBarge. “This impacts the fish habitat, spawning areas and other species. The reduction of soil erosion is an important factor, and implementing practices to control that is just as important as managing and reducing the loss of nitrogen and phosphorus nutrients. Reducing the loss of soil sediment reduces the movement of nutrients, as many nutrients attach to soil particles that are eroded from fields.”
Water soluble nutrients have garnered much attention in the water quality and nutrient management arena. “Soluble Phosphorus is another big area of concern,” said LaBarge. “The reason soluble P is a concern is because it is nearly 100% bio-available to the algae in the lake, and that promotes algae growth. If the phosphorus is attached to the soil particles that settle out in the river or streams, it is only 3-40% available to plants and algae.” Phosphorus loss is still an issue regardless of how it leaves the fields and moves in the water, but not as immediate when tied to soil particles versus being dissolved in the water.
Nitrogen is the other nutrient that is very concerning when water quality issues and the Lake Erie Algal Bloom is discussed. “Nutrient concentration, in particular the Nitrogen component in potable water resources is a health concern. When there are high nitrate levels in the river water that is used by communities as a water source, the EPA requires those communities to put out a warning to the residence, and that often hits the media and sheds a poor light on agriculture,” said LaBarge. “Ammonia in the water is also a problem for wildlife because it is very toxic to animals and high levels can lead to fish kill.”
The other two issues are tied together. They both involve excess plant growth. Hypoxia in the Gulf of Mexico is one. “Hypoxia is the condition when plants grow and then die and sink to the bottom and organisms break them down and the decomposition process takes oxygen out of the water and causes a dead zone. Two thirds of the water in Ohio goes south to the Ohio River and on to the Mississippi and the Gulf. The nitrogen and phosphorus in that water contribute to that hypoxia,” said LaBarge.
The second is the Lake Erie algal bloom. “The algal bloom is problematic because of concerns about the toxins that can be released from the bloom and get into the drinking water system,” said LaBarge. “The depth of water in the Western Lake Erie Basin is about 24 feet out to the islands, so the shallow water warms quicker and is conducive to increased plant and algae growth with the higher nutrient load. Wind speed and direction impact mixing of the water, and the location of the bloom.”
Phosphorus is the driver of the algal bloom size. “The phosphorus content in the water coming out of the Maumee River into the western basin is constantly feeding the bloom and causing the bloom to grow until late in the season when the water temperature changes and causes the bloom to die,” said LaBarge. “Nitrogen is the other component. Nitrogen in the water drives the micocystin production from the bloom and bloom toxicity. We can have an intense bloom with minimal toxicity if the nitrogen level is low.”