Keep nutrients in place

by Sally Colby

Farmers and manure haulers are committed to following nutrient management standards, and make every effort to keep up with requirements. To meet the need for ongoing nutrient management education, the inaugural edition of the Mid-Atlantic Manure Summit was held in conjunction with the annual PennAg Animal Ag Expo.

Rob Meinen, Penn State University senior extension associate, reviewed the foundational aspects of manure that help producers make sound decisions. He noted that the Four Rs stewardship program – the right source, the right rate, the right time and the right place – is still one of the best guides for managing nutrients.

Meinen said the nitrogen cycle can be confusing but it’s important to understand and can be simplified. He encourages producers to think about what’s happening as manure leaves the animal and becomes part of the manure nutrient balance sheet for the farm.

“We’re taking the soluble fraction and the organic fraction,” said Meinen. “That organic fraction is not available to crop and has to be broken down.” Meinen added that the organic fraction starts out as protein, lipid or carbohydrate in the ration and is passed through the animal without being completely processed.

The other portion of manure taken to the field is the soluble fraction, which is more available to the plant because it’s in solution “We want the nutrients to end up in the plant,” said Meinen, “so they have to be in a soluble form to either flow into the root or flow across the boundaries of the root then into the root itself.”

Meinen further explained nitrogen cycling, and said if every nitrogen molecule goes to the crop and is removed through the crop, there would be no water quality problems. “That’s the desired goal,” he said. “We need to think about nutrient placement as opposed to manure application. We place manure in the right place and at the right time so it goes to the crop and isn’t lost.”

Part of the cycle includes volatilization, which is one of the ways nitrogen is lost. Meinen said volatilization accounts for the largest loss when manure is land applied. This loss results in higher acidity, making soil tests and liming programs critical.

Leaching losses of nitrogen occur when the soil is exposed to excess water, through rain or irrigation, and levels exceed what the soil can hold. As water moves through the soil, nitrate (NO3) in the soil moves with water. “If water goes below the root zone, we’ve lost it,” said Meinen. “We want to manage systems to hold water at the root zone. We can do that through increases in soil organic matter to hold water. If we keep water out of the cycle, the other loss is denitrification. It’s a small loss – for every pound we lose of nitrogen through ammonia, we lose just a fraction of a tenth of a pound. It’s a smaller loss, but it’s still a loss.”

Another loss potential occurs with runoff. When water leaves the field, nitrogen leaves with it. Manure is applied in a soluble form, but if there’s a runoff event, even if the runoff water is clear, it’s likely to contain soluble nitrogen. In addition, nitrogen is lost when soil or manure particles leave the field.

Dealing with phosphorus presents a similar challenge – the phosphorus molecule must be in solution for plant uptake. Meinen uses the phosphorus cycle to illustrate his point, showing the carbon-based organic inputs (animal manure, plant residue) that require breakdown. “Some phosphorus will come onto the land in manure in a soluble form and available to the plant,” he said. “We rely on microbes to do a lot of that work.”

Phosphorus in a soluble form is easily leached and goes where the water goes. “If water goes below the root zone, we lose it,” said Meinen. “If water runs off the field, phosphorus can be lost. Runoff from the fields can occur as particle form as orthophosphates. It’s a continual process that continues throughout the growing season.”

Meinen said the foundation of manure management is matching the crop and manure, but making that match isn’t easy. “If we perfectly match the availability of nitrogen to the crop needs,” he said, “we will always over-supply phosphorus, and therein lies the problem managing manure.”

It’s a fact that excess phosphorus accumulates in the soil. Without the appropriate management steps such as soil sampling, a more conservative application rate is necessary. If the soil sample indicates excess phosphorus, the application will require extra nitrogen because the phosphorus-based rate will be lower.

Crop uptake of nutrients is a critical aspect of manure management. For example, corn requires high nitrogen during growth. The optimal time to apply nitrogen is at PSMT, before the crop starts rapid growth. “Can nitrogen be held until it’s really needed?” said Meinen. “Think about microbial populations in soil – we need the microbes to work for us, to turn organic carbon-based nitrogen into ammonium and then drive that cycle. To do that, the microbes will proliferate and function best when temperatures are warmer and at correct moisture.”

Testing manure is important because the nutrient content of manure varies from species to species and from one storage facility to another. Sampling protocols should be followed to avoid errors and inaccurate results. Meinen added that testing manure is essential for an accurate representation of what’s in that manure, but also because it’s the economically, agronomically and environmentally responsible thing to do.

Cover crops are a valuable tool to keep nutrients in place. Managing residue and keeping carbon-based material in and on the soil create an effective nutrient trap, and applying manure over a cover crop in fall helps capture nitrogen. “Nitrogen is captured in organic material where it will be immobilized and held,” said Meinen. “In spring, there’s more growth and more nitrogen is captured. The cover crop is harvested or killed, and nitrogen is still in the roots and stems left behind. Now it’s in organic form and held there, and can be released when microbes are cranking up when the summer crop needs it.”

Transport and source are critical in manure management. The sources of nutrients in the field can be manure, soil or fertilizer; the difference is in the transport. “I can bring a truckload of fertilizer or manure and dump it on the floor,” said Meinen. “If it rains, it isn’t going anywhere because there are roots. That’s a large source, but zero transport. If we dump a teaspoon of that fertilizer or manure in a parking lot, there’s a little source but large transport.”

Transport factors include erosion, runoff, leaching and volatilization. The goal is, and should always be, to manage soil, manure and fertilizers to hold nutrients in place where plants can use them.