Scientists continue research into best disposal of crop residue

Soil scientists spent nearly 18 years studying the impacts of field crop burning. The practice is utilized by grain farmers in the Arkansas Delta to clear fields of plant refuse, kill weeds and other pests, and prepare the seedbed for next year’s planting.

Some in the region complain, however, about the large plumes of smoke that fill the skies in the fall causing problems for those with respiratory ailments. Others complain about the smell, and the smog created. At times, the smoke can engulf streets and roads making it more hazardous to drive.

What scientists with the Arkansas Agricultural Experiment Station discovered was that burning causes fields to lose nitrogen and sulfur, displace phosphorus and potassium, and results in less crop residue as food for bacteria and fungi that help build soil aggregates and create slow-release nutrients. Those weren’t the only negative impacts and it could be hitting farmers in their pocket books as input costs continue to rise.

Trent Roberts, professor of soil fertility and testing for the Arkansas Agricultural Experiment Station and a soil specialist for the Cooperative Extension Service, recently ran the numbers on lost nutrients when burning crop residue. The value has increased exponentially over the past 18 months with the spike in fertilizer prices that began in 2021.

According to Roberts’ calculations at current fertilizer prices, a high-yielding rice or corn crop that produces about 5 tons of residue contains nutrients that are worth about $275 per acre: $85 per acre in nitrogen; $30 per acre in phosphorus; $150 per acre in potassium; and $10 per acre in sulfur.

With an estimated 1.15 million acres of rice planted in 2022, burning the crop residue would mean a loss of about $316 million in nutrients, Roberts said. Carbon locked in crop residue is also released back into the atmosphere as carbon dioxide when burned.

“When you burn residue or bale it and haul it off, you’re taking all the carbon off with it, too,” Roberts said. “There is a place where burning residue makes sense and is needed, but those situations should be taken on a case-by-case basis and not burning residue purely out of convenience.”

Kris Brye, professor of applied soil physics and pedology with the experiment station, said carbon and organic matter are critical to soil aggregation, the positive condition of soil particles clumping together. Soils with stable aggregates are less prone to erosion and can maintain greater levels of soil organic matter and total carbon, Brye explained.

“If you’re going to start managing for improved soil carbon, it’s going to take a semi-drastic change in philosophy and management practices,” Brye said. “You have to give it a little bit of time to equilibrate. Microorganisms have to adjust to new circumstances, and sometimes that’s hard to afford the necessary time to equilibrate. But those that do, I think, will swear by the benefits because there is a cascade of positive effects.”

The benefits, Brye said, include better nutrient cycling because more carbon is in the soil, a bigger microorganism population that can reduce the amount of fertilizer, and improved water infiltration.

Building soil health through carbon is more difficult in the South, where microbes work overtime in the warmer winters, Roberts said. Arkansas farmers will need almost quadruple the amount of organic matter on their fields to manage for improving soil carbon, Roberts said.

Microbes that eat the crop residue are more active in the South than in the North during the winter. Roberts said southern farmers “have to really pile that residue on there.” He said that crop residue plus chicken litter applications or cover crops are a start.

“You have to be patient,” Roberts said. “Even if we don’t see big increases in soil carbon, we see benefits. So don’t get discouraged by the fact that maybe your soil health score or your soil organic matter is not increasing. If you’re implementing those soil health practices like keeping the residue and reducing tillage, even though you may not be moving the dial, it is having a positive effect.”

Jarrod Hardke, extension rice agronomist for the University of Arkansas System Division of Agriculture, said there are only a few options open to farmers to prepare their fields for the next crop — fire, water, or tillage.

“Rice residue is actually allelopathic to rice, meaning it releases toxins as it breaks down that will make young rice sick,” he said. “If you have a lot of this residue still present next year, when we try to grow the rice again, we can injure or in some cases, kill, seedling rice.”

In drought years, farmers sometimes bale the stubble to sell as forage when livestock producers will pay top dollar for any available forage. Hardke cautioned that growers have sometimes found that the cost of restoring nutrients to that same soil in the spring made the venture ultimately less lucrative. Hardke said only about 40% of Arkansas rice fields see fire and only in years when conditions are conducive to burning. What stubble remains after burning is incorporated into the soil for further decomposition, adding nitrogen, sulfur, phosphorus and potassium back to the soil.

Tillage is the practice of plowing stubble into the ground. Without being able to burn, farmers left with large amounts of stubble may be forced to perform “what can be an excessive number of tillage passes that not only have their own emissions, but can decrease soil quality over time,” not to mention adding to the expense of a crop by increasing use of high-priced diesel, Hardke said.

“Holding water on that soil during the winter is not always a positive environmentally but can be good for the field and the residue management,” he said, adding that the downside is flooding can send loose stubble off the field, dam up ditches, block drainage and cause problems downstream.

If farmers do decide to burn they are encouraged to follow the Arkansas Department of Agriculture’s Voluntary Smoke Management guidelines.

“Bothering people in neighboring communities isn’t anyone’s goal,” Hardke said. “Keep in mind, all these farmers and everyone involved in this burning and management, they live right there, too. They’re not interested in putting themselves or their families through the annoyances either.”