While many U.S. farmers use continuous no-till when growing crops year to year, some may want to conduct a one-time tillage for problems such as controlling difficult weeds, breaking a compacted soil layer or reducing the risk of phosphorus loss.

Using no-till farming can increase yield, reduce erosion, improve soil quality and reduce cost and time requirements due to fewer field operations. Some caution that a one-time tillage could destroy the soil quality gained by using no-till farming.

However, University of Nebraska-Lincoln (UNL) scientists found that one-time tillage does not destroy the agronomic and environmental benefits gained by continuous no-till, or the absence of tillage.

While the Institute of Agriculture and Natural Resources scientists found the one-time tillage did not reduce soil organic matter, soil physical properties and yield, it did not have a positive effect on soil properties or yield. One-time tillage also reduced the risk of phosphorus runoff. Research is continuing to determine if carbon sequestration and soil organic matter eventually will be affected.

"Therefore, one-time tillage, conducted once in 10 or more years, might be justified to correct a problem in the field, but otherwise should be avoided unless the ongoing research finds a significant impact on the potential for carbon sequestration and increasing soil organic matter," said Charles Wortmann, UNL soils scientist. "If one-time tillage is conducted, it should be when soil temperature is 45° F or lower."

Researchers found that nutrients and soil organic matter were well redistributed with plow tillage to reduce stratification of available nutrients, but disk and chisel tillage did not effectively redistribute nutrients.

"A one-time chisel or disk tillage did not effectively redistribute nutrients while mini-moldboard plowing had an intermediate effect," Wortmann said. "Deep inversion tillage is needed for effective redistribution of nutrients."

Scientists also found there was not a significant increase in carbon dioxide emission from the soil with tillage compared to continuous no-till.

Soil organic matter concentrations were reduced by 24-88% in the 0- to 1-in. depth and increased by 13-381% for the 2- to 4-in. depth for the various tillage operations. Tillage did not affect soil organic matter in the top 12 in. of soil by 24-32 months after tillage.

A one-time tillage did affect soil microorganisms. Fungi that decompose crop residues were increased while other microbial groups were decreased by tillage. Most microbial groups returned to the no-till levels two or three years after tillage.

However, mycorrhizae, which colonize plant roots and are valuable in water and nutrient uptake, had not recovered to no-till levels by two or three years after tillage. This decreased mycorrhyzal colonization did not result in decreased plant phosphorus uptake while root phosphorus concentration was increased by tillage. Higher root phosphorus concentration following plowing is the likely cause for the delay in mycorrhizae recovery to continuous no-till levels.

Other findings include: Grain yields and soil aggregate stability were not affected by tillage treatments. Water infiltration was increased with plow tillage compared with continuous no-till at one location but decreased at the other. A one-time plow tillage reduced dissolved phosphorus loss in runoff at both locations and total phosphorus loss at one location.

Infrequent, one-time plow tillage can reduce surface soil phosphorus and the potential for phosphorus loss in runoff, but may reduce mycorrhyzal colonization of the roots with possible reduction in phosphorus uptake with some low-phosphorus soils. One-time tillage is not likely to increase yield in the short term. One-time tillage reduced nutrient and soil organic matter stratification, but it did not affect soil organic matter content.

This IANR research was funded in part by UNL's Agricultural Research Division and the International Sorghum and Millet Collaborative Research Support Program, or INTSORMIL.

These research findings were published in three papers in the July issue of Agronomy Journal.

The Institute of Agriculture and Natural Resources includes UNL Extension, the Agricultural Research Division and the College of Agricultural Sciences and Natural Resources. A 2007 study by an independent organization found IANR annually returns at least $15 in benefits to Nebraskans for every dollar of state support, making it a primary engine for economic and social sustainability ( atworkfornebraska.unl.edu).