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Myths and Facts about Residue Breakdown

Crop residue serves an important role in protecting soil by preventing soil erosion during rain events or high winds. Residue also plays a significant role in enhancing the soil biological community by providing sources of organic carbon and nitrogen for its energy or food needs.

To understand how residue breaks down, you need to understand the biological and chemical activities that decompose residue and how these are influenced by environmental and soil conditions, including air and soil temperatures, soil moisture availability, soil pH, oxygen, and type of microbial community. In agriculture, annual cropping systems and other ecosystems management can influence the factors that are critical to the process of residue breakdown. 

There is a common belief among many farmers and agronomists that the physical change in crop residue structure by tillage can accelerate breakdown by cutting residue into small pieces or burying residue.  Also, there is the belief that the application of nitrogen fertilizer on crop residue after harvest can speed up the process of residue breakdown.  According to research by Mahdi Al-Kaisi, agronomy professor at Iowa State University, both assertions may not be correct.

Tillage effects
Recently, Iowa State University conducted a study to examine the effect of three different tillage systems, including deep tillage, strip tillage, and no till, on residue breakdown of both Bt and non-Bt corn residues.  The results of this three-year field and laboratory incubation study show no significant differences in the breakdown or percent of residue that remained among the three tillage systems of Bt and non-Bt corn hybrid decomposition.  Also, in these studies after 12 months, there was no difference between tillage systems or Bt and non-Bt residue hybrids breakdown in the field, where 34% to 49% of the corn residue still remained on the soil surface.

Nitrogen fertilizer application effects
Corn residue decomposition was evaluated by applying three N rates, 0, 30 and 60 pounds of nitrogen per acre, to corn residue immediately after harvest. Specific amounts of corn residue were weighed and placed in nylon mesh bags and left in the field immediately after harvest for decomposition evaluation. The rate of residue decomposition was evaluated every three months for the entire year.

The results showed that corn residue decomposition increased with time with smaller amounts of residue remaining after each evaluation period. There were no differences in the rate of residue decomposition as a result of different N rates.  These results imply that applying N fertilizer to facilitate residue decomposition is not effective. The timing of N application for corn residue decomposition immediately after harvest, as practiced, may not be an effective strategy, as the soil and air temperatures decrease over time after fall harvest.

The same results were observed with laboratory evaluation. Corn residue samples from the field were incubated in the laboratory under constant temperatures of 32° and 90° for approximately 30 days each. The rate of corn residue decomposition under laboratory conditions followed a similar trend as that in the field. Similar to the results of the field study, no differences in residue decomposition with different N rates were found. The laboratory study results confirmed the field results and demonstrated the role of temperature in controlling corn residue decomposition rather than N rate, in which a slower rate of residue decomposition was observed at the low temperature and increased at the higher temperature without any effect of N application on residue breakdown.

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