If you harvest the leftover parts of corn plants to make cellulosic ethanol, are you depriving the soil of nutrients that it needs for long-term health? 

It’s a reasonable premise — while corn biomass is a renewable resource, humans seem to have a propensity for exploiting renewable resources beyond renewability (see also: fisheries, aquifers, and I’m sure you can think of more). How is soil health affected by harvesting corn stover (leftover cobs, leaves, husks, and stalks) instead of leaving it on the field?

The answer, it seems, depends on how much you harvest. New data from an Iowa ethanol project supports the assertion that stover can be harvested in a way that does not hurt long-term soil quality. According to a recently updated set of data analyzed by Iowa State University and funded by ethanol giant POET, conservative harvesting levels (about 25% of above-ground crop residue) at an Emmetsburg, Iowa pilot project showed no yield loss over successive years and resulted in no significant difference in soil carbon. Some fields where stover had been partially harvested showed a moderate increase in productivity, possibly because less cover helps fields warm earlier in spring.

Brendan Jordan, who works on bioenergy issues at the Great Plains Institute in Minneapolis, emphasized that acceptable removal rates will vary depending on geographic location, soil characteristics, slope, crop rotation, and more, but indicated that the POET study’s results appeared to be consistent with other studies he has seen and that the 25% harvest rate was on the conservative side relative to other projects. 

He also mentioned that corn produces a very high amount of residue, more than many other crops, and therefore it is possible to get high yields of stover while still leaving a significant amount on the fields.

This information is supported by other research as well, including studies from Cornell and the University of Nebraska, which indicates that while full-scale harvesting of corn stover may deplete nutrients, partial harvesting generally does not harm the long-term viability of soil.

Photo by Andy Arthur via Creative Commons

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