Tillage or Herbicide for the Environment? – part one


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Andrew McGuireGUEST AUTHOR:   | Agronomist | Washington State University Center for Sustaining Agriculture and Natural Resources

Andrew McGuire is an agronomist working in the Columbia Basin’s irrigated cropping systems. His current focus is on helping farmers build soils, save money, and maintain yields through high residue farming systems and cover cropping.

This piece originally appeared on the WSU Center for Sustaining Agriculture and Natural Resources blog. It appears here by permission of the author. Part two can be found here.


Last summer, I visited an organic farm in the area. The farmer showed me various parts of his operation, one of which was a field that he had planted to a species of perennial grass that produces an abundance of deep roots. We dug a hole and confirmed it; a dense fibrous root system had formed after two years of growth. The farmer’s goal in planting this grass was to build up the soil before vegetable production. When I talked to the farmer again this fall, he was trying to figure out how best to go from the grass to vegetables. There could be two options for doing this.

The first is to till the grass crop in order to kill it. This would most likely require disking the soil three times or plowing and then disking, to kill the grass and break up the sod that is turned up by the first tillage pass.

The other option would be to spray out the grass crop with an herbicide. One pass through the field and the grass would be killed completely if done right.

If the goal of growing the grass was to build up the soil, which is the best option? Tillage, we know from research, would break up the physical soil habitat built up over the two years, disturbing the microorganisms living there. It would also disturb or destroy larger soil fauna, such as earthworms. This physical destruction, combined with the flush of oxygen that comes with intensive tillage, would burn up much of the organic matter added by the grass. The tillage would also eliminate soil cover and leave the soil in a loose state that predisposes it to future compaction.

Spraying out the grass with an herbicide would leave the soil’s physical habitat intact. The root mass that we dug up last summer would be undisturbed and the surface would covered by the dead grass leaves, controlling wind erosion and reducing evaporation.

In terms of the goal of building soil, the second option is plainly better than the first. However, some may argue that herbicides are toxic, that they may be a detriment to soil organisms, or that they could pollute the environment. If we used the herbicide glyphosate (the active ingredient in Roundup) its toxicity is low compared to other herbicides (the EPA considers glyphosate to be non-carcinogenic and relatively low in toxicity). The toxicity concern is limited further because we are not spraying a crop that is going to be harvested and the chemical is not persistent in soils. Any detrimental effect to soil organisms would be minimal compared to the obvious effects of tillage. Glyphosate also has a small leaching potential. For building soil, the choice is clear; spray out the crop.

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However, these tradeoffs between using tillage and an herbicide do not fit in the black and white ideology of the organic standard. Although the standards claim to make soil quality/health a priority, in this case the standard’s dogmatic ban on synthetic pesticides wins out. In the end, this organic farmer, who is trying to do a good thing in building his soil, is hamstrung by ideology. He will end up tilling the soil and losing much of what he was trying to accomplish. And there are wider implications.

To give the consumer a clear, black and white choice, organic marketing strategy offers a black and white world where all human-made pesticides and fertilizers, and all genetically modified crops are bad, regardless of their value to farmers or to sustainability. Even limited use is prohibited because it would blur the marketing lines. This fear-based marketing strategy requires these complete bans.

However, I believe that in a rational, science-based system, abuse or overuse of certain tools does not invalidate their use. Such a system would give priority to farmers’ efforts to better conserve and build soils over important, but lesser concerns. It would facilitate development of “near-organic” no-till systems in Western Washington, where the difficulty of killing cover crops under organic standards make this near-to-impossible to implement now. I think this would be a better way to steward our soils.

Part two can be found here.



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