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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.
In a past post, I argued for the use of an herbicide instead of tillage to kill a soil-building cover crop. My post was mostly observation of the damage of tillage on the soil as compared to the lack of damage, at least visually, from the herbicide. But others suggested that herbicides may not be as benign in the soil as I portrayed them. Here is the latest science on the topic.
A series of reviews have been published on the effects of herbicides on the soil, starting with Bunemann et al. in 2006. They concluded, “The herbicides generally had no major effects on soil organisms.” More recently, a review by Rose et al. (2016) found, “Overall, the majority of papers reported negligible impacts of herbicides on soil microbial communities and beneficial soil functions when applied at recommended field-application rates.”
These were conclusions covering all herbicides, but there could be significant effects of specific herbicides. How about that singularly useful, yet most hated chemical in agriculture, glyphosate (the active ingredient in Roundup)? Since, in the previous post, I suggested glyphosate be used instead of tillage to get maximum soil building from the cover crop, we should see what the research says about it.
Here we can look at a meta-analysis, not just a review. A meta-analysis combines data from multiple studies and re-analyzes the combined data. Nguyen et al. (2016) looked at the results of 36 studies and found “Notably, field application rates [of glyphosate products] had no significant effect on SMR [soil microbial respiration] or SMB [soil microbial biomass].” They did find effects when applied at higher rates, but that is why we have the EPA and pesticide labels. Rose et al., reviewing the specific findings on glyphosate, observed “Numerous studies have found that glyphosate applied at standard application rates has little impact on the microbial biomass in soil, and stimulation rather than inhibition is more commonly observed.” They report that recent research, “the first to use next-generation sequencing”, found no significant effects of glyphosate on the structure of the microbial community. Another recent study, Newman et al. (2016), found “no overall effect of glyphosate on bacterial community diversity.” Rose et al. concludes, “To date, there is little evidence to suggest that long-term, repeat applications of glyphosate to soil causes negative shifts in soil microbial communities or functions.” While not conclusive, this evidence does not raise any red flags about the use of herbicides and their effect on the soil.
How do herbicides compare with other pesticides in their effects on the soil? The Bunemann et al. study reviewed all agricultural inputs and so could glean an idea of the differences between them: “Among the pesticides, herbicides appeared to have the least significant effects on soil organisms, whereas some insecticides and especially some fungicides proved to be quite toxic.” While another review, Imfeld and Vuilleumier (2012), finds that “the literature on the effects of pesticides on soil micro-organisms suggests that they only have minor or transient effects when they are applied at the recommended doses.”
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While we are on the topic of the effects of inputs on the soil, let’s look at synthetic fertilizers. They too are implicated in “killing the soil.” We are again fortunate to have a recent meta-analysis of 107 data sets from 64 long-term trials (duration of the trials ranged from 5 to 130 years, averaging 37 years) from around the world (Geisseler and Scow 2014). They concluded “that mineral fertilizer application led to a 15.1% increase in the microbial biomass above levels in unfertilized control treatments. Mineral fertilization also increased soil organic carbon content. From the Bunemann review, “There was little evidence for significant direct effects of mineral fertilisers on soil organisms, whereas the main indirect effects were shown to be an increase in biological activity with increasing plant productivity, crop residue inputs, and soil organic matter levels….” Nitrogen fertilizer, in particular, is often beneficial to soils because nitrogen supply often limits natural plant production. When we add nitrogen in agricultural systems, productivity increases, more plant material (biomass) is produced and so, in time, soil organic matter increases. This then increases microbial levels in the soil. This suggests that fertilizers, rather than “killing the soil” can sometimes enhance it.
Even where there is a toxic effect because of high concentration of fertilizer, such as with banding, these effects are both spatially limited, and “decrease within a few days or weeks in aerated soil due to nitrification and plant uptake.” Geisseler and Scow state, “[ammonia] concentrations at most times and in most locations, are likely far below levels toxic to microorganisms.”
The problem with nitrogen fertilizer, specifically ammonia-based fertilizers, is that they can change the acidity of the soil. When the soil pH drops below 5 or so, then both microbes and crops are negatively affected. Farmers lime the soil to keep pH above this threshold.
On to tillage effects on the soil. The NRCS Soil Quality Team describes tillage as a soil catastrophe:
Tilling the soil is the equivalent of an earthquake, hurricane, tornado, and forest fire occurring simultaneously to the world of soil organisms. Simply stated, tillage is bad for the soil.
For soft targets like earthworms, tillage implements are the multifunctional appliances of the farm field, slicing, dicing, and blending. Depending on the appliance settings, tillage can kill up to 80% of the earthworms in a field (Krogh et al. 2007). Glyphosate, in contrast, does not affect many worms at all, but has been found to cause specific types of earthworms to lose weight (no word on whether they can keep it off). Furthermore, the residues maintained by use of herbicides can decrease the effects of herbicides on earthworms (Rose et al. 2016).
The physical effects of tillage are evident. Aggregates are disrupted, soil structure is broken down, crop residues are buried, and the soil is left unprotected. Tillage can even make the effects of pesticides worse, “enhanced contact between pesticides and nitrifying microorganisms in ploughed soil increased the potential for the ecotoxicological effect” (Jensen et al. 2014).
Comparing tillage to herbicides, “Carter et al. (2007) found that glyphosate effects on soil biological properties in a 3-year potato rotation were periodic, inconsistent, and considered to be ecologically negligible compared to greater effects of tillage on soil structure” (reported in Rose et al. 2016)
So, herbicide or tillage? For some, this evidence will not matter. Their ideological bias (McGuire, 2016) against all uses of synthetic fertilizers and pesticides will win out. But for the rest of us, I believe the evidence is plain. Herbicides allow us to maintain a protective cover on top of the soil, protecting the soil from erosion, and maintaining the soil structure and microbial habitat. Tillage destroys all these. Ultimately, the question is: what do we want to protect? If protecting the soil is the first requirement for sustaining agricultural production, then clearly tillage is not our first choice if other, less damaging tools, like herbicides, are available. The tradeoffs between herbicide use and tillage favor herbicides.
Bünemann, E. K., Schwenke, G. D., & Van Zwieten, L. (2006). Impact of agricultural inputs on soil organisms—a review. Soil Research, 44(4), 379–406.
Geisseler, D., & Scow, K. M. (2014). Long-term effects of mineral fertilizers on soil microorganisms – A review. Soil Biology and Biochemistry, 75, 54–63. https://doi.org/10.1016/j.soilbio.2014.03.023
Imfeld, G., & Vuilleumier, S. (2012). Measuring the effects of pesticides on bacterial communities in soil: A critical review. European Journal of Soil Biology, 49, 22–30. https://doi.org/10.1016/j.ejsobi.2011.11.010
Jensen, J., Petersen, S. O., Elsgaard, L., & Krogh, P. H. (2014). Pesticide Interactions with N source and Tillage: Effects on soil biota and ecosystem services. Poster session presented at Global Soil Biodiversity Initiative (GSBI), Dijon, France.
Krogh, P. H., Griffiths, B., Demšar, D., Bohanec, M., Debeljak, M., Andersen, M. N., … Cortet, J. (2007). Responses by earthworms to reduced tillage in herbicide tolerant maize and Bt maize cropping systems. Pedobiologia, 51(3), 219–227. https://doi.org/10.1016/j.pedobi.2007.04.003
McGuire, A.M. (2016). The Ideological Threat of Organic Farming. Invited video presented at the International Annual Meeting of the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. Session: Sustainability Challenges in Organic Agriculture, Nov. 7, 2016, Phoenix, AZ.
Newman, M. M., Hoilett, N., Lorenz, N., Dick, R. P., Liles, M. R., Ramsier, C., & Kloepper, J. W. (2016). Glyphosate effects on soil rhizosphere-associated bacterial communities. Science of The Total Environment, 543, Part A, 155–160. https://doi.org/10.1016/j.scitotenv.2015.11.008
Nguyen, D. B., Rose, M. T., Rose, T. J., Morris, S. G., & van Zwieten, L. (2016). Impact of glyphosate on soil microbial biomass and respiration: A meta-analysis. Soil Biology and Biochemistry, 92, 50–57. https://doi.org/10.1016/j.soilbio.2015.09.014
Rose, M. T., Cavagnaro, T. R., Scanlan, C. A., Rose, T. J., Vancov, T., Kimber, S., … Van Zwieten, L. (2016). Impact of Herbicides on Soil Biology and Function. In D. L. Sparks (Ed.), Advances in Agronomy (Vol. 136, pp. 133–220). Academic Press.