Comparing effects of herbicides, fertilizers, and tillage on the soil

Is this better than an herbicide for the soil? Photo: United Soybean Board.
Is this better than an herbicide for the soil? Photo: United Soybean Board.

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.

What is this herbicide application doing to the soil? Photo: A. McGuire.
What is this herbicide application doing to the soil? Photo: A. McGuire.

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.”

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.

Which is better for worms? Tillage or pesticides? Photo: S. Shepherd via wikimedia commons.
Which is better for worms: tillage or pesticides? Photo: S. Shepherd via wikimedia commons.

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.

7-5-18 Update

What is the effect of glyphosate (Roundup) on soil bacteria and fungi? Older studies were hampered by their inability to grow most soil microbes in the lab. What do the newer genetic tools tell us? It does not have much effect, especially compared to other factors. For bacteria, in “wheat grown in Pacific Northwest soils across multiple years, different locations, and soils with different histories of glyphosate use.” “Only a small percentage of the bacterial groups were influenced by glyphosate…”

For fungi: “cropping system, location, year, and root proximity were the primary drivers of fungal community compositions, and that glyphosate had only small impacts on fungal community composition or diversity.”


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.

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.

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.

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.

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.

Schlatter, Daniel C., Chuntao Yin, Ian Burke, Scot Hulbert, and Timothy Paulitz. 2017. “Location, Root Proximity, and Glyphosate-Use History Modulate the Effects of Glyphosate on Fungal Community Networks of Wheat.” Microbial Ecology, December, 1–18.

Schlatter, Daniel C., Chuntao Yin, Scot Hulbert, Ian Burke, and Timothy Paulitz. 2017. “Impacts of Repeated Glyphosate Use on Wheat-Associated Bacteria Are Small and Depend on Glyphosate Use History.” Applied and Environmental Microbiology 83 (22).

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.


9 comments on "Comparing effects of herbicides, fertilizers, and tillage on the soil"
    1. You are right about the field application rates. Do you know of any evidence of widespread over-application of glyphosate?

      The patent is for an entirely different use, ingestion by mammals, including people, to kill the protozoa that cause malaria and other diseases. That is very different than spraying it on plants and looking for the effects on the soil.

  1. Thank you for writing this article, it’s nice to have an objective look at some of the inputs that go into raising a productive crop.

  2. To me, this looks like comparing two crappy solutions, in order to make your preferred crappy solution look better. There are better solutions. First of all, glyphosate disrupts the shikimate pathway, which causes death in plants, algae, and a wide spectrum of necessary fungi and bacteria. When you destroy these very important fungi and bacteria, things stop working properly. Humans exposed to glyphosate have an extremely disrupted gut and microbiome, which leads to diseases and vitamin/mineral deficiencies. Likewise soil exposed to glyphosate becomes damaged to the point that natural mineralization and decomposition processes are inhibited or disrupted greatly.

    The better solution here is crimping, by rolling over the grass with a roller with cogs or gear teeth every inch or so, to crimp and crush the grass every inch in order to kill it, and then use a seeder which can cut threw this mulch layer.

    1. Joseph, crimping and rolling is a good way to kill some cover crops, if the timing and conditions are right. However, it will not kill a perennial crop; to do that you have to use either tillage or an herbicide. As far as all your claims about glyphosate, you provide no evidence. The evidence that I do provide does not find such dire consequences of using glyphosate.


        This paper draws information from nearly 30 years of research studies on the matter, and claims that exposure to glyphosate doubles a persons risk of developing lymphoma.

        This study relates to the way glyphosate effects humans and causes disease.

        There are options of precision depth control with tillage, application of natural desiccating herbicides, tarping, grazing, regular rolling and crimping between rows, there’s a lot of options and ways to solve problems without poisoning the product and planet.

  3. Chemical pesticidal agents are not selective. Applications of chemical pesticides are intended to control invertebrate pests that are harmful to various crops and other plants. However, because of the lack of selectivity, the chemical pesticidal agents exert their effects on non-target fauna as well, often effectively sterilizing a field for a period of time over which the pesticidal agents have been applied. Chemical pesticidal agents persist in the environment and generally are slow to be metabolized, if at all. They accumulate in the food chain, and particularly in the higher predator species. Accumulations of these chemical pesticidal agents results in the development of resistance to the agents and in species higher up the evolutionary ladder, act as mutagens and/or carcinogens often causing irreversible and deleterious genetic modifications.

    Quoted from this monsanto patent:

    Andrew, we are what we eat, human and animal genes literally change based on what is consumed over their lifetimes, and within 3 generations those genetic changes are apparent. By feeding humans and animals crops poisoned with chemical pesticides, such as round-up, you are effectively supporting and perpetrating this crime against humanity.

    You’re old enough to remember “tobacco science”, right? Of course Monsanto will say it’s safe.

    1. Joseph, thanks for your comment. However, you paint with too broad a brush; there are a wide range of properties of pesticides. Some pesticides are very selective, some only affect plants, some fungi, some only certain types of insects. There are no pesticides that, if used according to the label, will sterilize a field. Some pesticides persist, but some degrade rapidly. Some persist in the food chain, but others do not.

      The focus of my post is limited to the effects of pesticides on the soil.

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