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Glyphosate and the Soil Microbiome with Tim Paulitz

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Episode Transcription:

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Drew Lyon: Hello and welcome to the WSU Wheat Beat Podcast. I’m your host Drew Lyon and I want to thank you for joining me as we explore the world of Small Grains production and research at Washington State University. We have weekly discussions with researchers from WSU and the USDA-ARS to provide you with insights into the latest research of wheat and barley production.

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Drew Lyon: My guest today is Timothy Paulitz. Dr. Paulitz is research plant pathologist with the USDA-ARS Wheat Health, Genetics, and Quality Research Unit in Pullman. He has been located in Pullman since 2000. He specializes in soil born root pathogens of wheat, barley, canola, and rotation crops both fungal, such as rhizoctonia, pythium aphanidermatum, and nematodes such as the cereal cyst nematode. He is also interested in soil microbiomes and using next generation sequencing to look at the effects of farming practices on bacterial and fungal communities. Hello Tim.

Tim Paulitz: Hi Drew.

Drew Lyon: A topic that’s gotten a lot of attention on the Internet here lately has been glyphosate and it’s effects on various things — human health, soil health, how it might affect the microbes in the soil. What are some of the claims that you’ve heard being made about the harmful effects of glyphosate?

Tim Paulitz: Yeah, well its interesting Drew. I mean glyphosate’s been around for about 40 years. Probably the most widely used herbicide in the world. And has generally been considered safe for a few reasons. One is that it very quickly binds the soil particles so it’s not moved in the water or in the ground water. It also has a site or mode of action of an enzyme that’s only present in bacteria fungi and plants and not in animals. So the toxicity of it in that respect has been fairly low. But for the last I’d probably say at least 10 years or maybe longer there’s been a lot of things on the web that make claims of harmful effects of glyphosate. About 10 years ago there was a professor, a retired professor named Don Huber from Perdue that made a series of claims. He even wrote a letter to the Secretary of Agriculture about his concerns and then there was also a researcher, Robert Kramer from USDA in Missouri that did a lot of work in this area. And so what I’ll try to do is maybe cover some of those claims and a lot of these just came up in an article this last November in No-Till magazine asking the question what harm is glyphosate doing to your cropping system. So there’s a lot of claims that have been made. One of the ones that had a lot of controversy when Roundup Ready soybean first came out what this idea that Roundup Ready soybeans were more susceptible to fusarium diseases and others. That there was somehow an inherent susceptibility and somehow glyphosate increased diseases. And the thing about a lot of this research is that again they just did not have the technology at the time to really answer these questions. So it’s not that I think that they, you know, came up with erroneous things based on whatever. But I think that so for example in the case of the Roundup Ready soybean, they did not have isolines, so they didn’t have lines where you have the gene and didn’t have the gene so you’re really comparing lines with different genetic backgrounds and coming to those conclusions. So again, if you look in the literature you’ll see papers and, like a lot of things, that go both ways some papers claim increase disease some papers claim decrease disease so, you know, it’s not one thing out there. But certainly all the claims that I’ve seen of diseases really haven’t been substantiated. Another claim that they made was that glyphosate, because it can act as a chelator, was causing nutrient deficiencies in soybean, specifically manganese deficiencies. And that has also been disputed with different scientific studies and I think that it really boils down to the fact that glyphosate in the environment we’re talking about parts per billion or parts per million. And so even if you had the glyphosate out there, even if it was chelating, the amount of those nutrients in the soil are going to be many, many times greater than the concentrations of the glyphosate you’re using. And of course the other thing is we don’t have Roundup Ready wheat unlike soybeans so that argument wouldn’t hold there. There’s been a lot of other arguments such as that somehow the DNA from the plant could be transformed into bacteria and cause problems. That one hasn’t really found any evidence. So in general, when looking at all those claims, I think the one that does have the most importance is the green bridge. And this is the idea that when you’re spraying out the weeds that they can then be a reservoir for soil born pathogens. So if you plant too soon after you spray, then you’re going to have increased disease. So that effect is real and I would call that maybe an indirect effect via the plant. And then you’ll also see a lot of controversy in terms of health claims. And I’m really not qualified to make comments on those. I’m not a physician but a lot of that focuses on possible effects on the human gut microbiome. There’s been a lot of studies which have detected low levels of glyphosate in certain food products that’s well below the threshold for regulation, but it’s still there nevertheless, and that’s getting some people concerned. But my research, which we’ll talk about in a minute here, really focuses in on the effects on the soil microbes. And so that’s really what I have the qualifications to look at.

Drew Lyon: Okay, so let’s talk about that research. You’ve been recently investigating how glyphosate effects soil microbes. What have you been finding?

Tim Paulitz: This is a question that has been posed by growers, especially direct seed growers, for the last 10 years. And again they read things on the web that talk about that glyphosate may be harmful to beneficial soil microorganisms, that glyphosate could increase disease, that glyphosate could chelate things like manganese and cause deficiencies. And at that time I had to tell them that — and I looked at all the data that had been published up to that point — I had to say that we really can’t answer that question on microbial communities. And part of that was because microbial communities are extremely complex. You know, one gram of soil or a pinch of soil may contain billions of bacterial cells and thousands of bacterial species and thousands of fungal species. But at the time the only thing we could really do is to try to culture them out, grow them in the lab, and then identify them. And so you’re really only looking at maybe 1% of the total community was out there. So about seven years ago new DNA technology was developed that enabled you to take a single soil sample and sequence thousands of DNA sequences and then specifically identify the organism that produced those sequences. And so now we could go into communities and look at all the bacteria, identify thousands of species, thousands of species of fungi. So then we really had the tools to try to answer this question. So about four years ago we initiated this study where we went to four long-term direct seed growers in the different rainfall zones. So we had one field near Genesee, a field near Colfax, a field near St. John, and then one out towards Ritzville to try to cover all the cropping systems. And we went to those fields and we sampled a field that had been in glyphosate use for over 20 years. And adjacent to that was a CRP (conservation reserve program) that had never had a history of glyphosate and so we wanted to compare the two communities, one that had seen glyphosate and one that hadn’t. So we brought that soil into the greenhouse, split it up into two treatments. One treatment will plant wheat and then spray it out with glyphosate. The other one will just cut the crowns off at the end of the experiment. And what we then do is sample the soil and also the soil around the root, which we call the rhyzosphere soil. So we did that through six different cycles. So we would plant the wheat and then at the end after three weeks kill it and then same and then go back again and replant. So we have kind of six successive cycles of this. And then I’m not going to go into a lot of detail on the technique and everything, but we essentially found was that if we compare these communities to each other, the biggest driver that affects them is the location. So obviously the communities in Genesee look very different from the communities in Ritzville. The next biggest thing was comparing the agriculturally cropland to the CRP (conservation reserve program). And as you would expect they’re very, very different communities, different plants growing in them. And we also found even a big effect of communities if you’re looking at the soil versus right next to the root. And that’s because the plant is exuding compounds that feed the bacteria in a very, very different community. But then when we looked at glyphosate, the effect that it had, it was very, very minor, much, much less than any of these other factors. And then we asked the question, OK of all those thousands of species of bacteria that we have out there, are any of them significantly increased or decreased by glyphosate use and we found less than 1% of them were. In effect most of them were increased by glyphosate use. So one of the things we thought about is this because microbes can utilize the glyphosate as a food source and break it down. But we think rather it’s really a greed bridge phenomenon because in the glyphosate treatment you have a lot of dying roots and these bacteria like to live on dying roots. Another interesting thing we found is that there seems to be more responsive bacteria in the fields that had a history of glyphosate use compared to those that didn’t. And this kind of makes sense too because microorganisms can adapt to different things. So the take home message from that was we really didn’t see any major effect of bacteria. And then we did the same thing with fungi and found pretty similar effects. And then finally we did this one interesting experiment where we wanted to see what happens to the fungi in a root as it’s dying after you’ve treated with glyphosate. So we sampled the roots before and then after at various times one week, two weeks, three weeks. And we found what we’d expect with the glyphosate and that there was a green bridge effect where there was a species of pythium that really liked that dying root and started to increase. And then over time other fungi would colonize them and start to decrease it. So that was another interesting study. And the last thing I’ll mention is that we’re also doing something called network analysis. So this not only asked who’s out there, but what are their relationships with each other. Are there competitive interactions? Are there negative interactions? Positive? In doing that we did find some subtle effects on the networks where even though we didn’t find big differences in the abundance, some of the treatments did change the network structure. So in other words how they interact. And this kind of also makes sense because if you have a dying root you can have organisms competing with each other with a negative interaction, which might not happen without the dying roots. So I think again the fact that we have dying roots really explains a lot of these phenomenon that we’re seeing. But this I think is the best study so far out there and the first one that has really used this technology to get at this question. Now there’ another group in Beltzville, also a USDA group, that is doing a similar study with corn and soybean and they’re coming up with pretty similar results although they’re a bit further behind than we are in terms of publishing. So the take home lesson seems to be that glyphosate is really having very minor effects on soil microbes. And I would say that the risk to the growers are probably higher for either green bridge, and then also the development of resistant weeds, and of course Drew I’m sure can speak for a long time on trying to keep from developing glyphosate resistant weeds. So that in a nutshell is kind of our findings.

Drew Lyon: That’s very interesting work and I hope I can have you back on to talk a little more in detail on that because I think there is a lot of interest out there on this and like I said a lot of, let’s say, not necessarily reputable information on the web. And so getting some real science behind this I think is going to be very valuable. Thanks, Tim.

Tim Paulitz: Okay, thanks Drew.

Drew Lyon: Thanks for listening to the WSU Wheat Beat podcast. If you have questions for us, that you’d like to hear addressed on future episodes, please email me at You can find us online at You can also find us on social media on Facebook and Twitter @WSUSmallGrains. Subscribe to this show through iTunes or your favorite podcasting app. The WSU Wheat Beat podcast is a production of CAHNRS Communications in the College of Agricultural Human and Natural Resource Sciences at Washington State University. I’m Drew Lyon; we’ll see you next week.

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