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“This information, you have to use it, but apply it according to the areas where you farm. It's not black and white information. It never will be.”

— Marisol Berti, Endowed Professor, North Dakota State University

In this episode of the Cover Crop Strategies podcast, brought to you by SOURCE® from Sound Agriculture, Marisol Berti, Endowed Professor & Plant Sciences Graduate Program Coordinator at North Dakota State University, discusses her multi-year research on nitrogen and cover crops and attempts to answer the question — can we count the nitrogen credits of cover crops for the following cash crop?   

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Full Transcript

Mackane Vogel:

Welcome to the Cover Crop Strategies Podcast, brought to you by Source from Sound Agriculture. I'm Mackane Vogel, Associate Editor at Cover Crop Strategies. In this episode, Marisol Berti, endowed professor and plant sciences graduate program coordinator at North Dakota State University discusses her multi-year research on nitrogen and cover crops. And attempts to answer the question, can we count the nitrogen credits of cover crops for the following cash crop? So I'm here with Marisol Berti, and I just want to ask you first before we get into some of your research, just to tell our listeners a little bit about who you are and what you're up to these days.

Marisol Berti:

Yeah. Hello, I'm Marisol Berti, I'm a professor at North Dakota State University. I've been here since 2009. I work in forages research, cover crops, cropping systems, and related to soil health and environmental impacts of all the systems involving cover crops and forages. And so my research is production, so I always say my lab is in the field. I don't have a building lab, so mainly in the field. And the idea is to try to get how we still grow our crops but protect our resources.

Mackane Vogel:

And so recently at the Soil Health Summit in Minnesota, you gave a presentation about cover crops and scavenging nitrogen and resources for the following crop. And so I think that's primarily what we're going to talk about today. So can you just give us a little bit of an overview of the study and the research and some key points?

Marisol Berti:

Yeah, one of the main questions that comes with cover crops is, can I assign any nitrogen credits to cover crops for my next crop, especially when corn the next crop? And we've been doing research since 2016 and trying to answer that question with different studies, but also for the presentation at the Soil Health Summit, I also used data from other researchers from other parts of the country to try to set the point of really what's happening. More than just say works or doesn't work, I try to explain what the nitrogen cycle is and try to understand how the nitrogen moves, where we can lose it and where we can gain.

Mackane Vogel:

All right. So first question is, what makes this question so difficult to answer and why might it take several years to come to any conclusion about something like this?

Marisol Berti:

Yeah, well we studied for a long time. We still don't have a definite answer, but we at least know some things. Why is it difficult is because it is a very complex system with many moving parts when it comes to nitrogen. Nitrogen moves in the soil quickly and it's transformed by bacteria in the soil to different forms. And so that's why my presentation explained the nitrogen cycle first. We have some inputs of nitrogen, like if we use cover crop legumes, we get nitrogen fixation. If we have cover crops or not, legumes, we have whatever those color crops take up, and then when they die or they're terminated, then the nitrogen goes in the soil as a form of residue.

So we have that input. If there's manure application, of course we have the manure. And the other main input in most crops is nitrogen fertilizer. So those are the inputs. Once it gets in the soil, a lot of things happen to the nitrogen. And from the fertilizer, if it's urea, it comes in ammonia form. Ammonia doesn't last too long in the soil. Ammonia ion is transformed by bacteria in the soil and a process called nitrification that transforms that form to nitrate, and nitrate moves in the soil with water.

So all these forms of nitrate and weather comes from the residue where it comes from fertilizer, once it's in nitrate form, can be easily lost from the system by leaching if you got excess water and especially sandier soils. Or also for another loss, people don't count much, but it's important, is by the nitrification, which is a loss when there's water logging in the soil, nitrogen can be converted to nitrous oxide and released as a gas in the soil. So that's why it's so complex, and then all these processes that happen with soil microorganisms, all these conversions depend on many factors, especially temperature, moisture.

In the time that takes those residues on the cover crops to convert to a form of mineral nitrogen that a plant can uptake and use, it varied the timing that is released. We've seen in the research that cover crops can take up a lot of nitrogen if there's nitrogen in the soil. We've measured up to a hundred pounds in a crop. But once that crop goes in the soil, we've never seen a hundred pounds nitrogen credit. So it's because it's another process that happen. If it's not converted to nitrate, it can be immobilized in the soil by the microorganisms. That means it becomes part of organic matter.

The body of the organisms, especially when the residue has a high carbon nitrogen rate, that means which [inaudible 00:06:17]. So we have cover crops, they're dry and they're going to have a high carbon nitrogen rate. Then the microorganisms are going to use residual nitrogen to try to eat that high carbon material. And that's what's called immobilization. That means it's in a form that is not used [inaudible 00:06:38] plants. It's not that it's not there.

That's why I say if a plant you measure and it has 100 pounds per year, there's 100 pounds that are going in the soil. The problem is, are they in the form the plant can take them up? So that's where I went back to the cycle many times. That's why it's so complex, the question, and there's so many factors, and that's what I focus on, the different factors that change mineralization rate, that can give or don't give us some nitrogen to the plant and be able to count those nitrogen credits.

Mackane Vogel:

So with so many variables, what was your strategy for... How many different types of cover crops did you experiment with and what was some of the methodology?

Marisol Berti:

Yeah, we have, like I said, more than one experiment going. We have different cover crops. We have grasses, brassicas, and legumes in those experiments. And we have, so some are winter hardy too, like rye in winter camelina, we have winter wheat. We expect them to survive for the next year. So it means if those crops have nitrogen, they're not going to release it until they're terminated in the next spring. While things like radish and the legumes will winter kill, so as soon as they're winter kill, that nitrogen is going in the rest of the soil.

But what happens here, usually that happens when it freezes, and microorganisms need temperature and water to do that. So they're not going to work on residues during the winter. They're frozen just like everything else, and they're dormant, they're not going to be working. So nitrogen will sit there in the fall if it freezes soon. Now if you get enough frost and it rains, you could lose it in the fall too. It depends where you are and what soil you have. So we try different crops with different functions and different root types, different crops.

And one of the things is the nitrogen in the cover crop is going to come two sources. Either it is a legume that fixes nitrogen or from the soil, and plants take up nitrogen according to their growth. So I always say, "A plant that grows is going to have a lot more nitrogen in it." If there's nitrogen in the soil. So I did chose some slides where if there's no residual nitrogen in the soil, these cover crops grow very, very, very small unless they have their own. So if you get a very small cover crop, you're not going to have much nitrogen credit.

So there is a correlation between how much nitrogen you take and the plant grows. So this is important because it's important the planting day that you do these cover crops. I know if you have a better window, like after wheat, you might be able to plant early and get more biomass. The more biomass you get, the more nitrogen those plants are going to take up and if they're legumes, they're going to fix. But in corn saving system, which is very common in the region, it is very hard to do that because you don't have time after the crops to plant them. And there's other people that work on interseeding, all kinds of things.

But it's hard to get a crop, a lot of biomass. So this is very important. I cannot get credit if I don't have a lot of biomass in the fall or the spring. That's the main thing. Plants grow at different rates. We don't recommend any warm seasons because they're probably going to kill very little and won't do anything. So any fall planting, we don't recommend any warm season crops. Only cool season crops. Some are going to grow much faster than others, like rye, grasses, and some cool season legumes like peas, they grow very fast and produce a lot of biomass.

Clovers, unfortunately for our studies, it might be different in other regions, they grow too slow and so they don't accumulate enough biomass for really accounting for any nitrogen. So once we planted those crops in the following year, we planted corn on the same, plus we had the cover crops, but then we had all those plots with cover crops were split on nitrogen rates. So we had five nitrogen rates, no nitrogen, and four other rates of 40, 80, 120 and 160 pounds per [inaudible 00:11:26] of nitrogen. I know some farmers use more than that, but North Dakota, that's the limitation.

So the reason we use rates, because this is the only way you're going to know if there's credit. If there is credit, the check treatment with no nitrogen rate should be similar to the one where you're putting fertilizer, because then you assume that nitrogen came from what it was in the cover crop. If you don't see differences, you're probably not getting the credit. That means the plant is using. If you see the same response on the check with any of the cover crops, that means the corn is using the nitrogen from the fertilizer, not the cover crops. And what happened is we did not find much difference.

Even with the legumes, we had a little bit of different with these, but very tiny and almost not significant at the lower rates, zero 40. What was more impactful is we saw a decrease in yield in all nitrogen rates when we have winter camelina or winter rye. And so what happens is when you do it, and that's why there's so many factors that play a role in this, this is the results of four years of study. But what happened is we have very dry springs when we did this study, which is not always the case. So what happened is the winter crops both, and my student measure [inaudible 00:13:03] water, the winter crops use too much water.

And so they affected the stand establishment of the crop and that carry on the rest of the year because we had less plants on those plots. We did this with sugar beets too. And so it really showed that at the beginning of the season, cover crops can have a negative effect in a dry spring in this area. So instead of seeing the nitrogen credit, we actually saw a loss of yield because of the high water use. So recommendation there is if you're using cover crops to survive the winter, wherever you are, you have to be watching those cover crops.

And if you see this is dry spring, you got to terminate them as soon as you can because it will affect it. So unfortunately you can't see the nitrogen. Now we saw a little bit of difference with four HP and this is what has been observed in other places. And I always mention, and I mentioned this too, is in Wisconsin Dr. [inaudible 00:14:14] has done very similar research with radish, with carrots, [inaudible 00:14:22] some clover, and he found the same results. So it shows some of the graphs from his results. He showed the same thing.

He showed radish as a previous crop, and then he put also four or five rates of nitrogen for corn, radish reduced yields. And it's interesting because radish is even in winter kills and so doesn't use water, but we still saw a decrease in it. And I explained that in my presentation, why would that happen. And then when he planted crimson clover, he saw a positive. So there was a nitrogen credit of about 40 pounds when he add crimson clover in Wisconsin. Which crimson clover doesn't do good here in North Dakota, but it does much better in other states.

So he had that credit. So farmers said, "Oh, this is bad news." I said, "One thing you can do," and this is a recommendation of Dr. Francis, his extensions of fertility here, "Is whatever rate of nitrogen you're going to put on your corn, just make a strip with a maximum rate that you usually use, I don't know, 160, 200 pounds, and then leave the field with maybe 40 pounds less. You're not going to get much than 40 nitrogen credits from a cover crop, and leave it with 40 less. And then you can look at about week 6, week 8, you can see there's a difference in color and there's time to see differences between the strip and the rest.

If you see a difference and your field looks yellow, your strip, you go and do a second application because that means you're not getting the credit, it's not getting there. So what happens here, here is what I explained. The nitrogen is going in the soil, but the time between the [inaudible 00:16:17] of that nitrogen and the crop and the time the corn needs, it can happen two things. It's either too slow released, which happens when you have hard carbon nitrogen ratio, when you have rye. That's exactly when it's going to take a long time to be released in the soil and it's going to be released after the corn is already picking up the nitrogen.

The corn takes most of the nitrogen it needs by the time it's gasoline, so it's already on the plant. So if the nitrogen in the soil is released after that, the corn can no longer use it because it might take a little more, but most of the nitrogen uptake rate of corn is very fast in the last stage before gasoline. So the plant needs to have the nitrogen in the plant to be able to move it to the cuffs as the grain develops. And so that's one thing that happens with high carbon nitrogen ratio.

Now the other thing that happens with very low carbon nitrogen ratio like radish or [inaudible 00:17:24], and this is other studies I show and this explains what happened with Dr. [inaudible 00:17:31] research radish and [inaudible 00:17:34], they release it too fast. And I show a graph that their winter kill and then they release it and two weeks after they were killed, they released 70 pounds of nitrogen. So all the nitrogen pretty much was released in the soil and it's converted to nitrate in those two weeks. So it was released about two weeks early before the corn even start taking it up.

It was already gone pretty much. And the spring, if you have rain or you have other conditions, you're going to lose it. So what happens when radish is the winter kill crop, you still see a reduction in yield, you don't see that nitrogen, it's probably because it's already gone. And they demonstrated that with [inaudible 00:18:23] too. It's a legume, had a lot of nitrogen, but most of the nitrogen was released like when the corn was in B1 stage. It was still in the hull, it wasn't taking it up. So it can be lost, doesn't have to be lost. Again, it's going to depend if it rains, it's going to depend if you have a sandy soil, how much of the nitrogen you lose.

Mackane Vogel:

We'll come back to the episode in a moment, but first I'd like to thank our sponsor, Source from Sound Agriculture for supporting today's podcast. If you want to make your fertilizer plan more efficient, source it. Source from Sound Agriculture optimizes the amount of crop nutrition supplied by the microbes in your soil, providing 25 pounds of nitrogen and phosphorus per acre. It's cost-effective and easy to use. Just throw it in the tank and spray and seed. If you want to unlock your crop's potential and increase ROI, there's only one answer, source it. Learn And now, let's get back to the episode.

Marisol Berti:

So this nitrogen credit thing is a play that you don't know where it's going to go because it depends on all these many factors. The good thing is we know what factors affect this, so we can more or less start estimating what's going to happen. So that's why in general, in most parts, you don't recommend a cover crop like radish alone because it's going to release. And the presentation was about nitrogen, but it can release a lot of phosphorus and actually phosphorus causes a lot of problems in water quality. So that's why you recommend mixes.

It's because you want to have different types of crops, so you don't lose it all in a very short time before the crops needs it. Now, anything that you do... As you improve your soil health, and this is what happened with farmers that have been in no-till and cover crops for many years. So we did it in a soil that hadn't had been in no tills for a long time. It's newer to tillage and new to cover crops. If you do the same study in a soil that already has high soil health, probably we could have got a different response because the soil is already cycling, it has a lot of microorganisms.

It's moving things much faster than a newer soil where you didn't have these biomass and diversity of organisms, even if you don't get nitrogen credit right away, doing cover crops and no-till are good practices, good management because in the long-term, you'll be able to recover that. I know some farmers that they can reduce their nitrogen rate to corn in 50 pounds just because they're having nitrogen cycling. What we don't know is that nitrogen coming from the crop the year before or maybe two years before, but then the microbes are moving.

And the other thing too, because the microbes are, you have a much better soil health, it's less likely you're going to lose it because if there is a molecule of nitrate there and there's a lot of microbes, they're going to use it. So soil health really becomes an important thing here because all these responses and things crops is when do I get my money back? And nitrogen credits is one of the easy things that I can get some money if I can reduce the rate. Unfortunately, it doesn't happen right away. It doesn't happen when you just start with cover crops.

It doesn't happen every year because of all these factors that depend on it. If you get a dry spring, even a farmer that has a very good soil of health, you might not get the credit because there's not enough moisture for the microbes to release it. Microbes need moisture too, moisture and temperature. So this is a play between all these factors, and we don't have control of most of them. What we have control is try to cover our soil so we don't lose soil by erosion. Increase the soil health, which is going to improve aggregation.

It's going to, the better health is going to keep the nutrients in the system and cycling them every year. So once you get that sort of health, you might be able to start benefiting of that nitrate credit. Unfortunately, it's very hard for us because somebody asked me, "Can we try to measure this nitrogen credit two years from now?" It will be very difficult because of the mobility of the nitrogen. It moves so much, it changes forms in the soil [inaudible 00:23:26] loss. And so it will be very hard to follow. I wish it had a good tracer.

There's some studies that you can put a 15 in isotope and try to trace it and see if you can see where it shows up. But it is not easy to do that over time to really see where your nitrogen from the cover crop went and where did it end up? Did it end up in the water, in the air, or where did it go in all this cycle? But I think it's very important to have a good picture of what's happening, and that's why I focus more on all the factors that are affecting nitrogen and they affect the ability of a residue to release or not release that nitrogen.

But that's the recommendation of trying to put a strip with more and then the rest of the field in maybe that year you are getting the credit and you don't have to use it or you'll have to do a second application, which many farmers do anyway. They do a second application of nitrogen or B6 or somewhere there. So that's pretty much in summary what I talk about on this. I think I had a lot of fun doing this presentation trying to put information. So it was more not just presenting research data, but more trying to teach about this. So it was more of a class than anything.

Mackane Vogel:

Wow. Well, that's a lot of great information. One of the key takeaways I got from all that is with all these factors, it's important to just control what we can control. Things, like you said, adding cover crops, reducing tillage. I'm curious, did you guys look at all into whether or not something like grazing and livestock has any effect on nitrogen?

Marisol Berti:

That's a very good question, and no, we don't. I work with forages, but I don't do grazing trials. One of my colleagues, [inaudible 00:25:27], she works in that and she's been doing grazing of cover crops. And that's a great thing, if you can integrate livestocks in your system, you are going to increase the nutrient cycling right away because that's a different thing. Right now, the rest of the cover crop goes through a cow, and so the nitrogen is going to be ready. It doesn't have to be converted by the microbes in the soil.

You are already getting direct nitrogen in urea and the urine of the cow or the rest of the manure that they're putting right there, so they're spraying it on your field. Well, a lot of people talk about the basic soil health principles, and one of them is integration of livestock. Very important one, because if you have that ability, I know a lot of people don't have animals, but there's a lot of things going on and some people that you can rent land for grazing. There's people willing to move cattle, that's happening. Neighbors move cattle to graze in other fields where they have cover crops. It depends.

Fencing is a limitation, water availability for the cows when they're grazing, those are things you have [inaudible 00:26:41]. But your whole system and nutrient cycling changes completely once you have livestock. But one thing, livestock is different than having manure. Dr. [inaudible 00:26:53] also, he did work with manure application, and what he saw is if you put manure and cover crops, you won't see the nitrogen credit because the cover crop takes it up and then it doesn't give it back right away. Now if you put manure alone, you might get the nitrogen credit from the manure.

So that's why, again, all those factors are playing, but having cows in there, because you're already getting that process. You're getting the nitrogen already in process form, and so it's much more likely you're going to get more credits. So if you have the ability to integrate livestock, it's a very good idea. I know it's not for everyone, but even if you cannot do that, there's a lot of things you can do. And I still encourage cover crops. If somebody say, "Well, maybe we shouldn't put cover crops," I still encourage to do it.

There's going to do so many other things for you. The thing is, it might not be directly in joint credits, but you're improving soil health. Your field is going to do better, it's going to be more resilient. If you have a drought or if you have things like that, people that's been doing cover crops and no-till and you get a dry year, they do better than those that are doing tillage because healthy soil also conserves water. And so you have a better capacity of the soil to conserve water and dry areas and dry years. So yeah, there's so many benefits. The thing with nitrogen is that it would be the easiest way to monetize and try to pay for your cover crop.

And where again, my, I don't know, $30 I spent on cover crops back, if we could have the nitrogen credit would be the easiest way to justify it. But it's really hard to do that because it varies every year. And the other thing is the other benefits, maybe you don't see the money there, but in the long term it will pay for it because if you get a dry year, you're going to get better yield than if you didn't do cover crops. So there's so many of the benefits. The monetizing or the benefits of cover crops is what is difficult. It's difficult to do.

Now, the other thing is since you asked about livestock, when you have livestock, you can't pay for the cover crop, the animal gain weight that you get with the cover crop grazing and be able to save hay for the winter if you have cows grazing in the fall and even early in the winter, some people do that, all that saving and gain weight or at least not lose weight during the winter, that has a monetary value and a lot of other people that has livestock can recover the cost of the cover crop. If they're grazing it, that's a way to recover easily.

Mackane Vogel:

All right. Well, I just have one more question for you. I recently was talking with another ag extension person, Paul Jasa of Nebraska Extension, and he was telling me that even though he's worked with the extension for more than 40 years, he's still learning new things pretty much every day. And I just thought that was so cool. I know you mentioned that these studies are fun for you as well, and you get to learn a lot. So the last question I have for you is just, what was one thing that was either really surprising to you that you learned during this study or just something that really stuck with you?

Marisol Berti:

That's a good question. Well, I always say, "We do all this research to answer questions, but we get more questions than answers." Every time we do a research, we got a hypothesis. We have the treatments to try to answer this question like we did with this one. Do we have nitrogen credit from these cover crops? And then you get all these other questions, what about the water? What about all these other things? So to me, what it is, it's so hard to control the factors. When you do greenhouse studies, you can control stuff, but in the field it's not like that, and that's what it is for the farmers.

So that's the reality, because I could do this in the greenhouse and maybe I get perfect results, but that's not reality. For me, it wasn't much a surprise because we knew what's going to happen because we've seen that it is hard to get those credits, but we learned a lot. We learned a lot about management of the cover crops, water management of them, [inaudible 00:31:32] establishment problems. We learned a lot with the sugar beets too. We didn't know how sensitive the sugar beets it is to this. Sugar beets was worse than corn. Sugar beets really got affected by all the winter crops, rye, winter wheat, it had a lot of negative effects because of the water use in the summertime.

One of the years that we did that, we have more of a wet spring and we didn't see as many differences. So water is really important to evaluate. So I think we learn a lot of things. My PhD student learn a lot of work and things that doing these studies take a lot of time too. It's a lot of sampling. You just see the summarized data I show in one graph of that, that takes a lot of hours of work to get that data. It's not perfect because nothing of this is perfect when you have so many factors. And I agree with the comment of the other Extension person, we learn every day. I've been doing this for 30 years working on research, and every day I learn something new.

Every day we do experiment, we learn more, and then we get more questions and we try to learn more. But that's the fun of it. You are learning and learning what's happening and trying to do it, and hopefully we can get some answers so it can help farmers because that's really the objective, to do research and supply research and store gear to the farmers so they can adopt practices that will help their soils. But also the bottom line, we don't want them to do things that are going to make them lose money. The thing is, we have to adjust them as they go, but I really encourage the cover crops, but I learn every time.

We never know what's going to happen with these things. We can guess what, but that's a surprise is what we get. I think we would've been surprised if we would've got a response because we really weren't expecting it. But what happened here, that necessarily doesn't happen everywhere. That's why you have to do the strip and test it. These are different soils. We are in a much shorter season area in North Dakota. It gets drier than other parts.

But what is important is to learn how these things are working so you can manage it and see. Out east they do get a lot more nitrogen credits for the cover crops because they have a lot more rain too. And so there's a lot of things that go into it. So this information, you have to use it, but apply it according to the areas you have. It's not a black and white information. It never will be. It's not a yes or no information. So it is what it is, and that's what's going to happen in the farmer's fields.

Mackane Vogel:

Big thanks to Marisol Bertie for today's discussion. The full transcript of the episode will be Many thanks to our sponsor, Source from Sound Agriculture for helping to make this cover crop podcast series possible. And from all of us here at Cover Crop Strategies, I'm Mackane Vogel. Thanks for listening and have a great day.