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Larry Phelan biological buffering

The objective of regenerative farming systems is to develop soils with a robust microbial community which can supply all of a crops nutritional requirements without the need for added fertilizers. The pathway to getting there is to harness the photosynthetic engine every day of the year possible and cycle as large a volume of carbon as possible as a food source for soil biology.

While on this pathway, one of the most valuable things we can do is, in Michael McNeill’s words “stop poisoning your soil!”

One very significant way many farmers poison their soil and inhibit their microbial community is with nitrogen fertilizer applications. The less carbon that is in the soil, the less biological buffering the soil has, the more pronounced the damage from nitrogen applications is to the soil microbial community. In a previous post I described how to buffer nitrogen applications so as to not have this damaging effect.

In this conversation, Larry Phelan desrcibes the capacity of soil organisms to absorb the excessive nitrogen that is so often applied, and then release it later in the crops development cycle.

John: What you’re describing, in essence, if I’m understanding you correctly, is the capacity of biology in the soil to absorb large amounts of nutrients that are applied and to contain those nutrients within their cells and then release them over a period of time. Is that what you’re describing?

Larry: Yes. So, even in this artificial situation that we created—where we put inorganic fertilizer into a soil that had an organic history—even in that situation, that organic soil from that organic farm had enough carbon lying around that those microbes could actually use that inorganic form of nitrogen, in combination with the carbon that was there, and then bring that into that microbial community. As they then die off and you have other organisms that are feeding on those microbes, they then allow for the mineralization of some of these nitrogenous compounds, and then it becomes available to the plant.

To follow up on this, we studied the dynamics of nitrogen across the growing season in these organic and conventional corn fields. Just as we would have expected, when you look at a conventional system, where in the spring the farmer is putting down relatively high levels of soluble fertilizer and really not much carbon, other than maybe some plant residue, you see huge fluctuations in terms of the availability of that nitrogen. So, of course, before fertilization, nitrogen levels are very low, so the farmer applies the fertilizer. Now they shoot way up, well above what the plant can use, and then over the course of the growing season that nitrogen declines.

But if you look at that same pattern in an organic system, what we found was that the levels of soluble nitrogen in the soil solution generally were lower overall, and they also didn’t vary much during the course of the year. That plant was getting this constant supply of nitrogen throughout the growing season. What we found when we compared these farms was that, overall, there was no difference in terms of the production at the end of the year between the organic and the conventional farms.

2020-06-25T09:24:20-05:00July 8th, 2020|Tags: , , |

The degree of protein synthesis determines insect susceptibility

Plant absorption of ammonium instead of nitrate influences the degree of insect susceptibility. Excessive nitrogen in any form results in excessive soluble amino acids within the plant that enhances insect development and population growth.

From the podcast interview with Larry Phelan.

John: I’d also like to understand the insect attraction piece a bit better. You mentioned that when you have this surge of nitrogen supply on the non-organic farms from the nutrient application period, you have an increased attraction because you have a nutrient-rich food source. Why can insects not utilize plants that don’t contain high levels of amino acids as a food source? They would also contain some level of amino acids, wouldn’t they?

Larry: Yes, all plants are going to have some levels of free amino acids, and they can also digest protein—let’s keep that in mind too. But it’s going to take energy for that insect to digest that protein.

Furthermore, a lot of plants have defenses that involve the inhibition of enzymes that break down protein in the insect. These are called proteinase inhibitors. And in terms of what we call inducible defenses, this is one of the responses of many plants to insect attack. They start the production of these proteinase inhibitors in order to reduce the insect’s ability to digest that protein. It basically knocks out those proteinases in the insect. The insect doesn’t necessarily starve, but it slows its development way down.

Well, in that situation, if you’ve also given that plant these high levels of nitrogen and it’s accumulating these free amino acids, that now short-circuits that plant defense system. So, in other words, the insect doesn’t need those proteinases as much because it’s getting these free amino acids that it doesn’t have to break down.

John: Got it. That was a piece that I had always not fully understood. I’ve heard it described that insects don’t have the capacity to digest complete proteins and that they’re dependent on soluble amino acids as a food source. And that never quite made sense to me. It made sense from a theoretical perspective, but it didn’t make sense to me that plants might have no amino acids versus one plant having very high levels of amino acids.

Larry: Yeah, it’s a matter of degree. And even proteins vary in terms of their digestibility for insects too. And, of course, insects evolve enzymes that are going to be most effective in digesting the proteins they’re likely to encounter, depending on what host plant they feed on. 

How the form of nitrogen influences insect feeding

The form of nitrogen influences not only the pathogenicity of soil borne fungal diseases, but also susceptibility to insects.

From the podcast interview with Larry Phelan.

John: As you were looking at these plant-insect dynamics in the field and developing your hypothesis of biological buffering, what was something that surprised you?

Larry: The one thing that was particularly exciting to us was when we took the next step and tested this idea of mineral balance resulting from this biological buffering of organic matter. We started growing plants hydroponically so that we could vary the proportions of different nutrients. The hypothesis we were testing was actually that when the plant was in good mineral balance, you would get both good growth and resistance to insect attack. And then as the plant moved out of balance nutritionally, you would see plant growth go down and insect performance actually go up. And then, ultimately, as the plant was way out of balance nutritionally, we expected the plant to not grow very well and the insects not to do very well either because of the poor host plant.

We tested this with a number of different combinations of nutrients, and the one that was most dramatic—that actually supported this prediction—was looking at soybeans in which we varied the ratios of ammonia to nitrate in the plant. We provided it all the nutrients that it needed in constant levels for all the plants. What we were testing was different ratios between these two different forms of nitrogen. And what we found was that as we increased the amount of ammonia up to about 30 percent, we saw the best plant growth. That ratio of 30 percent ammonia and 70 percent nitrate is where we got the best plant growth.

And then, when we looked at insects, we plucked the leaves off of these plants and then fed them to insects to see how the insects grew. The particular insect we were working with was the Mexican bean beetle. When we fed these leaves to the Mexican bean beetle, we saw just the opposite response. In other words, where the plant was out of bounds nutritionally and not growing very well, that’s where the insects grew the largest and that’s where we saw the best survivorship. But as we moved towards that 30 percent ammonia level, where the plants were growing their biggest, insect survivorship dropped from about 90 percent down to about 30 or 40 percent.

It was a very dramatic effect—even more dramatic than what we were expecting. When we followed up on this and measured the levels of free amino acids in these plants, it was consistent with the prediction. In other words, those plants that were not growing as well, that were out of balance in terms of this ratio, had much higher levels of free amino acids relative to that 30 percent ammonia plant.

2020-05-05T05:16:18-05:00May 12th, 2020|Tags: , , , , , |

Nutritional differences in insect susceptible plants

Healthy plants are completely resistant to insects.

We have observed this to be true in the field on many occasions. Over time, more connections come to light describing the scientific reasons for how this occurs.

Larry describes their pioneering research seeking to identify the nutritional differences between insect resistant and insect susceptible crops in our podcast episode here.

So, if you have an imbalance—if you have too much nitrogen relative to potassium—what happens is you get the buildup of free amino acids in those plants. And insects love free amino acids. They’re a very digestible source of nutrients for them because they’re also highly limited by nitrogen.

 

John: With some of the original research that you did—comparing the insect pressure on the organic farms versus the nonorganic farms—this is actually something that I often hear from organic growers, but I haven’t uncovered a lot of research where people have actually tried to do a comparison and evaluate the differences. What were the differences that you were seeing? What really stood out for you?

Larry: Again, keep in mind that this was twenty to twenty-five years ago. There’s been a lot more research in organic systems now. But at the time, organic farming was considered very unscientific, and more New Age, and not amenable to large-scale production. And so I was working with a lot of organic farmers who were basically doing their own research—not necessarily replicated research as we would do at the university, but trying different things and seeing what worked on their farm. This is the context in which we got started.

I actually received a lot of criticism at the time—that what I was doing was not very productive, or not the direction I should be going as a non-tenured faculty member. So, we went to these organic corn and soybean farmers, and the ones we worked with had animals integrated into their system as well. First of all, we just did a census of their corn, looking at the levels of European corn borer damage in their fields versus their conventional neighbors. And, as I’m sure you and many of your listeners have often heard, I kept hearing this idea that if we have healthy soil, then we’re going to have a healthy plant and that insects don’t like healthy plants—that’s why they don’t see the damage.

Although that in itself is not a very scientific statement, we could reformulate it as a hypothesis that we could test empirically. And so we collected soils from these organic farms and then went right across the street to a conventional neighbor, collected their soils, and brought these soils into the greenhouse and planted them to corn. And we fertilized each of them either with an organic fertilizer, like manure or compost, or with a chemical fertilizer.

What we were interested in figuring out was, first of all, whether the insects could tell a difference in these plants. And if they could tell a difference, was it associated with that short-term effect of the type of fertilizer we used or that long-term effect of that history of management and how that impacted the soil community. And so, after setting these plants up and letting them grow to a certain stage, we released European corn borer females that had been mated into the greenhouse. All of this was replicated and randomized.

And we just let them loose to see where they would lay their eggs. And what we found very consistently—we actually repeated this experiment, I think, with four or five different pairs of soils from farms—was that if the plants were growing in a soil from an organic farm, irrespective of the fertilizer we applied, they received relatively few eggs. Whereas if the plants were growing in a soil from a conventional farm, sometimes they would receive a lot of eggs and sometimes they would receive only a few eggs.

This gave rise to this concept that I call biological buffering. The way we envision this—and this was our working hypothesis—was that in those organic systems, where you have a recurring influx of organic matter into those soils, either in terms of cover crops or plant manures or animal manures, you create this soil community that is beneficial to the plant. And when nutrients then go into that system, they get absorbed by this microbial community and then they release those nutrients very slowly over time. As a result, we hypothesize that those plants are in better mineral balance than when you’re putting down high levels of nutrients.

Why this would be important is because plants are almost always limited by nitrogen levels. They don’t have mechanisms for dampening the levels of nitrogen that they take up. They’re going to take up whatever they can get. And in this context of putting down inorganic, highly soluble nitrogen sources, often these plants are taking up much higher levels of nitrogen than they are really set up to deal with.

We then hypothesized that in this situation, those plants would tend to accumulate the simple compounds. When you have an imbalance of nutrients—let’s say nitrogen and potassium—if you think about those two elements, nitrogen is of course very important in terms of protein synthesis and potassium is important in terms of converting amino acids into protein. So, if you have an imbalance—if you have too much nitrogen relative to potassium—what happens is you get the buildup of free amino acids in those plants. And insects love free amino acids. They’re a very digestible source of nutrients for them because they’re also highly limited by nitrogen.

In situations where you have these imbalances, that plant becomes very nutritious for the insect; whereas in the plant that is in better mineral balance—one that’s getting its nutrients relatively slowly—that metabolic machinery of the plant is able to act more efficiently. As amino acids and sugars are produced in the plant, they are more immediately converted to the less digestible and more complex building blocks of the plant, like proteins and starches and cellulose and that sort of thing.

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