Hello again everyone and welcome to Week 4 of our course. We're already almost halfway through. If you recall our last lecture we talked about some of the sources, we talked about the cycles and some of the sources of nutrients particularly nitrogen and phosphorous in the system and for nitrogen we talked about the various inputs that we can possibly have in a farming situation, from fertilizers to manures, to even deposition from our atmosphere. And we talked about different kinds of fixation processes, that being industrial that makes our fertilizers. And biological that plants do to fix atmospheric nitrogen from the air in forms that would be available to plants for assimilation. I want to continue a little bit now with some of the cycling part of this. So once we have the nitrogen in the system. Then what happens to it? Where do these nutrients go once they're in the agricultural production system and what happens to them? How do we manage that process? The main forms of nitrogen that our agricultural plants use directly would be ammonium, which is a positively charged ion. Recall cation exchange process, so this positively charged ion It can be attracted to those negative sites on the soil particles and it can be held there. So, in those soils where we have fairly high calorine exchange capacity, its an effective way to keep nitrogen in the system and keep it from being lost for example, due to leaching. Nitrate is an effect, probably the more often assimilated nitrogen form, by plants. It's a negatively charge, ion and therefore, not attracted to the soils and it's most, most likely to be lost by, by leeching that is if the plants don't have a chance to grab it first. So if we look at soils and how they might relate to our forms of nitrogen we would note and recall that sandy soil was having low cation exchange capacity are not going to hold very much ammonium. So even though we might supply nitrogen and ammonium forms, for example, from our fertilizer or it ends up in that form through some processes that we'll talk about now those sandy soils cannot hold very much ammonium. Plant uptake is really our ultimate goal for our fertilization processes also called plant assimilation. This is our big picture best management practice. This is our goal. We want most we would like to have all but we rarely ever get all of our fertilizer to be assimilated or taken up by the plant but we want as much as possible to be taken up by our plant because that's the, that's the form of, of nitrogen that we are supplying. We've spent money on that fertilizer and we would like to get the biggest return on that investment. And also we want most of our nitrogen to go into the plant and not stay around in the soil and be subject to risk to losses for example through leaching. Fortunately, we can measure how much of our nitrogen fertilizer actually ends up in the plant and we use the term Fertilizer Use Efficiency. That means the proportion of the nutrients that we applied as fertilizer that actually can be found it the plant. It might be, in the case of corn, it might be in the stalks, or in the leaves, or in the corn grain, but we're very interested in the amount of nitrogen that gets into our crops, because that, once the nitrogen gets into the plant, it can do its job in terms of plant growth and that's what we're interested in, in terms of yield and return on investment for our fertilizer. There are some other synonyms, I guess you would call them for crop nutrient uptake, or assimilation. Sometimes you'll see the terms crop nutrient removal, or crop utilization or nutrient harvest. These are all synonymous, with crop, nutrient uptake, or assimilation. We'll return to this topic, especially in terms of the, the term called crop, nurture removal a little bit later on when we talk about fertilizer management, and fertilizer BMPs. There are a couple of considerations that impact fertilizer and nitrogen use efficiency. For example, the health of the plant is important for its ability to take up and use nitrogen so for example, if we have plants or crops that are being hampered by diseases or other afflictions, insects, this can reduce the nitro-use efficiency. So if we apply amount of nitrogen that would be appropriate for healthy a growing and developing crop then for a crop that's diseased then we, we probably will not see the nutrient use efficiency under that situation. Another factor that effects nutrient use efficiency obviously is how much of that fertilizer gets lost in the system iIs leached through the soils, or is lost through some gaseous transformations that can happen in our soils. So if a portion of our fertilizer is lost through those mechanisms, then our efficiency is obviously going to be reduced. If you look at some crop uptake values I've presented a few of them for you. It's kind of an interesting table. If you look at corn grain, if we are if we have a crop that yields 180 bushels of corn grain per acre, that crop likely will contain 170 pounds per acre of nitrogen in that grain. The plant itself, what's called the stover of the plant that's left behind and most likely in that field will contain about 70 pounds of nitrogen and you can see the amounts of, of phosphorus expressed as PTO five in this case, that would be taken up by the crop. If you look at tomato, for example, for a crop that produces 30,000 pounds of fruits, tomato fruits per acre, those fruits will contain 50 pounds of nitrogen. Tomato fruits on the whole comparing to the actual tomato plant itself contains relatively small amount of nitrogen. And then you look at crops like soybeans and peanut and alfalfa hays you can see the large amount of nitrogen contained in those crops. And where does this nitrogen come from? Well in this case these are ligands that can fix nitrogen from the air. But they also can take up a, nitrogen from our fertilizers if we are going to apply fertilizers. In most cases, relatively small amounts of synthetic fertilizer are applied to these kinds of crops because they can fix so much, nitrogen from the air. So they contain a significant amount of nitrogen. So where this becomes important is that, that particular nitrogen that's contained in these crops, if that part of the plant is left behind as crop refuse and tilled into the soil, then that amount of nitrogen is returned to the soil and must be considered a source of nitrogen that we should be aware of, and manage so that we don't lose it from the system. How do scientists measure how much of the nitrogen fertilizer that we apply actually ends up in the crop, because, recall our nitrogen Cycle. There are several sources of nitrogen and they're available to that growing plant. And it's very difficult to tell whether the nitrogen that is in the plant came from manure. Came from nitrogen that was associated with organic matter that mineralized during the season. Or from fertilizer, or from atmospheric deposition, for example. And this is one example, the picture of a crop that uses the so-called difference method. You could also follow nitrogen 15 isotope that's applied as fertilizer and follow that tracer into the plant. In this particular picture, I'm showing you. Corn you see the darker green plants that have been fertilized and the lighter green plants that have not been fertilized. The not, the lighter green plants are finding nitrogen a small amount of nitrogen at least from the soil. So if you analyze the dark green plants and subtract the nitrogen found in the lighter green plants you would have an estimate of the amount of nitrogen, that from fertilizer that was taken up by the darker green plants. So there are ways some of em more sophisticated and may, perhaps more detailed than other scientific methods for measuring the amount of fertilizer nitrogen that enters the plant. And some scientists might be interested in the nitrogen from the fertilizer that ends up in the, the plant itself. In this case in the top picture the corn plant. The leaves and the stalks, perhaps even the roots. You might want to go to the effort of digging up the roots and measuring the nitrogen in, in the roots. We'll talk a little bit about nitrogen budgets in an upcoming lecture. You might be interested also in the nitrogen that's actually in the article of commerce in this case the grain. And it might be interesting from the regard from the standpoint that, that grain might be sold off the farm. And represents a, a, an important output of nitrogen from the farm. And we'll talk about that a little bit later on when we consider budgets because that might, that's not a negative environmental. Impact but that is an output and I like to refer to it as an export as opposed to an environmental loss aspect. So there are various reasons for being interested in the nutrient content of plants but not the least of which would be from the farmers point of view of figuring out how much return on investment that farmer is, is receiving from the fertilizer. In a typical agricultural scenarios, the fertilizer use efficiency might range from as low as 20 to 30% to as high as 70%. So that begs the question, what happens to the rest of the nitrogen that we put on as fertilizer. We found only a portion of it in the plant. Typically for walking-around knowledge, you might think of fertilizer efficiency as about 50%, and you'd be pretty close for most of our crops. So we call this other nitrogen unaccounted for nitrogen, we really don't know where it went. We know looking at the nitrogen cycle we have some suspects there, but we really don't know which one or more of them are at play here and what the quantity of nitrogen that was lost to either or several of those fates. But we can look at these different phase and guess or estimate which one might be important, and in fact, we can actually determine methods to quantify and measure the amount of nitrogen from our fertilizer that perhaps ended up, in those other pools. Mineralization is one of the other parts of the nitrogen cycle that's very important in the overall nitrogen nutrition of our crops. It generally refers to the conversion of organic forms of nitrogen into mineral forms. Mineralization also called ammonification is the process that moves or converts organic nitrogen from our planet residues for example into a mineral form, ammonium nitrogen. The ammonium form then, can be further converted or transformed by soil microbes in a two part process to nitrate. So we would have ammonium. The ammonium ion remember is positively charged. And if we're on soils that contain a lot of cation exchange capacity, that ammonium can be held against losses. And we can, in those farming scenarios, we can actually improve our, fertilizer efficiency, because the ammonium will be held at least for a short period of time. Nitrite, on the other hand, once the ammonium is converted, to nitrate, and that process goes on in our soils normally. But once its in the nitrate form that negative ion will not be held in the soil particles and is subjective leaching. Hopefully, the nitrate, when its converted and its in the soil, pore water, its available for plant uptake and hopefully our production systems are set-up such by the time it ends up as nitrate. The plants are ready to absorb it and we risk a smaller portion of it to losses and especially to leeching and we'll talk a little bit more to about how we do that. Nitrification is another process is, is this process that converts ammonium to nitrate, and it's interesting that in most of our warm soils for example our sandy soils in Florida this whole process of converting ammonium to nitrate can happen relatively quickly less than 30 days sometimes even as fast as a couple weeks in these kinds of soils. So this conversion is, is relatively rapid. And in the case of our sandy soils in Florida that cannot hold on to much ammonium forms of nitrogen. This conversion it happens fast and pretty quickly our nitrogen is in, in a form that can easily be leeched particularly with heavy rain fall. So, we hope that we've put out the fertilizer, in a timely fashion so that plants can grab as much of that fertilizer as possible. Now, nitrogen can go back the other it is in a, a another pathway in the soils. It can also In addition to being taken up by plants, the nitrate ion obviously can be used by soil microbes in their own growth. In this particular way we call it immobilization because that nitrogen is not available for plant uptake because it has at least been temporarily immobilize as it were by the soil microbes. That's a good thing because we need soil microbes in the soil to help perform a lot of the conversion processes and soil microbes are very important to good quality soil. But they do require, nutrients, and nitrate being one of them, so if we are putting out fertilizer, some, a small portion of that fertilizer will be used By these soy microbes. As part of this process it's important to point out that this carbon to nitrogen ratio is important. So if we apply organic materials that has a high carbon content for example, some composts that are Have a lot of woody material in them. These microbes will actually use nitrogen from wherever they can obtain it in the soils and that might, if we put out compost and we've also put fertilizer out, some of our fertilizer may go to supply the growth of those microbes as they decompose that high carbon compost for example. So some of our Unaccounted for nitrogen might be might be in this particular part of the, the nitrogen cycle, this particular pool that of the microbes. Denitrification is another process that goes the other direction and certain microbes in the soil can use nitrate in their respiratory process and. It through this through this process and can convert nitrate all the way back to, to Nitrogen gas and that Nitrogen gas then can go back up to the atmosphere architectural setting. Denitrofication is important in some agriculture settings. You need some certain situations to support denitrification. You need a low oxygen soil situation, anaerobic soil conditions. To do this obviously you need the nitrite ion in the first place. So waterlogged soils are an important suspect for supporting denitrification process. You also need a source of carbon for the growth and development of the microbes that are doing the conversion. And so waterlogged soils, that are relatively high in available carbon, are highly suspect soils for denitrification process. And in some scenarios 5% plus or minus of our nitrogen fertilizer might be converted and lost in these scenarios. So, some of our nitrogen that was lost in our example where we had a relatively low, say 50% nitrogen use efficiently. That part, that we're curious about where it went. And some of it might have gone back into the atmosphere through the denitrification process. Denitrification process by itself isn't necessarily a negative for the environment because all we are doing here Adding nitrogen back to the atmosphere. But it is could be in certain farming scenarios a very significant loss of nitrogen fertilizer from the economic side of the budget for the for the farmer. Denitrification turns out To be a very challenging process to quantify. There are several different ways to quantify denitrification. I've given you a couple of them here. But all of these processes are very, very difficult to do and take specialized equipment. So denitrification is usually estimated as part of that unaccounted for nitrogen in, in the overall nitrogen budget. But it can be measured if you have the right kind of equipment to, to measure denitrification. An interesting aspect of denitrification is the so-called denitrification wall. This is used sometimes in environmental areas where you, you know you have nitrate contamination of a particular water body. But you also have the certain prerequisite situations that we just mentioned for denitrification. You have water log soils or high water table, you have organic matter and you have nitrate entering into the system. So this picture depicts how denitrification wall might be set up And in this particular case you take the, you, you would use a fairly high carbon material, say wood chips or saw dust. And build a wall that, that nitrate, nitrate laden water must pass through. And those microbes in there then will use the nitrate coming in as effluent from say a, you know, say a, a, a heavy application of fertilizer or in a manured situation, and those microbes that use the nitrates that's coming in from that system for their respiration process as they decompose the, the, the carbon, the high carbon material. And in that process then, the nitrite is converted to nitrogen gas and lost from the system. Which is a good thing, because in this particular scenario, you can see that this wall has been erected between the source of, of nitrite and the receiving water, or the water body. And there are several areas in the world that are leading areas of study on denitrification walls and is receiving a lot of interest and research in the last decade or so. Just as an interesting positive utilization of a natural soil process. Nitrogen can also be lost as ammonia from the system due to volatilization. This happens when we apply ammonium fertilizer materials in soils that are high, have high pH. In this particular case, that ammonium fertilizer is converted into ammonia gas and, and lost from the, from the farming scenario. So if you have the right kinds of conditions, for example, a very, soil with relatively high pH 7 or above, you might have the correct conditions to promote losses of fertilizers through this process. You can you can reduce the possibility of losses through this process by incorporating ammonia fertilizers in the soil or by irrigating, shortly after that to move the, the fertilizers materials into the soils. And these loses like denitrification because we're talking about a gas. These loses are rather difficult and a challenge to, to measure and quantify. But nonetheless in our example that we started out the lecture with this might be another pathway for losses. Urea has a possible, is a popular nitrogen fertilizer. The urea is enzyme converts urea fertilizer to ammonium and if you're applying urea fertilizer to a high pH soil some of that urea fertilizer could be lost in this volatilization process and in some farming scenarios a not too significant amount of nitrogen can be lost. Some of our unaccounted for nitrogen might be in this particular pool if we had applied urea fertilizer. Some of our unaccounted for nitrogen may be in this particular pool. Here's a picture that compares a couple scenarios where volatilization may or may not happen. And if I told you that the top picture was a potato crop on slightly acidic soil where the farmer was applying ammonium fertilizers through a center pivot irrigation. And compare that to a scenario in the bottom, where fertilizer might be applied to this particular soil. Which is essentially limestone soil found in extreme southern Florida. Which of those scenarios might be more conducive for volatilization. I think you would pick the one at the bottom because you're applying fertilizer for example to a very, very high pH soil and likely setting up the scenario for losses at least for a portion of that fertilizer to volatilization. The one above even though you were applying ammonium fertilizers, the soil pH is only slightly is, is slightly acidic. And furthermore, you're incorporating the fertilizer with the irrigation. So when we talk about a nutrient fate we look at a few of the different fate and pools that we might find nitrogen, particularly starting from an example, fertilizer application to a crop and measuring how much of the fertilizers. Was actually assimilated by our plant and finding out that it was less than the total amount that we apply and then we asked the question, where might of some of that fertilizer be? And as we go through the list of different fates, we can prioritize the likely candidates for where some of that unaccounted for nitrogen is, just by knowing just a little bit about or soils and our cropping situations, and then once we home in on some of the likely candidates, even though it's very, very difficult to do we can actually set up experiments to quantify the amounts of nitrogen loss by these various pathways. So nutrient cycles, describe how nutrients get into a farming production system, for example, and how they cycle in that system and how they might be removed or lost from that system. And keep in mind that you know I like to describe or separate out the outputs in terms of losses to the environment which would be a negative output verses an export which are the nutrients that are associated with our crops and animals. For example, that we would sell off the farm, those nutrients would leave our particular farming scenario and not be subject to losses through the environment. For example that we would sell off the farm, those nutrients would leave our particular farming scenario and not be subject to losses through the environment. They might though become as we will acknowledge later on as we look at budgets, those same nutrients that are exported from our farm. Will become someone else's problem to deal with as food, as it enter the food system. The fertilizer we've acknowledge in most of our farming scenarios is the major way that nutrients get into our cropping systems, it's the largest input. Particularly, in areas such as we have in Florida and the Southeastern part of the United States where we farm on relatively sandy soils that do not hold large amounts of nutrients. In other places, the relative amount of nutrients that can be supplied from the unfertilized soil can be rather large and represent a significant portion of the nutrient that the crop needs. And I, I really think it's important that we understand the different fates and flows of these nutrients in our agricultural systems because once we have an idea about the fates in flows, then we can develop and prioritize our best management practices on those pools that are large and also very important in terms of potential losses to the environment. And also which are very important to the economics of the farm, because farmers want to have the, the biggest return on investment. And so if we know where, where nutrients are coming from, where they're likely to go, then. Then we can help out in both the economic and the environmental aspects. So knowing all of the different parts of the cycle I think is very important as we further go through the course and develop and think about nutrient management practices we'll probably recall back to these different parts of the nutrient cycles.
