This course will cover the agricultural and urban water quality issues in Florida, their bases, land and nutrient management strategies, and the science and policy behind the best management practices (BMPs). Students will learn to evaluate BMP research and analyze its role in determining practices and policies that protect water quality.
Nutrient Mass Balance
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Del curso dictado por University of Florida
Sustainable Agricultural Land Management
45 calificaciones
De la lección
Best Management Practices
For week 4, we will dig deeper into nutrient fates and talk about nutrient mass balance.
Conoce a los instructores
George J. Hochmuth, Ph.D.Professor
Urban and Agricultural Environmental Sciences
Welcome back everyone. If you recall, in our last lectures, we
talked about nutrient fates and flows in an agricultural, setting.
Today I want to talk a little bit about taking that information to the next level,
and seeing what kinds of things we can do with that information, that'll help us
devise methods that we call BMPs to manage those nutrients on the farm.
So we've identified the fates of nutrients right, in our farm , in our landscape.
How can we use this information? What can we really do with this knowledge
about where the, for example, where the nitrogen might be at any one time on the
farm? And how can we use this information to
hopefully improve our farming efficiency? So, the approach that people are working
on now, is called a nutrient mass balance. So what is a nutrient mass balance?
It's really simply an analysis, of the, the pools of nitrogen, for example, on the
farm, where those pools are, in what quantities, and what are the likely fates
of those pools? So it involves a quantification aspect,
and it also involves an analysis aspect because we want to minimize the amounts of
nutrients leaving the farm, especially those that end up in the environment.
And we also want to help the farmer maximize the efficiency of the nutrients
that the farmer purchases and brings onto the farm to grow the crops.
So we've got two components. We have this return on investment aspect,
that we're all interested in, because we want to help the farmer, stay as
profitable as possible and produce high quality, crops.
And we also want to help the farmer do that in a way, so that nutrients that they
are using on the farm, do not end up to any large degree in a place where we do
not want them. If you think back to some of the fates,
inputs and sources and outputs that we talked about over the last couple lectures
you might be able to find some of those in this particular picture.
This would be a flow, a nutrient flow, and in a water system, or a watershed that's
dominated by agriculture. So for example, see how many of them of
our sources and fates and flows that, that you can pick out.
For example, you've got fertilization going on, over here, you've got the
potential for los, gaseous losses happening here.
This is obviously a farm that's producing animals, so manure is going to be in the
question. We're pointing out runoff in another area,
so many of the, the fates and flows that we talked about previously, are depicted
on this picture. So if we take it to the next step, the
obvious question would be, can we quantify the amounts of nutrients, particularly in
this case, nitrogen and maybe phosphorous, that are associated with these various
sources and inputs. I like to think of nutrients being found
in four pools, in the environment. There are nutrients that are imported, to
the farm, for example, fertilizer, manure. If we have an animal operation, we're
probably bringing in feed, to a certain extent.
And if we have water bodies flow across our farm, those might potentially be
sources of nutrients that will come on to our farm.
There are nutrients also on the farm that is cycled or built up in our soils.
For example, when we apply manure to our crop fields, that manure needs to be
mineralized as you might recall from one of our previous lectures.
Those nutrients then are made available to the crop, the crop takes up the nutrients,
and some of those nutrients would probably be returned to the soil after we harvest
for example, the grain in a corn crop, and leave the, the plant behind.
So some of these nutrients that we bring on to the farm ultimately end up in a,
sort of a cycle on the farm. Nutrients can also be lost from the farm,
and here I like to divide the losses into two categories.
One I call exported, this isn't a bad loss to this, to this farm, because these are
the nutrients that are associated with the crops that the farmer produces and would
sell, off the farm, or the animals that are produced on the farm and sold off the
farm. In both cases there are nutrients that are
associated with those products that then leave the farm situation.
There are also nutrients however, that do get lost to the environment.
For example, leaching or run off, and there are also gaseous losses as we, as we
pointed out several times. So there are pools, these four pools, im,
inputs or imports. Those nutrients that enter in to some kind
of cycling or realization, sometimes, some people call it build up, on the farm, and
then also, two ways that nutrients can be removed or taken off the farm.
So in a mass balance approach, we try to quantify these pools.
We try to figure out how much nitrogen would be associated with the crop that's
taken off the farm or fed to the animals. The amount of nutrients that might be
returned to the soil, through crop refuse. The amounts of, of nutrients that might be
applied, to the fields and fertilizer and manure, and if we can we would like to
quantify the nutrients that might potentially be lost.
For example, in leaching and runoff we've already talked about, lysometers and the
scientific approaches to quantifying some of these losses.
Gaseous losses, nutrient, losses through, runoff and leaching are very challenging
to do because it does take some sophisticated equipment and some time and
some resource investment. So we tried to determine these quantities,
and then we try to look at the relationships, and figure out what amounts
are in the various pools, and are these nutrients where we would expect them to
be, and how are they in proportion to each other.
For example, we would like for as much of our fertilizer to go into the crop, to be
taken up by the plant, and result in yields, so that we have optimum yield.
And we're also interested though in figuring out how much of the nutrients
might be subject to losses through leaching or runoff.
So for example, we might have two major inputs on our farm, might be fertilizer
and manure, and some major outputs from a farm.
We hope one would be plant uptake and export, we may have some leaching or
runoff, and we may have some gaseous losses.
So once we quantify some of these pools or at least estimate them, we can then figure
out which ones are important by their size and by their threat to either the
profitability of the farm or the environment, and that's when we can set a
path to develop practices to minimize losses.
So crop uptake, as far as these budgets have been, is relatively straightforward
and easy to do. And most budgets you find in the
literature, do involve an analysis of nutrient inputs in terms of fertilizer or
manure, and the outputs measured by how much of the nutrients are actually taken
up by the crop and exported off the farm. Other types of exports or losses are more
difficult to determine, and there are still many studies in the literature, but
they are not as numerous as simple input crop uptake analyses.
So these other losses are sometimes very difficult to find good studies in a
literature that have actually directly measured leaching or gaseous losses.
But there's a lot of interest in this particular area, and research is starting
to increase. We have these drainage lysimeters that we
talked about before, that we can quantify, leaching.
So if we were really interested in a particular farming scenario, where
leaching might be, hypothesized to be a serious potential contributor to nutrient
losses, we might install some drainage lysimeters to research that idea and to
test and see if there are nutrients that are lost below the root, the root zone.
If we can find them, then we can sit down with the farmer and determine some of the
strategies that might be a well to institute to, to reduce the amount of
nutrients that are lost. We've done a lot of work with drainage
lysimeter's all over Florida, particularly in Northern Florida on our entisol's that
are highly prone to leaching. These are some drainage lysimeter's that
we installed on a, on a, a row crop farm to take a look at the potential for
nutrient losses in the soils. So in this nutrient mass balance approach,
we're trying to quantify parts and of these four, major, pathways, the budget,
is the groundwork, what I refer to as the budget, that's really the, the
quantification or the math, to understanding nutrient fates, and
developing strategies. So, the budget part is to partition the
pools of nutrients on the farm into pools that we can then quantify.
The balance then, in my estimation, is the analytical part.
We have the budget, we know some of the major pools.
We know something about the proportion of the nutrients, that can be found in those
pools, and now we need to, to analyze those pools and determine which are, which
are important. Which have the most amounts of nutrients
in them, and are likely candidates for at least sitting down and double checking
about our production practices, so that we can look at them and determine whether or
not we might need to make changes. So we want to ask questions about how
these balances, how these pools relate to each other.
Which ones are larger, which ones are small, which ones are easy to, to deal
with? Some of these pools of nutrients, for
example, the gaseous losses, may be very difficult for the farmer uh,, to deal with
so we need to make this kind of analysis. And we're really looking again at 2
approaches. We want to help the farmer reduce the loss
of nutrients, because that's money off the bottom line and we also want to identify
those loss pathways that might be potentially negatively affecting the
environment near that farm. And both of these are very of, of strong
interest to everyone in that watershed. And I just want to remind us that this
does involve some analysis through the triple bottom line approach because
economics, the environment and what society wants and, is very important to
consider as we go through this, this mass balance, analysis.
Mass balance is not really, not new. In fact, Baussingault as, far back as the
middle 1800s, set up some of the first Nutrient mass balance studies, in France.
He was fertilizing various types of crops, both legumes and non-legume crops with
manure. And he found out that rotations that
involve legumes were, were some of the more productive, and so he knew that the
rotations with legumes were contributing more nitrogen to the system.
And he theorized that this extra nitrogen came from the air.
But of course he had to wait a few more years for biological nitrogen fixation to
be described. But I thought it was very interesting that
a study this old and a scientist that was able to partition out the various sources
of nitrogen, the manure, the various crops that he was growing on the, on the farm,
and calculate up how much nitrogen was associated with each of those parts of
this budget. Let's look at some nutrient mass balances
for today's world. Here's one that would involve an
intergrated farm, this might be a dairy farm.
And you can see that the green on the left, those are inputs, so you can see the
various kinds of inputs that in a budget you would want to quantify irrigation
water even. There's some places in the world where
there's a significant amount of nitrogen in the irrigation water, so if the farmers
is using irrigation, it would be logical to want the quantify the amount of
nitrogen that's coming to the crops through the irrigation water.
Then the outputs, the yellow in this particular case, they're called managed
outputs, but these are equal to what I call exports.
So if we're selling meat or milk off the farm, or crops, or maybe selling the
manure off the farm, those are nutrients that are leaving the farm.
And then the red are pathways where nutrients are lost, they're either stored
in the soil or they might they might cross the farm boundary as runoff or, or
leaching. So you can visualize a farm and all of the
nutrient fates and flows in this fashion, with inputs, and exports, and losses, and
potentially recycling or, or build-up on the farm.
At the watershed level you can expand this out, and to do a good job on budgets and,
and balances, you really need to define the perimeters of the area that you're
working in. Is it a farm, is it a small watershed, a
big watershed, or is it even just a crop production system, perhaps just a field?
So you can take this approach to just about any level that you want.
So how do we measure, how do we get the numbers, how do we quantify these parts of
the budget? Well there's lots of different sources out
there, and sometimes it's easy to get the numbers out of already published sources
without having to go to the effort or the cost of recreating, or actually directly
measuring. So existing databases, are one good source
about soil types, about the nitrate content in soils, maybe even stream flow
and, and load of nutrients that are in a particular stream, the atmospheric
deposition. You can find databases that will tell you
how much nitrogen or phosphorus is being deposited on the land from the atmosphere.
Farm records are an excellent source, if you're working with farmers that are used
to keeping close records on fertilizer purchases, or manure inputs and
application rates. They know to the drop of how much water
they put on their crops, how much feed they brought onto the field, they can tell
you their crop yields the amounts of products that are sold off of the farm.
If you can get access to those kinds of numbers those are excellent sources for
information in a budget, because their real farm numbers and farmers typically
relate very closely to their own numbers. We can also measure some of these things
directly, and sometimes we may find that the available information is just not
satisfactory for our purposes, and we need to measure.
So we can measure the runoff and leaching loads that are leaving the farm.
Although it's very difficult as we've already acknowledged, we can measure
gaseous losses. We can measure cycling and immobilization
and mineralization. For example, we can do studies if it's a
farm, it's using manure, we can conduct studies to find out how much of a
particular nutrient nitrogen and phosphorus that's being made available or
mineralized from the, the manure applications that we make to a particular
crop through the season. We can also analyze plants.
It's a lot of work, it's a lot of cost but we can take plants and, and we can break
them down into their article of commerce, for example the corn grain and the plant
material, the so called stover, of corn crop and the roots.
And we can, throw them, we can take them through the lab and analyze the, the
nutrient content. And in fact the more things that we can
directly measure will make our budget much more accurate and meaningful.
But sometimes things are just too expensive and involve too, too, in,
involve equipment that we might not be able to use in a farm setting.
So sometimes we use we rely on already published information and make it
applicable to our farming scenario. Here's an example of a budget for corn
that's in your Meissinger, reference. And in this particular case the scientists
applied different rates of fertilizer, to corn.
And you can see on the picture here, as the nitrogen rate increased, and on the,
the y axis, you'll see the amounts of nutrients.
In this particular case, nitrogen taken up by the corn crop.
And what are some thing that you might notice, when you look at this particular,
figure? What they've done is, they've analyzed the
various pools of nitrogen associated with growing the corn.
The first one on the bottom, would be the corn grain yield, that's very important.
And what do you see as your eye moves across from the, the low rates of nitrogen
to the high rates? You find out that as you increase the
nitrogen rate above about 225 kilograms per hectare, the yield starts to level
off. Same thing happens with the stover, the,
the corn plant itself. So those are two large pools of nitrogen
that we find on this particular farm. But the interesting thing is, that after
we apply, apply a certain level of nitrogen and we add more to that, then the
crop yield does not respond to it. The other interesting thing on this slide
is that at that appropriate, what we might, will call later on, we'll call best
management practice fertilizer application.
You see that most of your nitrogen is found in the, in the article of commerce,
our, our plants, and smaller amounts are associated with losses.
But as you get out to high rates of nitrogen, a lot of the nitrate, a
significant portion, in fact most of it, is associated with either residual nitrate
that stays in the soil after the crop is removed, or in various loss, pathways.
And if you summarize, if you take data from that particular figure and summarize
it, you can see, how things add up. So at the recommended rate of nitrogen 140
kilograms per hectare, for example, is captured in the grain, and about the same
amount, in the right hand column with a higher rate of fertilizer.
So even though we added more fertilizer in the higher scenario, we haven't captured
more of that nitrogen in the, the corn grain or the stover, to any significant
point. But where the nitrogen ends up or, is over
here in these categories. So this is one example of how someone
might do a balance or a budget for a crop production scenario.
And these kinds of studies are very important to illustrate to us and to the
farmers where their nitrogen might reside, particularly, if they increase fertilizer
rates above those ergonomically desirable levels.
So what do you learn from a little study like this?
Extra fertilizer above the recommended rate, in this case, 225 kilograms per
hectare did not give us a return on investment.
Remember we want to help farmers make efficient use of their nutrients.
So an advice, somebody might advise this farmer, if they were using more than 225
to reduce the amounts, amount of nitrogen, extra fertilizer really just, in this
particular study, led to more nitrogen lost, and more nitrogen left behind in the
soil. So in that particular case, it didn't
result in, in increased crop. And so then we ask ourselves how is the
triple bottom line at work in this particular study?
And I'll leave that for you to, to ponder. Another aspect that we might learn from
studies like this, plants are really good scroungers, as it were, for nutrients.
They have good root systems, and if they're healthy they can take nutrients up
very efficiently from the soil. And so you see that the fate lines are
very closely packed in that figure at the appropriate nitrogen rate.
But those fate lines spread out, particularly in the loss categories as you
increase the fertilizer rate above the ergonomically appropriate rate.
So the farmer in that particular case, if they were using 300 or 350 pounds of
nitrogen, for example, can save money and also protect the environment.
This figure sort of summarize this whole concept, that at the, the appropriate
rate, which is the red line, as you reach that appropriate rate prior to that your
crop yield increases and responds to added fertilizer in a positive fashion.
But you do reach a point on the response curve where the response levels off, and
increasing rates of fertilizer beyond that, contribute to low nutrient use
efficiency by the crops. And also, probably, and of, of a concern,
would be the potential for losses of those extra nutrients, to the environment,
because if the crops don't take them up, they either stay in the soil, or they are
lost to the environment. I think fertilizer recommendations need to
embody more of this nutrient mass balance approach.
It's very difficult and it's very expensive to do, and we typically have
looked at simply the response of crops to increasing fertilizer rates, and measured
crop yield and crop quality. And only significantly at least in the
last several two or three decades have we tried to embody more of the measuring of
these different pools on the farm, and looking at how our fertilizer inputs
relate to crop productivity and nutrient loss from the farm.
And here's another, example that I think illustrates some of the things that we've
already talked about in the course. This happens to be rice production, and
you can see that they've used, urea fertilizer.
And remember, we talked about urea fertilizer, and the issue of
volatilization from this fertilizer. In this particular research they applied
this, nitrogen fertilizer in two fashions, surface application and incorporated in
the soil. And what did they find?
If you look at the blue numbers, those are the numbers that are associated with the
treatment where the fertilizer was incorporated in the soil, and you see that
more of the nitrogen ended up in the plants, in that case.
And down here at the bottom, more of the fertilizer ended up in the grain, and, but
on the other hand, with the surface application, a lot of the fertilizer, the
nitrogen ended up being volatilized away. So, in this particular study, the
different placements had a big impact, and so, if a farmer incorporated the nitrogen
in the soil, they're going to benefit, because more of that nitrogen ended up in
the plant, and less of it ended up being lost to gaseous losses.
So it's just another illustration in a crop production scenario, how you might
evaluate the economic and environmental, impacts and fates, due to different,
fertilizer placement, methods. So in a mass balance approach, we want to
know the inputs and the outputs, and we would like to do some calculations and
quantification of the different pools. And we're going to sit back with that
information and do an analysis, of that balance, to determine where those where
those pools are, and how much of our nutrient is in the various pools and will
determine which pools we need to take a, a closer look at and maybe devise some
better management strategies that we would recommend to the farmer.
This might involve some changes in culture practices.
And in the bottom line, what we are really doing is trying to refine our BNPs, and we
hope that we can do it in a farm scenario. Many farmers are very interested in this
particular topic, I find just in my work here in Florida, our farmers are extremely
interested in this. Fertilizer prices have increased, and the
environmental issues have becoming more and more important, and farmers are keenly
interested in learning more about the faiths of nutrients ah,on their farm.
And in our particular situation here in, in Florida, this triple bottom line is
very important, because we're farming in areas where we have a lot of very pristine
and important natural resources, for example our springs here.
And we're farming, and we're growing crops, we're producing food, and we're
using fertilizers and irrigation, in these scenarios, and so, farmers, as I
mentioned, they're very keen to learn about, better practices, because they want
to adopt them, on their farm. So if we look at a few take homes, the
nutrient budget approach or balance approach, takes a look at all of the
inputs and outputs as, as many as we can on farm.
And we would like to quantify them, and we can find information, published
information, or we may have to measure some of these ourselves, to develop, these
budgets. And I think more effort needs to be put
into refining our fertilizer recommendations in agriculture, taking
into account measurements or estimations of these different pools, particularly
those that represents losses from the economic side of the triple bottom line
and the environmental aspect. And if we can do this, if we can
understand more nutrient budget's and fates and flows of nutrients on our, on
our farms, we can do a lot to help farmers invest their inputs on the farm in more,
in ways that will have a higher return on investment.
And also we can do more to protect against the potential for, for losses of these
nutrients from the farm.
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