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 Fates, Part 3
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Del curso dictado por University of Florida
Sustainable Agricultural Land Management
46 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
Hello everyone, and welcome back to our course.
If you recall in our last lecture, we talked about the various nutrient sources
and fates and flows on a farm. And we looked at crop uptake and we looked
at different ways that nutrients move on the farm.
And we acknowledged that plants that we fertilize, particularly with nitrogen
usually do not take up all of that nitrogen fertilizer.
In fact, somewhere between 30 and 70% is about as good as most of our crops had
been measured. So we asked where would the rest of the
nitrogen that we applied go. And we looked at different fates for that
nitrogen. And we asked our self, would this
particular fate be important for losing nitrogen.
I want to continue that discussion a little bit today with you.
Before we do, I know that some of you might be asking.
Well, what can we do about some of these losses?
And we're going to cover this topic a little bit more in detail when we get to
best management practices, but I thought I would talk a little, just to take a minute
or two to mention a few of the things that we have developed, the scientists have
developed to reduce some of the losses and the potential for losses of nutrients.
For example remember we talked about urea as a popular fertilizer And the fact that
urea could be lost to volatilization if we apply it to surfaces particularly in high
pH soils. Well the urease enzyme that does the.
Conversion can be inhibited by certain inhibitors, so farmers can use urease
inhibitors to reduce the loss of nitrogen from urea fertilizers.
Likewise, there are also inhibitors to slow down the conversion of ammonium forms
of nitrogen to nitrate. And remember we talked about the fact that
you ammonia forms of nitrogen can be attached or associated with the soil
particles. And in that case, if we can keep the, the
nitrogen in that form, we can keep it associated with our soil particles for a
longer period of time before it's converted to nitrate.
And when it's nitrate, then it can be taken up by the plant or lost to the
environment by leaching. There are other management strategies that
farmers can use, for example split applications.
Portioning out the amount of fertilizer for the season in small.
Doses through the season so that the large, so that the total amount of
nitrogen is not risk to anyone of these loss pathways during the season.
Denitrification could be a factor on some farms and in the case of vegetables,
growers use raised beds to help minimize flooding of those soils which would be
conditions for, for denitrification losses.
I want to touch on a couple more pathways that are very important on farms, because
these are serious pathways for nutrient losses, both phosphorous and nitrogen.
The first one, nutrient runoff, is a very important one on agricultural settings,
particularly farms that are on rolling topography, where the land is sloped.
Nutrients that move offsite from these farms do so usually with water.
Although, we will acknowledge that wind can also be a factor in moving nutrients
offsite. Here are some scenarios shown in these
photographs where in the spring up north with the melting snow and ice can set up a
situation where surface runoff can, can happen.
And that runoff can carry sediment, some of our soil with it, and nutrients that
might be associated with that sediment. The middle picture shows you a picture
here from Florida where heavy rain has eroded the topsoil from parts of this
field. And this particular run off would contain
soil, obviously the water is colored with the soil and the nutrients.
And as I said, wind is the factor in moving soil around.
Particularly sandy soils that are very loose.
Also irrigation is a factor. As your going to see as we go through this
lecture and the ones coming up, water and irrigation management are extremely
important on farms for managing nutrients. Most of our nutrients will move with
water. And so, if we're in an irrigated
agriculture, the farmers are in control of irrigation and can do a lot to minimize
the losses of soil and nutrients through excess irrigation.
This picture shows excess of irrigation on a crop, and you can see that some of the
water is already starting to flow out of the field.
Also recall that we talked a little bit about the tile drainage used in the
midwest or in the upper midwest of this country.
And the benefits that farmers and the environment get from tile drainage because
if we can drain the soils more effectively they can then hold more water from rain.
For melting snow for example and the surface runoff is minimized.
But we also acknowledge that nutrients can get in these tiles particularly nitrogen
that might leech through the soil and end up in the tiles.
So farmers have devised with the help of scientists to figure out ways to minimize
the nutrient load in these in these tile drains and in the subsequent ditches.
And if you recall back, we talked about the idea of a denitrification wall, where
we take advantage of some natural processes.
Some natural fates of nitrogen, of nitrate in, in soils denitrification, for example.
So here are some, here's a picture of a potential best management practice for
these tile drained fields. Where these tiles instead of draining
directly Into an open ditch first go through a what's called, a biofilter here
so that the nitrates can be removed from the tile drain water before it's put into
the ditch. Another way to reduce nutrient and
sediment loss from these fields, from erosion, is with a constructed wetland.
This picture shows you a constructed wetland that has plants growing in it.
So, the runoff would be deposited in the, the constructed wetlands, the plants would
take up nutrients and the water that flows out of the constructed wetlands then would
be cleaned to some degree, before it flows back into the ditches and on to a natural
water body. In this particular video here, we show a
constructed wetland on one of our research centers that catches the runoff from the
fields and also from the greenhouses. These kinds of water bodies are very
effective in removing nutrients and, and pesticides from the runoff from
agricultural fields. Another approach is with vegetated ditches
or grass waterways. Vegetated ditches have been the subject of
considerable research in this country in the tiled drained areas.
Basically, instead of cleaning out the ditches, which you may have noticed in the
previous pictures of those ditches in the tile fields, they would be planted with a
plant material that would then slow down the run off into the ditches but also,
keep it there for a long enough period of time for plants to remove nutrients from
the run off. So, so far in taking a look at fates we've
touched on these different areas of potential fate for nutrients in a, on a
farm. And we've talked a little bit about
runoff. There's another on that I want to touch
on, and that is leaching. We've already mentioned this several times
through the course so far. Leaching is simply the downward movement
of water-soluble nutrients through the soil profile by water.
And so the water can be either from rainfall or irrigation.
It becomes an economic and environmental problem when the nutrients move below the
root zone. So if water moves nutrients from the
surface down into the root zone, that's a good thing because then the plant has
access to those nutrients. But when sufficient water is added and
moves the nutrients below the root system then they might be on their way to a
groundwater area or to run off. The soil water holding capacity that we,
we talked about before is an important factor in helping determine how, how much
water the soil can hold before it fills up so much that it moves below the root zone.
And also we question what kinds of nutrients would we be most concerned about
with leaching, and in our very sandy soils for example those that we have in the
southeastern part of this country nitrate leaching would be very important because
remember those sandy soils have very low cation exchange capacity.
And nutrient-holding capacity and low organic matter, and so mobile nutrients
like nitrate and even potassium can move with the leaching front.
For example, here's a very sandy soil. Remember our entisols, and I've shown you
both the leaching of nitrates, which we're not too surprised by, because nitrate
isn't going to be attracted to the cat-ion exchange capacity on the soil, but it
might be surprising to see potassium there.
Well, in these sandy soils potassium, the cation exchange capacity is probably so
low that at rates of fertilizer that agriculture would be using only a small
portion of that potassium may be held on the cation exchange capacity, because the
total volume of exchange capacity is so small.
So even in these soils, we talk about the likelihood that potassium as well as
nitrate can be leached. So what effects nutrient leaching?
What are some of the factors that we would think about or consider.
Obviously the nutrients we need to know a little bit about the soil that we're
working with, particularly the soil texture.
The organic matter content of the soil because remember, organic matter is going
to help hold water in the soil so we may hold more of that water against leeching.
Before finally the whole system is exceeded and we, and we have leaching.
Irrigation management is a huge factor in whether or not we'll have leaching.
And we'll talk a little bit about some of the, the methods to manage irrigation so
that we can mitigate against large leaching losses.
And obviously, the way we manage our nutrients, particularly those nutrients
that are likely to leach in very sandy soils like nitrogen and also the weather.
Now, we can't do much about the weather, but, we can also anticipate the kinds of
weather events that we might see during the growing season and develop our
fertilizer and irrigation management, strategy to anticipate some of those
potential challenges. Leaching is an interesting process because
it, it is one that we can understand and, and measure and a considerable amount of
research has been done on leaching in this country.
For example you can measure the amount of leaching that goes on in a field if you
have the, the, the appropriate equipment. Lysimeters a device for collecting water.
In the soil, under the bed or under the, the, yea, below the root zone is a good
approach to do this. And these are simply devices that in one
way or another collect the water that's moving through the soil profile, and you
can then analyze that water for the nutrient content.
There's a couple of approaches to using lysimeters.
In this particular picture here, I'm showing a suction lysimeter.
This is a long tube with a porous ceramic tip on it.
And this, this long tube is buried in the ground down to the, the, the full length
of the of the tube. The plastic tubing, tubing that is
sticking out of emerging from the, the top are the sampling tubes that we would come
along and sample. The idea is that you draw a vacuum on this
apparatus and that brings in the soil pore water through that porous ceramic tip and
it collects inside the lysimeter. And you can collect a sample of that pore
water and analyze it for, for nitrate or for other nutrients that you may have
interest. The, the dr, the suction lysimeter is very
effective tool, but as you can imagine, it has some limitations.
For example, if you're interested in getting the total amount or volume of
nutrients that move below the root zone the suction lysimeter, is limited because
it will sample a small portion of that. Water and you will not know where the
volume came from specifically, but you can analyze the concentration in the soil pour
water. A drainage lysimeter, or a gravity
lysimeter, is a more effective tool. In this picture, I'm showing you Drainage
Lysimeter being installed in the ground below what will be the root zone.
In these particular case, these basin's then can collect the lead shape which is
moving downward below the root zone. And, if you have a reservoir that the
basin drains into you can collect the total volume of leachate that moved into
that basin directly from above and so you can measure the volume and you can measure
the concentration of the nutrient and in that way you can calculate load.
Here's a research field with students sampling the leachate in a reservoir from
drainage lysimeters that are buried below the surface.
They're testing the amount of nitrogen that moves below the corn root system in a
field where dairy manure has been applied. And the two large white pipes sticking up
above the ground or the sampling ports for bringing all of the, the leachate out from
the reservoirs from below. I want to touch on this particular issue
because I think it's very, very important in managing nutrients in an agriculture
setting, and that is the relationship between leaching and irrigation
management. To get a good idea of how it's related if
we can understand our soil and what kind of soil type we are working with and the
likelihood of leaching, then we can look back at our irrigation management If we
know the crop water needs, then we can manage our irrigation so that we do not
apply more water than the crop needs at any one particular time.
So if we combine the crop water needs with the soil water holding capacity, then we
can schedule irrigation and time them very effectively.
And we also need to know something about the irrigation system and how efficient it
is as well. And we'll talk a little bit more about
this later on, when we talk about irrigation, in particular.
But the, the take-home lesson is that irrigation amounts that exceed the
water-holding capacity will move, and if they move below, the root zone, then
they're going to be taking nutrients. And those nutrients not only represent a
financial or monetary loss to the farmer, but also a potential environmental impact.
Here are a couple irrigation systems that we'll talk about later.
Centerpivity irrigation, in the top picture.
Understanding something about the uniformity of water application and the
rate of application of these systems are very important to managing that water
application appropriately, so that we keep the water and nutrients in the root zone.
The picture on the bottom with the blue rings is simply a drip-irrigated bed with
plastic mulch and we injected a blue dye through the drip-rrigation tubes so we
could see it in the soil. And it's a very instructive way to use
with farmers. To show them how their irrigation
management is, is happening. In this particular case you can run the
irrigation for different lengths of time, and you'll demonstrate to farmers where
their water and thus their soluble nutrients, for example, nitrate would be
in the bed. In this particular case, you see several
things. First of all you see that the uniform, you
see that the uniformity of the drip irrigation system, by the uniformity of
the blue rings. You also see, at least in this particular
length of operation of the irrigation system, how far the wetting front from the
drip emitters progressed down through the soil profile.
In this particular case, you would not want to run the irrigation system much
longer, or you would run the risk of moving.
The water and the nutrients deeper in the soil profile and possibly below the root
zone. But anyway, these are two examples,
particularly the one with the drip irrigation of how you can demonstrate
water management efficiency. And I, I just want to bring up this point
because I think it really is extremely important.
As you go through the course you'll, you'll think about some of the principles
and some of the themes and I think definitely this is, this is one that we
should be left with when we finish the course.
It's really about the water. If we can manage water then we can talk
about managing nutrients. It's very hard to work this system the
other way around. It's very hard to get growers.
To change fertilizer management strategies if we don't consider what they're doing
with their irrigation programs and this is particularly important on our sandy soils
because as we've already acknowledged things can really move fast in, in those.
So for a few take home lessons, at least for this particular lecture, you know,
and, and thinking back to the last couple. It's important to, to get a hold of the
idea that really we're after improving our efficiency in nutrient management because
if we can do that, and more of our nutrients end up in the crops that we're
growing, then less will be available for losses to the en, to the environment.
Runoff and leaching are two of the most important avenues for significant nutrient
losses that also involve significant potential for the negative potential for
the environment. Other gaseous losses, for example might be
significant to the economics on the farm, but in the case of say, for example,
volitilization may not be that negative to the environment.
Water is very important because water drives these processes these losses on the
farm, leaching and runoff. These two losses patterns, leaching and,
and runoff also illustrate the importance of managing nutrients and water.
Together on the farm. You really can't do one without the other.
They are, they are linked and as we'll talk about a little bit later on in more
detail. There are many ideas for, for reducing
nutrient losses. We've known about some of these for many,
many years. Particularly soil conservation practices
after the Dust Bowl. And we'll talk a about some of those
practices as we go forward
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