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

Nutrient Fates, Part 3

农业可持续土地管理

与讲师见面