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 Management, Part 3
Loading...
Del curso dictado por University of Florida
Sustainable Agricultural Land Management
59 calificaciones
De la lección
Water for Agriculture
For week 7, we will wrap up nutrient management and then move to water for agriculture. We will dive deeper into the water of Florida and then begin the topic of irrigation management.
Conoce a los instructores
George J. Hochmuth, Ph.D.Professor
Urban and Agricultural Environmental Sciences
Hello again everyone, and, and welcome back.
And to start off our seventh week of the course, I want to continue our discussion
about nutrient management and fertilizer management, best management practices, and
I want to talk today about the fourth of our four r's.
I've entitled this right material. Some people call it right source, right
fertilizer we can supply these nutrients in several different ways to our crops,
process minerals and salts, that we might have available to us.
Super phosphate, potassium chloride, potassium nitrate just for three examples.
We might also choose particularly for our intensively grown crops although this is
starting to change and is even being used on corn.
But using control release fertilizers these are Coded products or some kind of a
more complicated material organic compound that has nitrogen associated with it.
We'll talk a little bit about each of these and then also organic sources.
Such as compost and, or, or manures. Our fertilizers have many, many times,
come from or derived from stan, standard minerals and salts that we, may mine, from
the ground and process into various. Fertilizer materials, for example
phosphates. They can also be manufactured salts Some
of these materials may have more than one nutrient in them.
For example we might supply nitrogen and phosphorous from diammonium phosphate or
some kind of ammonium phosphate material. Often these materials are blended together
to supply some of all of the three major nutrients, N, P and K.
So remember, when we have a soil test performed and we have a recommendation for
the nutrients that we need we may need all three of those nutrients and the
fertiziler company or manufacturer can select the right kinds of basic materials
and blend them together to give us a fertiziler that will supply all of those
nutrients. I've given you just a few examples of Some
of the basic materials yh, that we would use to supply phosphorus, nitrogen and
potassium, and even magnesium. We haven't talked about some of these
other nutrients for example sulfur and, and magnesium in this course.
But just to show you that there are several sources of most of these nutrients
that we consider essential plant nutrients, and that in many cases we
supply, as fertilizer to our crops. So, if you went through a list of those
materials, like on the previous slide, could you find out what is the nutrient
content of those materials? So for example if you had potassium
nitrate, what is the potassium and nitrogen content of those materials?
Fortunately for those of us today the internet is a, is a great place to search
out and find information about that particular topic.
But it's important to know, because knowing the concentration of the
particular nutrient in a particular fertilizer can help us determine the cost
and the, the economic efficiency. Of those materials because different
fertilizer materials have different cost associated with them and so it's the pound
per actual nutrient that we're concerned with on our farm.
And we'll, we'll take a look at these. I'll post some, some questions just to
give you, those of you that are not familiar with this area, some opportunity
to practice. The right sources or the right material
there are several factors associated with it.
You want to select materials that supply the nutrients that you need.
If you do not need potassium, for example, if the soil test says you do not need
potassium, then selecting a nutrient's solubility, selecting nutrients that
supply phosphorous and nitrogen would be the way that, that you would go.
Cost is a factor and again the cost per, the cost per pound or cost per unit of
actual nutrient, plant nutrient is what is important.
Not so much the cost of the actual fertilizer material itself.
You might be concerned about the nutrient release patterns, particularly as you
think about incorporating controlled release materials into your fertilizer,
blends. Also soluability of the fertilizer is, is
critical if you're thinking about disolving it and injecting it into an
irrigation, system. So you would want to select a nutrient
that would be highly soluable, in, in that case.
That may drive the price up a little bit, but it's where you need to be because
you're thinking about saving money by injecting fertilizers through the
irrigation system. I just keep in plant, keep in, in mind
that when it comes to the plant a nitrate ion that's in the, in the soil pour water
near the root of this The plant that we may have in mind.
That plant really, tells no difference between that nitrate ion if it came from,
several different kinds of fertilizer materials.
A nitrate ion from potassium nitrate is the same as the nitrate ion from ammonium
nitrate and it's the same as the nitrate ion from the mineralization of organic
matter. The plant can't tell the difference, so
that means that you think more about the fertilizer material.
And the consequences of the different, sources and selecting of the different
sources to supply those, nitrate ions in that particular example.
Controlled release fertilizers are, have been around for a long time in, in various
forms and are becoming more and more popular today and I think for a couple of
reasons. One is the increased efficiency in
nutrient utilization with controlled release fertilizers.
And also the cost of these materials is starting to close in on other sources of
nutrients. So more and more farmers are getting more
and more interested in controlled-release fertilizers.
And there are several different approaches to this.
I've given you a couple of them here. They can be complex mate, molecules that
take a some time for the soil microbes to degrade them and release the nutrients for
example nitrogen. They can also be coated.
In the old days we had sulfur coated products.
We still have sulfur coated products. We're getting, more and more sophisticated
and knowledgeable about coating fertilizer materials, with various polymers, to
control the release rate of nitrogen, for example.
But the basic idea is to have a fertilizer material that acts almost like a small bag
of fertilizer and apportions out the nutrients from that particle from that
control released particle more in tune with when the crop is able to take them
up. So now we put out fertilizer, and that
fertilizer material acts like the time release, capsule, as it were, and we
don't, we, we now do not need to go through the field several times to apply,
split applications of, of fertilizer. Controlled release fertilizer materials
work very well for some of the more mobile nutrients.
And for example, here in Florida nitrogen and potassium, particularly nitrogen is
probably the most often used material, or fertilizer material, in a controlled
release Fashion and controlled release products are a way to increase,
potentially at least, increase our nutrient use efficiency, because
theoretically if we choose the right product with the right release timing
pattern, we can schedule the release of those nutrients from that part of them
when the crop requires them. Now I know I'll probably be condemned for
not showing all of the different controlled-release products but I did want
to let you know that there are lots and lots of different products out there.
Many of these are internationally Available and some of them you might have
been around long enough, that you might even recognize the names.
In the middle of the picture, is one of these more complex, nitrogen containing
materials. It's not coated, as the, the, sulfur
coated urea in the lower left hand. But nonetheless, it acts almost like a
slower releasing, material. Because it takes time for the microbes to,
dia, to, decompose, that material, and release those nitrogen, molecules.
In the lower left hand corner I've shown you a, an example of a coated material
where urea, which is soluble so if we used urea, uncoated urea, it would it would be
solubilized very rapidly. And, converted to ammonium and then to
nitrate very rapidly. And then opens up all kinds of different
scenarios for losses of the, the nitrate. And perhaps in sandy soils, even the
ammonium ion. In this particular case we're protecting
the urea. And because it's coated, the urea can be
cybalized by moisture moving in and out of that, particle or that coating.
And, and allowing the urea to, to move out and into the soil on a slow released sort
of time release, controlled release fashion.
So the release mechanism differs depending on the particular product, at least to
some degree, but the general idea on some of these Polymer coated or materials is
shown here in this particular picture. So the fertilizer on the inside is
solubilized as water moves into the, the particle and then that solubilized
nutrient solution, as it were. Then moves out in very, very slow fashion
into the soil surrounding that particle. And if there's a root in the vicinity,
then that root can take up those that released nutrient.
And there's always a cache of nutrient left behind in the particle, to be
released. And the thickness of the coating and the
type of coating would govern the rate and the timing of the, of the release.
Here's an example, all of these controlled release products are very highly studied
as far as their release pattern. In this particular case here, you can see
that almost 50 percent of the nutrient is released in the first three or four weeks.
And then the release pattern tails off, as time goes on.
So if you know something about the particular release, release pattern like
this, then you can, decide, what product is most, most efficacious, as it were, for
your particular crop. So if you have a crop that grows very fast
and needs more up front, release. Then you would select that kind of a, of a
product. Here's an example of the advantages or
some of the positive attributes of controlled release products.
In this particular case here we're talking about potatoes.
And the current recommendation at least here in Florida for a For potatoes, is 200
pounds of nitrogen for the season, split applied during, during the season.
So this particular slide here shows you that about 80 to 90% relative yield and
this experiment was acheived by. The current recommendation 200 pounds
split applied during the season. Here are some examples of controlled
release products that were applied in different splits as it were.
So, for example here's a 100 percent. Applied it one time and then here's
something 30, 30, 30. And different kinds of ways.
Here's 100% controlled release put out at planting.
The take-home lesson here is that at, the other point to, to remember here is that
all of these controlled-release products were applied at 150 pounds.
50 pounds less than the current recommendation.
And you can see there are several if not most of these controlled release
fertilization approaches that can equal the yields achieved with 50 pounds more
from ammonium nitrate. So the take home lesson is that with the
control release products as shown by this research you can put 'em out in different
fashions. At a reduced rate and still get the same
potato productivity. So if you look generally at some of the
advantages that people are starting to recognize with controlled release
fertilizers the time and the fertilizer can be released closer to the, to the crop
need. Remember our crop growth curve.
The, the idea that we can release nutrients in more in tune with the way the
crop grows should increase our fertilizer efficiency.
And if we can do that, we might be able to cut back on the total amount of nutrient
that we're applying to that crop. So even though Controlled release
fertilizers might by themselves be more expensive.
The fact that we can reduce the rate and reduce the number of applications during
the season, those applications might, come, come together to at least close the,
the, expense gap, associated with, some of these, products.
So, I, just to mention, we don't address gaseous losses to any great degree in this
particular course. But nonetheless, they're becoming more and
more important as, scientists study agricultural systems, The there's been
some studies, some recent studies that show that controlled release fertilizers
can actually help to reduce the amounts of gaseous losses, such as Dinitrogen oxide
or, or, or nitrogen oxide. And some of these are dangerous gasses for
the for the environment. And because I think mostly because you're
more efficient that plant has a chance to use those nutrients, particularly that
nitrate ion as it's being released from the, the controlled release particle.
So while you're gaining efficiency and nutrient uptake You're also reducing the
chances of introducing more of the these greenhouse gases into the, into the
atmosphere by going to some of these more efficient ways of fertilizing crops.
And so just recall back when we looked at nitrogen trans, transformations.
We talked about gaseous losses and some of those are sort of innocous to the
atmosphere whereby nitrogen by dinitrogen but also can be a problem with some of
these other oxides that, end up in the atmosphere.
A few more words about, manure sources, as, manures as sources of nutrients,
Livestock waste are the way we mostly think about this, particularly poultry
manure and, dairy, manure. On a local basis you may have other kinds
of manure sources available, to you. These manures because they are organic
materials do contain pretty much a complete suite of nutrients that are
important for crop plants. We're mostly interested in for fertilizer
purposes, mostly interested in. N, P and K but these manures also supply
other nutrients like calcium and magnesium for example.
They're relatively low concentration fertilizers and so therefore they, they.
Add to the costs of production because, of the trans, largely because of the
transportation. So if you have something available very
locally, then they can be, effective and efficient and economic, economical sources
of the nutrients. They behave I put in quotes, slow release.
It depends on the mineralization rate. Remember these organic materials have to
be decomposed and the nitrogen, the organic nitrogen that's in them converted
eventually into amonium And nitrate for the crop to take up.
And we've already alluded to the fact that these materials can be applied to our
crops as solids or as liquids. Here's a picture of manure, poultry manure
application on a Eastern shore of Maryland.
Remember we talked about the Chesapeake Bay.
And so best management practices have been developed in, in that area of this
country. Particularly targeting, utilizing poultry
manure on the farms, as a source of nutrients.
And in that particular case then, along with that, best management practices for
how to apply and when to apply, these, nutrients.
So that these materials so that the nutrients don't end up in the water body.
And like with the controlled release fertilizer is having a release curve
mineralization is the release pattern for the nutrients in, in the manure.
So for example on this Particular picture here you can see, that the nutrients, in
this case nitrate is released very rapidly, within a short period of time, 3
or 4 weeks again. And then the release, the further release,
tails off. So if you had manure and you're going to
be using it for, crop production then data like this would indicate to me that you
need to be very careful because mineralization happens very rapidly and
those nutrients can be released from that manure fairly rapidly and you would need
to time the manure application fairly close to when you're going to have a crop.
In the field for taking up the nutrients. Sometimes this becomes a little bit
difficult for growers to be able to do, but nonetheless this is an aspect of
manure utilization that we all need to be aware of.
So you need to know the source of the, of the material.
These, these manures can be analyzed for their nutrient content so that you know
how many pounds of phosphorus, for example, per ton of manure that you're
going to apply, and we've already talked about best management practices for
storing it when we talked about the NRCS. Agency having [INAUDIBLE ] funds to help
farmers store their manure under cover so that the, the nutrients do not leach out
from the manure if it's stored outside uncovered.
And you need to go through some mathematics to calculate out the proper
rate. And you need to know something about the,
the mineralization of the nutrients from that manure so that you can time their
application. And, we've talked a little bit about
incorporating, manures, either in, in a tillage system where you might plow them
down, or incorporate them mechanically or in a no till situation, where you might
actually inject the material, into the ground.
A few other a, aspects about fertilizer management that, that would fit into some
or all of these, these Rs that we've talked about.
Plant tissue testing and crop monitoring I know some of you might be interested in
this aspect because once we have the crop in the ground and we've, and we're doing a
good job with our fertilizer management, we want some kind of a system, so that we
can determine, some feedback mechanism from the crop as to how well we're doing
with our fertilizer management. The, most commonly used practice is
through some kind of plant tissue testing, either using dried leaves or harvesting
leaves from a random sampling throughout the, field, drying them and analyzing them
in the lab for their nutrient content. And there's lots and lots of laboratories
Commercial and, public laboratories through land grant, universities that can
do the, actual analytical work. Recently fresh petiole sap testing, has
come about. And some universities, including, the
University of Florida has done a lot of work on calibrating, fresh petiole sap
test. Testing I'll show you that in a second
crop sensors have come about now and are getting better and better at detecting
nutrient deficiencies by looking at and basically looking at the color, of the
crop particularly for chlorophyll as an indication of how healthy the crop is.
We also have in-season soil testing for nitrogen.
Remember we talked about nitrogen being mobile in most of our soils and we usually
don't use a pre-fertilization soil test for nitrogen.
But if you're on the eastern shore of Maryland and you're using a lot of,
manure, on your farms, and you know that, that manure is mineralizing all through
the season. It would be nice to have a way of testing
the soil, to get an idea of how much of that nitrogen is being released at any one
particular time, in the season, and whether or not you would need to.
Supplement it with a side dress of, other types of fertilizer or maybe not, make an
application of, supplemental fertilizer, as the case, might be.
Here's an example of, you know, how We've already looked at a curve like this, with
crop response to fertilizer. Here in this case on the x-axis we have
nutrient content. And as, as the crop grows the nutrient
content increases. In the, in the crop as we add more and
more fertilizer to it. And we'll, we, we'll reach a critical
point here where this is the maximum concentration of a particular nutrient in
the leaves, for example here, the leaves of a tomatoe.
That'a called the critical value. So below that would be an indication that
the crop is deficient in nutrient. Above that we have adequacy.
We may even have a zone out here where we're well beyond the critical point and
we Probably over fertilized in this situation.
So a, a tissue test can help determine where on this curve you are, and that's
something that growers are very, very interested in.
And many growers will take tissue samples through the season And develop quite a
data set on, on their nutrient status of their crops and they can make good
decisions about fertilization practices based on looking at the, the results from
tissue sampling. There are other sensor material equipment
out there to take a look at crop growth And some analytical techniques, sort of
labratories for, for the farm. The lower right hand corner is the, Is an
ion specific electrode that we had been using for pedial sat testing.
And this is an example where during the, during the season you can take a leaf
sample from the plant and using the pedial and squeezing the juice, or the sap out of
the pedial you can use this ion specific electrode and tell the concentration of
nitrate for example or potassium in that sap.
And so this is a way that a farmer can do an on the spot testing, without having to
collect samples and send them away to, to the lab.
Here's an example of, just for field tomatoes how this petiole sap, works.
Now these nitrate numbers here will be different than you would get from a lab if
you sent them/g. Them, a leaf sample to be drug down and
tested for, for nitrates. This is the fresh sap, and these numbers
are going to be considerably lower because we're working with fresh sap, which is
largely water. But you can see that early in the season.
We want to see higher concentrates of nitrate nitrogen in the petiole sap and as
the crop develops the sap numbers go down. So the fact that they decrease during the
season is, is normal. What we want to know is, eh, are the, the
leaves in, in the right range. And we can do the same thing, for
potassium. This is a particularly useful test for
those, farmers, particularly farm, vegetable farmers, that are fertigating
through their drip irrigation system. So in that case, they can make rapid
changes in their fer, fertilizer injection programs based on a, a sap quick test.
So these are very handy and a lot of growers, particularly vegetable producers
are using these. And just to give you an example of the
fact that this system works here, these are watermelons.
This was a, on-farm demonstration test that was done in north Florida.
So we had several farmers, that we selected.
They were all growing watermelons. We used, sap testing on them.
And the, at this particular, results here. We tested through the season.
But this particular set of results were pulled just before, harvesting.
And our recommended range for watermelons near harvest is 600 to 800 parts per
million and I think if you look down through this set of data it doesn't take
long to realize that those farmers that were maintaining the nitrates in their
petioles in and around this range were the ones that had the highest yields.
The farmers that were maintaining very high concentrations this late in the
season were the ones that didn't get the little didn't, didn't get the gold star,
although it's not gold star, it's a black star.
So the ones that got stars were the, the growers that were actually Maintaining
lower nitrate concentrations. So it gives just, just goes to show that
over fertilization does not necessarily result in an increased yield.
So it's a really neat on the spot test to see where you are in terms of fertilizer
management. As I mentioned also soil nitrate tests
are, are available and this is an approach to use a similar kind of look at the
actual nitrate's nitrogenous in the soil. These are soil samples that are pulled and
analysed in a lab for nitrate. Nitrogen in the soil, and if you pulled a
sample, say, partway through the season and you had put on manure, on a corn,
crop, and you knew that you were probably, most of that manure was being mineralized
Being taken up by the crop and now you are at a point in the crop sequence where you
needed to make a decision about an application of nitrogen.
So a pre-sidedress, that's why it's called pre-sidedress is, before you make a
decision to sidedress with actual, with additional nitrogen, you run this test
determine, to determine if you really need that sidedress application.
And there's been several studies on this particularly in New England on manuered
soils and here's an example this is the this is the calibration so, if you had if
you had a Presidedress Nitrate Test of greater than 25 parts per million nitrate
nitrogen the probability that you're going to get a response to added fertilizer is
very low. If you had a Preset Presidedress Nitrate
Test that was less than 21 parts per million.
Then it's a higher probability that you're going to get response to your nitrate
application and you might want to go forward with it.
And these are the data so as you go out further on this x axis, the nitrate
concentration increases in the soil. And you can see that as you go past about
25 parts per million here, the probability you know, of responding is, is very low.
And it's in this range here where you're going to get a response to fertilizer when
the soil is, is less than 25 parts per million of nitrate.
And also, some other technologies that are out there for making us more, helping us
become more efficient with nitrogen fertilizer, nitrification inhibitors and
urease inhibitors. Inhibitor, so think again back to the, the
nitrogen transformation lecture we did several lectures ago and the pathways of
how nitrate or nitrogen forms in the soil or are converted back and forth.
And it just so happens that you can intervene in some of these pathways with
various inhibitors. And technology has come along to provide
materials that we can mix with our fertilizers.
And keep a particular, Nitrogen in the form that we actually applied it if that's
our, our desire. And I'll show you how that works, how, how
that's come about. The overall goal with using these,
products and mixing them with our fertilizers is to gain efficiency.
And reduce the losses of these nutrients to the environment.
Remember ammonium forms of nitrogen in fertilizers.
In some cases this is how we apply a nitrogen fertilizer.
It just so happens that many times it's a less expensive way of applying.
And also remember on those soils that have a lot of cation exchange capacity they can
hold on to the ammonium form. So we would like to keep that fertilizer
in that form as long as possible because once it's nitrified to nitrate then it
could be subject. To leach and there's, there's a product
here, Nitrapyrin that can slow down that conversion of ammonium to nitrate and
likewise recall urea and the transformations that urea undergoes in the
soil. There are materials that we can use to
slow down that conversion of urea to, to ammonia and I've given you one of the
examples, here. So we can, we can interrupt, these, these
pathways and maintain the fertilizer material, or in a form, that we would
desire it to be in for a longer period of time.
And these are used, practically by farmers to increase the efficiency of, of
fertilizer. And then finally you know there's, in the
computer age, there's a lot of information that growers have access to, equipment
that growers have access to help them become more efficient.
Recall that not every farm has very uniform optimally uniformed land.
So there's a lot of variability out there in the fields as far as their nutrient
content. And wouldn't it be good if we could
identify those areas, and then as we apply nutrients to that field, we can change the
rate of application of nutrients across that field depending on some, some
database that we developed, that describes the nutrients, contents, across that field
and then when we drive the fertilizer applicator across it changes the rate of,
fertilizer application on the go. So for example you might have some fields
that have this kind of variability in them and you may have this variability
described for example it might be a high PH or a low PH or a low phosphorous area.
Well, it probably needs a different fertilizer treatment than say this area of
the field. So you can do that, if you have enough
information about those different, places in the field and so here's a, here's just
kind of a, a graphical example of theoretically this tractor is going
through the field applying, in this case liquid fertilizer.
Because you see the coulter and the knives.
And here the different colors are different kinds of say for example,
nutrient content in those soils. And so this machine then, can adjust the
amount of fertilizer that it's applying Depending on the nutrient content in those
areas so that the ones that need fertilizer get fertilizer and the ones
that already have enough for example get less or none and this is very effective
when you're talking about managing ph with lyming materials for example.
It can also be used to measure yield. So if you've done this process and you
have variably applied, the nutrients, to a field, you can then at harvest time,
figure out through the use of this same technology, to Collect data on the yield
over those same spots that you've identified.
And so you can then start to couple how well your fertilizer management program is
by looking at the yield responses and you can make adjustments.
So this is a practice that is offered through a lot of private consultants.
We've been working with some of them here in Florida on peanut fertility,
particularly with liming and also with potassium.
So here's an example of where the field has been described, broken out into grids,
they call it grids. And then as you go through and apply maybe
the fertilizer or the lime and then you come back at the end of the season and you
harvest. And you can build a, a terrific database
with the idea that eventually you can get very, very efficient about how you apply
your fertilizers and, and, and really becomes and get really high a return on
investment. Here's a picture of a, a machine, a
planter machine going through the field, and it's set up to apply different kinds
or different amounts of fertilizer on the go, based on the mapping of the nutrient
status of that soil. Very, very, very, very important
technologies. So if you take home from this particular
part of the course, we're focusing mostly on the fertilizer materials.
We've identified the fact that, that fertilizer sources can vary we have lots
and lots of options out there for fertilizer material so we can mix and
match the fertilizer material and the amounts to suit the nutrient needs for our
crops. And[noise] to reflect what's required out
there by the soil tests. Technologies are developing on controlling
and managing, nutrient release. Through controlled-release fertilizers.
Plant tissue testing is, very, very sophisticated.
We have now. Techniques and technologies to test crops
through the season anytime on the spot as we go through the season and make, fine
tune adjustments in our fertilizer program.
And we have new, technologies that are already in, in place and more coming along
that farmers, take advantage of some of the natural processes that are, are
undergoing, in our fields. And control, the form of nitrogen for
example, that's in our, in our field. And improve or tip the balance in favor of
crop uptake. As opposed to nutrient leaching.
And if we just summarize this whole aspect of the four Rs and fertilizer.
Management and utilization. We started out talking about soil testing
and the important role that soil testing, plays in efficient, nutrient management.
Not only to, in terms of return on investment of our fertilizer dollar, for
the farmer. But also coming up with strategies, to
help the farmer make efficient use of fertilizer so that less is lost, to the
environment. Fertilizer rate is very important but it's
not the only part of a good fertilizer management strategy.
It's, it's one of the four Rs. And I think understanding the four Rs and
determining how to how to Put them into action on our farm is very important,
because we'll improve the fertilizer efficiency on the farm, we'll minimize and
reduce greatly the risk to losses of nutrients, to the environment, and really
those are two of the factors that we're very, very concerned about, particularly
today. So that should summarize most of the
material that I wanted to cover on. On nutrient management.
Those at least in this particular state are what we embody in our nutrient
management, best management practices. So the four R's are a very important.
Theme throughout our best management practices, whether you're talking about
corn or soy beans so-called row crops or sugarcane or our vegetable crops, these
are very, very important aspects of nutrient management.
Explora nuestro catálogo
Inscríbete de manera gratuita y obtén recomendaciones personalizadas, actualizaciones y ofertas.
Coursera brinda acceso universal a la mejor educación del mundo, al asociarse con las mejores universidades y organizaciones, para ofrecer cursos en línea.
© 2019 Coursera Inc. Todos los derechos reservados.
