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.
Agricultural Soils
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Del curso dictado por University of Florida
Sustainable Agricultural Land Management
68 calificaciones
De la lección
TMDLs & Nutrients
In week 3, we will discuss TMDLs and agriculture, agricultural soils, and the first part of nutrient fates.
Conoce a los instructores
George J. Hochmuth, Ph.D.Professor
Urban and Agricultural Environmental Sciences
Hello everyone, welcome back. I want to shift gears a little bit now in
today's visit with you and, and talk about soils.
I think soils are obviously very important to agriculture, but maybe not everyone has
looked at them to enough detail to really understand their advantages and challenges
that they present. Here's a video of a big field in northern
Florida, and I think one of the things that I notice about this particular field
just by looking at the surface, is the, the sandy nature of, of the soil.
So we're going to talk about different kinds of soils and some of the advantages
and, and disadvantages that these soils have.
There's a, there's a very nice video at this website for the novices of us about
soils. What soils are, how in the United States
we classify soils and, and how we determine what soils might be good for
agriculture, or for building a home, for example on.
So you might want to click on that, it's about, i think its about a half an hour or
so, but it's, it's probably well worth particularly if you're new to the idea of
soils and soil descriptions. So what is a soil?
There's lots and lots of different definitions of soil.
It really depends on what your particular approach and need would be.
The Soil Science Society of America, has a couple parts to a definition of soil.
If we were interested mostly in the agriculture end of things, we would be
particularly interested in that unconsolidated mineral or organic material
that is on the surface of the earth that we grow plants in.
If we're looking a little more geological about it, we would be interested more down
deep, and how that soil was formed, and what kinds of layers or horizon's that
soil has, and a little about the physical and chemical nature of that soil.
So there's different approaches to it, and we're going to try to look at both of
those as we go through the, through the course.
I also want to take this time just to call your attention to this idea of soil
taxonomy. The USDA NRCS, remember who the NRCS is
now, and what they go back to. The NRCS Is our agency, our federal
agency, that's very concerned about natural resource conservation's, and we'll
talk a little bit about the, the beginnings of NRCS a little bit later on
in the course. But the NRCS has this, this web-based soil
survey. And so you can go into this web survey and
look around in there and determine the, the type of soil that you have in your
backyard maybe, or on your farm or on your neighbors farm.
And you can, you can actually pick a, an area, a field, that you want to learn
about, and you can go in here and learn a lot about the soil that's in that field,
for example. The soil has a lot of properties that
affect water and nutrient management. And of course in this course we're very
interested in nutrient and water management.
So, for water management, properties of the soil, such as the soil texture, it's
hydraulic conductivity the water holding capacity and its natural drainage
characteristics are, are very important. For nutrient holding we might be
interested in the organic matter content, the cation exchange content of the soil,
maybe, maybe the PH. So let's take a look at some of these
characteristics properties of soil, and think about how they might affect us as we
try to manage nutrients and water on our farm.
Soil texture is the relative proportion of sand, silt and clay just simply,
straightforward that's what it is. The texture affects, water holding in the
soil and nutrient holding, particularly those cations or positively charged ions.
The picture at the bottom simply shows how you might get a rough idea of sand, silt,
and clay, by collecting the soil sample mixing it with plenty of water and shaking
it up, and just watching how the different fractions settle out, sand on the bottom,
and the finer clay particles on top. And, It may take several hours, if not a
couple days for this process to happen. The soil textural triangle, many of you
have probably seen this, maybe even have learned about it or studied it in courses.
This is a, a pictorial way if you know the proportion of, of sand, silt, and clay,
you can get an idea of the textural class for that soil.
What do you notice about this, this triangle?
Notice the proportion of the triangle that has clay in the term, terminology.
Much of the triangle, that's because clay is a dominant force, reactive force in the
in the soil. And you can see sand and loamy sands
occupy a small corner of the, the triangle over to the left.
So if you know the, the portion of sand, silt, and clay you can use this triangle
to find out where your particular soil would be, as far as its texture.
If it's a loam that means it's got mixtures of sand, silt, and clay, of
different, different proportions. Loams are very highly desired for
agriculture, because they embody many of the, they embody the attributes of all of
the, the different portions, the sand, silt and the clay.
Here's a sandy soil that you can see is, is possibly predict would be highly prone
to, to leaching, of nutrients, because it does not have a high water holding
capacity, that we'll talk about. It might be one of those sandy soils that
blows around in, in the spring. So knowing a little bit about the textural
class of our soil, can give us some ideas about its thee, it's performance as a, as
a soil for crop production, and how we might, It, it might be easy to manage that
soil, or it might have some, some difficulty in managing it.
Soil organic matter is another important property of soils.
Basically, anything that was once alive, from freshly deposited plant residues,
remember we saw we've seen so far plant residues on the surface of the soil, all
the way down to highly decomposed organic matter in the soil, that is more or less
stable organic matter in the soil. It can range from less than, actually less
than 1% to nearly 100%. And we'll, we'll look at some of those
types of soils that have a range of organic matter content.
If you remember back a couple pictures ago, that particular soil profile had very
low organic matter, surely less than 1% in there.
The organic matter is a mixture of living and recently dead material.
These can be organisms, microorganisms that are in the soil, or it can be organic
matter that came from a previous crop residue, or organic matter that we
actually add to the soil, for example through livestock waste.
Organic matter does two things for our soil.
It adds water holding capacity, and it also nutrient holding capacity.
If you can envision a lot of organic matter holding on to water, almost like a
sponge would. Here's organic matter from a previous crop
that's still on the surface, this cover crop has been tilled into that residue,
and that organic matter achieves a couple goals for this farmer.
One is, it protects that soil from erosion, for example, and also that
organic matter will eventually decompose, and those nutrients will be released back
into the soil to support the growth, of what is a cover crop in this particular
case. Now soil organic matter as it's
decomposing it some of that soil organic matter will go back to carbon dioxide and,
and be lost from the system. And this oxidation can be rather, rather
rapid in warm humid environments like the southeastern United States and, and our
soils here in this state of Florida. But this mineralization or decomposition,
also adds those nutrients that were once associated with that organic matter, and
they can be, those nutrients can be added back into the soil, to be taken up by
another crop. And that's one of the keys with organic
matter, particularly if you're adding it, to take advantage of those nutrients, you
want to make sure that you have a crop, growing, that can take up those nutrients,
otherwise once they're oxidized or mineralized from the organic matter, they
could be lost to the environment. So there's good news and bad news about
organic matter in the soil, and we just need to understand going back to our talk
about the processes in the in the, in the soil, and we'll learn a little bit more in
detail about those. But if we understand how that organic
matter is decomposed in the soil then we can institute management practices to take
advantage of those nutrients. Soils that tend to stay wet for long
periods of time during the year are soils that tend to have a little more organic
matter because it doesn't oxidize as rapidly.
Hydraulic conductivity is an important aspect of soils, and it's basically the
ability or measurement of how water moves through the soil.
Sandy soils would have a relatively high hydraulic conductivity, because they would
drain fast, they have large pores, and the water can move through those soils.
Clayey soils on the other hand, with small pore spaces drains, water would drain
fairly slowly through those. Here's a picture of a field several days
after a heavy rainstorm. So what would you say about the hydrolic
conductivity of this particular soil? Water holding capacity is another
characteristic of soil, and this is the ability of the soil to hold water against
gravity. So, if we put water on a soil, and then we
let it drain then the water holding capacity would be descriptive of the
amount of water that's left in that soil after all of the excess has drained by
gravity. Water holding capacity is related to
mostly to the, the smaller fraction, smaller size fractions the sand, I mean
the silt and the clay, and the organic matter, in the soil.
Remember, sand, because it's fairly large in size would not have a very high
water-holding capacity. So we're very interested, if we want to
have a soil that has a good water-holding capacity, we're probably looking for a
soil that has organic matter, and a significant amount of silt or clay in it.
Water-holding capacity is important, because it can determine how we manage our
irrigation system. So, for example for us in Florida with
very sandy soils, we know that the water holding capacity is fairly small, our
bank, as it were, of water is very small. So, we're more likely going to irrigate
several times with small, short irrigation cycles, to keep replenishing the water in
the soil. If we irrigate it for a long period of
time, and fill that water holding capacity up and kept on irrigating, then we know
we're going to be losing water due to gravitational forces, and If that water
has nutrients in it, then it could go below the root the root zone.
Soils have ranging from about a quarter of an inch to two inches per foot of soil.
So if you had a foot of soil, a depth of soil that was a foot, and you put on the
equivalent of say, a half an inch of water on the top of it, and that soil had a
water holding capacity of a half an inch, then that water would move down and
essentially fill that soil up without dripping or losing water from the bottom.
If you put on more than a half an inch, then you would be catching some water
dripping out the, the bottom. So that's kind of a description of how the
water holding capacity works. Cation exchange capacity is another,
property of, of soils. It's a measure of the soil to hold onto
positively charged ions, called cations. For us in farming this means ions such as
calcium, potassium, ammonium nitrogen, magnesium, these are positively charged
ions, and they're also important plant nutrients.
So cation exchange is important to us as a farmer, because that would tell us that if
we had a fairly large cation exchange capacity, that would tells us that we have
the capacity to hold on to significant amounts of these nutrients, and that's
important. Cation exchange capacity is largely
import, imparted to the soil by the clay and the organic matter in the soil.
Sandy soils do not have very high cation exchange capacity, because they do not
have a large amount of, of clay and organic matter.
So soils that are loamy in nature, or have silt and clay in 'em, remember back to
the, the textural triangle, those kinds of soils are going to have relatively high
cation exchange capacity, much more than sandy soils, and those would be more
fertile, so called fertile soils, because they're holding more of these positively
charge ions. So generally, the higher the CEC equates
with a little higher fertility in that soil.
Here is an illustration on how cation exchange happens by the virtue of the word
exchange. These cations can be exchanged for each
other on the soil the clay, or the organic soil particle.
And once they're exchanged, and those particular ions are in the soil solution
they have a couple fates, major fates, one would be taken up by the plant, hopefully,
or, they could be lost. In sandy soils, remember we talked about
high cation exchange capacity, but in sandy soils, we would have relatively low
cation exchange capacity, so there's not a lot of room, for holding cations like
potassium and magnesium. So sometimes these even though they're
positively charge ions, they can be lost to, to leaching, because there is not
enough cation exchange surfaces to, to hold large amounts of those nutrients.
So if a farmer put on a large amount of potassium fertilizer on a sandy soil, some
of that potassium fertilizer could be lost to leaching.
So what kinds of soils might we encounter? I just want to take you through a few of
the, the major soils that we have in Florida, that we use for agriculture.
I want you to keep in mind that this whole issue of soil taxonomy you know, there are
some differences about how specialists, approach this.
And so just keep in mind that we're talking about, some fairly general, kinds
of, aspects here. And, but it, but it should be good enough
for you to at least know a little bit about these kinds of, soil orders, so that
at least you can ask some questions about the soil that might be present in your
particular area. The soil orders here are those that are
used by the USDA NRCS here in the United States, and there are other countries that
have soil classification systems, they, they may not be exactly the same as we use
in this country. And there is a, a, there is an effort
internationally to try to harmonize some of the information in databases from one
country or another, but for this, for purposes of this course, we'll use the
USDA approach. So the soil order is the highest level in
this taxonomic classification system. There are actually six categories that go
all the way down to the series. The order gives an idea about the, the
soil that you're working with, and especially about it's development, the
processes that led to that particular soil being the way it is.
There are 12 orders in the scheme in the USDA scheme, and there are several
thousand series. And the soil series would be more like
what we would deal with on a more local basis.
A particular farm may have one major soil order, but maybe many series scattered
around the farm from field to field. Here's a picture of the different orders
as they would show up in this country, and you can see the, some of these orders
dominate the landscape in this country. For example, in the Midwest the Molisoles,
the dark green colors and the eastern east coast, Piedmont and Plains areas with the
orange opasol's. You can see these similar types of orders
do present themselves around the world, you can see the different kinds of colors.
Again, you know, looking at the United States, you see the major soil orders.
The green ones in the midwest, and the yellows on the southeastern and the east
coast really show up as being dominant soil orders.
And you can see those same orders scattered about the world.
Florida has seven of the, the 12 orders. So Florida quite variable in our, in our
soils, and as I've traveled around the state working with farmers on soil
fertility over 30 years, it's amazing how different farms can be because of the, the
soils that you're dealing with. So up on the panhandle, which is the, this
area up here, we call the panhandle. The red colors are the so called ultisols,
and then down on the peninsula you see we have entisols, and we have spotisols, and
then we have the organic soils down here in the everglades areas, those are the
soils that were drained for farming. We'll go through a few of these.
Here's the so called spotisols, this is kind of an interesting soil type, it
generally forms in, in very wet areas, with high water tables, we'll come, come
back to that, that picture. Sometimes people call these soils the so
called, Flatwoods, because they, they appear in our state in very level areas
that have high water tables particularly during, in a natural state, the water
table might be very high in these soils. Agriculture areas, we drain these soils so
that we can grow crops, and we manage the irrigation, the water table in those
agriculture areas where we're using spodosols.
These soils tend to be very sandy, as all soils do in, in the state of Florida in,
outside of the Everglades area, which are organic soils.
Most of our soils tend to be dominated by, by sands.
The spodosols has a highly leached, sandy layer, just under the surface, and, that's
called the E horizon, and the clays and metal oxides have been basically leached,
down from this, sandy soil over the years and years, and have accumulated in a,
what's called a BH, horizon. This is sometimes called the spodic,
layer, in these soils. This layer may have a slightly, a slightly
lower, hydraulic conductivity, because of the clay and the organic matter that's
accumulated there. Here's a picture of a spodosol, one of the
series, now remember there's, you know, several to many series of spodosols,
here's one called the Immokalee series in southern Florida.
You can see how whiten and bleached sandy the soil is.
Over many, many years the clay and the, metal oxides and the organic matter has
been moved down and accumulated in this layer in the in the profile.
So these different layers, the whole, the whole picture here is called a soil
profile, and then these layers are called horizons.
So you hear terminology such as A horizon, E horizon, B horizon, in these soils.
As I said before, these spodosols are used in, in Florida for agriculture because,
they can be easily irrigated, they're very sandy soils.
Here's sweet corn growing on a spodosol in southwestern Florida.
And you can see that farmers have learned over the years to bring water in through
these ditches and canals, and effectively raise the water table in that, field.
And sometimes, the spodic, layer, in the, in the soil, can, can, in some cases,
enhance the lateral movement of this water, so that it fills the field nice and
level with the water table. And so you can probably picture if you
have house plants, that you pour the water into the tray under the house plants, and
let it sub-irrigate, that's essentially what we're doing in this with taking
advantage of the spodosols being under high water table conditions.
We effectively raise the water table, so that we can irrigate the crops, and its a
relatively inexpensive way to irrigate, but on water use efficiency, and we'll
talk a little bit about this later on it, it's not very it's very efficient in, in
the use of water. Histosol's, these are the soils that were
drained in the Everglades area and you can see, and that they're uniformly consist of
organic matter and decomposing organic matter.
It's a very thick soil, sometimes these soils can be several meters thick but over
the years as we drained and farmed them, the organic matter just like the organic
matter in your compost pile will decompose and oxidize.
These organic soils were formed under wetland conditions, years and you know,
thousands of years of wetland plants living and dying and accumulating in,
under those wet conditions. And remember, organic matter doesn't
decompose as fast when it's in a wet environment.
Sometimes we call these muck soils because they can be kind of wet and sticky and
mucky. When these soils are drained, as we
learned to do all over the, the world, really, and to grow crops on 'em, when
they're drained they become very, very fertile soils, and particularly, are good
for crops like vegetables and sugar cane, and even sod.
And thinking back to our discussion about some of the properties of soils, if you
have a lot of organic matter in there, you have a lot of nutrient holding capacity,
and also water holding capacity. Now on the other hand these soils do
oxidize when they're exposed to air, they start decomposing and they will subside.
And that's because this organic matter is being oxidized and converted basically to
carbon dioxide. Now this oxidation does release the
nutrients that are in that material and so, fertilizer needs on, for crops grown
on these histosols, is considerably less than the fertilizer needed to grow the
same crop, on a sandy soil, for example. A soil that we'll talk about, in a bit,
where we need to add, nutrients, because the low organic matter on content.
Here is a picture up at the top of some of histosol's in the Everglades area of the
state of Florida. And its kind of interesting, here is a
picture here, we teach a summer course and we take the students travelling around the
state to visit different agriculture areas.
This is at our research center in southern Florida in the Everglades area, that
concrete post was put in back in about 1920, and at that time, the soil was at
the, at the top level of this post. And you can see over the years the soil
has subsided, oxidized and has disappeared.
And in fact in many areas of that agriculture area over years and years of
farming those soils in, even in the agriculture fields have subsided.
To a level where there's, there's maybe only a meter or so of the soil left from
an original several meters of soil. Another important soil is the ultisols.
Ultisols have a defying clay aspect to em, a dominating horizon, these are found on
the Panhandle of Florida, and all up down the Piedmont area, the eastern part of
this of this country. Here's an ultisol.
There are all kinds of different types of ultisols.
This is not you know you'd see ultisols that look quite a bit different, they'll
be very, very red in color. But you can see the different horizons of
this ultisol. So there's a loamier soil at the top, that
would be where we would grow mostly be, be doing agriculture, and then below that the
clay has accumulated, and it has a very thick and extensive clay horizon.
Here's a redder ultisol from the panhandle where the tree is tipped over.
There's also a, a field from the panhandle of ultisols.
So the looser topsoil, that tends to be a little more sandier in nature, is very
highly erodible in these soils. So, here's a case, we'll talk a little bit
later on in the course about conservation practices.
These soils can be highly erodible, because the topography is not level.
Entisols tend to be fairly sandy in nature, they also may have a weak or very
little diagnostic subsurface features or horizons.
They tend to be low in fertility and have low water holding capacity, because,
because of the sandy, very deep core sandy nature, so they're highly prone to
leaching, although they can be very good agriculture soils.
If you manage the inputs of nutrients and organic matter and manure for example, and
water these soils can be very productive they warm up very early in the spring
because of their sandy nature and they drain very well, also and those are two
desirable traits from a farming practice. Here's an Entisol showing sort of the
uniform, sandy nature down through the, the profile.
And an inset of our students, they're actually harvesting potatoes from a potato
farm that was just getting ready to harvest potatoes.
And this farm is, sits on top of some wide expansive very, very sandy soils entisols.
Entisols are all over this country. Here's a map showing where some of the,
the entisols make a big proportion or a large proportion of the, of the soils.
They're used in this state to produce all kinds of crops.
Here's a picture in the bottom of citrus production, on the, our sandy soils.
Here's a picture of a series, a Tavares series of entisols, and you can see, you
know, there's not too much that you can define as far as horizons in these soils.
So soil is very, very important, to our decisions about, managing it.
Here's a picture here, that shows two aspects to farming, in Florida, very, very
sandy soil and an irrigation system. And so, we should be able to start putting
together some of our thought process about a farming scenario like this.
How do we manage nutrients on this, on this farm, because of the sandy nature of
the soil? How do we manage that irrigation system to
apply water? Member our water holding capacity of these
sandy soils, our water bank as it were, is kind of small.
So hopefully this farmer is using that irrigation system to apply small amounts
of water, on a fairly frequent basis. Heavy and large amounts would likely push
nutrients and water down below the root system.
So, for our soil discussion, here, soils, I, I think probably are arguably one of
the least understood aspects to, to water quality protection.
How many times do we talk to farmers who really have, not a very extensive
understanding of the soils beneath their feet, and what's in those soils and how
those soils maybe beneficial or challenging to managing inputs such as
nitrogen and, and water. So I think understanding the soil or
knowing a little bit about the soils that you're dealing with, if you're an advisor
or you're a farmer you know, maybe after this course you'll be interested in going
out and finding what you can about the types of soils that you work with, or
about the soils that you work with, that your farmers have.
So that you can help them understand a little bit more how to manage those soils,
and get the return on investment, for their inputs for their crops, and also at
the same time you know, go a long measure towards protecting the environment.
Most agriculture areas are composed of a variety of soil, soil series, and some of
these may have some pluses and minuses. So that you may have to make, or a farmer
may have to make some decisions about how to manage part of the farm verses, as
opposed to another part of the farm. So i will leave you with a question, you
know, what types of soils do you have in your area?
I know here in Florida we have a variety of em, and just about everywhere you go
they're different, and it really causes a headache when you try to help farmers
devise best management practices, when these soils can vary all over the state.
So what kinds of soils do you have in your, in your state, in your country, in
your, in your home area, and what do you know about those soils, and where can you
get information about the soil? So I'll leave you with those thoughts.
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