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.
Soil Testing, Part 1
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Del curso dictado por University of Florida
Sustainable Agricultural Land Management
69 calificaciones
De la lección
Soil Management
For week 5, we will look at soil management in detail and begin to discuss soil testing.
Conoce a los instructores
George J. Hochmuth, Ph.D.Professor
Urban and Agricultural Environmental Sciences
Good day, everyone and, and welcome back. Before we get started today, I want to
just sort of recap a little of where we've been.
We've looked fairly intensively at nutrient fates and flows on a farm where
they come from and, and where they go, and a little bit about nutrient cycling.
We've also taken a, a good look at soils and the role they play in agriculture and
the fact that maybe we need to know a little bit about our soils before we start
dealing with developing best management practices.
We also took a look at some of the bigger scale soil conservation, soil management
best, best management practices. Today and in our next lecture, I want to
dig down a little bit deeper no pun intended to take a look at some best
management practices pertaining to nutrient management.
And to get started, I want to spend some time today and our, in our next lecture
talking about a very important part of nutrient management, and that is soil
testing. We've tried to understand a little bit
about best management practices. And part of that is understanding how much
fertilizer our crops are going to require. We don't want to put more fertilizer on
than they can use because that has negative implications for the bottom-line,
economically, for the farmer, and also potential problems with the environment.
And most everyone, I think, would agree, that a first step to determining a
nutrient management or a fertilizer management program on a farm is a good
soil testing. I think this soil testing applies to most
all of the crops that would, we would be growing or thinking about from an
agronomic or horticultural standpoint on our farms.
Why do we do soil testing? What is the purpose of it?
What are the parts of a good soil test and how do we develop a good soil test?
These are some topics I want to, to cover today.
So, what in fact are we really soil testing when we do a soil test?
I put this picture up here to sort of illustrate because it confirms some of the
things that we've already gone over in the course.
We understand a little bit about soils. And so now, if we take a look at this
picture we want to try to understand where our crops are deriving the nutrients that
they get. Some of these nutrients can be derived
from the native soil and that's the part that we want to understand quantitatively
of, with the soil test. And then, we supplement that native soil
fertility with, with fertilizers. So, we want to understand a little bit
about the soil's ability to, to provide nutrients to the overall nutrient
requirement for our crop. Some of these nutrients can, in fact come
from the soils, and in some cases, those amounts can be fairly significant.
The first step, obviously, is the soil sample.
And I've given you some pictures here to sort of summarize the general process.
There are tools that many of you that are doing this already know about.
If this is new to you, you can see some of the tools that are used in the profession
to get a soil sample. If you look at the bottom picture, you'll
see the so-called soil sampling probe. It's about one inch in diameter, and you
push it into the ground and it takes out a core of soil down to the depth, usually
six inches or 15 centimeters is what we're, we're shooting for.
If you don't have these kinds of tools, then a trowel or a shovel will work just
fine as long as you use it as is illustrated in the top cartoon.
Soil testing is usually practiced on a management scale.
So, we typically have some kind of an idea about a part of a field or a field or a
part of a farm that constitutes a management unit that we're going to be
instituting some kind of uniform crop production practices on.
And that would be our soil sampling unit. On large farms, this usually is about 20
acres. On a smaller farm, it may be smaller than
that and we want to be sure to collect a representative sample.
Our soil sample should be, to the best of our ability, representative of that of
that field, or that block or that management unit.
If we have several of these then we want to take a soil sample from each one of
them and we want several samples taken or several probes as they were taken from
each of those man, management units so that we can get some good amount of
representation of that management unit. Discontinuous areas or fields or parts of
fields that we know are a little bit different from the rest of the field.
Maybe we want to sample those separately to try to understand the differences.
Typically, we're taking a soil probe or a soil sample about 6 inches deep.
This is done because that is thought to be representative of the tilled area of the
field, so if we're incorporating fertilizers in the soil it captures for
many of our horticulture crops at least, it captures a significant part of the root
zone. So, the standard protocol for most soil
sampling procedures is to sample to a depth of 6 inches.
Now, soil sampling, we'll talk a little bit more about this as we go through the
next, this lecture and the next. Soil sampling works best for what we call
non-mobile nutrients. These are nutrients, that under most
situations do not move in the soil are less likely to be leached or held in the
soil either on the cation exchange capacity that we've talked about, or as a
precipitated like in the case of calcium and, and phosphorous.
So, soil testing works well for these, because the soil sample that we pulled
today and analyzed today will represent that nutrient status in the soil for some
time to come, at least through the cropping season.
Mobile nutrients like nitrogen we, most, most universities and most commercial labs
do not do a pre-fertilization or pre-crop soil sample for nitrogen because they
understand that with one rainstorm that nitrogen that we found today might be gone
tomorrow. So, we manage nitrogen in a little bit
different approach and we'll talk a, talk about that as we go forward.
Here's a picture that shows a field or a farm that has a little bit of variability.
So this person decided to sample these areas of this farm separately.
So, you see sample one, two, three, and four.
And you can get an idea the kind of pattern that his person walked over the
field to collect the, the samples. There's no set way to do this.
It's really logic and, and a decision that the soil sampler needs to make before they
embark on walking or riding around the field to collect the, the soil samples.
In a general rundown, most soil sampling and soil testing procedures goes like what
would be described here on this slide. The samples are collected in the field.
We've talked about the importance of collecting a good sample that's
representative of the field to the right depth.
Those samples then are likely to be put in a soil testing lab bag that you obtain
from the, the soil sample facility or the laboratory.
The soil, once it's received at the lab will be dried, usually air dried and then
sieved if there's some organic matter or pebbles or gravel in it to get a uniform
sample of the soil. That soil in the bag then is mixed and sub
sampled for the actual laboratory analysis.
So, several sub samples may be taken from that one bag.
The soil is then mixed with what is called an extractant, and we'll talk more about
extractants because it's a very important part of this whole process.
But basically, an extractant is a chemical that has been determined for that lab for
that farming region for the types of soils to be appropriate to mix with the soil
because that chemical, it's usually a chemical solution and when mixed with the
soil, it will extract, as its name implies some of the nutrients from the soil.
Once extracted in the supernatent from that mixture is then sampled and analyzed
to determine the concentration of phosphorous or potassium, for example,
that's in that soil and then the lab will generate from those results, a
recommendation as to whether that number is low, medium, or high and what
fertilizer recommendation needs to go with that.
And we'll talk a lot about that particular process.
Here's a lab technician in the upper left-hand corner taking soil samples from
those bags. You can see the tray of bags below his
left hand. And he's transferring a sample or a scoop
of those, of that soil into these vials, that or bottles that will then be mixed
with the solution and he's adding the extractant in the lower right-hand
picture. That tray of samples then will be, in
typical fashion, will be placed on a shaker and they will they will be shaken
for a set period of time appropriate for that particular soil analysis procedure.
In the old days these analysis labs were very time consuming.
The equipment was not advanced as it is today.
And typically the lab technicians analyze one nutrient at a time, for example,
potassium. And then, if the farmer wanted also to
know about phosphorus, that was a separate analytical procedure.
Today, there's a lot more sophistication in these labs and typically, these
extractions have the capacity to extract several nutrients from the soil sample,
and our machines or analytical equipment has the capability to analyze
simultaneously several if not many samples, dozens of samples at, at a single
at a single time. So, you can quickly get results from a
soil sample for a suite of nutrients that the farmer might be interested in.
Now, a little bit about these extractants, because it's important to understand what
an extractant is and what it isn't. It's typically a chemical solution
designed to extract or remove a certain portion of, of nutrients from the soil
sample so that those concentrations can then be determined.
Soil testing does not measure the total amount of nutrient in the soil, and that's
a very important point to make. The extractant removes or extracts a
portion of the nutrients, a portion of the phosphorous that's in that soil.
There are many types of extractant out there.
And I'll just say, at this particular point, we'll, we'll repeat it later.
If you're practicing soil testing, you want to make sure that the laboratory
employs an extractant that was developed for your farming area, for your types of
soils in particular. Not all extractants are correlated and
calibrated as we will talk about a little bit later for your particular farming
scenario. So, if you took a sample from your sandy
soils and sent it to a lab far away to have it analyzed, they may not be able to
give you valid results. So, just keep that in mind and we'll
reiterate that a little bit as we go through.
These extractants vary in their chemical composition and because of that they are
designed to, to extract different kinds of nutrients or elements from the soil and
perform under different kinds of soil conditions.
High pH, low pH, sandy versus heavy soils, for example.
If you took the same, took one soil sample and sent it to several labs that employ
different extractants, you would get different numbers back.
And this confuses a lot of people because they, they need to understand that
different extractants will, by definition, give you different numbers.
So, you need to, to know that the laboratory that you're proposing to use
sample and analyze your samples has, is employing the proper procedures that were
developed for your growing situation. Just an, as, as an example here's the so
called Mehlich-1 extractant. There are several Mehlich examples
extractant that have been developed over the years.
The Double-acid, this is another name for the Mehlich-1 extractant so-called Dilute
Double-acid extractant. It's a mixture of dilute acids,
hydrochloric and, and sulfuric acid, and the ideal is that this dilute acid will
extract or remove a certain portion of the nutrients from that soil when it's mixed
with the soil in the, in the lab. It will extract some of those nutrients.
The idea is that, that extractant, whether we're talking about Mehlich-1 or Bray-P1
or several of the other extractants extract some of the nutrients in the soil.
And that amount of nutrients that are extracted have, are related to the amounts
of nutrients that the, the, the crop or the soil can provide to the crop during
the season. And that's very, very critical.
You want an extractant that is correlated with a crop response and we'll talk a
little bit about that. Where does the nutrients come from that
the soil extractant is finding in the soil?
Well, they come from a variety of sources. For example, we've talked about some of
these when we talked about nutrient, nutrients in, in soils.
For example, phosphorus can be extracted from a soil because the extractant
dissolved some of the phosphorous from the minerals that were present in the soil.
And that amount of the phosphorus that was extracted from the minerals is related to
the amount of phosphorus that the crop will see and benefit from during the
growing season. Also, recall cation exchange capacity.
Some of these extractants can exchange some of the nutrients and measure them
from the from the, the particles in the, in the soil.
Here's another important part of understanding soil testing.
The number that we get from the lab after they have gone through this process and
they come up with a number that they measured with their analytical equipment
in that soil extractant solution, or mixture, is called an index, a soil test
index. This is the amount of nutrients that are
in the soil, that theoretically and hopefully the crop will benefit from, be
able to see as it were and take up during the season.
That's the whole purpose of soil testing. So, this has to be carefully thought out
and carefully designed. The index is not a measure of the total
amount of nutrient. So, for example, phosphorous, the soil
test index may only measure, extract, and measure a tiny fraction of the total
phosphorous that's in the soil. Much of the other phosphorous that's in
the soil may not ever be available to the crop anyway.
So, why have a extractant that would extract a large amount of phosphorous when
some of that phosphorous is not even going to be available during the season to the
crop? These indices, these index values need to
be interpreted. So, when I have a number that says, I have
30 parts per million of phosphorous in my soil, I need to know, what does that mean?
Is that very low, low, medium, high or very high?
And we'll talk we'll talk about that. These index values are usually expressed
as parts per million or milligrams per kilogram of soil.
Now, some labs express these index values as pounds per acre.
Sometimes, you'll see that very commonly. You have to understand that the pounds per
acre is also an index in other words, you can't do arithmetic with that index, that
pounds per acre. So,for example, if the pounds, if the soil
test index is 100 pounds per acre. And you think you might want to apply a
150 pounds of, of pea you can't do a subtraction and say, well, there's 100, so
I only need 50. That 100 may actually be a low or a medium
pea soil and you need to actually add more.
So, be very careful of the units in these in the soil test reports.
Some of us in this area would probably just prefer that the indices are not even
presented. Just, all the real, the farmer really
needs to know is low, medium, or high and a fertilizer recommendation.
So, I put this, I, I present this to you because most soil testing labs do present
the, the actual index number. But just be careful and make sure you
understand what that number means. So, these index numbers, for example, for
the Mehlich-1, need to be interpreted, and most labs have developed a scheme for, for
pigeon holing, as it were, these in, index numbers.
So, for example, a very low index means that you're going to get less than 50% of
the appropriate or maximum or expected yield from that crop if you do not
fertilize that soil. And so then, you can see that as the index
number gets bigger and bigger, that means there's more nutrient in that soil that
can be extracted, and theoretically, more nutrient that would be available during
the season for that crop. And you actually reach a point where you
have enough nutrient in the soil that you would not expect to get a response to
fertilization and we'll talk a lot about this as we go forward, particularly in our
next lecture. Now, there is two parts to developing a
soil test making it appropriate for your farming situation.
One is called correlation and we've kind of eluded to this a few minutes ago.
The question is, does the soil test extractant correlate with crop response?
So, a useful soil test, something that's going to be of value to the farmers, needs
to be correlated with crop response. So, what do I mean by this?
A high-extracted index means that, that soil is capable of producing high crop
yields without fertilizer. On the other hand, a very low or low soil
test index means there's very little of that nutrient in the soil, in the native
soil so that if you do not fertilize that soil for that crop, you will get low crop
response. So, high extracted nutrients are
correlated with high or associated with high crop response and low in, indexes
associated with low crop response. And that's the correlation.
You want that relationship to be as strong as possible, otherwise, the soil
extractant is not able to sort of give you the, the real world picture on the
nutrients in the soil. We can show it here graphically.
Here's a picture of some data from several experiments.
And so, so what this researcher did here was to take a crop, I think it was corn,
and grow that crop on soils of different extractable levels of phosphorous.
And so, you know, if you're fortunate enough to find soils in a research area
that have different levels of phosphorous in them, you can do studies like this.
And so, the soil test index across these experiments ranged from 0 to, to almost
60. And you grow the crop and you look at the
crop response. And so, in this particular case, this
extractant is highly correlated with crop response because at the low when soil had
very little this phosphorus in it, we got low yields.
And as the phosphorus the native phosphorus in the soil increased as
determined by our extractant, then we were, we get more and more, we approach,
approach are 100% relative yield. And as we increase, as we find soils that
are even higher and higher in phosphorus, the yield stays at a 100%.
So, this tells us that low, soils that have low content of phosphorus are going
to give us low yields if we do not fertilize them, and soils that have high
numbers are associated with high crop yields.
That means, that, that soil test extractant is correlated with that crop
response, and we have a chance now of making that extractant work in terms of
predicting fertilizer needs. To go to that step most scientists call
this phase of developing a soil test, the calibration phase.
So, we need, we don't only want to know that low numbers give us low yields, now,
we want to know how much fertilizer do we need to add to those soils that are low
because we want a 100% of our yield. And so in that zone of response, if you
noticed from the, the figure that is the area that we are concerned with now,
because that's the area when we get those certain index numbers, we wan't to know
how much fertilizer do we recommend to the farmer.
So, calibration is this process that will help us given enough research be able to
confidently tell a farmer, if you have a soil test index that is x, you need to add
y amount of fertilizer to bring that soil up to a level that will support optimal
plant growth. And so to do this, we work in that zone of
response where the crop yields were increasing as the, as the, the native soil
level in native phosphorus level in the soil also increase.
So, we want to set up experiments on those kinds of soils.
And s, here's a picture, our same our same graph with our same data.
And I've divided it into the non-responsive and responsive, by the red
line. So, those soils that are testing, say,
above 35 parts per million are not going to respond to added phosphorus.
We're already there. The soil has enough to supply the needs
for the crop. To the left of the red, this is where we
need to do some more research and divide that zone of response into smaller areas
for example, we might call this very low, low, medium, and then high to the right of
the red line. Now, we may not know this right away, but
for discussion purposes, I think we can sort of integrate in our minds the
parallel thinking process here. So, we'll call those, or we'll predict
that they, they're going to be called very low, low, and medium.
And so, we're going to go out and we're hopefully going to find some soils.
They're getting harder and harder to find low and very low soils, for example.
So sometimes, this kind of research, at least preliminarily, is done in, in
greenhouses. And so, I've titled this the law of
diminishing returns, and you'll see why in a second.
So remember, back to our soil test interpretation levels.
So, if we have a soil as depicted here that is has a very low number, five, let's
call it, very low number. We don't know exactly if 5 is truly very
low, it could be low. But we're going to find that out as we go
through this research. So, we have a soil that has a very low
number, 5, and we grow a crop on that soil.
And if you look at the x-axis this time, we're adding different amounts of
phosphorus fertilizer, in this case. And we'll do a replicated study and we'll
analyze it statistically and properly. And you'll see that the data from 3
experiments is presented on this slide. It looks a lot like the other, the
original correlation chart that we gave you.
If you look over to the left, where we do not add phosphorus to this, some of our
treatments are going to be a zero check, no phosphorus, we get about 20% of our
yield. Well, that falls in to the category as I
presented here, very low, less than 50% of our yield is expected.
And then as we add more phosphorus to this, this crop, you can see the yields
start to increase. And we reach a point up here in the green
oval, where we're now at 100% of our expected yield, and it looks like maybe
120 or so pounds, a P2O5. Now, I'm expressing this in P2O5.
You can do the conversion to P by multiplying by point 44.
So, with this experiment here on a really low P soil, we've, we've achieved a very
high response. And now, we can start piecing together the
puzzle. But we're going to need some other pieces
to go along. So, we hope we can find a soil that has a
little greater number than 5 I've chosen 15 here.
And so, we do the same experiment on this kind of soil and we apply different rates
of P2O5 across the x-axis. We would expect by our by our chart to
expect about 50% to 75% relative yield. And, in this particular case, you can see
we're getting about 55 or 60 percent relative yield when we do not add
phosphorous fertilizer. And as we add phosphorous fertilizer we
get the same kind of increase in yield, up to a point.
But now, pay attention because the maximum response now is lower, about 80 to 100.
And the reason is, is because this soil can supply a portion of that crop nutrient
requirement of phosphorous from the native soil.
We know that because at 0 phosphorous, our check plots, we did get a significant bump
in yield compared to our previous experiment.
So now, we have another piece of the puzzle.
And hopefully, we can find some soils that are testing high.
In a lot of agriculture areas, particularly in, in this country, many of
our agriculture areas are high because we've been growing crops and fertilizing
them for many, many years. And phosphorus, as you know, now know, in
most of our agriculture soils does not leach and so it builds up in these soils.
So, here's an example of where we've done the exact same kind of experiment.
We've added different amounts of phosphorus, the same, same experiment.
And when we omit phosphorous from our plots, we get basically almost a 100%
relative yield. And so, that means that this soil can
supply most, if not all, of the phosphorous requirements for that crop.
So, just to illustrate with these three simple charts, the process that scientist
go through. Now, they don't restrict themselves to
three charts and three experiments at each site.
Typically, for a well-calibrated soil test extracting and procedure, you would want
to look at different crops, you'd want to look at maybe different types of soil in
your region and may be over different years.
But I think by now, you get the idea of how the research is done to calibrate a
soil test procedure and also how detailed and rigorous the research is.
So, when you're done going through this process, or even along the way, you start
developing some preliminary charts like this, and here's our categories, very low,
low, medium, high, and very high. And now, you see from the previous
research, very low, 150 is probably the amount of phosphorus that we need to add
as fertilizer, that soil cannot supply hardly any of the nutrient requirement.
And then as the soil test index, index increases, we reach a point out here where
that soil is interpreted as being high or very high and can supply most, if not all,
of the nutrients. Now, this chart also has potassium and
magnesium on it. The same kinds of procedures and, and
testing would be done to calibrate a soil test extractant for those nutrients.
So hopefully, you understand a little bit more about the intense amount of research
that goes behind a well calibrated soil test procedure.
And how we do the research to develop and calibrate that extractant so that we can
now use the numbers, the index, and we can be fairly accurate as far as making a
fertilizer recommendation for a grower. And also, you, you should, you should see
how important it is to do this type of research as locally or regionally as
possible. Now, soil testing, as we've mentioned
before, works for our non-mobile nutrients like phosphorous, potassium in most
situations. There are situations where it's not that
accurate. And a good example would be on our very,
very sandy soils with low cation exchange capacity and taking potassium as an
example. A soil test may be of limited value in
those kinds of farming scenarios for determining potassium.
So, why not for nitrogen? Well the main reason is that nitrogen is
fairly mobile in most of our soils. And if we did one of these soil test
procedures before we were going to fertilize, by the time we put the nitrogen
fertilizer out there, the soil nitrogen status may have probably has changed.
So generally, nitrogen is treated on a little different basis.
And we'll and I'll show you how as we go through, the process here.
So, you need to know a little about your farming scenario, the soils, and you need
to ask hard questions at the soil testing lab.
They should be able to tell you about how their extractant is correlated and
calibrated. And maybe even should be able to bring you
some research papers that document how that process was done for that particular
soil test procedure. Now, when we talk a little bit about soil
tests and fertilizer recommendations, we focus mostly on the soil test itself and
I've showed you how soil tests are developed and how they're correlated and
how they're calibrated. And so, what the farmer is interested in
is I have a soil sample, I'm going to give it to you, you analyze it, and you tell me
how much how much fertilizer I need to add to that soil.
And it's usually fairly straightforward. But there are some things some processes
that some labs go through with the soil test index, even if it's a high index.
And this all gets down to sort of a philosophical approach to interpreting
that index and making a fertilizer recommendation from it.
And we're going to talk a little bit more in detail about that in our next lecture
because even though we have an index and even though the lab says, I have a
calibrated soil test. And at high I'm going to recommend 0,
there's still are a significant amount of farmers, we're all human beings, and
sometimes, it's really hard to say, yeah, I believe that, I'm going to add 0.
Because of the huge financial risk that farmers have in growing crops, fertilizer
is a relatively small input. And we'll talk a little bit about these
philosophical differences. And it'll, it'll lead, I'm sure to some
significant discussion out there on the internet.
So, what are some take-home lessons from this part?
Hopefully we've all been convinced of the importance of soil testing and developing
a good nutrient management program. Without a soil test, we really are
swinging in the dark as far as how much fertilizer to put on our crops.
And if we don't know how much fertilizer our crops really need, then either we'll
under shoot it and get a poor crop response, or will overapply and with the
potential problems with the environment. And we're moving into a realm now, had
moved into a realm now, where there's a lot of concern and a lot of attention put
on the environmental side of the triple bottom-line, and soil testing fits in very
nicely to a triple bottom-line, discussion.
So, the recommendations from our soil test are going to tell us how much fertilizer
we need to add to that crop so that we can get our maximum yield on that soil.
The management of those nutrients is going to be a whole other scenario that we'll
talk about in a couple lectures because that's important.
Knowing the rate is one part of a BMP. Knowing how to use that amount of
fertilizer properly is another part of it. Soil testing must be conducted carefully.
If you haven't given much thought to soil testing in your farming scenario, if
you're a grower and would like to know more starting with the scientific support
structure in, in your country, in your area for example, the extension service in
this country would be a good place to start.
Also, visiting that soil testing lab that's actually doing the analytical work
on your sample is very important. I find it I find it interesting that many
farmers do not really know what happens to that soil sample once it leaves their
farm. Where did it go?
How is, is treated? How is it analyzed?
What are the data and the research that backed up the, the fertilizer
recommendation that I received? And so, how the soil test extracted, was
developed, correlated and calibrated is incredibly important to practicing good
fertilizer management on the farm. Research is required.
There's been a lot of research done in this particular area over the years,
particularly in this, in this country. Unfortunately there is more that needs to
be done in this regard.
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