15.5 Precision Composite Soil Sampling Explained

We unpack why “precision composite soil sampling” isn’t a contradiction and show how GPS-marked points turn an old method into a reliable trend tool. We cover workflows, gear, timing, and when shifting results should push you toward zones.

In this episode we discuss 

• Pre‑GPS composite sampling and its limits
• Marking exact GPS points for repeatability
• Keeping post‑harvest or spring timing consistent
• Tracking year‑over‑year P, K and pH trends
• When composites signal a move to zones
• Benchmarks versus wider composite coverage
• Core counts for quarters and half sections
• Gear tips: external GPS pucks and apps
• Faster fieldwork once points are mapped
• Starting with GK Field Mapper and ADMS exports

More info about GK Field Mapper here: https://gktechinc.com/gk_field_mapper/

Tune in next time for a tiny bite of knowledge from GK Technology, where we have a map and an app for that!

https://gktechinc.com/

Episode Transcript

Sarah (00:00):
And now it's time for a Tiny Bite of knowledge.
Jodi, we are going to talkabout the biggest oxymoron on
precision agriculture ever.

Jodi (00:18):
Ooh, what's that?
Usingfoam markers on a tractor with auto steer
?

Sarah (00:25):
Close, but no banana.
It is talking about compositesoil sampling that's precision.
Precision composite soilsampling.
Have you ever heard of thatidea before?
I don't know.
Have I?
It sounds a littleintimidating.
Is what does it involve?

(00:46):
Well, it involves a GPS forone.
I think I can deal with that.
Well, let's back up and talkabout what a composite soil
sampling is and what we used todo with soil sampling before we
got into precision soilsampling.
Did you ever soil sample beforeprecision agriculture was
around Jodi?
And if so, how did you do it?

Jodi (01:07):
No, because uh I think uh what is it, the first GPS
installed inside of a tractorwas like 2003.
So I was a very young girl whowas not soil sampling at that
time.
Gold, but dang.
To answer your question, I havesoil sampled before.
Actually, you know what?
I take that back.
I have never soil sampledwithout marking my points
exactly where I took a core.

(01:28):
But I want to hear yourperspective.

Sarah (01:31):
Well, here is age before beauty and all that.
Here we go.
Let me explain to you how weused to do it in the old days
when we used to walk three milesuphill to school and three
miles uphill back home again,and we couldn't tell because we
didn't have LiDAR to tell uswhere a uphill was.
But anyway, back in the olddays before we had GPS, the way

(01:53):
that we would soil sample is youwould pull into a field, let's
say it's a quarter section, youknow, 160 acres, for example,
and what you were trying to dowas get an average uh soil
analysis value for all your soilsampling parameters.
So you would soil sample, youwould take approximately 20 to

(02:14):
25 cores in the field randomlythrough the field, but in areas
that were quote unquote thoughtof as average.
So you would stay out of yourheadlands, you would stay out of
that low pothole, out of theditches, those kinds of things.
If you knew where an oldfarmyard had been where there

(02:34):
had been a lot of manure orweird things going on, you would
stay out of that spot and soilsample the the rest of the field
without that rich area ofmanure.
And that way you would get anaverage value.
So when we start talking aboutcomposite soil sampling, it's
kind of like the oldestfashioned way of doing soil

(02:58):
sampling ever.
You don't need a GPS, you justvisually look across the field
and you try to figure out wherethose average spots are and you
go and pull them, take 20 to 25cores in a quarter, you get them
back, and you get that averagesoil test.
That's how we did it in the olddays.

Jodi (03:17):
Sounds wonderful.
But it sounds like there's amore precise way to do this.
Yes.

Sarah (03:23):
There are many people today where composite sampling
maybe is still a fit, but thereare more precise ways of doing
composite soil sampling than theold days.
You can go out with a GPS andfind those spots on the field
where you want to pull thosecores from, those 20 to 25

(03:44):
cores, and you can designatewhere they should be pulled from
either ahead of time or whenyou're in the field.
You can mark them as you go.
Some people might put thosepoints on a map before they go
out there.
Then what you can do is soilsample every year in that same
spot.
So go back to that same pointevery year.

(04:05):
And theoretically doing that atthe same time of the year.
Now, when I'm talking the sametime of the year, I'm talking
about like post-harvest orspring, but let's not do spring
and post-harvest.
Let's try to be consistent withthe general time of the year
when we're pulling those soilsamples.

Jodi (04:23):
That is that's such a great, great way to do it.
Um, I mean, it sounds like, andand having worked at a soil
testing laboratory, one thingthat's really important about
looking at soil test results isthat it's really nice when you
can compare from year to year,right?
If you're asking the question,like, am I building soil test
phosphorus over time?
Um, am I, do I seem to reducein phosphorus over time?

(04:47):
Do I seem to reduce inpotassium over time or maybe
increase?
If you're not pulling the coresin the same areas of your field
from year to year, and when Isay same areas, I mean from
those same precise points thatare marked, it's really hard to
say whether or not thatdifference from year to year is
because of a fertilizerpractice, something that you've
done or haven't done, or if it'sjust because you've you've put

(05:11):
or drawn a core from someplaceoutside of where you did before.
So adding, simply adding thatprecise, the precision component
of sampling in the same placefrom year to year adds so much
value in terms of what thosetotal test results can tell you
about a field.

Sarah (05:30):
That's so true, Jodi.
You know, there's places aroundhere in the Red River Valley of
North Dakota that are prettyflat and pretty consistent
across the uh across thelandscape.
But uh, and and compositesampling actually works fairly
well in some of those fields.
And some of those fields youstill need to think about doing

(05:50):
um zone management, even whenthere is the same soil type in
the same field and it's allflat.
But if you use precisioncomposite soil sampling where
you soil sample in the samepoint every year, if you get a
different value, for example, ofyour soil pH or your phosphorus

(06:12):
from year to year, that can bea trigger to tell you that you
should probably start thinkingabout adding in some other
precision soil samplingconcepts.
It might be time to think aboutdoing some zones, just trying
to figure out if there'ssomething different out there.

Jodi (06:27):
That's such a great point, Sarah.
So, another thing to thinkabout too is there's a form of
soil sampling or a moreprecision coded version of
composite sampling where you setbenchmarks out in a field where
you measure the same area, youchoose where your benchmark is
going to be, and then you samplethat same spot from year after

(06:50):
year after year.
The one challenge with this islike, even though you might have
benchmark spots that you decidethat represent the whole field
and you're going back to thatsame spot year after year, the
challenge in that is that it'shard to pick one spot or even
like three spots that reallyrepresent what, like, how do you
choose one benchmark spot thatyou're coming back to every year

(07:12):
that represents the wholefield?
So it's another way to do it.
But if you ask, I mean, ifyou're gonna be the person that
picks the benchmark, and anybodythat doesn't know the field
isn't gonna be what the one thatwants to choose that, by doing
more cores across a wider area,like you would in composite
sampling, right?
Like you recommend 15 to 20cores in every zone or every, I

(07:35):
don't know.
What is your how many cores doyou like in your composite
sample, Sarah?

Sarah (07:39):
For a for quarter section, I usually like to pull
20 to 25.
If it's a half section, I'm upat like 30 is usually pretty
standard, 30 to 35, somethinglike that.

Jodi (07:53):
Yeah.
So just, I mean, spreading outthat the spatial variability,
even though you're only getting,you know, one result with a
composite or a benchmark withthat composite where you've got
those 30 different soil samplingpoints that are coming together
for the average, you're reallyspreading out that variability
and getting a more average rateor an average soil test result.

(08:13):
So just something to thinkabout when you're considering
these things.
But yeah, I mean, if if you'resomebody that's soil sampling
and you know, you're doingcomposites and you just want to
level up the value of what thosesoil tests are coming back as
and what they're helping you oryour customers do in terms of
soil fertility, get out thereand get a GPS.

(08:34):
Yes, most phones and a lot oftablets will have an internal
GPS, but if you get like anexternal GPS puck, like a Garmin
Glow or others that you canconnect to your devices, those
tend to be more reliable.
And you can go ahead and startmarking those solo sample core
points so that you can come backto those every single year.
And not only do you get thebenefit of hopefully stabilizing

(08:57):
those soil test values overyear or year over year, so you
can do some comparing andcontrasting, but also it can
hypothetically speed up the timethat you're in the field.
If you know where your pointsare going to be, instead of
having to decide in the fieldwhere you're gonna sample, it
can speed up your soil samplingprocess too.

Sarah (09:15):
Actually, Jody, I think that's a very important point
for this particular podcast.
Once I started marking pointsin the field, um, the first time
I would lay out my samplepoints, um I would create those
points when I was in the fieldand I wanted them to be well,
you know, good, good points thatI was laying down.
And so the first time that Iwould lay those soil sample

(09:36):
points out, it would take me alittle bit longer.
But then the years that cameafter that, boy, it sure sped me
up.
I knew exactly where I wassupposed to be going.
And, you know, up here in thenorthern plains and North
Dakota, it actually gets dark,you know, pretty early in
November.
And so it actually has helpedme work a little bit longer into

(09:58):
the night, like, you know,eight o'clock at night, nine
o'clock at night, if I needed toget something done before we
had some weather coming orwhatever, and it was getting
dark outside.
So it's always nice when you'vegot that computer there and you
know where you're supposed tobe driving.

Jodi (10:12):
Absolutely.
So speaking of like advances intechnology over time and moving
from, you know, non-precisioncomposite sampling to precision
composite sampling.
And if you're somebody that'sbeen sample soil sampling for a
while and you want to not have acomputer in your vehicle, or if
you're if you're looking tostart collecting these points

(10:35):
where you're pulling samplesfrom and you're doing composites
and you've never done thatbefore, really, really great
place to start is to downloadthe GK Field Mapper app.
Um, just the free version.
You can record your soilsampling points, you can keep
them in your phone, you can savethose soil sampling points for
future use and you can come backto them year after year.

(10:56):
And if you're somebody that'salready has your points marked
in ADMS, you can get GK FieldMapper where you can just export
those points that you've got inADMS saved right onto your
phone, onto your iPad, your yourAndroid tablet, whatever you've
got, and have those ready to goand be out there making precise
composite samples.

Sarah (11:16):
You can embrace the oxymoron of precision composite
soil sampling.

Jodi (11:22):
Oh, I love that.
So do I.
Because at GK Technology, wehave a map and an app for that.
Tune in next time for a tinybite of knowledge from GK
Technology, where we have a mapand an app for that.