2.5 Tiny Bytes: Plant Nutrients and How They Move (or Don't) in the Soil

Jodi and Sarah explore the crucial distinction between mobile and non-mobile nutrients in soil. Understanding how these nutrients behave not only aids in effective soil sampling but also enhances soil fertility management, providing fertilizer users with the knowledge they need to maximize returns on their fertilizer dollars. 

They cover
• Mobile vs. non-mobile nutrients
• Nutrient behavior based on soil charge 
• Cation exchange capacity (CEC) and nutrient adhesion 
• Sampling strategies for mobile vs. non-mobile nutrients 

Episode Transcript

Jodi (00:00):
And now it's time for a tiny bite of knowledge.
Hey Sarah, have you everthought about why certain
nutrients are only tested on thetop zero to six inches of soil

(00:21):
samples, yet there's others thatare tested on both the zero to
six and the six to 24 inchdepths?

Sarah (00:27):
Yes, Jodi, I actually lay awake all night at night and
just can't get good sleep.
Help me with this.
I need to be able to sleep atnight.
Why are we soil samplings forcertain nutrients in the zero to
six and others all the way downto two feet?

Jodi (00:44):
Well, Sarah, I've got some great news.
We're going to talk all aboutit in today's episodes of Tiny
Bites, where we discuss mobileand non-mobile nutrients.

Sarah (00:55):
What is a mobile and what is a non-mobile nutrient?
Help me out.

Jodi (01:00):
So it comes all back to the name, right?
When you think about non-mobilenutrients, those nutrients,
those stay pretty much in place,relatively in place, and then
you've got mobile nutrients that, as water moves through the
soil, water is going to carrythose nutrients away, and so
that's really the big differencebetween our non-mobile
nutrients and our mobilenutrients.

(01:21):
But why?

Sarah (01:23):
would some nutrients not move with the water?
What causes?

Jodi (01:29):
that.
So one of the really big thingswe think about in soil,
fertility and just soil ingeneral is that soil is
negatively charged.
It's the cation exchangecapacity, and the composition of
its negatively charged claysplus negative charges on organic
matter contributes to thesoil's overall net negative

(01:52):
charge.
And so when we look at thesenutrients in the soil they are
either positively charged, likepotassium, or negatively charged
, like nitrate like potassium,or negatively charged, like
nitrate.
And if you've got a positivelycharged nutrient in the soil
it's going to stick to the clayand into the CEC, but if it's

(02:13):
not, it won't stick.

Sarah (02:15):
So it's just like the concept of a magnet, then.
Is what you're saying right?
So like, if you put a positiveand a negative end together,
it's going to bind together.
And yet if you put a negativeand a negative together, those
two's going to bind together,and yet if you put a negative
and a negative together, thosetwo ends are going to repel away
from each other.
In other words, if you've got anegative ion, like a nitrate,

(02:35):
and you've got a negativelycharged clay, those two are not
going, they're going to repeleach other and that nitrate can
move through the soil.
So is nitrogen a soil mobilenutrient then?

Jodi (02:46):
Nitrate is a soil mobile nutrient.
Yes, and there's differentforms of nitrogen, but in
general nitrate is the mostcommon form of nitrogen in the
soil and, for the most, partiumwhich is positively charged and
in that instance it can actuallybind to the cation exchange

(03:16):
capacity or those negativeparticles in the soil.
Yes, yes, and even though, likewe might hear about, oh, if I
have a CEC of X in the soil,that means I can apply a bunch
of anhydrous and not worry aboutit changing.
But that's not really the case.
Sure, there's some amount ofanhydrous or like the NH4 plus

(03:38):
molecule that's going to stickto the CEC, but it's not a
permanent sticking situation.
That's going to be transformedinto nitrate over time and it's
just going to depend on howquickly that is based on, like
soil, temperature and otherfactors.
But it's not permanent and sothat is also a little bit change
back into nitrate and flow awaywith the groundwater.

Sarah (03:59):
So let's talk for one second about which nutrients are
soil mobile and which nutrientsare not.
Are are soil immobile, sonitrate, sulfate, chloride,
those are all negatively chargedions and so those are soil
mobile.
But when you take a look atitems nutrients specifically I'm

(04:21):
thinking off the top of my headlike potassium and phosphorus,
are well known to be soilimmobile.
And phosphate is kind of aninteresting cat.
Okay, because that is actuallyphosphate.
It's a negatively charged ionbut it actually adheres to the
clay, it absorbs to the claysand it actually sticks on there.
It's very that one is actuallyvery immobile because of its

(04:44):
adhesion to the clays.
But potassium is kind of aninteresting cat.
Potassium is definitelysomething that is binding to the
cation exchange capacity.
Specifically, where we get thenegative charges on the cation
exchange capacities, it's on theedges of the clays and it's
from the organic matter.
It is not from the sands.
The sands are actually prettyinert.

(05:06):
So this is important to thinkabout because if you are in a
soil that is actually very sandy, with no organic matter, it's
actually possible to get thatpotassium to leach through the
profile and and you can't holdon to it.
As a matter of fact, in some ofthe potassium trials that Dr
Franzen had done at NDSU here afew years ago.

(05:27):
He mentioned that in some ofthese soils it wasn't actually
going to be possible to bebuilding those potassium levels
and not to even try, but ratherto make sure that the potassium
needs for the crop being raisedduring that year were met.

Jodi (05:41):
Yeah, so I mean another way to think about mobile versus
non-mobile nutrients.
If you're going to a lab andit's got options to what am I
going to test for?
Nutrients on the on the bottom6 to 24, the nutrients that
they're going to test are goingto be those nutrients that do
flow through the soil or aremobile, and those nutrients that
are only tested on the top 0 to6, they're only going to test

(06:04):
for those.
Well, they're going to test foreverything, but the ones that
don't make it across, both arejust going to be your non-mobile
nutrients, right.
But the ones that don't make itacross, both are just going to
be your non-mobile nutrients,right?
Those nutrients they tend tonot really go down the profile
very far.
So it's not going to be worthyour time or the laboratory's
time to test those non-mobilenutrients farther down in the
profile.

Sarah (06:25):
Absolutely.
And when you think about gridsversus zones, oftentimes we have
actually found that the soilmobile nutrients are predicted
better by zones, and oftentimes,when we are grid sampling,
we're focusing more onnon-mobile nutrients, such as
your P and K, and this isincreasingly important because a
lot of times when we're gridsampling, usually most people

(06:48):
are doing a zero to six profile.

Jodi (06:52):
But thinking about what Sarah mentioned before about
landscape position and thinkingabout how water moves across a
landscape.
So if we're looking to testmobile nutrients like nitrate,

(07:17):
that nutrient versus, you know,gridded squares across a field,
so think about water movement.
That's how you can think aboutthose mobile nutrients, whereas
there's a little bit of adifferent process for all those
other non-mobile nutrients tothink about where they are
located across the field.
Nutrients to think about wherethey are located across the

(07:39):
field.
To wrap this up, what'simportant to keep in mind is
that not all nutrients arecreated equal.
Some are mobile and move withthe water, like chloride,
nitrate, sulfate and some arenon-mobile and they stay pretty
tight to the soil, likepotassium and phosphorus, like
potassium and phosphorus.
Tune in next time for a tinybite of knowledge from GK

(08:02):
Technology, where we have a mapand an app for that.