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 

Chapters

• 0:00 Mobile vs Non-Mobile Nutrients in Soil
• 6:52 Movement of Nutrients Across Landscapes

Transcript

Jodi: 0:00

And now it's time for a tiny bite of knowledge. Hey Sarah, have you ever thought about why certain nutrients are only tested on the top zero to six inches of soil samples, yet there's others that are tested on both the zero to six and the six to 24 inch depths?

Sarah: 0:27

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 at night. Why are we soil samplings for certain nutrients in the zero to six and others all the way down to two feet?

Jodi: 0:44

Well, Sarah, I've got some great news. We're going to talk all about it in today's episodes of Tiny Bites, where we discuss mobile and non-mobile nutrients.

Sarah: 0:55

What is a mobile and what is a non-mobile nutrient? Help me out.

Jodi: 1:00

So it comes all back to the name, right? When you think about non-mobile nutrients, 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 carry those nutrients away, and so that's really the big difference between our non-mobile nutrients and our mobile nutrients. But why?

Sarah: 1:23

would some nutrients not move with the water? What causes?

Jodi: 1:29

that. So one of the really big things we think about in soil, fertility and just soil in general is that soil is negatively charged. It's the cation exchange capacity, and the composition of its negatively charged clays plus negative charges on organic matter contributes to the soil's overall net negative charge. And so when we look at these nutrients in the soil they are either positively charged, like potassium, or negatively charged, like nitrate like potassium, or negatively charged, like nitrate. And if you've got a positively charged nutrient in the soil it's going to stick to the clay and into the CEC, but if it's not, it won't stick.

Sarah: 2:15

So it's just like the concept of a magnet, then. Is what you're saying right? So like, if you put a positive and a negative end together, it's going to bind together. And yet if you put a negative and a negative together, those two's going to bind together, and yet if you put a negative and a negative together, those two ends are going to repel away from each other. In other words, if you've got a negative ion, like a nitrate, and you've got a negatively charged clay, those two are not going, they're going to repel each other and that nitrate can move through the soil. So is nitrogen a soil mobile nutrient then?

Jodi: 2:46

Nitrate is a soil mobile nutrient. Yes, and there's different forms of nitrogen, but in general nitrate is the most common form of nitrogen in the soil and, for the most, partium which is positively charged and in that instance it can actually bind to the cation exchange capacity or those negative particles in the soil.

Jodi: 3:20

Yes, yes, and even though, like we 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 about it changing. But that's not really the case. Sure, there's some amount of anhydrous or like the NH4 plus molecule that's going to stick to the CEC, but it's not a permanent sticking situation. That's going to be transformed into nitrate over time and it's just going to depend on how quickly that is based on, like soil, temperature and other factors. But it's not permanent and so that is also a little bit change back into nitrate and flow away with the groundwater.

Sarah: 3:59

So let's talk for one second about which nutrients are soil mobile and which nutrients are not. Are are soil immobile, so nitrate, sulfate, chloride, those are all negatively charged ions and so those are soil mobile. But when you take a look at items nutrients specifically I'm thinking off the top of my head like potassium and phosphorus, are well known to be soil immobile. And phosphate is kind of an interesting cat. Okay, because that is actually phosphate. It's a negatively charged ion but it actually adheres to the clay, it absorbs to the clays and it actually sticks on there. It's very that one is actually very immobile because of its adhesion to the clays. But potassium is kind of an interesting cat.

Sarah: 4:49

Potassium is definitely something that is binding to the cation exchange capacity. Specifically, where we get the negative charges on the cation exchange capacities, it's on the edges of the clays and it's from the organic matter. It is not from the sands. The sands are actually pretty inert. So this is important to think about because if you are in a soil that is actually very sandy, with no organic matter, it's actually possible to get that potassium to leach through the profile and and you can't hold on to it. As a matter of fact, in some of the potassium trials that Dr Franzen had done at NDSU here a few years ago. He mentioned that in some of these soils it wasn't actually going to be possible to be building those potassium levels and not to even try, but rather to make sure that the potassium needs for the crop being raised during that year were met.

Jodi: 5:41

Yeah, so I mean another way to think about mobile versus non-mobile nutrients. If you're going to a lab and it's got options to what am I going to test for? Nutrients on the on the bottom 6 to 24, the nutrients that they're going to test are going to be those nutrients that do flow through the soil or are mobile, and those nutrients that are only tested on the top 0 to 6, they're only going to test for those. Well, they're going to test for everything, but the ones that don't make it across, both are just going to be your non-mobile nutrients, right. But the ones that don't make it across, both are just going to be your non-mobile nutrients, right? Those nutrients they tend to not really go down the profile very far. So it's not going to be worth your time or the laboratory's time to test those non-mobile nutrients farther down in the profile.

Sarah: 6:25

Absolutely. And when you think about grids versus zones, oftentimes we have actually found that the soil mobile nutrients are predicted better by zones, and oftentimes, when we are grid sampling, we're focusing more on non-mobile nutrients, such as your P and K, and this is increasingly important because a lot of times when we're grid sampling, usually most people are doing a zero to six profile.

Jodi: 6:52

But thinking about what Sarah mentioned before about landscape position and thinking about how water moves across a landscape. So if we're looking to test mobile nutrients like nitrate, that nutrient versus, you know, gridded squares across a field, so think about water movement. That's how you can think about those mobile nutrients, whereas there's a little bit of a different process for all those other non-mobile nutrients to think about where they are located across the field. Nutrients to think about where they are located across the field. To wrap this up, what's important to keep in mind is that not all nutrients are created equal. Some are mobile and move with the water, like chloride, nitrate, sulfate and some are non-mobile and they stay pretty tight to the soil, like potassium and phosphorus, like potassium and phosphorus. Tune in next time for a tiny bite of knowledge from GK Technology, where we have a map and an app for that.