¶ Mobile vs Non-Mobile Nutrients in Soil
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 ?
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 ?
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 .
What is a mobile and what is a non-mobile nutrient ? Help me out .
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 ?
would some nutrients not move with the water ? What causes ?
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 .
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 ?
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 .
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 .
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 . 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 .
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 .
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 .
¶ Movement of Nutrients Across Landscapes
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 .