It’s clear that there are a lot of farmers who consider soil science to be an intimidating or highly technical topic, and it can be. And I think we realize that, for most farmers, there's so many things to keep track of these days as far as seed genetics and herbicide programs and so forth. 

Nutrient Management Specialist / University of Minnesota Extension
Carlson brad
Extension Educator / University of Minnesota Extension
Soil Scientist / University of Minnesota

Three members of the University of Minnesota Extension’s nutrient management team recently released a podcast episode discussing four common soil test myths that are important for producers to be aware of. Guests on the podcast included Dan Kaiser, nutrient management specialist at the University of Minnesota; Carl Rosen, soil scientist at the University of Minnesota in the department of soil, water and climate; and Brad Carlson, extension educator at University of Minnesota Extension. The podcast was hosted by Jack Wilcox, communications generalist at University of Minnesota Extension.

Myth No. 1: The Mehlich-3 test is a better test for farmers to consider

KAISER: That's one question I get from particularly a lot of state agencies because across the country, particularly the NRCS, would like to see a singular soil test being used in all states across the country. And the Mehlich-3, if you look at it, you look at all the options we have, we recommend either the Bray P1 or the Olson test as we have a significant amount of information really to back those up in terms of that they predict crop response. The Mehlich-3, what it's meant really to do is to be a test that you could use across soils, particularly going from acidic to calcareous soils, and there's some advantages to that. The main issue though is we haven't been able to get it to work on all the soil types.

ROSEN: The Mehlich test is just one of many tests that can be used. It's what we call more or less a universal extraction, and the one advantage of that is that you can analyze all the essential nutrients at once, particularly phosphorus, potassium and some of the micronutrients. That's the one advantage of it. But you run into the same problems mentioned already with high calcareous soils. You're still going to end up, in most cases, needing a separate analysis on those particular soils. I think the bottom line is that you need to know what the test is that you're running and you have to be sure that you know there's good correlation with what those numbers actually mean.

Myth No. 2: I can predict my nitrogen requirement with the cation exchange capacity test

CARLSON: The cation exchange number is a part of the soil test report. Farmers have long seen that number and thought, "Well, what do I do with that?" It's an index that correlates very strongly to the amount of clay in your soil. Clay particles are negatively charged, and therefore the more clay there is, the more ability there is to hold cations. We actually have separate nitrogen recommendations just simply based on soil texture.


KAISERWell, and that's one of the things too, if you look at a lot of the early information regarding this. I mean the majority of it or all of it was geared towards anhydrous. If you're looking at any of your other nitrogen sources, I don't think there's a whole lot of bearing there. Urea is completely different. You apply urea, its neutral molecule, it isn't held on your soil and it won't be potentially held until it converts to ammonium. At that point, if things are going well, it's going to essentially convert over to nitrate relatively quickly.

Myth No. 3: K base saturation is the better way to predict potassium

ROSEN: Normally we base our potassium recommendations on the amount that's extracted. The amount that's extracted is expressed as parts per million. This can be converted to an equivalent or milli equivalents per hundred grams, which is what the saturation is based on. And the cation exchange capacity is what we call the total amount of cations in that soil that the soil can hold on to. The K base saturation is what percentage of that cation exchange capacity is occupied by potassium. There are recommendations that have been made on that. It usually is hard to adjust your base saturation to any large degree. It also comes into the myth about having an ideal cation saturation ratio between potassium, calcium and magnesium, and trying to adjust those gets very difficult.

Usually, it's not related to yield in most cases. Our recommendations are primarily based on the amount there in the soil in parts per million as opposed to milli equivalents per hundred grams or the amount that's occupied relative to the total amount of potassium in the soil. It's not a very good predictor in most cases. If you start trying to adjust these ratios, it can be very expensive, so we don't use it in our recommendations. There are some labs that report it, but they'll also report parts per million. In most cases, the recommendation is based on parts per million potassium, not on the base saturation of potassium.

KAISER: I think the myth here is that you need, on some of our high clay soils, a 2% K base saturation – are just hard to get that to move with the amount of other nutrients that are in the soil. You've got to put a lot of potassium on, more than what it's going to be economically feasible or where it's going to be economically justifiable for the crop. I've been starting to look at some of my data where we can get some CEC values to look at the base saturation just to see where it is. And if you look at the majority of the data, I've got a lot of nonresponsive sites that are anywhere from 1% to a 2% K base saturation and that's what a lot of people see. If you look at a lot of the research around that, there just is no relationship to it.

Myth No. 4: I need to run an analysis of all micronutrients

KAISERThe problem with all that data is not all of it really has any meaningful value to it. One of the issues. If we don't have a good correlation or any good response trials for micronutrients, getting a value back really is pointless because a lot of times you'll get a number back. Maybe a lab will report it as low, which I don't exactly know where they get their information from. The saying that it's low really doesn't mean anything.

ROSEN: For some micronutrients, just the simple pH measurement might tell you more about potential availability than the actual number that you get with a micronutrient extraction, for example, iron. If you're in highly alkaline and calcareous soils, that's all you really need to know that you're probably going to run into some iron issues, particularly if you're growing soybeans. The other micronutrients, manganese and copper, again, mostly are important on organic soils, high organic matter soils, that might be a place where you would want a test there. And high pH will also indicate potential manganese problems as well. Those are things that we look at in the test. Getting an actual number may or may not mean anything for some of these tests. And pH may actually be more important.

Final thoughts

KAISER: Well, I just want to reiterate, when you're getting a report back, just don't get too much. It's really, I think, the easiest way to do it, is to simplify things down and get only what you need because it can be really confusing if you get too much on a soil test report.

ROSEN: The best recommendations are going to come from research that's conducted locally or regionally. And that's where you should be getting your information from.

CARLSON: I'll just wrap with, don't overcomplicate this. It's just simply not that difficult for the vast majority of farmers.