Potassium & Soybeans

University of Nebraska – Lincoln Author

09/01/2020 Added

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Dr. Brian Arnall looks at research study results looking at the application of potassium and its impact on soybeans yield.
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[00:00:16.110]Hello, my name is Brian Arnall,
[00:00:17.753]and I'm with Oklahoma State University.
[00:00:20.270]My position is
[00:00:21.120]precision nutrient management extension specialist.
[00:00:24.140]I get to work with all things nutrients
[00:00:25.880]and the crops that we grow here in Oklahoma
[00:00:28.160]and get to play with precision ag.
[00:00:30.410]Today, I wanna talk about potassium
[00:00:32.127]and potassium application in our summer crops.
[00:00:35.750]Of recent, there's a lot of interest in K fertility,
[00:00:38.600]we've seen a lot of symptomologies of potassium deficiency.
[00:00:42.610]There's also a lot of interest and curiosity
[00:00:45.230]about the methods we use for soil tests K recommendations.
[00:00:50.530]I'll say one, we have a lot of history
[00:00:53.210]on soil tests K recommendations.
[00:00:55.380]We understand there are weaknesses in there.
[00:00:57.880]A soil test is just trying to extract via chemical methods,
[00:01:02.110]what the roots can see.
[00:01:03.860]A lot of current research
[00:01:05.000]is evaluating the drying methods we use
[00:01:07.750]going in soil testing lab.
[00:01:09.770]So don't think we're not trying to advance science
[00:01:13.590]when it comes to soil test potassium.
[00:01:16.020]But I will say a lot of the things I learned in the field
[00:01:18.720]as an agronomist and extension specialist
[00:01:21.090]is telling me it's not just the soil test,
[00:01:24.110]which is leading us to the conditions we might see.
[00:01:27.190]Taking a soil sample to give the test recommendations
[00:01:29.530]still has great value as what I see.
[00:01:32.760]One thing to look at it is this,
[00:01:34.690]if the soil test shows low values,
[00:01:38.520]your probability of response is gonna be high.
[00:01:41.500]If your soil test is high,
[00:01:42.900]you're probably relative response is lower.
[00:01:44.850]You need to remember a soil test's value,
[00:01:47.190]when we look at it as a sufficiency range.
[00:01:49.520]For Oklahoma state,
[00:01:50.660]it's 125 PPM soil test K is efficiency.
[00:01:54.430]That means a crop should have enough
[00:01:56.210]to get through its entire lifecycle without a deficiency.
[00:01:59.300]As you go closer to zero, the sufficiency drops.
[00:02:02.440]This isn't a true sliding scale,
[00:02:04.990]this scale, this soil tests index,
[00:02:07.210]it's an indices.
[00:02:08.920]This look at how much is in the soil
[00:02:12.100]doesn't give us an absolute value on return.
[00:02:15.480]It really, I like to think of it as a probability scale.
[00:02:18.960]If 125 PPM is our optimum point or our sufficiency scale,
[00:02:24.990]if I'm at 125% and if I'm going to the Blackjack Tables
[00:02:28.790]or if I'm going to Vegas,
[00:02:30.330]I'm not going to put a lot of money down
[00:02:32.410]that says that if I apply potassium, I will get a response.
[00:02:35.930]Does it mean there is no chance of a response?
[00:02:38.980]No, it does not mean that,
[00:02:41.600]It means the probability is low.
[00:02:43.630]If I'm at 120 remembering 125 is zero 500,
[00:02:47.731]120 PPM does that mean I absolutely will get a response
[00:02:52.010]if I apply fertilizer K?
[00:02:53.500]No, the probability is there
[00:02:57.140]but it's still very low
[00:02:57.973]'cause you're close to that threshold.
[00:02:59.890]It's as we go closer to zero,
[00:03:01.860]moving from 125 PPM to a hundred PPM,
[00:03:05.080]my probability to get an economic response
[00:03:07.690]out of applying potassium fertilizer is increased
[00:03:09.990]as we get to 50.
[00:03:11.630]I'm going to blame money down
[00:03:13.080]that my probability to get increase of,
[00:03:16.360]a economic increase to potassium is going to increase.
[00:03:20.580]As I get near zero, if I'm in single digits,
[00:03:23.340]I going to go to Vegas,
[00:03:24.420]I'm gonna lay all my money down on the tables
[00:03:26.750]and say, you know what?
[00:03:27.583]I'm gonna apply potassium fertilizer
[00:03:29.480]and I'm gonna make money off of it
[00:03:31.350]'cause your probability is extremely high.
[00:03:33.490]So think of it as a sliding scale of probability.
[00:03:36.540]At 150, my probability of response to potassium fertilizer
[00:03:42.530]is not there,
[00:03:43.500]it's above critical threshold, I'm not saying it won't.
[00:03:45.890]If I'm below, it means that my probability is increasing
[00:03:49.560]as the closer I get to zero.
[00:03:51.700]That is a soil test sign.
[00:03:53.900]How do we manage that in Oklahoma?
[00:03:55.630]It's a little bit of a twofold issue.
[00:03:57.600]If I'm at a lower value crop or a lower yielding region,
[00:04:00.750]I'm at sufficiency.
[00:04:02.110]I do not apply if that value is above 125
[00:04:05.157]and my probability is lower return,
[00:04:07.550]I'm looking at a 20 or 30 bushel of wheat crop,
[00:04:09.900]I'm not going to do that,
[00:04:11.260]I'm going to farm as it is.
[00:04:13.240]But however, if I'm growing at 250, 300 bushel of corn crop,
[00:04:17.100]if I'm growing a five bale cotton crop,
[00:04:19.210]at that point,
[00:04:20.250]then I start looking at, okay,
[00:04:21.540]not only am I looking at sufficiency,
[00:04:23.170]I'm looking at replacement
[00:04:24.700]because I'm removing so much potassium.
[00:04:28.500]A soybean crop and a cotton crop
[00:04:30.200]are some of our largest K users.
[00:04:32.470]In fact, a high yielding cotton crop
[00:04:34.250]can remove as much as three to four pounds of potassium
[00:04:37.260]per day, during bolos.
[00:04:39.559]A high yielding soybean crop
[00:04:40.810]can be pulling one to two pounds very easily
[00:04:42.810]during peak fill time.
[00:04:45.380]So we have some high demanding crops.
[00:04:49.560]Recommendations is still pre-plant incorporated,
[00:04:52.280]get it in there or pre-plant broadcast
[00:04:54.620]and no till we get good incorporation.
[00:04:58.640]The place I'm standing right now though,
[00:05:00.120]go into effect about what we can do in season.
[00:05:04.100]So right now I'm standing in the alley
[00:05:06.490]of two trials, of two plots.
[00:05:08.630]This is trial for Dr. Loughlin,
[00:05:09.990]our cropping system specialist,
[00:05:11.470]looking at residue management in double crop soybeans.
[00:05:14.880]I look at these
[00:05:15.940]and you see in my ally that I'm standing in,
[00:05:19.050]we have significant potassium deficiencies right here
[00:05:22.330]and go to either side,
[00:05:24.450]the plots do not show me very much potassium,
[00:05:26.640]you can find small symptomology.
[00:05:28.950]That test and deficiency that we're looking for
[00:05:31.790]in these plants,
[00:05:32.980]you have the margins of the leaf
[00:05:36.010]that have a yellowing cirrhosis,
[00:05:38.380]you have the margins of the older leaves
[00:05:40.270]that are actually turning to chronic.
[00:05:41.650]Necrosis means you have death of tissue,
[00:05:44.260]so the more you have that impact,
[00:05:46.380]the more tissue death you're actually seeing.
[00:05:48.750]On the newer growth you'll see the chrosis
[00:05:51.000]and you get up here on the really new growth
[00:05:53.290]and you see just a slight yellowing of that leaf margin,
[00:05:57.470]that's very classic potassium deficiency.
[00:06:00.380]But if we go over into the plot area or near the plots,
[00:06:03.230]we see that, you know, we have nice, good leaves,
[00:06:07.610]we have very, no symptomology right on this plant
[00:06:10.360]of potassium deficiency.
[00:06:12.000]What was the difference?
[00:06:13.280]Well, the interesting thing about these plots
[00:06:14.990]is that we'd watch it progress
[00:06:16.630]from early vegetative stages.
[00:06:18.890]We'd seen it go, everything looked good
[00:06:20.850]and in a three day timeframe
[00:06:22.530]the crop went from looking perfect,
[00:06:25.170]to potassium deficiency symptomology around are one.
[00:06:28.950]We were able to get out,
[00:06:29.850]our loft was able to get out within a couple of days
[00:06:32.160]and apply 0060 over the top with rainfall.
[00:06:35.880]So this crop on either side of me,
[00:06:37.860]if you look right or you look left,
[00:06:40.070]you're going to see a nice green crop.
[00:06:42.410]It's because of a side dress potassium application of 0060,
[00:06:46.340]just dry fertilizer.
[00:06:47.660]Within a matter of five days,
[00:06:49.180]those potassium deficiencies went away,
[00:06:51.460]we completely recovered,
[00:06:52.760]which means just because you see a K deficiency in a field
[00:06:56.390]does not mean you're behind
[00:06:58.360]and it doesn't mean that you have to go
[00:07:00.190]and do something extreme with a very high cost product.
[00:07:03.180]Dry potash worked well here.
[00:07:05.870]Now I wanna talk about these plots a little bit more
[00:07:08.130]and talk about what we're seeing.
[00:07:09.900]The soil test in this site is a little low.
[00:07:12.050]We're at about 110 PPM potassium,
[00:07:15.300]which means yeah, the probability of response, it's there
[00:07:19.130]but not extremely high.
[00:07:21.070]What I see in this field
[00:07:22.650]is a combination of what I'm seeing on a lot of fields
[00:07:25.260]in Southern Great Plains,
[00:07:26.620]is that we have a combo of soil by plants.
[00:07:29.750]You gotta remember our soil test,
[00:07:31.200]that 110 PPM at this location
[00:07:33.510]is looking at what could the plant physically get near,
[00:07:36.150]what's available by the soil?
[00:07:38.160]It does not tell us one extremely important aspect.
[00:07:41.370]Potassium is still a relatively immobile nutrient
[00:07:44.860]and the plant has to seek it out via of its roots.
[00:07:48.360]What I believe we're seeing on this field
[00:07:50.090]and many other fields in Oklahoma,
[00:07:52.180]is the fact that we have stunted roots
[00:07:54.250]for one condition or the other.
[00:07:55.980]In this field if we dig down,
[00:07:57.564]I'm gonna dig down and look at these roots,
[00:08:01.550]take a look what they look like
[00:08:03.376]and I'm going to suspect
[00:08:05.720]that this root of this deficient plant,
[00:08:08.320]is not gonna be what we want.
[00:08:09.610]I'm getting a restrictive layer already,
[00:08:11.420]I'm seeing how it lifts up with this no till,
[00:08:13.680]we have a restrictive layer that.
[00:08:17.503]Get in here, pull this plant up.
[00:08:21.670]To be honest, this is a perfect example.
[00:08:25.410]We have a root that went down that seed
[00:08:27.710]and it's going sideways.
[00:08:29.200]We do not have a lot of exploration,
[00:08:31.900]we do not have a lot of fibrous roots to get into that K.
[00:08:36.240]So this is, I see this very, very often
[00:08:38.830]is if we have a clay film on the seed wall
[00:08:42.880]or something where it's going down
[00:08:44.630]and what you see as this root goes down
[00:08:46.840]and it traces the row,
[00:08:49.410]so it's following that row restriction roots.
[00:08:51.290]So, even if we had enough potassium out here,
[00:08:55.230]we're getting deficiencies because the roots
[00:08:57.220]can't access it.
[00:08:59.440]Let's go look at another place in the field
[00:09:01.570]where it had some extra nitrogen
[00:09:03.640]and it's growing better than this bulk areas
[00:09:05.330]that was limited.
[00:09:06.650]So as I moved to another part of the field,
[00:09:08.460]this is an experimental field of ours,
[00:09:10.450]that we manage the inside.
[00:09:11.900]So a lot of the area that we just came from,
[00:09:13.750]was very nitrogen depleted.
[00:09:15.890]Over in this area you see these plants,
[00:09:18.580]bigger plants, healthier plants are growing well,
[00:09:21.470]this is not heavy in potassium.
[00:09:23.410]This had early season nitrogen on the wheat crop
[00:09:26.240]and so we have residual and what we get is a healthier plant
[00:09:29.760]with bigger, better roots.
[00:09:31.670]And so I truly believe in this field
[00:09:34.430]that we have an impact of residual nitrogen,
[00:09:37.870]impacting the soybeans,
[00:09:38.990]allowing them to grow better, have better rooting systems,
[00:09:42.610]rooting structure, getting better access to the potassium.
[00:09:46.660]Again, I'm saying this over and over again,
[00:09:48.540]as we start digging into
[00:09:49.980]and discussing this potassium issue in the field.
[00:09:53.270]Are our soil tests perfect?
[00:09:55.160]Absolutely not, our soil test are not perfect.
[00:09:57.960]We're using chemistry to estimate biology.
[00:10:01.090]We're doing the best we can with that,
[00:10:02.860]it gives us a great starting point.
[00:10:04.430]If you're extremely low,
[00:10:05.840]you know, you need potassium.
[00:10:08.410]But what we can't forget about
[00:10:10.590]is the physiology and biology
[00:10:12.180]of the plants we're working with.
[00:10:13.780]Are we working with plants
[00:10:15.570]that are not able to put on enough root
[00:10:17.410]to support the biomass?
[00:10:18.850]In a cotton crop,
[00:10:20.030]what we see as we go into higher yielding cotton,
[00:10:22.990]as the demand for potassium in a four
[00:10:25.200]or five yield cotton crop goes to three and four
[00:10:27.210]and five pounds of K per day.
[00:10:30.260]Then that crop when we have a K stress
[00:10:32.760]and such a heavy blooding period,
[00:10:35.020]that crop cannot access enough potassium
[00:10:38.110]to support that bolo.
[00:10:39.890]I also see it in big yielding, high yielding beans,
[00:10:43.330]80, 90 bushel beans.
[00:10:44.550]If we have any restriction in root growth whatsoever,
[00:10:48.060]we start seeing potassium deficiencies
[00:10:50.030]especially during the time when we started having pod fill
[00:10:52.740]and the K demands of that crop start increasing.
[00:10:55.690]So, just because you have a high soil test potassium value,
[00:10:59.560]and you're getting K flashes and K deficiencies,
[00:11:02.970]it does not mean per se,
[00:11:05.680]that we are doing wrong on soil testing.
[00:11:08.730]It might be that their crop just can't access
[00:11:11.330]the potassium that's in the soil, the soil test seas.
[00:11:14.120]We're learning more about this,
[00:11:15.740]we're looking more to end season applications,
[00:11:17.780]we're looking to see how we can take advantage of this
[00:11:20.540]and how we can treat this economically,
[00:11:23.130]to maximize yield and maximize nutrient efficiencies.
[00:11:27.020]Again, I'm Brian Arnall with Oklahoma State University.
[00:11:29.460]I appreciate your time.

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Potassium & Soybeans

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