25 - 2020 Soybean Management Field Days - Soil Water Use Considerations with Cover Crops in Irrigated Soybeans
Soil Water Use Considerations with Cover Crops in Irrigated Soybeans - Aaron Nygren, Nebraska Extension Educator, Cropping Systems discusses the following: Wetter soil profiles typically seen in irrigated fields may present an opportunity for cover crop use to reduce deep percolation of water and nutrients. He also looks at water use by a cover crop in an irrigated soybean system in the eastern half of Nebraska is likely to be replenished by rainfall during May and early June in most years.
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[00:00:06.889]This is Aaron Nygren with Nebraska Extension.
[00:00:09.540]Wanted to talk a little bit about soil water use
[00:00:11.580]considerations with cover crops.
[00:00:13.790]So the stored soil water, that's often just been one
[00:00:17.080]of the costs that we've associated with cover crops.
[00:00:19.000]You know, it's a plant, it's gonna use some water.
[00:00:21.510]We've been doing some research that says
[00:00:22.910]that maybe that isn't as big of a cost,
[00:00:24.690]especially when we look at an irrigated system.
[00:00:27.530]So with Nebraska being the leading nation
[00:00:29.740]or leading state in the nation for irrigation,
[00:00:31.920]that's something we really need to focus on
[00:00:33.430]in our soybean irrigated system.
[00:00:36.099]So one reason for this is that it's pretty easy
[00:00:39.050]to leave those fields fairly wet
[00:00:40.440]at the end of the season.
[00:00:41.492]We talk about trying to dry them down,
[00:00:43.326]but it's just a challenge
[00:00:44.550]with irrigation management
[00:00:45.530]to really get that soil dried down.
[00:00:47.410]So we store a little bit of extra water there.
[00:00:50.140]One of the side effects of that is that we leave
[00:00:51.740]very little room for off season precipitation.
[00:00:55.190]And then at the same time with that,
[00:00:56.430]we get a lot of runoff and we also get some deep percolation
[00:00:58.980]of water through the system.
[00:01:01.433]Now, if we looked at a dry land system,
[00:01:03.500]that would be a different story.
[00:01:04.830]So that with this talk, we really wanna focus more
[00:01:07.026]on the irrigated system.
[00:01:08.689]So dry land fields we typically will dry those down
[00:01:11.415]quite a bit more
[00:01:12.248]especially even down to a six foot depth,
[00:01:14.230]which we're not gonna do very often
[00:01:15.470]with an irrigated system.
[00:01:17.364]So when we talk about soil water,
[00:01:19.630]we have to really think about how that moves
[00:01:21.160]through the soil.
[00:01:21.993]So it seems common sense to say
[00:01:23.920]that water recharges from the top to the bottom.
[00:01:26.770]But you know, we don't always think of that.
[00:01:28.294]So if we look at the top foot, if we have that top foot
[00:01:31.094]above field capacity,
[00:01:33.100]we're gonna start moving some water that excess water
[00:01:35.360]above field capacity is gonna leach
[00:01:36.690]down to the second foot.
[00:01:39.230]You know, it just gonna move down over time.
[00:01:40.792]So if we have these feet, top two feet above field capacity,
[00:01:44.825]we can move quite a bit of water.
[00:01:47.060]So the range is anywhere from 0.09 to 10 plus inches
[00:01:50.890]of water per day, that can actually deep percolate
[00:01:53.315]and drain through that soil.
[00:01:55.130]So that's a considerable amount.
[00:01:57.660]Now, if we think about a typical soil,
[00:01:58.994]eventually we might have something that's maybe
[00:02:00.910]at field capacity.
[00:02:01.990]So in this case, our third foot is at field capacity.
[00:02:05.500]That's still gonna move some water,
[00:02:06.651]but maybe that's down about 900 of an inch per day.
[00:02:10.170]So that helps us out a lot.
[00:02:11.795]Ideally though what we would have as somewhere
[00:02:14.130]we would have a dryer layer.
[00:02:15.680]So if we have something at 50% of available water,
[00:02:18.239]that really slows down our deep percolation
[00:02:20.900]down to an inch or less per year.
[00:02:22.850]So think about trying to manage your soils
[00:02:25.350]in ways that gives you some dried layer
[00:02:27.200]in there to help reduce that percolation.
[00:02:30.488]At the same time,
[00:02:31.910]if we think about one that's completely filled up
[00:02:33.416]or you're the guy that cusses a rain
[00:02:36.195]because, you know, he'd kept it so wet
[00:02:37.940]with irrigation that it's just running off the field.
[00:02:40.273]If we have that completely wet all the way down
[00:02:43.196]that water is just gonna keep moving down.
[00:02:46.010]So here we have all four feet above field capacity.
[00:02:49.690]We're gonna deep percolate a lot of water.
[00:02:52.270]The downside of deep percolating
[00:02:53.610]all that water is any nutrients that are mobile
[00:02:55.490]like nitrogen or sulfur,
[00:02:57.100]are also gonna move with that soil water as well.
[00:02:59.372]So that's what we wanna avoid in Nebraska
[00:03:02.080]because we do have some issues with high nitrates and areas.
[00:03:05.250]So this is something with irrigation management
[00:03:06.597]if we could cut down on moving that much water
[00:03:09.850]through the soil
[00:03:10.683]that would really help us out.
[00:03:13.380]So this is our recommendation.
[00:03:15.430]We always recommend at the end of the season
[00:03:16.930]to try to dry down an irrigated field
[00:03:19.470]to 40% available water.
[00:03:21.398]We feel safe with that,
[00:03:22.472]that dragging it down to that level
[00:03:24.470]isn't gonna hurt your yield.
[00:03:25.950]And that would give us as much utilization
[00:03:27.940]of that water that we have
[00:03:28.870]in the profile to finish that crop out
[00:03:31.730]also which would help our economics by reducing pumping.
[00:03:35.460]So if we would reach this target, which I said is hard to do
[00:03:37.957]we would have some pretty good storage left
[00:03:41.240]in that field.
[00:03:43.110]So on a sand we'd have about 2.4 inches
[00:03:45.150]that we could store a loam 4.8,
[00:03:47.390]or a silty clay Loam of 4.3.
[00:03:50.400]Dry land field again, remember is gonna be vast
[00:03:52.760]drastically different than this.
[00:03:54.060]So we may have dried that down
[00:03:55.129]you know, a lot farther all the way down to six foot.
[00:03:58.760]So we would have considerable more storage there.
[00:04:01.860]So looking at these amounts, if we look at what kind
[00:04:05.280]of rainfall we get, it's definitely a challenge, okay?
[00:04:08.683]So we would have room, you know, up to 5.5 inches
[00:04:11.750]even on silt loams.
[00:04:14.010]We just don't usually get it that dry.
[00:04:15.530]So if we only have two or three inches of
[00:04:18.260]off-season to store off-season precipitation,
[00:04:21.333]what's gonna happen with all that excess water.
[00:04:24.080]So in a minute here
[00:04:25.150]we'll look at how much off-season precipitation we get
[00:04:28.320]and keep in the back
[00:04:29.153]of your mind that we only have maybe room for two
[00:04:31.080]or three inches of that in a lot of irrigated fields.
[00:04:35.290]So here we have the site that we're out here at Shelby.
[00:04:38.420]If we look at that monthly total precipitation,
[00:04:41.240]you know on the average for the season where the entire year
[00:04:44.050]we're about 28 inches a little over.
[00:04:46.720]But the main months that we wanna focus on
[00:04:48.540]is the off-season pre sip.
[00:04:50.090]So October through May.
[00:04:51.740]You may say, why May?
[00:04:53.240]We plant our crop in May, right?
[00:04:54.650]Well, remember that crop is pretty small.
[00:04:56.501]We're not using a lot of water,
[00:04:58.010]so we still get more rainfall
[00:04:59.350]than we have the crop actually use.
[00:05:01.700]So if we look at that, we have a total here
[00:05:03.590]of about 14.75 inches of water
[00:05:06.760]coming in the off-season.
[00:05:08.590]Now, if we only have room for two or three inches
[00:05:10.410]you can quickly see where we have some runoff
[00:05:12.049]or we have some deep percolation happening
[00:05:14.320]on some of these irrigated fields.
[00:05:17.391]So the four sites that we have this year are fairly similar.
[00:05:21.183]So Arlington is around 15 inches
[00:05:22.906]and then off-season 14 and a quarter
[00:05:25.640]or 14 and three quarter in the Shelby area
[00:05:27.220]like I mentioned.
[00:05:28.258]Elgin around 14.25, and then 13 inches in the Hildreth area.
[00:05:33.610]This is only looking through October through May.
[00:05:36.950]And a lot of situations we could also include
[00:05:38.668]at least probably half of June rainfall as well
[00:05:41.380]because we still have a fairly small crop.
[00:05:43.850]June is typically our highest rainfall month in Nebraska.
[00:05:46.630]So we may have excess precipitation
[00:05:48.710]that's gonna also help refill that soil profile.
[00:05:53.365]Now we know that not all of that's gonna run in,
[00:05:56.033]you know some is gonna run off,
[00:05:57.630]some is gonna evaporate naturally.
[00:05:59.390]So we can't count on all of that.
[00:06:01.310]But even if we think about capturing half of that,
[00:06:04.456]at six and a half to seven
[00:06:05.920]half inches of water, so we could easily deep percolate
[00:06:08.899]two or three or more inches on multiple fields
[00:06:12.080]across the area.
[00:06:14.570]So the take home points on this talk,
[00:06:16.080]remember that any water that is deep percolated
[00:06:18.594]is gonna move nitrates
[00:06:20.350]and other nutrients deeper into the soil profile
[00:06:22.440]which is something we wanna avoid in Nebraska,
[00:06:24.890]with some of the high nitrate areas
[00:06:26.160]that we have across the state.
[00:06:28.120]At the same time
[00:06:28.953]it may be advantageous to have
[00:06:30.440]that cover crop using the water from the profile instead.
[00:06:33.620]It's not only gonna help us out
[00:06:34.810]with our deep percollation
[00:06:36.000]but we also gonna get some soil health benefits as well.
[00:06:39.320]And the water that we used by the cover crop
[00:06:41.330]is likely to be replenished by rainfall
[00:06:43.220]during the month of May and early June,
[00:06:45.520]since that's typically when
[00:06:46.560]we have the highest rainfall amounts
[00:06:47.734]and the crop water use is fairly low
[00:06:50.130]in those months as well.
[00:06:51.900]So the next video we'll focus on planning considerations
[00:06:54.910]with cover crops and irrigated soybeans.
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